POTATO

BLIGHT

EPIDEMICS

Throughout the World

Agriculture Handbook No.174

UNITED STATES DEPARTMENT OF AGRICULTURE
POTATO

BLIGHT

EPIDEMICS

Throughout the World

By A. E. COX and E. C. LARGE

Plant Pathology Laboratory

Ministry of Agriculture, Fisheries, and Food

Harpenden, England

Agriculture Handbook No. 174

Published by

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

Washington, D. C: T^ ir»/n ^ ' ^" January I960

For sale by the Superintendent of Documents, Government Printing Office, Washington 25, D. C. Price $
FOREWORD

Among the important diseases of miajor food crops the late blight of potato is

one of the most difficult for us to understand and combat. Despite the intensive

studies that have been devoted to it, the conditions for blight occurrence are still

not thoroughly known. Frequently, a severe unexplained outbreak of the disease,

or its appearance in a region where it was previously unknown, revives problems

that had been considered already solved or raises new questions.

Observations wherever the disease appears throughout the world contribute

knowledge for better understanding and solution of these problems. This worldwide

account of recent epidemics of potato late blight provides a useful background of

epidemiological information. This Handbook is the first review of conditions associated with late blight outbreaks throughout the world. Its length is due to the

inclusion of the detailed information that is necessary to permit comparison

between regions and correlation with climatic data. The detail cannot be omitted

without lessening the precision and therefore the usefulness of the data.

We gladly acknowledge our indebtedness to the authors who undertook this

arduous task, and to others of the Plant Pathology Laboratory, Ministry of Agriculture, Fisheries, and Food, Harpenden, England, who made this study possible.

The inception of this collaborative effort was a joint consideration between the

Crop Plant Disease Forecasting Project of the Crops Research Division, Agricultural Research Service, United States Department of Agriculture, and the Department of Plant Pathology, University of Wisconsin, Madison. The program was

administered by the Wisconsin Alunrini Research Foundation with funds for conduct

of the work provided by the United States Government.

The results of this extensive study should be useful to plant pathologists

throughout the world and especially to the many plant pathologists of the Crop

Plant Disease Forecasting Project. It is hopedthatthe review will provide a basis

for additional studies on the impact that plant diseases have on crops and the subsequent economic conditions in various parts of the world.

^'^X "Z^X O^^^C^^^^fy^Cd^ '.(¿.^>Ly

Crops Research Division

Agricultural Research Service

U. S. Department of Agriculture

n
PREFACE

This study was made possible by the Wisconsin Alumni Research Fovindation, U.S.A., which

administered funds for the purpose. Through the efforts of A. J. Riker, University of Wisconsin,

who was especially interested in assembling world data about important epidemic plant diseases,

the authors were able to undertake a comprehensive review of recent epidemics of potato blight

throughout the world. A. E. Cox, agriculturist, and E. C. Large, plant pathologist. Plant Pathology

Laboratory, Ministry of Agriculture, Fisheries, and Food, Harpenden, England, under the direction of W. C. Moore, of that Laboratory, worked jointly in making contacts with specialists, in

assembling the data, and in preparing the report. The maps and diagrams were prepared by V. J.

Pearson, of the above-mentioned Laboratory.

The material on which this world study of potato blight has been based was derived in part

from published literature, but to a far greater extent it has been based upon unpublished information generously provided by leading workers in different countries and from England and

Wales, verbally or in correspondence withthe author s. The help sought by the authors has always

been most readily given and has varied greatly in kind. Special acknowledgments are nnade to

the library of the U. S. Department of Agriculture and to the following specialists:

A. Angus, Northern Rhodesia

G. R. Bates, Southern Rhodesia

F. Beran, Austria

W. Black, Scotland

R. Bonde, U. S. A.

P. M. A. Bourke, Irish Republic

C. J. Briejer, Netherlands

L. C. Callbeck, Canada

L. Caudri, European Plant

Protection Organization

H. T. Cook, U. S. A.

J. C. CuUen, England

H. M. Darling. U. S. A.

H. Darpoux, France

D. Delaney, Irish Republic

R. W. Dickey, U. S. A.

CM. Driver, New Zealand

H. Dubniak, Poland

P. Dumas, France

A. H. Eddins, U. S. A.

C. J. Eide, U. S. A.

S. Fish, Australia

C. E. Foister, Scotland

E. F^rsund, Norway

S, Fridriksson, Iceland

G. P. Georghiou, Cyprus

J. Grainger, Scotland

P. Grainger, Canada

E. Gram, Denmark

E. G. Gray, Scotland

M. Guntz, France

J. M. Hirst, England

D. Hoctor, Irish Republic

J. C. F. Hopkins, England

J. G. ten Houten, Netherlands

J. L. Howatt, Canada

W. G. Hoyman, U. S. A.

H. Johannes, Germany

P. Journet, France

M. A. Keay, Uganda

W. E. Kerr, Southern Rhodesia

D. Lihnell, Sweden

M. M. de Lint, Netherlands

T. P. Mclntosh, Scotland

R. McKay, Irish Republic

C. J. Magee, Australia

P. R. Miller, U. S. A.

A. E. Muskett, Northern Ireland

R. M. Nattrass, Kenya

J. S. Niederhauser, Mexico

L. C. Peterson, U. S. A.

J. E. van der Plank, South Africa

I. Polykov, U. S. S. R.

H. N. Racicot, Canada

H. Richter, Germany

E. A. Riley, Tanganyika

K. Scheibe, Germany

C. Bazan de Segura, Peru

J. H. Simmonds, Australia

P. W. Smallfield, New Zealand

J. D. Tallantire, Cameroons

H. J. Toxopeus, Netherlands

J. Ullrich, Germany

R. W. Vasudeva, India

E. Ventura, France

G. C. Wade, Australia

R. W. Wallace, Northern Ireland

J. R. Wallin, U. S. A.

J. M. Waterston, Cameroons

W. Welvaert, Belgium

V. E. Wilkins, European Plant

Protection Organization

m
CONTENTS

Page

INTRODUCTION 1

Potatoes as a World Crop 3

Some Factors Affecting Potato Yields 5

Life History of the Fungus on Potatoes 7

**Blight Weather'' -- 9

THE BRITISH ISLES 10

England and Wales - 10

Scotland 36

Northern Ireland 4-3

Republic of Ireland 48

CONTINENTAL EUROPE 60

France 60

Belgium 67

The Netherlands 71

Denmark 85

Iceland 89

Norway 92

Sweden 95

Western Germany 99

Austria 107

Poland 109

Union of Soviet Socialist Republics - - m

Cyprus 115

NORTH AMERICA 118

United States of America 118

Canada 140

Mexico 151

SOUTH AMERICA - 155

Peru -- 155

Chile 160

AFRICA 166

Union of South Africa 166

Southern Rhodesia 170

East Africa 171

Kenya - 172

Uganda 175

Tanganyika 177

Northern Rhodesia 177

West Africa 178

Page

ASIA- - - 181

India - 181

AUSTRALASIA 185

New Zealand - -- 185

Australia - 191

CONCLUSIONS - 196

The Definition of Climatic Conditions 196

Effect of Mean Temperature on Potato Cropping 196

Effect of Rainfall on Potato Growing - I96

Definition of Blight Epidemics I96

Determination of the Effects of Blight

Epidemics on Yield I97

Frequency of **Blight Years" in Different Regions I97

Incidence of Blight and Weather Conditions 198

The Duality of the Disease 199

The Disease in the Tubers 199

Compensatory Effects on Yield of

Blight Epidemics and Rainfall 199

Economic Effects of Blight Losses:

Tropical and Subtropical Regions - - 200

Economic Effects of Blight: Main

Potato-Growing Countries 200

Course of Establishment of Blight in

New Areas 201

Carryover of Blight From Season to

Season 201

Effect of Varietal Resistance in Modifying Blight Epidemics 202

Potato Blight Forecasting 202

Protective Spraying: Materials 203

Protective Spraying: Extent of Use- - 203

Protective Spraying: Effect on Control

of Blight in Tubers 204

Haulm Destruction 204

Control by *'Disease-Escape'* Methods 205

Indications of Subjects for Further

Inquiry 205

LITERATURE CITED 207

INDEX 220

IV
INTRODUCTION

Over a century has now passed since the

potato late blight fungus, Phytophthora infestans (Mont.) de Bary, found its way from

the New World to the Old, and first ravaged

the potato crops of Europe. Since 1845 the

fungus has been the subject of innumerable

studies: The commonly occurring stages of its

life history and the phases of its parasitism

upon its solanaceous hosts are now well known;

but, like the true cryptogam that it is, it still

contrives to conceal much from human scrutiny. After more than a century of search its

sexual reproduction by means of oo spore s

has only recently been found occurring on potato plants in the field--in Mexico. The means

by which it succeeds in producing new races

or strains elsewhere in tíie apparent absence

of a sexual phase remains a subject of disputation. The relations between sporulation

and spread of the fungus and the microclimatic

conditions within potato crops are now under

active investigation but are still very imperfectly known; real progress in blight forecasting has been made in recent years, but

forecasting is still upon an empirical basis.

A vast expense of money and labor goes each

year upon control measures, but our debt to

the partial blight resistance of some varieties

cannot be realized and the rationale of spraying in relation to the frequency and nature of

the epidemics is still imperfectly understood.

Of all the mysteries of potato blight that

will surely be engaging agriculturalists, plant

pathologists, and plant breeders for generations yet to come, perhaps the greatest, and

certainly the most economically important,

is the real extent and nnagnitude of the losses

that the disease causes among the potato

cultivations of the world. There is an intrinsic

difficulty in measuring the incidence and the

effects of any plant disease over extensive

areas, and the branch of plant pathology concerned with such work is still in its infancy.

It is only in very few countries of the world

. that there is sufficient field survey information to define the courses of the blight epidemics or to support even approximately

accurate estimates of the losses they cause.

At present, very little precise information is

[ available.

The objective of this study is (1) to explore

as far as possible the findings in those countries where special surveys have been nnade;

(2) to draw attention to the methods used; and

¡3) then to apply the results in principle for

, "he interpretation of such scattered informaion as is available in other countries, to

Provide the beginnings of a mid-twentieth cenury conspectus for the world. It cannot be put

ligher than that; a little has been done, more

emains to be done, and it will have to be done

n the field.

It would be idle ^:o attempt to de s cribe potato

blight epidemics except against the background

of the distribution and climatic requirements

of the potato cultivations in which they occur.

To do this for all countries, and for all the

differing regions in countries extending across

whole continents, could not be profitably attennpted in a couple of years. This study is

therefore confined to a limited number of

countries, and regions within those countries,

which among them probably experience most

of the types of epidemic that occur in the world.

For each country studied a fairly full section

or case study was compiled, after first setting

forth some very brief general observations on

the potato as a world crop, and on the life

history of the fungus, for the assistance of interested readers who maybe neither professional agronomists nor plant pathologists.

An indication of the world distribution of

potato growing is given in figure 1. In general,

principal areas of production were chosen for

the closest study. However, in view of the

recent introduction of potato blight intoanunnber of countries where it was previously unknown--a second wave of world dispersal

related to the speeding-up of communications,

expansion of agriculture and commerce, and

troop movements of this twentieth century--

some countries where relatively few potatoes

are grown but where interesting or alarming

epidemics have occurred were included. Some

countries are also included to exemplify limiting conditions both for the crop and for the

occurrence of the disease. A map showing the

world distribution of Phytophthora infestans

on potatoes is already available (Commmonwealth Mycological Institute (C.M.I.), 1954;^

the distribution of the fungus is now for all

practical purposes coincident with that of the

potato itself. Serious losses from the disease

on outdoor tomatoes have occurred in recent

years iñ many countries, but the world distribution and present status of tomato blight

has already been investigated (Miller and

O'Brien, 1955), and the disease on tomatoes

is referred to only where it is of particular

importance as a link in the chain of infection

of potato crops.

In the section on each country the extent

and distribution of potato growing, the importance of the potato in the country's food

supplies, the principal varieties grown, the

nature of the cultivations, and the times of

planting and harvesting are given first. Then

the cropping periods are related to the longterm average rainfall and average daily mean

temperature for each month of the year at

^ References to Literature Cited (p. 207) are herein indicated by the

name of the author (or authors), followed by the year of publication.
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representative weather stations. For this purpose a number of small **climagrams" were

constructed to show up as clearly as possible

similarities and differences in widely separated parts of the world. The explanatory

notes on the climagrams given in figure Z,

are important not only in helping to define

cropping conditions, but also, later, in revealing the mean temperatures and rainfall

associated with the several types of blight

epidemic. These climagrams show only the

mean temperature line. The diurnal range

of temperature also affects cropping and the

incidence of blight, but this and other climatic

factors are discussed in the text. Fuller

relevant information on climate is available

inKendrew (1937).

No attempt is made in the climagrams to

portray humidity conditions, although low temperature s in relation to latitude often reflect

the modifying effects of cloud and coastal or

mountain fog. Very detailed humidity observations at frequent intervals during the day

are needed to reveal the actual incidence of

**blight weather." For those few countries

where such observations have been made, the

findings, and the methods used, have been

described in detail; but only very cautious

inferences can be drawn from humidity data

available for the rest of the world. Average

humidities are among the nnost unreliable of

climatic parameters.

The incidence of potato blight in each country or region is determined by the frequency

with which **blight years** occur and by an

account of what constitutes a *'blight year**

in that particular locality, in terms of the

stage in the growing period at which the potato

plants are defoliated by the disease, and also

in terms of the consequent effect on total yield

and on blight infection of the tubers. Some

countries had considerable data; in other

countries statements were obtained that in

such and such years the disease was **severe**

on »'moderate" or ''slight,** with little to define such terms, except perhaps the results

of a few spraying trials and some clues from

scattered observations on the foliage. In some

other countries, as in England and Wales,

during recent years systematic surveys on

hundreds and sometimes thousands of crops

were nnade each year, and these surveys were

condensed and sumnnarized. In general, epidemics for the period 1947 to 1956, inclusive,

were reviewed; but where important series of

trials were made over sequences of years

before that, the earlier trials were sonnetimes

included.

Where possible the unrestricted epidemics

occurring on unsprayed crops--usually principal maincrop varieties--are described. Then

the modifying effects upon the epidemics of

any spraying or other control measure s adopted

in the country, with practical particulars of

those measures and the extent to which they

are used, are shown. Under control measures

not only protective and destructive spraying

are considered but also the far more subtle

and often almost unconsciously adopted measures that become more apparent from a study

of the blight epidemics themselves, such as

measures of disease escape, retardation of

infection by the avoidance of overlapping

cropping, and the like.

The work of the past quarter of a century

on attempts to breed blight-immune varieties

by taking advantage of the genes for resistance present in Solanum demissum and other

wild potato species; the consequent discovery

of new biotypes or races of the blight fungus;

and the further stages of attempting to defeat

these by search for "field resistance** and

other means, are considered briefly in the

sections for countries in which the work has

been done. The part played by partially resistant varieties, old or new, in modifying

the incidence of blight and reducing losses,

is a matter most germane to any study of

blight epidemics.

Finally, the main findings for each country

are summarized briefly at the end of each

section. In a final chapter, without attempting

a full interpretation of the body of the work,

the writers have set out some factors that

appear common to blight epidemics in the

world as a whole.

POTATOES AS A WORLD CROP

In such a study as this, which by its terms

of reference concerns only a single disease,

there is a danger of conveying the impression

that late blight is the major cause of low

yields in potato crops wherever blight occurs

and low yields prevail. This is entirely fallacious. Lack of rain, inadequate nutrition,

poor cultivation, and virus diseases from

3oor seed are often far more potent factors.

To put matters into perspective, it is necesîary to say something of the requirements of

he potato crop and its part in world agriculural economy.

Potatoes do best under moist, temperate

conditions, where the growing season is long.

The greatest concentration of potatoes occurs

in northern and central Europe (see fig. 1),

and the European continent as a whole contributes about 75 percent of the world's production. The maritime climate of northwestern

Europe is almost ideal for potato growing,

and it is from this area that the highest average yields (10 tons per acre inthe Netherlands

and Belgium) are recorded. In many European

countries potatoes form a large part of the

people's diet. In North Ameri-.a the crop is
CLIMACRAMS

construction line to indicate

when temf)erature line

rises above 50°F

RAINFALL

Inches

M J J A

MAINCKOPS

mean temperature line

ri Mean doily Max-f Min

70-,.ü$ual upper limit

tor potato aoppinq

60

50-^'ower limit

40

30

- length occupied by

these words indicates

spread of planting

and lifting

-rainfall histograms shaded

to emphasise growing period

and rainfall during that

period

-Month of the year

0 = December etc.

— alternative way of indicating

growing period

NOTES

The mean air temperature line is shown. The mean maximum ^empe^atu^e

line usually runs 5°-7^F above the mean line, and the mean minimum 5^~

y'F below it, in maritime areas. In areas with a continental climate the

mean diurnal variation is greater(±9° to ± 12*^F).

The'hump'of the mean temperature line above 50°F characterises the

potato qrowinq season. Its lenqth indicates the period during which

there is potato qrowth above qround. Its heiqht indicates favourability

or otherwise of main part of the qrowinq season for tuber production

Cupper limit about 70°F).

In the southern hemisphere December is made to occupy the place of June

in the northern hemisphere, so that'summer'is always in the centre of the

diaqram.

Lonq-term averaqe fiqures for each month of the year are qenerally showa

Portions of the diaqram for the croppinq season only in individual years of

special interest are sometimes shown below.

'Rainfair includes other forms of precipitation: snow,haiLetc.

Figure 2. —Explanatory notes on the climagrams used in this study.
relatively less important, and the short growing season and high summer temperatures over

much of the continent affect yield adversely.

Potatoes are a major crop in some South American countries, notably in Peru, parts of Chile,

Argentina, and Brazil. In countries on or near

the Equator they are grown at high altitudes, as

temperatures at low altitudes are too high.

Although the distribution of potato growing

is in close accordance with the distribution of

population in Europe, North America, and

South America, this is clearly not so in the

countries of Asia, in many of which rice is

the staple food. The largest acreages of

potatoes grown in the Far East are in China,

India, and Japan, but here it is only in Japan

that the crop makes an important contribution

to the diet of the people. Potatoes are only a

minor crop in Africa, where they are grown

principally at high altitudes. The acreage is

small in Australasia, but this is chiefly because of a low demand for potatoes due to the

small population.

Apart from their use for human food, potatoes are fed to livestock, mainly pigs, and they

are used in industry for the production of

potato flour (farina) and alcohol. The use of

potatoes for livestock is largely confined to

those countries where there is a large agricultural labor force. According to the importance of potatoes in the diet of the population,

the countries of the world can be divided

roughly into four groups. Those with a high

human consumption of potatoes (300-400 lb.

per head per annum) include a number of

European countries--European Russia, Germany, Poland, and France. In each of these

countries a large proportion of the total

potato crop is fed to livestock and used for

industrial purposes. Peru is one of the few

countries with a high human consumption of

potatoes that has no important alternative outlet for the crop. Countries with a medium

consumption rate (200 lb. per person) include

Great Britain, Holland, and Sweden. Most of

these have alternative uses for potatoes other

than the ware market, but in Great Britain

surplus potatoes can prove an embarrassment

and are only reluctantly taken for livestock

feeding. The U.S.A., Canada, and Australia

represent countries with a low rate of human

consumption (100 lb.). The ware and seed

markets are the only outlets for potatoes in

such countries, and at the present time the

crop is barely holding its ground in the face of

competition from other vegetables. Lastly,

there are a number of countries where potato

consumption is very low (generally less than

20 lb. per head per year). These include India,

Pakistan, and Mexico, where potatoes become

in effect a luxury crop and their contribution

to the national diet is negligible.

The present planting of potatoes throughout

the world is about 57 million acres (F. A. O.,

1956). This represents only about 12j percent

of the land area devoted to wheat, which is

the most important grain crop. There is little

international trade in potatoes. This is explained by the bulkiness of the crop, its

liability to deterioration during transit, and to

stringent important regulations. Nonetheless,

some important trade goes on in seed potatoes, notably from Holland, Canada, Ireland,

and Scotland. There is also some trade between countries on a snaall scale in out-ofseason potatoes; as, for example, the export

of early potatoes to Great Britain from a

number of Mediterranean countries.

SOME FACTORS AFFECTING POTATO YIELDS

Climatic conditions, daylength, soil type,

cultural treatment, and the incidence of diseases and pests all affect the yield of potatoes. A few of the principal factors have now

to be considered individually, as a part of the

background of our study, but their interrelationships must also be borne in mind. The optimum water requirement of a potato crop, for

example, will depend on temperature and soil

conditions, and climate profoundly affects disease and pest attacks.

TEMPERATURE. The work of Bushneil

(1925) is usually referred to when considering

the optimum temperature conditions for tuber

formation in the potato crop (Van der Plank,

1946; Burton, 1948). Bushnell found that maximum tuber production occurred over the temperature range 59°-64o F., but his experiments were conducted with plants grown at

constant temperature, a state of affairs very

different from that in nature. However, it

would seem that if Bushnell's limits are

slightly widened, say to 580-65°, this range

represents the usual condition occurring in

practice during the latter part of the potatogrowing season in those parts of the world,

notably northwestern Europe and northeastern

North America, where the largest potato

yields are produced. The many climagrams

given in following sections bring out this

point. The temperatures during the last 2

months before lifting are most important, as

it is then that the bulk of the crop is formed.

Potatoes are intolerant both of high temperatures and of frost. Bushnell (1925) found

that tuber production ceased at 84oF., and

Bald (1941) has said that in areas of Australia

where mean monthly temperatures exceeded

750 potatoes did not produce a profitable crop.

Thus, potatoes are not grown on any large

scale in any part of the world where the mean

temperatures during the hottest part of the

growing season is much in excess of 70°.

Again some allowance needs to be made for
amerences between continental and maritime

climates. To some extent, too, there is an

interaction between temperature and daylength

(Driver and Hawkes, 1943). Thus, short days,

of less than 12 hours, modify the effect of

high temperatures.

In most of the principal potato areas of the

world frosts mark the beginning and end of the

growing season. The mean temperature at

which frosts are likely to occur varies according to situation, but in practice planting

usually takes place in the month that m.ean

air temperatures approach 50° F,, and lifting

begins at about the same temperature, (See

fig. 2 and other climagrams.) In certain

frost-free areas, as in the Ayrshire early

potato-growing district of Scotland, potatoes

are planted, and a considerable amount of top

growth is made before mean temperatures

reach 50°; and in the Sierra of Peru the

whole growing season occurs at about this

temperature; but these areas are exceptional.

The importance of temperature is thus seen

to lie not only in its effect on tuber production but also in the effect on the length of the

growing season, and the length of the growing

season is often too short to allow maximum

potato production, particularly under continental conditions. Very roughly the potatogrowing areas of the world may be divided

into two main types: Those such as Aroostock

County in Maine, U.S.A., northern Sweden,

and the Leningrad district of the U.S.S.R,,

where the growing season is short (about 120

days) and there is insufficient time for late

or maincrop varieties to produce their maximum crop; and those, such as Great Britain,

Holland, Australia, and New Zealand, where

the growing season is long (160 days or more)

and late varieties die down naturally sometime before lifting.

RAINFALL. Although an adequate supply of

water is the most important factor affecting

yield in the potato, it is impossible to generalize about the plant's actual rainfall requirement. The distribution of the rainfall

over the growing season is important. A given

amount of rain evenly distributed over the life

of the plant may be just what is required, but

the same amount coming in a few heavy rainfalls may lead to waterlogging of the soil and

actual mechanical damage to the potatoes.

The rate of water loss fronn the soil both

by evaporation and drainage is also

relevant.

The greater part of the world's potato crop

is grown under rainfall without the aid of irrigation. The rainfall may be excessive, as

in the high valleys of Mexico, parts of Africa,

and the hill crop potatoes in India; under such

conditions, often characteristic of high altitudes, yields are generally very low. Rainfall

is generally adequate in maritime areas or

localities under maritime influence, as in

northwestern Europe and the northeastern

areas of North America, where the highest

potato yields are obtained, and fluctuations

in yield from year to year are not usually very

great. Rainfall is often insufficient under

continental conditions, as over a large part

of European Russia and the midwestern states

of the U.S.A, The crops then often suffer from

drought, and yields fluctuate considerably from

year to year.

Potatoes may also be grown wholly or partly

under irrigation, as, for example, inthe Mediterranean countries, western U.S.A., and parts

of South America and India. In these areas it

is channel irrigation that is almost universally

employed. The use of spray irrigation is confined to supplementing rainfall under more

temperate conditions.

DAYLENGTH. Work on the effect of photoperiodism on yield and maturity of the potato

has been fully reviewed by Driver (Driver and

Hawkes, 1943). Most of the world's potato

crop is grown at middle to high latitudes

under long-day conditions, and these favor

maximum vegetative growth. Under short-day

conditions in equatorial regions, top growth is

smaller, maturity is speeded up, and yield is

decreased. This latter statement has, however, been challenged by Mor eau (1944a),

working in East Africa. Particular potato

varieties have a definite photoperiodic response. Thus, a short-day variety is suited

only to short-day conditions, and when grown

under long-day conditions it will fail to produce

tubers. But many varieties are day-neutral.

An example is the Americanvariety Kennebec,

which outyielded all others in trials in Florida

and Iceland. In Florida a winter crop is grown

and the daylength may be as short as 9 hours;

in Iceland in June the daylength reaches 21

hours.

SOIL CONDITIONS. Most of the potato crop

is grown on sandy or closely allied soils,

which can be easily worked and which dry

out quickly after rain. This is particularly

important where the growing season is short,

as under continental conditions, because planting and lifting have then to be done quickly

and delays due to waterlogged soils cannot be

tolerated. Light soils, however, are generally

short of organic matter and their moistureholding capacity may be so low that the yield

suffers. Where the growing season is long,

soils of heavier body (medium and heavy silt

loams) often make the best potato soils, as in

the Netherlands and New Zealand.
UFE HISTORY OF THE FUNGUS ON POTATOES

HOW THE DISEASE SPREADS

It is not within the scope of this study to

review the immense world literature on the

biology of Phytophthora infestans. All that is

necessary is to explain a few technical terms

frequently used in the following pages and the

way in which the fungus lives as a parasite on

its potato host, with the requirements for its

survival and spread. It is in the light of this

knowledge that all accounts of late blight

epidemics, wherever they may occur in the

world, must be seen and interpreted.

The fungus survives the winter as living

myceliunn in potato tubers. When these infected tubers, planted or remaining in the soil

as groundkeepers, produce new growth in the

spring, the fungus, still in the form of living mycelium, succeeds in invading a few of

the shoots--a very few of them--and comes

aboveground within their tissues. When certain

necessary temperature and humidity conditions

occur, the mycelium of the fungus produces

hyphae, known as sporangiophores, . which

thread out, usually through the stomata of the

leaves and stem, and project into the air. On

these sporangiophores the microscopic sporangia of the fungus are formed. They become

detached and drift off when ripe, and they are

also dispersed by rain. Some reach healthy

potato leaves and stems nearby and if these

remain wet from rain or dew for a sufficient

time, the sporangia germinate. At rather high

temperatures they may germinate directly,

each sporangium putting forth a single germ

tube, but generally and under cooier conditions

they gernninate indirectly, the protoplasm

within the sporangium first breaking up into a

number of smaller swarm-spores, or zoospores, which swim about in the water on the

leaf, and may each produce a small gernn tube

of its own. After the spore has germinated

it may infect the potato leaf or stem on which

it is resting, and in this infection process the

germ tube of the spore penetrates the leaf

cuticle, or, occasionally, enters the leaf by

way of a stoma, and then initiates a growth

of the fungus mycelium in and among the leaf

cells. The invading mycelium debilitates the

cells on which it feeds, and as these begin to

decay it spreads peripherally into freshtissue.

The period from infection (successful penetration by the gernn tube) to the appearance of

, a visible blight spot, or lesion, on the leaf, is

known as the period of incubation.

. Given a recurrence of the same favorable

, temperature and humidity conditions that induced sporulation on the first infected shoots,

the new lesions will in turn produce sporebearing hyphae, and the first generation of

the fungus is completed. At this stage, the

number of blight spots on the originally infected plant and those nearby is generally quite

small, but a primary focus of the disease has

been established in the potato field.

From this focus of infection more spores

are disseminated; another crop of lesions covers a greater radius; these lesions in turn

sporulate with successive spells of favorable

weather, and after two or three such generations of the fungus, blight spots may be found

here and there, generally but lightly distributed over an entire field, and beginning

in other fields in the vicinity. Epidemic spread

may then be said to have begun.

At first the damage is negligible, for the

crop loses only a few leaves here and there,

but if the attack is not checked by protective

spraying or dry weather, there will be progressive decay of the foliage as the established lesions increase in area and new ones

develop. The course of the epidemic may be

rapid or slow, always according to weather

conditions, and the crop may be affected in

two distinct ways. (1) If the foliage is killed

prennaturely the yield will be reduced by what

is in effect a reduction of the growing period;

the earlier the haulnn is killed the greater the

loss of crop. (2) Andif living sporangia from the

diseased foliage reach the tubers themselves,

these also may be infected, causing a further

loss, either in the ground or in storage.

In wet weather, sporangia are washed down

from the foliage onto the soil. Normally the

soil cover protects the tubers to a great extent, but where the soil cover is inadequate

or the soil texture or moisture content render

it permeable by the sporangia, some tubers

will be infected before lifting. If, at lifting,

the tubers are exposed while the fungus is

still sporulating on partially diseased foliage

or if living sporangia are still present in the

surface layers of the soil, further infection

of the tubers will occur. This infection will

not be apparent at the time, but it will develop

in storage.

The infection of the tubers completes the

life cycle of the fungus. The vast majority of

the blighted tubers rot in the ground or during

storage, at least to such extent that there is

no living mycelium left in them by the following

spring. As far as possible, blighted tubers

are removed from seed stocks, and of the

few that pass undetected not one in a hundred

gives rise to affected shoots aboveground. The

first infected shoots among the crops are rare

and hard to find, but these, together with occasional infected shoots arising from groundkeepers, and fromi discards by clamp sites

or cull piles, are sufficient to start the blight

epidemics each year.

HOW HUMIDITY AND TEMPERATURE

CONDITIONS AFFECT THE FUNGUS

The temperature and humidity conditions

known or believed to be critical for each
phase of the survival and spread of the fungus

may be stated briefly as follows:

SURVIVAL OFMYCELIUM IN THE TUBERS.

It may be expected that where seed tubers

are subjected to temperatures of 86^F. or

over for any considerable time in storage,

transit, or in the soil, the fungus will not

survive in them (Jensen, 1887), There appears

to be no lower limit for the survival of the

mycelium; chilling or frosting that does not

kill the tubers does not kill the fungus. Van

der Zaag (1956) found that the mycelium was

killed when seed tubers were immersed for

1 hour in water at 110°. This treatment damaged the tubers themselves when carried out

in April, but not in December.

SPORULATION. In experiments under rather

artificial constant tenape rature conditions, i.e.,

without the diurnal variation of temperature

occurring in nature, Crosier (1934) found that

sporangia were never formedin an atmosphere

at less than 91 percent relative humidity,

and that humidity approaching 100 percent was

required for their abundant formation. In a

saturated atmosphere, temperatures of 62° to

72°F. were optimum both for rapidity and

intensity of sporulation. Sporangia commonly

appeared within 8 hours and were abundant

in 14 hours. At 46° to 59° sporangia were

fairly numerous in 24 hours and plentiful in

58 hours. At 37° only a few sporangia were

formed in 6 days. Sporangia produced at temperatures over 7 0° did not gernainate well.

The sporangia were found to lose their viability in 1 to 2 hours in very dry air (20 to 40

percent relative humidity) and in 3 to 6 hours

in air at 50 to 80 percent relative humidity.

Although the main sporulation conditions as

defined by Crosier are supported by the findings of the majority of workers, it must be

noted that the capacity of the fungus for some

restricted sporulation in the field under apparently adverse high temperature and low

humidity conditions was reported by Wallin

(1953).

LIBERATION OF SPORANGIA. In explorations of the spore content of the air at 6 feet

and at 18 inches above the ground in potato

fields during the course of potato blight

epidemics in England, Hirst (1953) has found

that rapid liberation of sporangia starts at

about 8 o'clock in the morning (0800 hours)

and reaches its peak at about 10 o'clock

(1000 hours). This is consistent with De Bary*s

(1887) suggestion that spore discharge occurs

with the hygroscopic twisting movement of the

sporangiophores exposed to rapidly changing

humidity, and indicates that liberation coincides with rising temperatures and the disappearance of dew and raindrops from the leaves.

The ripe sporangia may, however, be dislodged

and dispersed by rain at any time of the day or

night.

GERMINATION AND INFECTION. Crosier

found that free water or dew on the leaves is

necessary for the germination of the sporangia.

Direct germination, without the production of

zoospores, can occur over the temperature

range 48° to 79° F., but has a definite optimum

at about 75°. Little direct germination occurs

in 8 hours, and about 32 hours is necessary

for nnaximum germination in this way. Direct

germination therefore probably plays little

part in the spread of blight in the field.

Indirect germination, by zoospores, requires

free moisture, and a rather low temperature

(50° to 59°F., optimum 55°) for a period of

only 1/2 to 2 hours. It may also take place at

temperatures as low as 37° or as high as 70°,

but the time required is considerably longer.

After the zoospores have germinated a further period of at least 2 to Zj hours at 50°

to 77°F. is required for the infection process,

or penetration of the term tubes into the host

tissue. The whole process of indirect germination and infection thus requires a period of

Zj to 4j hours with free moisture on the leaves

and cool conditions over at leasthalfthis time,

INCUBATION. After penetration of the germ

tubes into the host tissue the fungus develops

most rapidly at 64° to 70°F., i.e., at about

the same temperature as that which is also

optimunn for sporulation. Under such conditions Crosier found that new lesions appeared

in 3 to 5 days. Thran (cited by Uhlig, 1955)

found that the incubation period could be as

much as 13 days at 53°, 6 days at 60°, and

2 to 3 days at 68°. Very varied figures have

been given by other workers, but in general

the period is 3 to 5 days.

SURVIVAL OF THE FUNGUS IN LEAVES

AND STEMS. Crosier found that continuous

temperatures of 86° F. checked the growth of

the fungus in the field, but that intermittent

temperatures of 68° to 86° during the day

were not particularly unfavorable to it, and

that in potato stems it would tolerate intermittent high temperatures up to 94°. In one

trial, one stem lesion out of 197 developed

sporangia on plants exposed to bright sunshine

and maximum temperatures of 110°to 115°for

6 days. This capacity of the fungus to remain

viable in stem lesions during hot dry weather

and to start sporulating again when more

favorable conditions recur has been noted in

England by Clayson and Robertson (1955), and

extensively investigated in the U.S.A. by Hoyman and Wallin (1957), who reported survival

of the fungus after exposure for short periods

to temperatures as high as 114°.
^BLIGHT WEATHERS

As it is a part of the rhythm of nature for

air temperatures to rise during the day and

to fall at night, it might be expected that the

blight fungus, preferring rather warm conditions for sporulation and rather cool conditions for swarm-spore germination, would

produce its sporangia during the day (given

the necessary high humidity) and release

them in the evening to germinate during the

cooler night and in the night dews. Although

there may be some sporulation during the day.

Hirst has shown, at any rate under English

conditions, that the peakliberation of sporangia

takes place in the morning. This renders the

parasite less efficient than it might otherwise

be, and imposes a natural check on distant

spread of the disease by airborne sporangia.

There is no such restriction, however, on

local spread by splash and drip dispersal

of the sporangia by rain.

Although high humidity plays an extremely

important part in the development of a blight

attack, so also does rain. Rain provides

water necessary for the germination of the

sporangia on the leaves, it raises the humidity

within the crop by wetting the soil, and it is

responsible for much of the local dispersion

of infection. Hänni (1949) and Uhlig (1955)

put wet weather very high in their lists of

conditions favoring the mass increase of

blight, and this is in accord with conamion

observation.

The interplay of diurnal changes of atmospheric humidity and temperature and the differing requirements of the fungus for sporulation, spore discharge, and germination,

together with all the divers effects of wind,

rain, and varying density of crop growth,

render it unwise to attempt to postulate

what will or will not constitute "blight weather"

from a mere consideration of the optimum

conditions for sporulation and germination

of the fungus as determined in the laboratory.

Very detailed micro-climatic investigations,

such as those of Johannes (1953a) in Germany and of Hirst at Rothamsted, are necessary to reveal what constitutes *'blight

weather" in nature and how it is, in fact,

related to any given combination of climatic

parameters.

' *Blight weather** is a convenient term

for weather that permits the increase and

spread of the fungus in the field. The problem of what constitutes a "blight weather

spell,** or "critical period,** has arisen in

attempts to forecast outbreaks. Progress in

this art has conne from making intelligent

guesses in the formulation of empirical rules,

and then trying them out. With successive

nnodifi cations and limitations, these rules,

set out in this study in the sections on the

Netherlands, England and Wales, the Irish

Republic, the U.S.A., and Western Germany,

now embody all we have of real knowledge

of blight weather; as a result, "fundamental

biological and meteorological .research'* is

laboring to catch up with the practical findings.

In the consideration of blight in relation to

weather, there are successive levels on which

attention may be focused. First, the microscopic level that is concerned with the behavior of spores in damp channbers; then

the close-up of blight lesion distribution in

small plots with meteorological instruments

among the plants; then the midview of epidemics in limited regions and attemipts to

forecast them; and finally the long-shot view

of the world as a whole. This study is primarily concerned with the world view of

blight weather and the broad climatic circumstances in which such weather occurs.
THE BRITISH ISLES

ENGLAND AND WALES

England and Wales, with a population according to the 1951 Census of about44n:iillions,

now produces 5 to 6 million tons of potatoes

each year, most of which are for hunnan consumption within the country. A small tonnage,

notably of surpluses, is fed to stock, but

very few potatoes are used for industrial

purposes. The potato acreage, which reached

over a million during the 1939-45 war, stood

at about 650,000 acres in 1954, of which

530,000 acres were maincrops (including

second-early potatoes), the rest being firstearly potatoes (M.A.F.F. [Gr, Brit. Ministry

of Agriculture, Fisheries, and Food], 1956).

The consumption of potatoes per head is about

200 lb. per annum.

Maincrop production is concentratedin a few

districts, with an even and relatively sparse

distribution elsewhere, as shown in the distribution map (for 1954) (fig. 3). The principal

districts for maincrops are the Fens and the

area round the Wash, where the crop is grown

on deep organic loams or on silt soils producing very heavy yields; the Vale of York,

where the chief soils are sandy loams; Lancashire and the Cheshire plains, mostly on

sandy soils of glacial origin. Important potato

areas are also in Bedfordshire, Essex, and

Kent, with London as their nearest market;

ENGLAND ft WALES

Each Dol rcprcscnU lOOO Acres

FIGURE 3.--Distribution of maincrop potatoes in England and Wales.

near Birmingham; and in Durham, supplying

the coal mining districts of the north.

The principal maincrop varieties are Majestic and King Edward. Majestic accounts

for about 45 percent of the maincrop acreage,

and King Edward (including Red King), which

is regarded as the quality potato, for about

35 percent. Other popular varieties are Arran

Banner, Arran Peak, Dr. Mclntosh, and Redskin, each contributing 3^ percent to the

total maincrop acreage. Very few really late

varieties are grown.

The planting of maincrops starts at the end

of March and continues into early May, according to the season. Whole seed tubers

are planted with the sets about 18 in. apart

in the rows, and the rows 27 to 30 in. apart

(28 in. is the standard width). Lifting usually

begins about mid-September, but most of it

is done in October. The growing season is

therefore rather long--about 160 days. The

average yield is about 9 tons per acre. Chitting

(sprouting before planting) is practiced fairly

extensively in early districts and in the Fens,

and this gives the crops a start of about 2

weeks extra growth at the beginning of the

season. A newly revisedbulletin on **Potatoes'*

(M.A.F.F. [Gr. Brit.], 1957) provides full information on potato cultivation in England and

Wales.

Early varieties are grown in most of the

principal maincrop districts; but the first

supplies for the early summer market come

from Cornwall, Pembrokeshire, the Scillies,

and the Channel Islands. The principal firstearly varieties are Arran Pilot and Home

Guard.

In 1954, Arran Pilot accounted for over

50 percent of the total first-early acreage,

and Home Guard for 21 percent. Other popular early varieties include Ulster Chieftain,

Ulster Prince, Duke of York, Eclipse, Ninetyfold, and Sharpe's Express. Epicure and May

Queen have diminished in popularity since

1945.

In Cornwall and Pembrokeshire the planting of early potatoes starts in early February

and continues into March, with lifting toward

the end of May or early June. The seed

tubers are planted 12 to 15 in. apart in rows

24 to 28 in. apart. Often they are grown on

the flat, without earthing up. Yields are from

3 to 5 tons per acre. Early varieties in the

other districts, notably parts of Lincolnshire, the Cheshire Plain, Bedfordshire,

Essex, and Kent, are planted later and provide the continuation of supplies of '*new

potatoes in June and July.

Seed-potato growing in England and Wales

is chiefly confined to the production of good

•* once-grown'* stocks--for which there is a

10
certification scheme. Most of the **new*' seed

Is imported from Scotland and Ireland, although the upland or coastal areas in Devon,

Cornwall, Cun:iberland, and parts of Wales,

where climatic conditions are unfavorable

for the spread of virus diseases, produce

small quantities of the higher grades of

certified seed. About 60 percent of the maincrop potato acreage is planted with certified

seed, almost all of which is imported from

Scotland or Ireland (Boyd and Lessells, 1954).

Key for the assessment of potato blight on the

haiilm

Blight

Percent

0.0 Not seen on field.

0.1 Only a few plants affected here

and there; up to 1 or 2 spots in a

12-yard radius.

1.0 Up to 10 spots per plant, or general light spotting.

5.0 About 50 spots per plant, or up to

1 leêiflet in 10 attacked. <'¡

25.0 Nearly every leaflet with lesions,

plants still retaining normal

form; field may smell of blight,

but looks green although every

plant ciffected.

50.0 Every plant affected and about

half of leaf area destroyed by

blight; field looks green flecked

with brown.

75.0 About three-quarters of leaf area

destroyed by blight; field looks

neither predominantly brown nor

green. In some varieties the

youngest leaves escape infection,

so that greenis more conspicuous

than in varieties like King

Edward, which commonly shows

severe shoot infection.

95.0 Only a few leaves left green, but

stems green.

In the earlier stages of a blight epidemic,

parts of the field sometimes show more

advanced decay than the rest, and this

is often associated with the primary

foci of the disease. Records may than

be made as, 1 + pf 25, where pf 25

means 25 percent in the area of the

primary foci.

Losses by potato blight in England and Wales

occur chiefly on the maincrop and secondearly varieties. Although the first outbreaks

of blight each year are usually on the early

varieties, these are lifted before sufficient

defoliation occurs to affect the yield, and

usually before the tubers are infected. The

following study of late blight epidemics in

England and Wales therefore refers essentially to the disease on maincrop potatoes

(taken throughout as including second-early

varieties), although, undoubtedly, blight foci

among the early varieties contribute to the

sources of infection of the maincrop in the

vicinity each year.

DELIMITATION OF '»BLIGHT ZONES"

During the second World War the need was

found for some precise but practical field

method of defining the course of blight on

potato crops in different seasons and in

different parts of the country. As a result

of this, an assessment key (as shown in column one) was worked out by a Disease Measurement Committee of the British Mycological Society (B.M.S., 1947).

This key was successfully employed in

local surveys (Moore, 1948) and for the study

of foliage protection in spraying trials (Large,

1945). In 1950, when a new section for plant

disease measurement and quantitative survey

work was started at the Plant Pathology

Laboratory (Large, 1953b, 1955a), the key

was put to use in national surveys. These

surveys, using a uniform plan, continued

for 7 years, are reported by Large (1956).

Thousands of records have been obtained

from all parts of the country; for England

and Wales, it is at last becoming possible

to define, measure, and compare the blight

epidemics occurring over a sequence of years

and to put an evaluation of the losses the

epidemics have caused upon a factual basis.

This is a realization, in some part, of plans

long laid by W. C. Moore, whose address

to the Association of Applied Biologists in

1949 on the significance of plant diseases in

Great Britain (Moore, 1949) provides the

background for all that follows.

Two points on the curves obtainedby plotting

progress of blight on a time scale are of

particular importance (Large, 1952a). The first

is the 0.1-percent stage, defining the beginning

of epidemic spread, or ''outbreak,'* for purposes of forecasting and the timing of spraying. The second is the 75-percent stage of

defoliation, at which time further tuber development is arrested (see fig. 8 and the section

on computation of losses). This latter stage

is discussed first, as the 75 percent maps

(75 to 90 percent in practice) provide the best

single picture of the severity and frequency

of the epidemics in . the several^ regions of

the country (fig. 4).

Of the 6 years illustrated, 1950, 1953, and

1954 were *'blight years,'* with losses of all

the September growth over most of the south

of the country, and losses of some of the

August growth also in the southwest and the

Fen district round the Wash, but with little

or no loss in the north. In the other years

losses of growth occurred in the west, but

only here and there in the other zones. From

a study of the maps (Large, 1958) it becomes

possible to divide the country into four main

**blight zones":

11
ENGLAND 6 WALES

FIGURE 4.--Zoning of England and Wales according to the date at

which the 75 to 90 percent blight stage was reached on unsprayed

maincrop potatoes in the years 1950-55.

Southwest zone- - -A southwestern and

west-coastal belt including the potato growing areas in Cornwall,

Devon, Somerset, Gloucester, Monmouth,

Glamorgan, Carmarthen, Pembroke,

Cardigan, Merioneth,

Caernarvon, and Anglesey; with an earlier subzone containing Cornwall and Pembroke.

Fen zone An area roundthe Wash,

roughly equivalent in

acreage to that of the Isle

of Ely and the Holland

division of Lincolnshire, but extending

somewhat into Hunts,

and West Norfolk.

Southern zone All the rest of the country south of Denbigh,

Cheshire, Derby, and

Notts., but including the

Ke Steven and Lindsey

Divisions of Lincolnshire, and also the

northern county, Cumberland, where the epidemics are similar to

those in the south.

Northern zone All remaining northern

part of the country.

These zones are indicated in figure 5, and

it will be seen how closely they represent

the mean pattern of shading--for time of

7 5-90 percent defoliation--in both the blight

years and the years with little blight in the

east. Naturally, these zones are not in fact

limited by county boundaries, but their areas

are here reckoned in equivalent county potato

acreages in order to arrive at the approximate percentage of the country's total maincrop

acreage that is contributed by each zone.

This, on the basis of the 1954 distribution

(fig. 3), is given in table 1.

FREQUENCY AND SEVERITY OF THE

EPIDEMICS ACCORDING TO ZONE

With the above zoning, and taking in the

years 1947 and 1949, which were dry years,

and 1948, a '*blight-year" rather similar to

1950, the frequency of epidemics of differing

degrees of severity, in the 4 zones, over the

10 years, 1947-56, was as shown table 1. The

mean dates at which growth was stopped by

blight are for unsprayed maincrops, and by

reference to figure 8 and the section of computation of losses of which it is part, it is

possible to proceed directly to estimates of

the mean loss in yield due to defoliation on

the unsprayed crops in each zone and for

England and Wales as a whole over this

period. These estimates are entered in the

table--probably the first of their kind ever

to be presented for an entire country.

This analysis indicates that if all the maincrop potatoes in the country had been left

unsprayed during the 10 years, the mean

annual loss due to premature defoliation by

blight would have amounted to about 8 percent; that is to say, 8 percent of the total

yield that would have been obtained had blight

12
ENGLANDS WALES

FIGURE 5.--Key map, showing blight zones in England and Wales.

been entirely absent. But in the southwestern

zone, where every year was a ' *blight-year,*'

the mean loss would have been about 20 percent; in the Fen zone, with 6 blight years in

10, about 14 percent; and in the large southern

zone, with only five moderately severe blight

years in 10, about 7 percent--less than the

average for the country as a whole.

13
TABLE 1. --Frequency and severity of potato blight epidemics in England and Wales, 1947-56

Maincrop

acreage

Number of years in 10 in which growth was

stopped by blight at—

Mean loss per

annum the 10

years to—

Zone

Mid-Aug.

28 percent

loss

End Aug.,

13 percent

loss

Mid-Sept.,

4 percent

loss

End Sept.,

no

loss

Zone Whole

countrySouthwest

Percent

8

13

51

28

5

0

0

3

2

5

0

0

1

2

5

0

3

3

5

Percent

20

14

7

2

Percent

1.7

Fens 1.8

Southern 3.7

Northern 0.6

England and Wales.... -- — -- -- -- -- 7.8

In the worst of the individual blight years,

such as 1950 or 1953, the national loss, on

the same basis, would have been about 16

percent; and in the years with least blight,

such as 1951, 1952, or 1955, a little over 2

percent.

COURSES OF THE EPIDEMICS IN THE FOUR

ZONES

Having established the blight zones of the

country by reference to the 75 to 90 percent

maps, we may now consider the development

of the epidemics within each zone. Figure 7,

given in the section on forecasting (p. 18),

shows the outbreak (0.1 percent) map, and

the regional blight progress curves obtained

for a **blight year" (1954). There is no constant relation between the outbreak date and

the date at whichtuber development is arrested.

The progress curves may be steep, or they

may be long-drawn-out, according to weather

conditions. Even in severe epidemics it is

rare for the attack to progress from the

0.1 to the 75 to 90 percent stage in less than

5 weeks, and in a lingering attack the period

may be 2 months or more.

Blight is not found at the 0.1 percent stage

in all the unsprayed crops in a region on the

same date; there is commonly a range in

time of about a fortnight, between the first

and the last appearance of this small amount

of blight. Table 2 gives the range of outbreak

dates for each zone for the 7 years, 1950-56.

TABLE 2. --Times of outbreak, and weeks of growth lost at the end of the season,

maincrop potatoes, 1950-56

Southwest zone Fens zone Southern zone Northern zone

Year^

Outbreak

Growth

lost Outbreak Growthlost Outbreak Growth

lost Outbreak Growth

lost

Weeks Weeks Weeks Weeks

1950-B... Early July 7 Early July 7 Late July 5 Esirly Aug.-

mid-Aug.

3-1

1931 Late Julyearly Aug.

3-5 End of Aug. 2-3 Early Aug.-

mld-Sept.

1-3 Late Aug.-

mid-Sept.

1

1952 Early Julyearly Aug.^-^ Mid-Aug.-

end Sept.

1-2 Early Aug.-

end of

Sept.

1-3 Early Aug.-

mid-Sept.

1-3

1953-B... Early Julylate July

6-8 Early July 5-6 Late Julyend July

5 Late Julylate Aug.

1-3

195^-B... Early Julylate July

7-6 End of July 5 Late Julyend July

5 End of Julylate Aug.

1-3

1955 Late Junemid-Aug.

5-7 End of Julylate Aug.

3-5 Late Aug.-

end Sept.

1-3 Mid Sept.-

end Sept.

0-1

1956-B... Late Julyearly Aug.

5-7 End of July 6-7 Early Aug. 2-5 Early Aug.-

late Aug.

2-3

'• B = blight-year in the Fen and southern zones.

14
"Early July'' signifies the first 2 weeks of

that month; ** mid-July'' the middle 2 weeks;

'*late July" the last 2 weeks; and **end-July"

the last week of July and the first week of

August. In this table the 7 5 to 90 percent dates

for the several zones and years are also

given, but they are expressed in terms of the

number of weeks before the end of September

at which this stage was reached. The resulting numbers are in fact good code figures,

of real biological significance, for expressing the severity of a blight epidemic, where,

as in England and Wales, tuber development

in crops free from blight normally ceases

by that date. By reference to the histograms

at the bottom of figure 8, or the growth

curve above them, these code figures may be

translated directly into probable losses in

tons per acre, or in percentage of potential

crop.

In the southwest zone outbreaks occur in

July every year, with frequent outbreaks in

June, and even in May on the early varieties

in Cornwall and Pembroke. In the Fen zone

they may occur early or quite late in July

even in the blight years; while in the southern

zone generally outbreaks are in late July in

the blight years, with a tendency to be earliest

in the coastal regions and latest in the central, inland area. In the northern zone, August

outbreaks are most common, with many fields

remaining unaffected until late August or

September. Outbreaks tend to be somewhat

earlier in Lancashire and the southern part

of the Vale of York than elsewhere in the

zone.

WEATHER CONDITIONS

Climagrams showing the long-term average

rainfall and temperature conditions in the

four zones are given in figure 6. The upper

diagram, for Durham, in the northern zone,

represents a district where the epidemics are

latest and least severe; while the second

diagram, for Penzance in the extreme southwestern subzone, represents conditions where

they are earliest and most severe. It will be

seen that even between these two extremes

there is no very great difference in mean

rainfall over the growing period. The principal difference is in the higher winter and

spring temperatures at Penzance, which, with

freedomfrom frosts inthe sheltered coves, permit the spring cropping of first-early varieties.

However, very great local variations in

both annual and summer rainfall occur in

England and Wales. The country is mountainous to the west and north. On parts of the

Pennine s, the Welsh mountains, and the high

moors of Devon and Cornwall, the annual rainfall exceeds 80 inches and it reaches 150

inches in a few places. Over most of the districts in which potatoes are grown, however,

and certainly in the principal districts, which

ENGLAND 6 WALES

. Northern Zone

DURHAM

RAINFALL

inches

J FMAMJ JASOND

. South-western Zone

PENZANCE

JFMAMJJASOND

. Southern Zone

RAINFALL 4

inches

RAINFALL

inches

FMAMJ JASOND

BLIGHT YEARS OTHER YEARS

-60°F r^^^'^'^ ^""^-ôO'F

50"F 4( 50° F

J J A S J J A 5

FIGURE 6.--Long-term average monthly rainfall and mean temperamres in relation to potato cropping for three zones in England and

Wales, with July to September means for five blight years and five

other years in the southern zone.

are at altitudes below 200 ft., the average

annual rainfall does not exceed 30 inches.

Rainfall and mean temperature figures for

the months of June, July, August, and September over the 10 years, 1947-56, are given in

table 3. Data for Cambridge have been chosen,

as the deviations from average at this station

are fairly representative of a very large

part of the important southern blight zone

and also of Fens. The years are grouped

into **blight years** and the **other years,**

in which the blight attack was late and caused

little loss.

To bring out a fact that disturbs all preconceptions about reduction of potato yields

in blight years, the mean potato yields for

England and Wales, for each of the years.

15
TABLE 3.--Rainfall and deviations from average temperature at Cambridge for June to

September, 1947-56, and mean national potato yields in '^blight years^» and other years

Year, average,

and mean

Rainfall Mean temperature Mean

Potato

June yield July Aug. Sept. June July Aug. Sept.

Inches Inches

2.2

1.5

7.7

2.0

2.7

3.5

Inches Inches

' 50

+2

+5

-1

-1

-3

62

0

0

-1

-4

0

°F.

61 '

+1

0

0

-2

-4

°F.

57

+3

-1

0

-2

+1

Tons/acre

Long-term average...

"Blight years":

1948

1950

1953

1954 ,

2.1

2.4

1.5

1.9

1.7

1.9

2.4

^.5

2.6

2.5

3.6

4.9

1.6

1.6

2.5

1.3

1.7

1.4

7.6

7.7

8.7

7.9

1956 8.3

Mean 1.9 3.3 3.9 1.7 0 -1 -1 0 8.0

"Other years":

1947 3.3

1.2

1.7

2.1

1.5

3.3

.8

.5

.5

.2

1.2

3.3

2.6

1.6

1.0

.2

2.2

2.3

1.6

+4

+2

0

+2

0

+3

+4

+1

+2

+2

+5

+3

-1

+2

+4

+3

+7

+2

-4

+1

5.S

1949

1951

6.6

7.9

1952

1955

7.9

7.3

M9aii 1.9 1.3 1.9 1.4 +1 +3 +3 +2 7.1

are added to table 3. The figures are taken

from the last published Agricultural Statistics

(M.A.F.F., 1956) with comparable estimates

from other sources for the years 1955 and

1956. The figures are for all varieties (early

as well as maincrop), but they serve well as index figures for the level of yield in each year.

The blight years are characterized by high

rainfall and average or slightly low temperatures in the months of July and August; while

the other years, with little or no blight, were

associated with low rainfall and temperatures

decidedly above the long-term average

( + 3°F.) in these months. These differences

are shown in figure 6.

The first and overriding economic effect

of the abundant rain and moderate temperatures in the blight years was to favor the

growth of the potato plants, thus raising the

yields above average despite whatever losses

there may have been from potato blight.

That abundant rainfall and moderate temperatures in July and August also favor the

development of severe blight epidemics is

well brought out in table 3, but this also shows

that a severe epidemic, such as that of 1953,

can occur with only about average rainfall

(Z\ in.) and normal temperature (about 61° F.)

in August. Such conditions can occur anywhere in the country, and they may or may

not accompany severe blight attacks. The

main differences responsible for the differing

general levels of incidence of blight in the

four blight zones are to be sought, not in

mean temperature and rainfall, but inhumidity.

HUMIDITY CONDITIONS IN THE FOUR ZONES

The southwest zone, in which severe or fairly

severe epidemics occur almost every year,

is exposed to southwesterly winds bringing

in moist maritime tropical air. In this zone

from June to September, there are 1,000 to

1,250 hours during which the relative humidity

is 90 percent or more. These are mean durations as found by Smith (1954, 1956a) in his

humidity surveys of the country over the

years 1950-54. Cumberland is also exposed

to this moist air stream; but most of the

southern blight zone is in the lee of the

mountains and high land to the southwest and

has a seasonal mean of only 750 to 1,000 hours

of such high humidity. In the northern zone

particularly, dry areas occur in Lancashire

and Cheshire in the immediate shelter of

the Welsh mountains; and also in the lee

of the Pennine s in Yorkshire, where the

count of high humidity hours is less than

750. This goes far to explain the low level

of blight incidence in the northern zone. In

the Fen zone, near the Wash, there is a high

water table in low-lying land, with exposed

water in many drainage channels. Undoubtedly

the Fen zone differs from the rest of the

south in high humidity due to these causes,

and, in comnnon with some other east coastal

areas, it is occasionally affected by fogs

comiing in from the North Sea.

Even such regional humidity differences,

however, do not entirely account for the

differences in frequency and severity of blight

16
in the four zones, and in particular districts

within the zones. These differences have been

studied over the past 7 years in terms of the

frequency and times of occurrence of Beaumont periods;2 that is to say, of humidity

periods of such duration and at such a temperature as to permit sporulation and spread

of the blight fungus itself. The recording of

these periods all over the country has been

primarily for forecasting purposes; but it

has also revealed the weather spells after

the outbreak, during which further spread is

favored and the epidemics can progress most

rapidly. (See fig. 7.)

FORECASTING

Since 1950, potato blight forecasting by the

Beaumont humidity-tempe rature rule, with

instruments in screens 4 ft. above ground level

at standard weather stations, has been investigated and developed throughout England and

Wales. Some 60 weather stations have taken

part in the work, and the validity of the forecasts (based on hourly records of humidity and

temperature) has been tested by hundreds of

observations of blight outbreak dates in the

potato fields in all parts of the country each

year (Large, 1953a, 1956). From this work it

has been found that with observations in

standard screens at weather stations and not

actually in the crops, individual Beaumont

periods are often at fault, but that **flushes**

of such periods, occurring at a number of

stations within a blight zone, as revealed by an

'^operations-chart'* (fig. 7), provide a reliable

basis for regional forecasting when an

additional working rule is introduced. This

additional rule is that in the southern. Fen, and

northern blight zones, **flushes" occurring before the last week in June are disregarded, and

that only **flushes" at or after that time are

to be taken as warnings.

This is anadaptationof the Beaumont scheme

to the particular circumstances within the

blight zones in England and Wales (other than

in the southwestern zone, where June flushes

are valid). This adapation reflects the fact that

although June flushes occur every year, and

they do undoubtedly permit the sporulation of

the blight fungus on the very few and widely

scattered infected shoots arising from blighted

tubers planted each year, they do not initiate

epidemic spread. The June flushes give rise

to small primary foci involving a few plants

only, and it is not until the potato plants begin

to meet the drills, reducing ventilation within

*By the Beaumont rule (Beaumont, 1947), spells of not less than

48 consecutive hours during which the relative humidity does not fall

below 75 percent and the temperature is not less than 5(P F, are

expected to be followed by blight outbreaks within 14 to 21 days.

Near-critical periods are those in which these conditions are not

broken for more than 1 hour in the 48.

the crop, and the next spell of critical weather

occurs, that distant or epidemic spread begins.

Epidemic spread occurs when blight at the 0.1

percent stage by the B.M.S. key (see p. 11) is

found over a considerable area in a field crop,

and this stage is taken as constituting an

^'outbreak'* for forecasting purposes in England and Wales.

Important studies have been made by Hirst

(1955) on the very early stages of the blight

epidemics in England and Wales. However,

there is still a cryptic and only partially explored phase in the development of blight

epidemics, which includes the whole of the

happenings in the fields between the inadvertent

planting of blighted tubers in April and ,the

first appearances of new infections at the'0.1

percent stage on the resulting foliage in July,

August, or even September. Whatever the true

story of this phase may be, forecasts based

on the occurrence of the first flushes of Beaumont periods in screens after the last week in

June are now enabling correct regional spray

warnings to be given year after year in England and Wales. Thus, forecasting in this

country is now moving out of its investigatory

stage into firm establishment in practice.

The maps (fig. 4) show plainly that it is chiefly

in the southern and Fen zones that forecasting

services are required, and it is here that the

scheme as now developed is giving its best

performances.

Figure 7 shows the operations chart for a

blight year (1954). There was the usual flush

in June, too early to herald the outbreaks. In

1954 it was the flushes about July 10 and July 17

that gave warning throughout the southern and

Fen zones of first outbreaks occurring over the

last week of July and the first week of August.

In the following year, 1955, there was no flush

of warnings after the end of June until about

August 13 in these zones, correctly indicating

the absence of outbreaks until the end of

August, too late to justify protective spraying.

In England and Wales the possibility of

forecasting by 90 percent humidity criteria has

also been investigated (Smith, 1956b). By

analyses over the 5 years 1950-54, it was

found that the substitution of 11 hours at 90

percent relative humidity for the Beaumont

humidity requirement would give some slight

improvement in the performance of the scheme

as now operated, but that the disadvantages of

a changeover would, in practice, outweigh the

advantages.

AMOUNT OF PROTECTIVE SPRAYING DONE

The percentage of the crops in England and

Wales in which protective spraying or dusting

is done is provided by records from a representative sample of some 2,000 crops each

17
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18
year on which crop-check weighings are carried out for the purpose of estimating seasonal

supplies.3 Table 4 shows the percentages of

the maincrops in the four blight zones which

received any protective treatment in each of

the years 1953-56.

TABLE 4. --Percentage of maincrop potatoes

that received at least one protective

treatment in England and Wales, 1953-56

Zone 1933 1954 1955 1956

Percent

16

55

4-0

4

Percent

16

49

44

8

Percent

19

51

38

4

Percent

Southwestern.••• 32

62

50

7

Fens

Southern • ,

Northern.

Average^ England

and Wales 28 32 28 39

On the showing of these figures the proportion of the crops protectively treated was about

the same in the year with very little blight

(1955) as in the blight years 1953 and 1954, but

there was an increase in the blight year 1956,

and the number of applications, not ascertained

in these surveys, was undoubtedly greater in the

blight years. The figures indicate that in recent

years only some 30 percent of the crops (miaincrop) in England and Wales were protectively

treated; the proportion reached 50 to 60 percent in the Fens and some parts of the southern

zone, notably Kent and Essex; 20 percent in the

southwestern zone, and only about 5 percent in

the northern zone. As regards variety, in 1956

about 60 percent of all the King Edward crops

in the survey were sprayed; 30 percent of the

Majestic; and also 30 percent of the other

varieties.

Thus, it will be seen that in England and

Wales, where more than half the crops are left

unsprayed, it is possible first to estimate the

loss caused by unrestricted blight epidemics

on unsprayed crops, as has been done in table

1, and then to proceed to examine the extent to

which this loss is, or could be, reduced by

protective spraying, in each of the now-defined

blight zones.

COMPUTATION OF BLIGHT LOSSES AND

GAIN FROM SPRAYING

The derivation of the method, now employed

in England and Wales, for estimating average

This work of crop check weighing is done by the Potato Marketing

Board, with whose kind permission these and other supplementary

figures were obtained for the Plant Pathology Laboratory in the

course of the **crop-check surveys" included in this study. A brief

account of the organization and scope of the surveys is given in a

paper by Large and Honey (1955) on the incidence of common scab

of potatoes in England and Wales.

losses of crop due to defoliation, from blight

progress curves for the haulm read in conjunction with bulking curves for the tubers, has

been given by Large (1945, 1952). The practical

use of this method is here briefly explained by

reference to fig. 8.

ENGLAND 6 WALES

PERCENTAGE

OF CROP ,

:i::::::—=^— -.-<--A ^^

^^

, • ," " 1

'A- ^' r—1 ^î ' ~i" "

^ T i ^~^ ' ^ X- 1 J^~^' > 1^ 1 ^

/ -t -à' * i Z .: 1 ^

^ ■■

t

^

y

~^ '■■ -rZ ih "^'

_i_

,

lO 20 in a lo r>o

AUGUST

72%

SEPTEMBER

^ I I

96% IOO%

BLIGHT

ON

HAULM

per cent

TONS PER

WEEK

(lO-Ton crop) I^Ib

8 7 6 5 4 3 2

WEEKS FROM END OF SEPTEMBER

FIGURE 8«--Mean growth curve for maincrop tubers in England and

Wales, showing percentage of crop obtained according to the date at

which blight reaches 75 percent on the haulm. Also the mean tuber

production per week in a 10-ton crop free from blight is shown.

Dotted growth curve shows deviation in late-planted crops.

The upper graph in this figure gives the

growth curve for maincrop tubers provisionally

taken as normal for England and Wales when

the haulms are unaffected by blight and die

down naturally at the end of September. The

curve is based on the results of fortnightly

liftings on 60 crops in the 6 years 1940-45

(Doncaster and Gregory, 1948), and is therefore a mean curve. In some years the course

of tuber development was relatively early and

in others relatively late. The curve is for

Majestic, the principal English variety, but

experience so far has shown that within practical limits, it may also be used for the other

early maincrop varieties grown in England and

Wales, notably King Edward (Rosser, 1957).

19
Increase in the crop of tubers is taken to

cease when blight on the haulm reaches the 75

percent stage; the slight increase, if any, after

that stage is offset by the slowing down of

growth before it, which begins at the 50 percent

stage.

Hence, when the blight progress curve on the

unsprayed haulm is as **A,*' in the middle

diagram, and reaches the 75 percent stage in

mid-August, only 72 percent of the potential

crop is produced and the loss (of potential

crop) is 28 percent.

If spraying retards the destruction of the

haulm by a little over a fortnight, giving a

progress curve as **B,*' with the 75 percent

point at the end of August, then 87 percent of

the potential crop is produced and the loss is

13 percent. The gain from the fortnight's

prolongation of growth over the second half of

August is therefore 28 minus 13, or 15 percent

of the potential crop.

Except in those very rare cases in which the

haulm is kept completely protected from blight

until the end of the growing period, the potential crop, were there no blight, is not known.

This can be estimated, however, from the

growth curve. If the yield from a sprayed crop,

with 75 percent point at the end of August is,

for example, 12 tons per acre, then 12 tons is

87 percent of the potential crop. And the potential crop is therefore 13.8 tons per acre.

Similarly, but with less accuracy, the potential

crop can be estimated from the yield of the

unsprayed areas.

The gain from a given number of days prolongation of growth over a given part of the

growing period will vary according to the level

of fertility of the field and the vigor of the

potato stock, in short, according to the potential

yield. Thus, to obtain a loss from blight, or

gain from spraying, in terms of tons per acre,

the percentages read off from the growth curve

are multiplied by the potential yield in tons per

acre. In the example given above, where the

blight progress curve on the unsprayed is as

"A,'* and that for the sprayed as'*B,*'the gain

from spraying, 15 percent, amounts to 1.5

tons per acre where the potential yield is 10

tons per acre, and to Z^ tons, where the potential yield is 15 tons per acre.

The histograms at the bottom of figure 8 show

the normal increment of yield produced in each

week, numbered backwards from the end of

September, for a potential crop of 10 tons. This

is perhaps the most general level of yield in

England and Wales, and the diagram is therefore useful for making quick approximate

estimates of gains and losses with little calculation. If, for example, in a bad blight year,

spray deposits are washed off rapidly by rain

and cannot be renewed in time, only one week's

growth (say week No. 6) is gained by spraying

on a potential 10-ton crop, and this gainamiounts

to only 0.7 ton. The unrecovered loss (weeks

1 to 5) is 1.7 tons, and the yield obtained is

8.3 tons. On a 15-ton potential crop all these

figures are one-and-a half times as much.

It must be remembered that the provisional

normal growth curve is derived from the mean

for a number of crops over a number of years.

Its valid use is therefore for estimating mean

losses oVer a number of years. Individual crops

in individual seasons miay vary considerably

in rate and timing of tuber production, and on

them the losses from blight and gains from

spraying must be expected to vary about the

long-term norm.

One of the principal causes of deviation is

late planting (Dyke, 1956), which tends not only

to reduce the potential yield but also to increase the weekly increments of growth in the

latter part of the growing period. As a guide to

the type of growth curve that may be expected

where the crop is planted late (or has suffered

retardation in its early stages), a dotted curve

is added to figure 8, representing the mean

course of tuber development in 16 late-planted

crops unaffected by blight in the southern zone

in the years 1955 and 1956. Where the growth

curve follows the normal course, about half the

potential crop is laid down by the end of July;

but on the dotted curve only about a third of the

potential crop is then present.

The prolongation of growth from spraying,

i.e., the mean separation in time between the

two more or less parallel blight progress

curves for sprayed and unsprayed, is, strictly,

best measured between the 50 percent, or

**half-decay,** points on the 2 curves (Large,

1945). As, however, the difference in time

between the 50 and 75 percent stages is commonly less than a week and the rate of tuber

bulking on the normal growth curve does not

vary greatly over so short a time, the simplicity of estimating prolongation of growth between 75 percent points on the curves for the

sprayed and unsprayed crops outweighs any

possible slight gain in accuracy that would

result from stepping back to the 50 percent

points for this purpose.

CHECKS OF LOSS ESTIMATES BY SPRAYING TRIALS

Since 1941, several series of spraying trials,

each over a number of years, have been carried out in various parts of England and Wales.

In these trials yields determined by weighing

the crop from replicated plots were related

to the blight progress curves for the sprayed

and unsprayed haulm. The **actual'* gains from

spraying were computed with the *'expected*'

gains, calculated from the 75 percent dates on

the haulm in accordance with the method and

example given in the previous section, using

the provisional normal curve for tuber development (fig. 8).

The first series was carried out by Beaumont and Large (1944) in Devon and Cornwall

(southwestern blight zone) during the yearr

20
1941-46. In the 4 years 1943-46, the effect of

two >yell-tiined applications of freshly prepared

bordeaux mixture (4.5.50), applied at about 100

gallons per acre, by 5- or 7-row horse-drawn

or tractor sprayers with drop lances, was

tested on uniform stocks of certified Majestic

at a dozen centers or more each year. In table

5 the mean results (some hitherto unpublished)

for each year are given for two groups: Cornwall and western districts of Devon where

blight is exceptionally severe (extreme western

subzone), and the rest of Devon where conditions are more representative of the southwestern zone as a whole.

TABLE 5.--Results of spraying trials with 2 well-timed applications of bordeaux mixtures on

Majestic potatoes in Cornwall and Devon, 1943-46

Year and area

1943:

Cornwall and western

Devon

Eastern Devon

1944:

Cornwall and western

Devon

Eastern Devon

1945:

Cornwall and western

Devon

Eastern Devon

1946:

Cornwall and western

Devon

Eastern Devon

Crops

Number

6

5

10

5

Date when blight

on unsprayed

haulm reached--

0.1

percent

stage

June 26

July 21

July 25

Aig. 8

June 25

July 17

July 7

July 18

75

percent

stage

Aug.

8

23

Aug. 20

Sept. 9

July 23

Sept. 19

Aug. 4

Aug. 23

Prolongation of

growth by

spraying

Days

17

21

18

12

18

21

15

18

Total yield

of

unsprayed

plots

Tons/acre

6.6

13.7

12.5

6.7

8.9

9.3

11.5

Gain from spraying

Actual

lifting

Tons/acre

2.7

2.9

2.8

1.1

2.4

1.6

2.3

3.2

Expected

from 75

percent

dates

Tons/acre

3,5

1.5

2.8

.7

2.9

2.0

2.6

2.2

Here the **expected** gains, especially when

averaged over the run of years, are in good

general agreement with the *'actual'* gains

as determined by lifting, which for all the labor

involved, were still subject to a standard error

of at least half a ton per acre. In the more

easterly part of the area (lower figures) the 75

percent stage of blight on the haulm was

reached about a week after mid-August in 3

years out 4, and in early September in 1 year

only (1944). This is in good accordance with the

frequency for the southwestern zone over the

later period of 10 years (1947-56) given in

table 1. The computed mean total loss of

potential crop by blight over the 4 years was

18 percent as against the mean of 20 percent

entered in table 1, and the gain from spraying,

averaging 2.2 tons per acre over the 4 years,

was about 12 percent. In these trials, therefore,

two carefully timed applications of one of the

most rain-re sistant spraying materials

(freshly made bordeaux mixture) saved about

two-thirds of the defoliation loss by blight.

The next series of trials was carried out by

Beaumont, Bant, and Storey (1953) during

1947-51, at Goole on the Humber (fig. 5). This

center is on the border of the northern and

southern blight zones, where blight conditions

more closely resemble those of Hertfordshire

in the southern zone than those in the greater

part of the north. The spraying was again done

with freshly prepared bordeaux mixture

(4.5.40), but with a knapsack sprayer. The two

well-timed applications were put on at the rate

of 200 gallons per acre on Latin-square plots

of certified Majestic each year. The relevant

results are shown in table 6.

In the two blight years, 1948 and 1950, the

expected gain from 3 weeks prolongation of

growth with blight at the 75 percent stage at

the beginning of September was again in close

agreement with the ** actual'* increase found by

lifting, with yields subject to a standard error

of about ±0.3 ton per acre. In the two dry years,

1949 and 1951, when blight caused no loss, there

was an indication of a loss caused by spraying.

This could not be attributed to wheel damage,

as the plots were sprayed by hand. It was

probably due to copper injury following the

insect damage common in dry years. Some

21
TABLE 6.--Results of spraying trials with 2 well-timed applications of bordeaux mixture on

Majestic potatoes at Goole in Yorkshire, 1947-51

Date when blight on Prolongation

of

growth by

spraying

Total yield

of

unsprayed

plots

Gain from spraying

unsprayed hauljn re ached--

Actual

by

lifting

Expected

from 75

percent

dates

Year

0.1

percent

stage

75

percent

stage

1947 Aug. 5

Aug. 13

Sept. 9

Aug. 4

Aug. 28

Sept. 28

Sept. 6

Sept. 28

Sept. 2

Sept. 28

Days

14

21

10

20

7

Tons/acre

14.9

10.3

15.5

13.6

14.4

Tons/acre

+0.1

+1.0

-.9

+2.1

-.9

Tons/acre

0

1948

1949

1950

1951

1.2

0

1.6

0

yellowing and scorching was observed on the

sprayed plots.

The third series of trials was that conducted

by Large, Storey, Taylor, and Yule (1954) over

the 6 years, 1948-53, at Terrington, south of

the Wash, and on silt land at the border of the

Fen blight zone (fig. 5). The variety was King

Edward, with good certified stocks in all years;

the replicated plots were large--up to an acre

in area and running the whole length of the

field; and the spraying was done on contract.

A 13-row tractor-hauled sprayer was used to

apply 6 lb. per acre of copper oxychloride

(50 percent Cu) at 100 gallons per acre, except

in 1949 when a proprietary burgundy mixture

was used. This high-volume spraying was

compared with low-volume spraying, using an

Agro air-blast machine with droplegs, and also

with dry dusting. The number and timing of the

respective applications were as shown in figure

9, which gives the blight progress curves for

the several treatments and years. In table 7 the

leading figures for the spraying treatments are

abstracted.

In the course of these trials separate

weighings were made from rows damaged and

not damaged by the passage of the sprayer

wheels. With 4 rows in 13 so damaged, the

mean loss due to this cause was 4 percent of

the potential yield. A deduction of 4 percent

from the expected gain has therefore been made

in the table. With this correction the *'expected" gains are once again in close agreement with the **actual" gains as found by lifting

the crop (yields subject to standard error of

to.5 ton per acre). In these trials there was a

statistically significant loss from spraying in

the two hot and dry **no-blight'* years, 1949

and 1952. This loss considerably exceeded the

loss due to wheel damage, and it was probably

caused by copper injury, as at Goole, but in this

case following both insect and wheel damage

in the hot dry season.

A fourth series of trials was carried out in

the west Midlands over the years 1950-54

(Rosser, 1957). Here the trial variety was

King Edward, the spraying was done with a

22

knapsack sprayer, and copper oxychloride

(50 percent Cu) was used at 5 lb. per 100

gallons per acre. There were always 2 and

sometimes 3 applications, timed as far as

possible in relation to Beaumont periods. The

trials were made each year in each of the 5

counties, Cheshire, Shropshire, Staffordshire,

Warwickshire, and Worcestershire. In general

the epidemics were earliest in Worcestershire,

and table 8 shows the leading results in this

county.

Here again there was very good agreement

between the expected and the actual gains, and

in all cases the expected gains were within

the standard error of those determined by

lifting.

Occasionally, in individual trials, the estinnated gain has varied significantly from that

determined by lifting, owing sometimes to a

greater effective prolongation of growth than

that indicated between 75 percent points on the

curves for sprayed and unsprayed where these

have not shown the usual parallelism; and

sometimes to exceptionally late bulking, to late

planting, or to other causes.

In a comparable group of 32 crops of Majestic and King Edward, all planted about the

same time--the first half of April--and all in

the southern blight zone, on which fortnightly

liftings were made in supplementary work on

the crop check surveys in 1956, 15 reached

the 75 percent stage, or were burned off,

shortly before the end of August, and 17 did not

reach this stage until mid-September. The

mean yields, in tons per acre, for the two

groups are shown in table 9.

The mean potential yield for both groups of

crops was about 12 tons per acre. The percentages of the potential crop present on the

several dates were, therefore, as shown in

table 10.

The percentages in table 10 corresponded

very closely to the normal growth curve in

figure 8, and the mean gain from prolongation

of growth from the end of August to midSeptember, 1.3 tons per acre, was in exact

accordance with expectation.
ENGLAND 6 WALES

lOO

BLIGHT

ON

HAULM

p«r cent SO

Jun€ July

-jd—fr

d = dusting

Q- atomising

w- wet spraying

t= top spraying

August September

7^ ^^-i:^—

October

Gain from

spraying I TON

per acre

1948

lOO

50

fd—75"

Tw&a

TT

NO BLIGHT

Loss from

spraying 2 TONS

per acre

1949

lOO

50

O

lOO

50

75 ^

Twaa

id dTTd"^ T

waa ^&Q^X I

^-^-^^ i BurntTWêC

Td fd I'd jrd

TWiO&a TwaaW

Gain from_

spraying 3TONS

peracre

I950

1951

1952

1953

HGURE 9.-Progress charts of blight on unsprayed. sprayed, and dusted King Edward potatoes in trials at Terrington.

near the Wash. 1943-53. showing dates of applications and approximate gains and losses from spraying.

23
TABLE 7. --Gain or loss from spraying King Edward potatoes at Terrington, 1948-53

Date when blight on

Prolongation

of growth

by

spraying

Total

yield of

Ijnsprayed

plots

Gain from spraying

Year

reached—

Actual

by

lifting

0.1

percentage

stage

75

percent

stage

Expected

from 75

percent dates

1948 July 30

(")

July 24

Mg. 30

(")

July 7

Aug. 13

Aug. 17

Sept. 22

(')

Aug. 13

Days

12

0

25

10

0

13

Tons/acre

16.3

14.8

14.0

16.2

18.9

16.6

Tons/acre

+0.9

-2.0

+3.0

-.1

-1.8

+2.6

Tons/acre

+1.9

1939 -.6

+3.0

-.6

-.7

+2.3

1950

1951

1952

1953

No blight.

TABLE 8.--Gains from spraying King Edward potatoes in Worcestershire, 1950-54

Date when blight on

Prolongation

of

growth by

spraying

Total yield

of

unsprayed

plots

Gain from spraying

Year

reached

Actual

by

lifting

Expected

from 75

percent

dates

0.1 percent

stage

75 percent

stage

1950 July 24

July 31

Aug. 7

July 24

July 20

Sept. 4

Sept. 20

Sept. 16

Aug. 24

Sept. 4

Days

22

10

14

3

16

Tons/acre

12.9

11.8

10.4

10.5

14.4

Tons/acre

2.3 + 0.9

.5 + .7

1.0 + .6

-.4 + .7

1.6 + .3

Tons/acre

1951

1.5

1952 .3

1953 .5

1954 .3

1.3

TABLE 9. --Meanyields of 32 crops of Majestic and King Edward potatoes in southern blight zones

as related to date blight reached 75 percent stage, 1956

75 percent

Crops

Total yield

blight stage

reached End of

July

lYiidAug.

End of

Aug.

MidSept.

End of

Sept.

End of Aug

Llid-Sept

Number

15

17

Tons/acre

6.1

5.6

Tons/acre

8.4

7.6

Tons/acre

9.9

10.3

Tons/acre

10.3

11.5

Tons/acre

10.3

11.6

24
TABLE 10. --Yields in table 9 expressed as percentages of mean potential yield (12 tons per acre)

Crops

Percentage of potential yield

L,

75 percent stage reached— End of

July

MidAug.

End of

Aug.

MidSept.

End of

Sept.

Vnñ of Au?

Number

15

17

51

47

50

70

63

72

S3

86

87

86

96

96

86

Ml ñ ->Sp"nt 97

Expected yield (see fig. 8) 100

Further work is required on checking the

normal growth curve and in adapting the

method of estimating blight losses for use on

individual crops. Enough has already been done,

however, to show that the present curve can

now reasonably be employed without further

delay for the computation of average blight

losses over a number of seasons, in the several

blight zones of England and Wales.

PROTECTIVE SPRAYING: MACHINES AND

MATERIALS

Freshly prepared bordeaux and burgundy

mixtures, although unsurpassed in point of

efficiency, are now very little used for potato

spraying in England and Wales. Their place

has been taken by copper oxychloride and

cuprous oxide preparations, usually applied at

the same copper dosage (2.5 lb. Cu per acre).

These preparations are much less resistant to

washing off by rain than the bordeaux and

burgundy mixtures (Large, Beer, and Patterson, 1946), and they save the need for a certain

amount of skill and labor in the preparation

of the spray fluid on the field. Unlike bordeaux

and burgundy mixtures , copper oxychloride and

cuprous oxide preparations can be used for

low-volume application. Generally, they give

nearly as good protection of the haulm as the

bordeaux and burgundy mixtures, except where

high retention is really needed to resist heavy

rain, as in the Fens in the years 1954 and

1956, when the performance of the newer

preparations was very disappointing.

Much of the more efficient spraying is now

done on contract with high-volume miachines

(applying 80 to 100 gallons per acre), and with

wide booms (11- to 15-row widths) to keep

down wheel-damage as much as possible. The

contract charge for such spraying, with 5 lb.

per acre of copper oxychloride (50 percentCu)

was about 45s. per acre per application in 1956.

Two sprayings are usually applied, three are

sometimes given.

Air-blast low-volume sprayers (Agro

sprayers) applying copper oxychloride or

cuprous oxide in 10 to 20 gallons of water

per acre came into fairly extensive use after

1945; These machines, with nozzles set well

down between the rows on droplegs, gave

about as good results as high-volume machines

applying the same materials. But owing to

the weight, complexity, and high cost of the

Agro sprayers, they have since been replaced

by low- or medium-volume machines of the

type introduced for the application of 2, 4-D

and MCPA weed killers on cereals. These

machines apply 20 to 40 gallons of spray

fluid per acre at pressures of 50 to 100 lb.

per sq. in. through fine nozzles. Such machines, which are light, relatively inexpensive, and often already on the farm, make

it easy for farmers to do more potato spraying themselves. The old difficulty of water

cartage is greatly reduced. But it is doubtful

if spraying the plants from above only, as is

usual with these machines, gives as good

protection of the foliage as spraying with the

traditional droplances (Large and Taylor,

1953). As yet the machines are often not

sufficiently adapted mechanically for the

particular and rather exacting job of potato

spraying; infinite trouble, as well as very

poor spraying, will result from blockage of

the fine nozzles unless the filters are kept in

good order. With copper oxychloride (50 percent Cu) at 400s. per cwt., the cost to the

farmer for materials only, is 18s. per

application.

A considerable amount of dusting is done in

the Fens. At one time 6 to 7 dustings were

given in the course of the season, but the

more usual number of dustings is now 3 or at

most 4. Dusting is now by no means cheap in

comparison with spraying. Copper dusts containing 20 percent Cu are the most popular,

and these are applied at 16 to 20 lb. per acre.

Such dusts cost about 170s. per cwt. in 1956,

so that the cost of materials per application

was about 30s. It is generally reckoned that

2 dustings are equivalent to one thorough

spraying in protective effect, but dustings may

do rather better than this if the applications

are well timed in relation to the blight attack.

Since 1950 a considerable acreage of

potatoes is now sprayed from the air in eastern

and southern England. Both helicopters and

fixed-wing aircraft are used. The standard

amount of copper oxychloride is generally

applied in 3 to 6 gallons of water per acre.

The contract charge for this air spraying has

25
been about the same as for spraying with

ground machines. Although many farmers

have been well satisfied with the results and

the wheel damage from the passage of ground

machines is avoided, there is still a lack of

critical trial data for assessing the efficiency

of this form of application.

Dithiocarbamates were not launched on the

market for potato spraying in England and

Wales until 1957, and little experience has so

far been gained of their use under English

conditions. If the need for frequent renewal

of the spray deposits (Large and Beer, 1946)

can be overcome, they may have a useful

field of application (Cox, 1957), especially in

the eastern parts of the country where some

copper injury (following insect attack) is caused

by routine spraying in the warmer and drier

seasons. In the wetter seasons when nnore

blight develops, copper could be substituted

for the later applications.

THE GAIN FROM SPRAYING

In estimating the net gain from spraying,

one difficulty is in making fair allowance for

the damage caused by the passage of the

sprayer through the crop. In the trials at

Terrington this danciage was found to amount

to 4 percent of the potential crop when a 13-

row sprayer was used. The corresponding

damage with an 11-row machine would be 5

percent, and with a 7-row machine as muchas

7 1/2 percent. But the crops at Terrington

were exceptionally dense and heavy; in lighter

and more upright crops the damage is obviously

less, but much spraying is done, especially by

farmers, with machines of quite narrow rowwidth, and care is not always taken to drive

the sprayer over old wheel tracks when repeating the applications. For the purpose of

the present estimates a mean figure of 3

percent is taken for loss of potential yield

by wheel damage.

There is also the matter of the copper

injury to the haulm in hot dry seasons. This

effect is certainly seen occasionally, but it

must not be exaggerated. In order that the

present estimates may be correct, at least

in principle, a small loss from copper injury

is assumed in the Fen and southern zones

only, in the years (3 in 10) when the 75 percent blight stage is not reached until the end

of September, if at all. The figure is put as

equal to that by wheel damage, thus, in these

zones and years, the total for loss by spraying

damage is estimated at 6 percent of the

potential crop.

As against this, the figures taken for gain

from spraying in the blight years are put at

their very best, and are such as can be

achieved only by efficient spraying, with two

well-timed applications in general, and three

in seasons with exceptionally heavy rain. It

is assumed, for simplicity of estimation, that

the net gain frona spraying (after deducting

the loss by wheel damage) is equivalent to

two clear weeks' prolongation of growth, at

whatever time in August or early September

it may occur, and that in effect the actual

prolongation is some days more than this to

compensate for the wheel damage. On this

basis the gains and losses, according to the

time at which blight reaches 75 percent on

the unsprayed field, for maincrops of 10-ton

potential yield are as shown in table 11.

For a 15-ton potential crop, which is the

most common level of crops in trials, and on

the better commercial crops on which most

spraying tends to be done, all the above figures

would be increased by a half. This increase

would give a net gain from spraying in the

worst blight years of about 2 tons per acre

and for the other blight years (75 percent

stage at end of August) about 11/4 tons per

acre, despite the losses by wheel damage.

Quite often, in individual cases, higher gains

from spraying, up to 3 tons per acre, are

obtained; but growers in the Fens, in such

years as 1954 and 1956, wo\ild have been

very content with gains of about 2 tons.

If the data from tables 1 and 11 are combined, the gains from spraying on maincrops,

over the 10 years, 1947-56--assuming the

TABLE 11. --Net gain or loss from spraying according to the date when blight reaches

the 75 percent stage on a potential crop yield of 10 tons potatoes per acre

75 percent blight on unsprayed

crops by—

Yield of

unsprayed

crops

Net gain

from

spraying

Unrecovered loss

By wheel

damage

By

blight

Potential

crop

Mid-August ,

End of August..•.

Mid-September...,

End of September<

Tons/acre

7.2 r

8.7

9.6

10.0

Tons/iacre

1.4

.8

.1

-.6

Tons/acre

0.3

.3

.3

.3

Tons/acre

1.1

.2

0

0

Tons/acre

10

10

10

10

26
TABLE 12. Gain from routine spraying over 10 years (1947-56) for maincrop potatoes

Enerland anri Walec in

Zone

Percentage

of

total

acreage

Mean net

gain from

spraying

Percentage

of

acreage

sprayed

Mean gain

for

zone

Contribution

to

national

gain

Southwes t Ö

13

51

28

Percent

11.0

2.4

.3

20

55

40

5

Percent

2.20

3.02

.96

.01

Percent

Fens

0.18

Southern .39

Northern .50

~~

England and Wales — — 30 — 1.07

TABLE 13. --Gain from spraying only where needed over a 10-year period in England and Wales,

1947-56

Zone

Percentage

of

total

acreage

Mean net

gain from

spraying

Percentage

of

acreage

sprayed

Mean gain

for zone

Contribution

to national

gain

Southwest 8

13

51

28

Percent

11.0

7.2

4.0

0

20 (10)

55 (6)

40 (5)

5 (0)

Percent

2.20

3.95

1.60

0

Percent

0.18

.52

.80

0

Fens ••

Southern..................

Northern •

England and Wales -- — 30 — 1.50

Number in parentheses refers to years in 10 in which spraying is needed and assumed done.

amount of spraying done had been at its recent

level over the whole period, that all of it had

been efficient spraying, and that it had been

done every year--would have been as shown

in table 12.

Thus, if routine spraying were done every

year on the percentage of the acreage now

receiving any protective treatment and all

this spraying gave the best results obtainable

in practice, the mean gain over a 10-year

period would be only about 1 percent of the

potential maincrop yield from the country asa

whole. This would represent a saving of about

one-eighth of the total defoliation loss due to

blight (table 1). With the actual mean yield

at about 7.5 tons per acre, the nnean potential

yield (were there no blight) would be about

8.2 tons per acre. The gain would therefore

be about 0.09 ton per acre, or a difference of

one point (0.1 ton) on the average yield for

the country.

If the price of potatoes is £11 per ton, this

gain, on a half million acres of maincrops,

would represent a gain of some 50,000 tons

of potatoes worth about £550,000. With the

mean cost of spraying of 2 applications by

contract at 45s. per acre on 30 percent of the

acreage each year, this gain would be obtained

at a cost to the growers of about £675,000.

If the spraying were done on the same

percentage of the acreage in each zone as that

shown in table 12, but only in blight years,

or, to be more precise, only in those zones

and seasons where the 75 percent stage is

reached by the end of August, then the analysis

of gains and the saving in spraying would be

as shown in table 13. Spraying on this basis

would increase the long-term gain to about

0.15 ton per acre, and through the avoidance of

spray damage in the **no-blight*' years it

would save nearly half (46 percent) of the

spraying over a 10-year period. Thus, there

would be a mean annual gain of 75,000 tons

of potatoes worth about £825,000 at a cost for

spraying of about £310,000. The justification

for forecasting is that it can help toward the

attainment of this better result.

In fact, protective spraying in England and

Wales is not done either wholly by routine

(table 12) or wholly according to need (table

13). Also, if the inefficiency of some of the

spraying is considered, the mean national

27
gain is probably nearer the lower figure of 0.1

ton per acre on the long-term average.

In a bad blight year, such as 1953, however,

the gain fronn spraying may amount to 0.35

ton per acre in the mean yield for the country.

Such a gain is very substantial, reckoned on

the whole acreage, but its value is lessened by

the fact (brought out in table 3) that in the

blight years the national yields tend to be

above average, as the rain that favors the

development of blight is even more favorable

for the growth of the crop. In 1953, the actual

yield was 8,7 tons, as against the average of

7.5 tons for the 10-year period, and more

potatoes than were gained by spraying must

have gone for stock feed at v:4 per ton. The

year 1953 was one in which the yield was

exceptionally high, but the mean yield for

the 5 blight years in the period 1947-56 was

8 tons per acre, as against a mean of 7.1 tons

for the other 5 years. The years giving rise

to shortage of supplies (and high prices) are

not usually the blight years in England and

Wales--they are the drought years in which

there is little or no blight.

Despite these national considerations, the

gain from spraying achieved by individual

growers, or groups of growers in particular

districts, is often very considerable. At present

prices a net gain of only about 8 cwt. per acre

will pay for two sprayings, and in a blight

year in some districts there is a likelihood

of obtaining 20 or even 40 cwt. increase for

this outlay.

Routine spraying (spraying every year) is

profitable in the southwest zone, and just

profitable in the Fen zone. In the large

southern zone, protective spraying is only

profitable if it is done according to need.

This implies a nice discrimination in selecting

the crops to be sprayed--those of the higher

potential yield and those of the more susceptible varieties, such as King Edward, which

gives the best return from spraying--and an

equally nice judgment of seasonal conditions.

The forecasting work in England and Wales

is designed to help in this latter connection,

by indicating the time at which the first

spraying can be given to the best advantage in

blight years and by discouraging spraying in

years when little blight is likely to develop.

BLIGHT IN THE TUBERS

Losses due to blight infection of the tubers

vary greatly from crop to crop in England

and Wales. Even in bad years for tuber infection more than half the crops suffer no loss,

or a very trivial loss, from this cause. But,

against this, a small proportion of the crops

always siiffer a heavy loss, even up to 2 tons

per acre or more, and now and then the loss

of an entire clamp is reported, owing to heating and secondary rots following blight. The

occasional severe losses tend to color the

picture and give the impression that the overall loss from blight in tubers is much greater

than is in fact the case; while the sporadic

and extremely variable incidence of the loss

renders it extremely difficult to assess by

survey methods.

Losses due to blight in the tubers may be

divided into two fairly distinct parts, namely:

1. Losses caused by infection of the tubers

in the ground before lifting and apparent

at lifting.

2. Losses caused by infection at or shortly

before lifting, and developing in store.

In some countries the second of these losses

may be by far the greater, for if the potatoes

are lifted when the haulm is still partly green

and with the blight fungus actively sporulating

on it and the lifting is done in wet weather,

very extensive infection of the exposed tubers

is to be expected and considerable losses

will occur in store. It is also possible that

some infection at lifting may occur from

spores surviving in the soil for 10 days after

the haulm is dead, as reported by Murphy

(1921). In England and Wales, however, most

of the maincrops are not lifted until October,

and by or before that time much of the haulm

has either been killed off by blight or by

normal autumnal dying down. It is generally

only in the late-maturing crops (notably in the

northern blight zone), in some crops kept

green unprofitably long by protective spraying, and in crops lifted early to supply an

immediate market, that there is much real

opportunity for infection at lifting to occur.

Even so, if the lifting is done in a dry spell

or the blight lesions on the haulms have

dried up, very little infection may occur.

Opportunity for the tubers to be infected

in the ground before lifting is not restrictedto

those comparatively few crops in which the

haulm is still partly green near lifting time.

Infection occurs wherever the soil permits

access to the tubers of spores washed down

by rain, and it persists over the entire period

during which blight is sporing on the haulm--

from the beginning to the end of the foliage

attack, and perhaps for 10 days afterward.

Moreover immature tubers are considerably

more susceptible to infection than mature

tubers at lifting (Boyd and Henderson, 1953).

Thus, infection in the ground before lifting is

responsible for the most widespread occurrence of blight in the tubers and possibly for the

greater part of all the loss from this cause in

England and Wales.

Losses Due to Infection Before Lifting

Weighings of blighted tubers carried out in

the crop-check surveys since 1953 now provide good data on the waste due to blight

infection before lifting in a representative

sanaple of some 2,000 crops in England and

Wales each year. Table 14 shows the mean

28
TABLE 14. --Mean losses of potatoes caused

by blight infection before lifting, England

and Wales, 1953-56

Blighted tubers 1953 1954 1955 1956

Hundredweight per acre

Percentage of crop..,.

2.5

1.3

3,4

2.0

0.2

.1

2.9

1.6

loases in all these crops over the country as

a whole in the years 1953-56.

Thus, in the three blight years, 1953, 1954,

and 1956, the mean loss was 2 percent; and in

the dry year with very little blight on the

haulm, the loss was negligible. The distribution

varied greatly according to region, as shown

by the maps (fig. 10) for 1953 and 1954. The

variation according to region was from less

than 1 cwt. to lOto 15 cwt. per acre. In general,

the losses were highest where the blight

attack on the haulm had been earliest and

most severe, with the important exception of

the relatively heavy losses shown in parts of

the northern zone in 1953. This was in accordance with past experience (Moore, 1946, 1948)

that late attacks of blight in the north may, in

some years but not in all, cause heavy losses

by tuber blight.

The frequency of occurrence of different

levels of loss among the crops is shown in

figure 11 for 1954, the worst of the 4 years

for blight in tubers. This diagram brings out

the salient fact about the distribution of losses

due to blight in tubers: The large number of

crops that scarcely suffer at all; and the few

crops m which the loss is high. In 1954

57 percent of the 2,109 crops in the survey

had no blighted tubers (or less than 1/2 cwt.

per acre) before lifting; about 25 percent

had 1 to 5 cwt. per acre; about 8 percent had

5 to 10 cwt.; and there were sharply decreasing

percentages at higher rates. Although the

mean national loss was only 3 1/2 cwt. per

acre, more than 100 crops in the 2,109 had

a loss of over a ton per acre, and two had

over 3 tons.

No firm association can yet be established

between the level of tuber infection and the

type of soil; heavy losses may occur

occasionally, and negligible losses very frequently on both heavy and very light soils.

But the level of loss certainly varies according

to the potato variety. In the 3 years, 1953,

1954, and 1956, the breakdown according to

variety of the mean losses in the crop-check

surveys is shown in table 15,

The mean loss was over twice as great

in King Edward as in Majestic. In Arran

Banner and Gladstone, the loss was over

six times as great as in Majestic. Indeed,

in the histogram for 1954 (fig, 11 ) of the 88

crops shown in which the loss was over 1

ton per acre, 23 were Arran Banner, 38 King

Edward, 7 Gladstone, and only 12 Majestic,

FIGURE 10.--Regional distribution of losses due to blight in tubers, 1953 and 1954.

29
ENGLAND 6 WALES

1 —57 PER CENT OF CROPS

-25 PER CENT OF CROPS

Each dot represenfs one crop

in rhe survey

Mean loss : 3¿cwt per acre

— 6 PER CENT OF CROPS

— 4 PER CENT OF CROPS

— I PER CENT OF CROPS

ifUMi

I -5 -lO -15 -20 -25 -30 -35 -40 -45 -50 -55 -60 -65

• BLIGHTED TUBERS CWT PER ACRE MEAN

FIGURE 11.--Frequency of différent levels of blight in tubers before

lifting in a survey of 2,109 crops in England and Wales, 1954.

despite the preponderance of crops planted in

Majestic.

The estimation of blighted tubers in the

crop-check surveys has not yet been done in

all types of season in which tuber infection

may be prevalent. The reports on the diseases

of the crops in England and Wales (Moore,

1946, 1948, and earlier bulletins) indicate

that only the very dry years (like 1955) in

which there is little or no blight on the

haulm at the end of September can be regarded

as ''no-blight" years for the disease in the

tubers. Years in which the 75 percent stage on

the haulm is not reached until mid-September

may be among the worst for blight in the

tubers; and years in which this stage is

reached at the end of August (blight-years

for the disease on the haulm) are most

frequently also bad years for blight in the

tubers, although sometimes they are not. It

is the slope of the blight progress curve,

rather than the 75 percent date, that affects

the amount of blight in the tubers. The opportunity for tuber infection is greatest when

the attack on the haulm is long-drawn-out,

least when it is rapid.

Losses Due to Infection at Lifting

The proportion of the maincrops in England

and Wales in which the tubers may be infected

from blighted haulm at lifting can be judged

to some extent from table 16, which is based

on observations made in the course of cropcheck surveys.

It is evident that in more than two-thirds

of the crops over this period of years, the

haulmi was dead well before lifting began,

and that in all except the 2 years 1951 and

1952, in which the 75 percent blight stage

was in mid-September, there was little growth

left in the rest of the crops.

The only direct survey evidence bearing

upon the annount of infection occurring at

lifting and developing subsequently in the

clamps, is that from a systematic sampling

of 700 clamps in all parts of the country

carried out by advisory mycologists in the

1942-43 storage season (Moore, 1948), In this

survey the average loss from rots in March " was about 6 percent--half of which was

attributed to blight. In only 38 (5.4 percent)"

of the clamps did the loss from blight exceed,

10 percent, although in several it reached 301

percent, and in one 43 percent. These figures [

indicate the same kind of distribution oi\

losses in clamps as that shown in figure 11,,'

for blighted tubers in the groundbefore lifting. ^ The year 1942 was one with much blight in'

September (75 percent stage about the end

of August in the worst-affected parts of the

south, and a week or two later elsewhere)

and the progress of the attack was fairly'

rapid. It was recorded at the time of the ,

survey that the losses from tuber blight had '

been somewhat greater in the previous year,

1941, when the attack on the haulm was more ' drawn-out. In some years, therefore, the losses

in clamps may be higher than in 1942, but in ^

some other years, such as 1955, the losses^

are undoubtedly less. It may not be un-^|

reasonable, therefore, to take the mean figure^

of 3 percent, carefully determined by accurate

sampling in 1942-43, as a provisional estimate ^

of the mean annual loss caused by blight in ^^

clamps in England and Wales.

This loss is made up of the tubers infected ^|

at lifting together with a fairly high proportion í

of the tubers infected in the soil before lifting. |^

Many of these latter, where they are obviously J

and severely blighted, are rejected when the

crop is picked by hand, and tiiey are not put ^

into the clamp. The proportion is quite in- ^

determinate, and must dependupon the weather j

at lifting, the type of labor employed, and

other factors. ^

30
TABLE 15.—Mean losses of potatoes from blight infection of tubers before lifting, according to

variety, 1953, 1954, and 1956

Variety

Crops in survey

Blighted tubers

lifting

before

1953 1954 1956 1953 1954 1956

Majestic

KT TiP" Edward

Number

1,019

379

120

41

Number

954

566

92

28

Number

1,005

591

62

16

Percent

1

2

4

8

Percent

1

3

9

10

Percent

1

2

11

17

Ä-rran Banner. ••••.••

Gladstone

All varieties 2,070 2,109 2,057 1.5 1.7 1.7

TABLE 16. —Percentage of potatoes lifted in relation to state of haulm before lifting, England and

Wales, 1951-56

State of haulm some

days before lifting 1951 1952 1953 1954 1955 1956

Dead

Percent

66

25

9

Percent

57

34

9

Percent

84

16

0

Percent

S5

15

0

Percent

56

AA

0

Percent

94

Little growth left

Much growth left

6

0

From a review of all the available evidence

it would appear that the mean total loss from

blight infection of the tubers for all varieties,

and for England and Wales as a whole, is

about 3 percent of the crop, and that this loss

is made up of about Z percent apparent at

lifting and 1 percent developing in store.

This estimated mean loss due to blight in

the tubers is less than half of the estimated

mean loss caused by premature defoliation

(table 1); but it is a more constant loss, and

it probably occurs in all except the very dry

••no-blight*' seasons; that is to say in 8 or 9

years out of 10.

CONTROL MEASURES

The principal measures for the reduction

of losses due to blight infection of the tubers

in England and Wales are (1) efficient earthing

up of the rows to insure good soil cover over

the tubers; (2) the postponement of lifting

until the haulm has been dead for 10 days;

and, (3) the avoidance, as far as is practicable,

of varieties like Gladstone, known to be very

susceptible to tuber infection.

Protective spraying of the haulm has some

effect in reducing tuber infection in the ground

before lifting, by keeping blight off the haulm

for about a fortnight in the early stages of

the attack and thereby reducing the period

during which washing down of spores can

occur. But in England and Wales protective

spraying is rarely employed for the purpose

of protecting the tubers by keeping blight

off the haulm altogether. This would often

require far more applications than the two

or at most three that are generally given, and

it would scarcely be economic. As it is,

protective spraying quite often prolongs the

growth of partially blighted haulm beyond

the stage at which useful increase of yield

can result, and thereby increases the amount

of tuber infection.

Cutting and removing the blighted haulm,

as a measure for decreasing the amount of

infection of the tubers, was advocated in the

19th century; indeed, alnaost as soon as the

disease appeared in Europe. However, the

measure was expensive and difficult to carry

out on any large acreage. In the early 1930*s

one of the chemical manure and sulfuric acid

companies in Norfolk carried out experiments

on the spraying of the haulm with diluted

sulfuric acid as a substitute for mechanical

destruction. The kill of the haulm was quite

spectacular, and by 1934 severallarge growers

in Norfolk and Lincolnshire who had been

persuaded to give the new process a trial

were hailing it with great enthusiasm. They

sprayed 3,500 acres in that year and claimed

that crops from unsprayed areas, on being

taken from the clamps, frequently contained

up to 50 percent of blighted tubers, wherea^s

those potatoes from the areas "burned-off**

with sulfuric acid were almost completely

free from the disease.

31
The first critical trial was carried out by

Bates and Martin (1935) under what can now

be seen to have been optimum conditions, on

a crop of King Edward at Terrington, near the

Wash. A 10 percent solution of B.O.V. (Brown

Oil of Vitriol, 77 percent H2SO4) was applied

to the green haulm at 100 gallons per acre on

September 16, 1934, when only a few plants

were yet affected by blight, and, as the yields

showed, almost the whole obtainable crop was

already under the plants. (See fig. 8 for expectation of increase of crop after

September 16.) After the date of spraying

blight spread all over the unsprayed plots,

giving opportunity for washing down of spores

for some 3 weeks, until the potatoes were

lifted on October 6, when much of the blighted

haulm was still green. The total produce from

the sprayed and unsprayed plots was stored

separately in the same clamp, and examined

for blighted tubers at the end of January 1935,

when the clcimp was opened. Table 17 shows

the results.

TABLE 17. --Percentage of King Edward tubers

with blight in clampfrom plots sprayed with

sulfuric acid, Terrington, January 1935

Treatment Total

yield Blighted tubers

Tons/

acre

11.9

12.0

+ .03

Cwt./

acre

9.1

1.6

+1.4

Percent

of crop

Unsprayed

Burned off with 10

percent B.O.V. ..

Standard error.

3.9

.7

These results were obtained under about

optimum conditions. The total percentage of

blighted tubers in the unsprayed crop was

about 4 percent, and the acid spraying, done

when there was still very little blight on the

haulm, reduced the figure to about 1 percent.

Since 1935, •'burning-off•• the haulm has

become firmly established in practice, and

indeed it is extremely popular among potato

growers. This is not only because it can,

in some circumstances, save a good deal of

blight in the tubers, but because it undoubtedly

facilitates lifting and kills weeds. It "cuts

losses," saves further concern about the

progress of blight in the crop, and, apart

from the lifting, it puts an end to the job for

the season.

Most of the acid spraying at the present

time is done by contractors, who apply 20

gallons of undiluted Brown Oil of Vitriol per

acre with special acid-resistant machines. The

contract price for this treatment in 1956

was about £4 per acre, the value of about

7 cwt. of ware potatoes.

In 1951, when there was a world shortage

of sulfur, soditun and potassium arsenite

mixtures were introduced for haulm destruction in England and Wales. Their use has

increased very rapidly since that time, despite

the availability of acid. In 1956 about half of

all the destructive spraying was being done

with arsenites and half with acid. The

arsenites, usually in the form of liquid preparations containing 10 lb. of As 2P3 per gallon,

are applied both by high- and low-volume

machines, at the rate of 1 gallon in 80 to

100 or in 20 to 30 gallons of water per acre,

respectively. The action of the arsenites is

slower than that of acid—the leaves take up

to a week to go brown, while those sprayed

with acid are blanched and limp on the day

of application; but there is little to choose in

point of eventual completeness of destruction

of the haulm. The contract charge for spraying with arsenites in 1956 was considerably

less than that for acid, being about £2 IBs.

Od. per acre. Arsenites have the advantage

that they canbe appliedby farmers themselves,

with sprayers available on the farm that are

often unsuitable for the application of acid.

The first systematic trials of haulm destruction at a nvunber of centers and over

several years, in England and Wales, were

those carried out by the National Agricultural

Advisory Service in 1951-53, for the purpose

of testing substitutes for sulfuric acid (Large,

1952b, 1952c, 1954). The destructive spraying

was done at each center at a time in accordance with practice in the district, and the

percentage of blighted tubers in the sprayed

and unsprayed parts of the crops was determined both at lifting and after a period of

6 weeks in storage. The mean results, with

acid, for 9 trials in 1951, 11 in 1952, and 11

in 1953, were as shown in table 18.

Despite the fact that many of the untreated

plots in these 31 trials were lifted while there

was still a little growth left in the haulm,

blighted tubers from the unsprayed plots

exceeded 3 percent after storage in only 9

trials and were less than 1 percent in 17 trials.

In the crops where heavier tuber infection

occurred, the destructive spraying showed

advantages varying according to the date on

which it was done, the progress of blight on

the unsprayed crop, the lifting date, and the

rainfall at lifting. All these variable factors

are illustrated for 1953 in figure 12, which

also shows the use made of blight progress

curves in the study of haulm destruction. As

a result of the trials as a whole it was concluded that destructive spraying, whether with

acid or with arsenites, as usually carried out

in practice, can at best save only about half

of the infection occurring in the ground before

lifting. Here and there, it may save considerable infection at lifting if the weather

happens to be wet and the haulm would not

32
TABLE 18.—Percentage of blighted tubers at lifting and after storage in spraying trials with

sulfuric acid, 1951-53

Blighted tubers

Treatment 1951 1952 1953

At

lifting

ACter

storage

At

lifting

After

storage

At

lifting

After

storage

An"!d SDraved «.•••.•..••.•>•••••

Percent

2.2

2.2

Percent

2.6

2.8

Percent

0.4

1.3

Percent

0.9

2.4

Percent

1.7

3.5

Percent

2.5

Untreated • •..••....•••.•• 3.7

otherwise have been dead in time* Similar

resiilts were obtained with mechanical haulmcutters or piilverizers, which are now available and are used on a relatively small acreage

ENGLAND 0 WALES

in England and Wales; and also by spraying

with certain tar oil fractions, which have not

come into any extensive use in England and

Wales but are nevertheless excellent nonpoisonous and noncorrosive haulm killers

(Large, 1952b, 1952c, 1954).

CUMBERLAND (Mojcstic) DURHAM CArranPeaO

July August Septcmb« 11*

II

3%

9% rtijf

i Bl 1 1 J 1 1

YORKSHIRE (Maicst c)

4 II

nTTrrTTTilml

1 1 L_^^J 1 1 1 I

LINCOLN (Glodttonc")

CAMBRIDGE (King Edword)

T"

m 11

.A 1 1

8%

20%

1 1 U^J 1 UJ U U I

KENT CMojestic)

1

^ Â

^^^rttílilÍ

T

1 %

;t JU

KDL

Beaumont period

% Haul m killed br blight lOO

Dote of A

50-

Blightcd tubers 4c %

%

Date of haulm lifting

destruction f

r Total roinfoll per week

FIGURE 12.—Percentage of bli^ited tubers in relation to progress of

blight on haulm, dates of haulm destruction and lifting, and rainfall

at 11 trials in England and Wales in 1953. Upper figures indicate

percentage of blighted tubers with haulm destruction; lower figures

Indicate percentage of bUghted mbers without haulm destruction.

GAINS AND LOSSES FROM HAULM

DESTRUCTION

The amount of haulm destruction now done

in England and Wales may be judged from

the percentages of the crops in the crop check

surveys on which farmers carried out this

treatment: 17 percent in 1953; 22 in 1954;

10 in 1955; and 30 in 1956.

In 1956 haulm destruction was carried out

on 47 percent of the 591 King Edward crops;

on 21 percent of the 1,005 crops of Majestic;

and on 26 percent of the 461 crops of other

varieties.

The mean weights of blighted tubers per

acre, before lifting, in the untreated crops,

and in the crops on which haulm destruction

was done, with or without protective spraying,

are shown in table 19.

Apart from the indication that , the mean

weight of blighted tubers was increased in

1954 by protective spraying without subsequent

haulm destruction, these mean figures from

some 8,000 crops provide no evidence of any

general reduction of blight in tubers as a

result of haulm destruction. However, haulm

destruction was probably done where it was

most needed, so that the figures are not inconsistent with the probabüity that there were

considerable gains on some crops.

Against this, reduction of blight in tubers

by destructive spraying of the haulm can

generally only be achieved at the expense

of some loss of the healthy crop. The loss

caused by stopping the growth of the haulin

too soon in destructive spraying can be judged

from the diagrams in figure 12 and the mean

worth of each week's growth in August and

September as shown in figure 8. If the haulm

is destroyed when blight reaches the 75 percent blight stage, then the loss of yiclf/^«

to shortening of the growing period will be

33
TABLE 19. —Mean weights of blighted tubers

per acre before lifting after various

treatments, England and Wales, 1953-56

Treatment

Mean weight of blighted

tuhers before lifting

1953 1954 1955 1956

Haulm destruction Cwt./

acre

3.2

3.4

2.8

2.3

Cwt./

acre

4.2

3.4

5.6

2.8

Cwt./

acre

0.02

.01

0

.40

Cwt./

after protective

spraying

Haulm destruction

only

Protective

spraying only...

No protective or

destructive

spraying

acre

2.7

3.1

3.1

2.4

negligible. However, there will then be little

reduction of the period during whichthe spores

can wash down into the soil, and therefore

little hope of much reduction in the amount of

blight in the tubers before lifting. If, on the

other hand, the destruction is done at the

25 percent blight stage or earlier, there can

be considerable reduction of the period during

which the spores can wash down, but there

will be a curtailment of useful growth.

This question of the crop sacrifice attendant upon haulm destruction was investigated extensively in the spraying trials carried

out on King Edward in all the counties of the

West Midlands over the years 1950-54 (Rosser,

1957). In each trial there were plots that

received no protective spraying but which

were burned off in early August (mean date,

August 8); at the end of August (mean date,

September 1); and in mid-September (mean

date, September 18). The mean total yields

and weights of blighted tubers in the 30 trials

were as shown in table 20. The expected reduction in yield from the premature killing

TABLE 20. —Effect of haulm destruction at various dates on total yield and on weight of blighted

tubers in trials on King Edward potatoes, England and Wales, 1950-54

Item

Mean date of haulm destruction

-Aug. 8 Sept. 1 Sept. 18 Sept. 30

Blighted tubers tons/acre

Total crop tons/acre

Percentage of potential yield

Expected percentage (fig. 8)

0.25 0.40

9.50 12.60

68 91

62 SB

0.60

13.40

97

97

0.65

13.80

100

100

of the haulm, as read from figure 8, is added

for comparison.

These trials provide confirmiationthat within

practical limits the growth curve for Majestic,

given in figure 8, is also validfor King Edward

in England and Wales. They also left no doubt

about the loss of crop involved when premature

haulm destruction is employed in attempts

to reduce tuber infection before lifting.Haulm

destruction in early August reduced the weight

of blighted tubers from 13 to 5 cwt. per acre

at the expense of a loss of 4 tons in the total

crop. Haulm destruction at the end of August

reduced the blighted tubers from 13 to 8 cwt.

per acre at the expense of 1 ton of the total

crop; and haulm destruction in mid-September

caused no significant reduction either in yield

or in the weight of blighted tubers.

In view of the loss of some healthy yield

that is almost inseparable from haulm destruction as normally practiced, and the low mean

level of blight in tubers in England and Wales,

as revealed by the trials and surveys, it would

appear that the mean national gainfrom reduction of blight in tubers by haulm destruction

may well be almost nil. For the individual

grower it nnay be very profitable now and then.

But as the reduction of blighted tubers has to

be 10 cwt. per acre to pay for the treatment

and for some few cwt. of sound crop lost, the

chances of its being profitable are not more

than 1 in 10 on King Edward and 1 in ZO on

Majestic (see frequency diagram, figure 11

et seq.). It is perhaps best to regard haulm

destruction in England and Wales as a farm

operation worth its cost in preparing the field

for lifting, especially needed when the crop

has to be lifted early, and sometimes paying

a bonus by reducing blight in tubers in other

circumstances.

RESISTANT VARIETIES

Potato breeding for blight resistance

throughout the world owes muchto the pioneering work done in England by the late Radcliffe

N. Salaman on the employment of Solanum

demissum as a source of blight resistance,

and the recognition by him and his coworkers

at Cambridge of the existence of biotypes of

the blight fungus in the early 1930's (Salaman,

1949). The progress made up to 1939, and

34
the prospects of success in breeding for

resistance from R-genes, and for what is now

called "field resistance," as seen by those

two great exponents of the rival schools of

thought, Salaman and Pethybridge, was presented in their lifetime and with their aid

by Large (1940). The influence of Salaman

and Hudson, and the exploration of Hawkes in

South America (Hawkes 1941, 1947) led to

the establishment of the Commonwealth Potato

Collection at Cambridge,^ to provide potato

breeders with new sources of R-resistant and

other genetically valuable material. Much work

on the fundamental problems of breeding for

^ blight resistance has been done, not only

by Black in Scotland (q.v.) but also by Howard

at the Plant Breeding Station, Cambridge

(Howard, 1953), and by Müller and others at the

National Institute of Agricultural Botany, also

at Cambridge (Müller, 1953; Müller, Culien,

and Kostrow icka, 1955).

Since 1945, six named R-gene résistant

' varieties, raised in Scotland and Northern

Ireland, have been submitted to trial at numerous centers in England and Wales by the

National Institute of Agricultural Botany

(N.I.A.B.). All six of these varieties are

attacked by blight on thé haulm before the

end of their growing period, and all six are

susceptible to blight in the tubers. The one

variety of the R3 genotype--Pentland Ace

(I95I), a second-early potato--has been found

to pulp badly on cooking. Of the five named

varieties of the Rj genotype, Craig's Snow

White (1948), a maincrop (comparable with

Arran Peak in point of maturity), has failed

because of its extreme tolerance of virus Y,

its late maturity, and its indifferent cooking

quality. Craig's Bounty (1943), also a maincrop,

has failed because of its poor quality and long

stolons. Ulster Torch (1955), another maincrop, is still promising, but has badly shaped

tubers. Pentland Beauty (1955), a first-early

variety, is all right so far.

Of the six varieties, only one, Orion (1947),

an early maincrop (comparable with Majestic

for maturity), has survived the long series of

registration and N.I.A.B. trials (which take

10 years from the production of the first

seedling by the breeder) to reach the stage of

recommendation by the N.I.A.B. in 1956. That

honor was shared with only four varieties for

lifting as maincrops: Majestic, King Edward,

Ulster Supreme, and Arran Viking; but it was

a restricted recommendation for special purposes, as Orion has yellow flesh and in England

and Wales it is suitable for chipping or processing only.

Potato breeding, specifically for **field resistance" to blight as defined by Müller (1953),

has scarcely yet started in England and Wale£;,

but breeding for as much practical resistance

as possible has been going on for at least

* Now at the John Innes Horticultural Institution, Bayfordbury, Herts.

three-quarters of a century. Such resistance

has always been one, among the many desirable

qualities that the breeders have striven to

obtain. A number of excellent new potato varieties, other than those few with R-genes, have

come into commercial cultivation in England

and Wales in recent years.

The testing of potato varieties is the responsibüity of the N.I.A.B., and Culien, in

charge of this work, has recorded the performance of these new varieties in trials

throughout the country (Culien, 1953, 1955,

1956). Home Guard (1943) and Ulster Premier

(1945) are among the best in the first-early

group; Craig's Alliance (1948), Craig's Royal

(1948), and Ulster Dale (1950) in the secondearly varieties; and Dr. Mclntosh (1944),

Arran Viking (1945), and Ulster Supreme

(1946) among the maincrop potatoes. Of these

last. Dr. Mclntosh and Ulster Supreme are

about equal to Majestic in point of susceptibility

to blight, while Arran Viking (as yet) is distinctly more resistant.

Apart from their long period of testing

before recommendation, potato varieties in

England and Wales, as elsewhere, undergo an

inexorable process of screening, in practice,

as a result of which the more blight-susceptible varieties fail to establish themselves or

pass out of cultivation, unless, like King

Edward, they have some other compensating

quality that renders them profitable to grow.

The very susceptible varieties, Up-to-Date and

British Queen, despite their unrivaled quality,

have almost disappeared, and Gladstone, very

susceptible to blight in the tubers, is going

the same way. On the other hand, it is undoubtedly largely because of its low susceptibility to blight in the tubers that Majestic

has reached and still maintains its place as

the predominant maincrop variety. The adoption of Majestic probab' represents the most

economically effective measure of blight control yet achieved in England and Wales.

The importance of this can be seen fromi

table 15. If an '*improved King Edward" could

be produced by the breeders, t combine the

shape, pink particolored skin, and accepted

quality of King Edward with the tuber resistance of Majestic, that achievement alone

would save the country at least 1 percent of

the present King Edward production, or about

14,000 tons of potatoes almost every year.

Another little realized, but nonetheless significant measure, adopted in England and Wales

for the reduction of blight losses, is represented by the local preference for Arran

Banner as the principal nnaincrop variety in

Cornwall and other parts of the southwest

where the earliest blight epidemics occur.

Arran Banner, with its high susceptibility to

blight in the tubers, can scarcely be said to

have miuch resistance to the disease, but

because of its heavy yield and early bulking

it can produce a good crop before blight

35
defoliation losses begin. This is an example

of control, not by disease resistance, but by

**disease escape/'

SUMMARY: ENGLAND AND WALES

Work on the definition and quantitative

measurement of blight epidemics, by their

effects both on the foliage and the tubers,

began in England in 1941. Since 1950 systematic

surveys designed to provide the necessary

information for the realistic evaluation of

national losses have been carried out over the

whole country each year. On the foundation of

this work it now becomes possible, for the

first time, to attempt apreliminary but closely

documented estimate of the present economic

significance of potato blight in England and

Wales. The attempt has here been miade, and

the many facets of the work, as well as the

methods used, have been described in some

detail. Briefly, in England and Wales, *'blightyears,** characterized by 75 percent defoliation of the unsprayed maincrop potatoes between mid-August and the end of August, occur

in about 5 years out of 10 inthe southern half of

the country, with later epidemics in the other

years. The mean loss by premature defoliation

on unsprayed crops over a 10-year period is

about 8 percent of the potential crop for the

country as a whole. Of this, less than one-half

could be recovered by efficient protective

spraying. In fact, only about 30 percent of the

crops arc sprayed, and the actual gain from

spraying does not amount to more than about 1

percent of the total potato productionon a longterm average. The mean loss due to blight in the

tubers is about 3 percent on the same basis.

This loss, on which destructive spraying,

practiced on a third to a half of the crops,

makes little or no impression, probably affects

the national economy more than the greater

losses by defoliation, which are in general

more than offset by high yields due to the

abundant rain in the blight years. The tuber

loss occurs in all except the few outstandingly

dry years, that is to say, in 8 or 9 years out

of 10, and 3 percent of the national production

represents some 120,000 tons of potatoes,

or the produce of 15,00.0 acres.

Although the mean national gain from protective spraying is slight, the gain to individual

growers in some districts and in some seasons

may be very appreciable--typically 2 tons per

acre where the 75 percent blight stage is

reached on the unsprayed haulm by midAugust, and about 1 ton where this stage is

reached at the end of August. Individual gains

from destructive spraying may also be considerable here and there, especially when

the crop has to be lifted early, but the chief

gain from this process is a saving of time

and trouble at lifting.

SCOTLAND

Scotland is noted for its production of highgrade seed potatoes, most of which are sold

to plant the ware acreage in England and Wales.

Seed potatoes are exported to a number of

other European countries, notably Spain, and

also to South Africa. In 1955, of the 135,000

acres of potatoes grown in Scotland, approximately 60 percent (80,000 acres) were for seed

production. Seed potatoes are grown throughout

the country. Consumption of ware potatoes is on

a level similar to that in England and Wales--

approximately 200 lb. per head per annum.

Average yields are around 8 tons per acre.

POTATO DISTRIBUTION AND VARIETIES

A large part of Scotland consists of mountainous areas unsuitable for arable cultivation.

Cropping is largely confined to the lower

ground, in particular the coastal and inland

districts running up the east of the country.

The largest concentrations of potatoes are

found in the three adjoining eastern counties:

Angus, Perth, and Fife. Elsewhere in the east,

Aberdeen, Kincardine, and East Lothian are

important potato areas. In the southwest in

Ayrshire there is an early potato district

confined to a narrow coastal strip with its

main center around Girvan. Figure 13 shows

the distribution of potato growing in Scotland.

Each dot = 1000 acres

FIGURE 13.--Distribution of potato growing in Scotland,

36
Although well over 50 different named varieties are grown, 7 varieties account for 80

percent of the total potato acreage. Three

of these, Ar ran Pilot (early), Majestic, and

King Edward (maincrop), are grown almost

entirely for export as seed. The remaining

four varieties, Epicure (early), Kerr's Pink,

Redskin, and Golden Wonder (maincrop) are

grown principally for home ware consumption.

A description of the varieties grown is given

in a publication on seed potatoes (Scotland

Department of Agriculture, 1?52).

SOIL AND CLIMATIC CONDITIONS

Most of the crop is grown on well-drained

sandy or silty loams. These are usually in

a state of high fertility, due to the system of

farming practiced, with its emphasis on a

balance between crops and livestock. Summer

temperatures are low: Mean monthly figures

at this time of the year do not usually exceed

60 F. Rainfall is usually adequate, although in

some potato areas, notably parts of East

Lothian, conditions in the summer may sometimes be too dry for the crop. A marked

feature of climatic conditions in the potato

growing areas is the variation, particularly in

rainfall and humidity, within short distances.

This results from the hilly or mountainous

nature of the countryside and the varying

distance of the crops from the sea.

Early potatoes are planted at the end of

February or early March, and lifted in June;

the maincrop season is from the end of April

to October.

BLIGHT EPIDEMICS: GENERAL

It would be unrealistic to attempt to review

blight epidemics for Scotland as a whole,

owing to variations in the severity of the

disease from place to place and the differences

in climatic conditions. Some division of the

country is necessary before the problem can

be approached. Unfortunately, it is not possible

to divide the country up into a small number

of climatic regions. The position is further

complicated by the growing of early and

maincrop varieties side by side in the seedgrowing areas, with variations in the incidence of blight on the 2 types of potatoes. For

the purposes of this study, a simple division

into 3 main regions has been made, based on

the 3 advisory regions of the Scottish Agricultural Advisory Service. These three

regions--the north, the east, and the west--

are shown in figure 14, and they can be described briefly as follows:

North region.--Includes 20 percent of the

total Scottish potato

acreage. Most of the potatoes are grown on relatively low ground near the

coast, from the south of

Kincardineshire to just

north of the Moray Firth.

Elsewhere, scattered

crops are grown in the

valleys, with some production on the islands,

notably the Orkneys.

East region.--62 percent of the potato

acreage. A very important

seed-growing area lies to

the north and south of the

Firth of Tay. Ware and

seed potatoes are produced

in the Lothians and Berwickshire.

West region.--18 percent of the potato

acreage. Potatoes are

grown throughout the area,

principally on low ground,

with concentrations near

Dumfries (seed), southwest Wigtownshire (early

ware), and Girvan (early

ware).

SiS'Ô ORKNEYS

SCOTLAND

FIGURE 14.--Advisory regions and bcalities of Scotland referred to

in text.

C. E. Foister has provided a summarized

account of potato blight in Scotland for each

of the years from 1946 to 1955. These accounts

have been built up from records supplied by

the three regional plant pathologists of the

advisory service and by the potato seed

37
inspectors of the Department of Agriculture

for Scotland. This material is not published.

Some additional information has been incorporated by Foister from annually published

accounts on the productivity of the principal

Scottish arable crops, given in the Transactions of the Royal Highland and Agricultural

Society of Scotland.

Table 21, which has been condensed from

Dr. Foister's account, shows the incidence

of blight on a regioncd basis over the 10-year

period.

TABLE 21. —Incidence of potato blight in Scotland, 1946-55

Mount of blight North Region East Region West Region

Moderate•••••••••••.•••. 1949, 1953

1948, 1954, 1955

1946, 1947, 1950,

1951, 1952

1948, 1949, 1950, 1954

1946, 1951, 1953

1947, 1952, 1955

1950, 1953

Slight

Negligible

1947, 1952, 1955

1946, 1948, 1949, .41,

1954

It is not possible to define exactly what is

meant by •'negligible," *'slight," and "moderate" attacks of the disease, but in the analysis given in table 21, the terms used refer to

the combined effect of the blight attack on the

haulm and on the tubers.

The most noticeable feature of table 21 is

the number of years in which the disease is

described as "negligible" or **slight." In the

north region, the disease was "negligible"

or ''slight" in 8 out of 10 years; in the east

region in 6 out of 10 years; and in the west

region in 8 out of 10 years. The east region

is the worst affected by blight during the 10

years under review, and this is the most important potato-growing area. Within the east

region blight is most serious in its effects

in the seed-growing areas near the Firth of

Tay, but the principal losses here appear to

be caused by tuber infection.

BLIGHT ON HAULM

Although reports of isolated primary foci

may be made very early in the summer,

generalized field outbreaks of the disease

in particular areas very seldom occur before

the end of July or the beginning of August.

According to Foister, blight usually appears

first in the southwest of Scotland, Gray,

Clouston, and Biggar (1952) have analyzed

first reports of blight for the north region

from 1943 to 1952. During this lO-year period,

these reports came between late June and the

third week in July, but they did not signify

general outbreaks. For the west region,

Grainger (1955) has given the times of the

first appearance of blight on his disease

phenology plots at Auchincruive over the

period, 1944-52. These were as follows:

MidJuly

1948

End

July

1944

1947

1951

1952

Early

Aug.

1946

1950

MidAug.

1945

End

Aug.

1949

Blight occurred every year, with outbreaks

at the end of July or in early August in 6 out

of the 9 years. It seems very probable that

the first appearance of the disease on the

plots at Auchincruive will precede generalized

field outbreaks on maincrop varieties in the

west of Scotland. Similar data are not available

for the east region, but, in general, field

outbreaks there are most unlikely to occur

before outbreaks in the west.

The most detailed information on the effect

of blight on the haulm comes from the west

region and has been provided by Grainger,

again from data obtained on the Auchincruive

plots. Table 22 sets out the dates on which

100 percent blight destruction of the tops

occurred, for 2 potato varieties, over the

period 1949-54.

TABLE 22,—Date on Which 100 percent blight of

haulm occurred at Auchincruive, Scotland,

on Kerr's Pink and King Edward potatoes, 1949-54

Year

100 percent blight on

haulm of—

Kerr's Pinic King Edward

1949 September 12

2S

27

16

17

20

September 16

1950 10

1951 28

1952 10

1953 10

1954- 3

Average date

(6 years) September 20 September 13

The 100 percent stage was generally reached

around the middle to the end of September.

The 75 percent blight stage will have occurred

some days before this, but, even so, the

evidence from table 22 is that, at this center,

annual yield losses through premature defoliation of the plants are very small.

38
For the north region, Elizabeth G. Gray has

informed the writers that seed crops are usually burned off before blight reaches the 75

percent stage, and blight on the haulm is not

often severe enough to affect yield, except in

coastal districts. In the east region, A. E. W.

Boyd has reviewed his observations* made

between 1949-56, from which he concludes that

the 75 percent date for Majestic inthe east and

southeast of Scotland is usually during the

first week in September; but he stresses the

variations in blight incidence throughout the

region*

In Scotland, it is unusual for the 75 percent

blight defoliation stage to be reached much

before early- to mid-September in any year.

If this is so, yield losses through defoliation

are likely to be small. The data for England

and Wales (fig. 8) indicate that when blight

reaches the 75 percent stage by mid-September, there is an average loss of only 4

percent of the potential potato yield. The

Scottish potato-growing season is somewhat

later than in England, so that the 75 percent

stage by mid-September probably indicates a

slightly greater loss than 4 percent, but this

does not happen every year. The lackof precise

information on these various points shows the

necessity for more detailed survey work,

although the variable topography and weather

conditions may render this rather difficult.

An appraisal of defoliationlosse s in Scotland

must take account of the predominant part

played by the seed potato crop. The practice

among growers is to destroy the tops of their

seed crops at the time, usually during the first

half of Septennber, when they consider that

they have obtained the maximum yield of seedsized tubers (2^ in. x l|in.). Thus, the defoliation losses by blight in seed crops are likely

to be very small indeed, as the majority of

these crops are burned off before the 75 percent

blight stage is reached.

If the acreage of early ware crops is added

to the acreage of seed crops, these two are

found to account for three-quarters of the

total Scottish potato acreage. It is therefore

on only a quarter of the total acreage (the

maincrop ware) that defoliation losses of any

importance are likely to occur, and even here

they have been shown to be small.

BLIGHT IN TUBERS

There can be no doubt that blight in tubers

is the problem which is of most concern in

Scotland, because of the importance of the seed

crop. Since 1953, figures have been obtained

in the course of crop-check weighings by the

Ministry of Food and Potato Marketing Board,

in Scotland as in England and Wales, which

give the weights of blighted tubers present at

lifting in a representative sample of 600 to 800

Scottish potato crops each year (principally

maincrop varieties). Table 23 shows the results for 1953-56. In 1953, 1954, and 1956 a

fair amount of blight was present on the

potato haulm, at any rate in parts of Scotland.

Over the 4-year period, the average amount

of tuber infection causedby blight before lifting

was under 2 cwt. per acre, which, expressedas

a percentage of the average potato crop, equals

about 1 percent of the actual yield. This puts

the problem into perspective. It means that in

most of the crops, even in blight years, tuber

infection in the soil is either absent or very

slight. The few crops in which really heavy

infection occurs, with a loss of up to 3 tons

TABLE 23. —Mean weights of blighted tubers recorded at lifting, Scotland, 1953-56

Region and county 1953 1954 1955 1956

North region:

Ross and Cromarty, Inverness, Caithness

Aberdeen, Kincardine

East region:

Perth, Angus

Fife :

Lothians, Berwick

West region:

Ayr, Lanark, Argyll <

Dumfries, Kirkcudhright ,

Scotland (all regions)

Number of potato crops in survey

Cwt./acre

1.6

3.0

4.4

3.7

1.0

2.8

Cwt ./acre

0.4

..7

2.8

1.4

1.6

2.4

1.6

Cwt ./acre

0.7

.3

0

0

0

2.3

1.0

.2

Cwt ./acre

0.2

.3

2.2

4.9

3.6

2.7

1.0

1.9

600 810 782 732

39
of potatoes per acre, tend to get an undue

amount of publicity with a consequent exaggeration of the problem.

It would appear from table 23 that tuber

blight infection is least important in the north

region, where in 3 out of the 4 years only

very small amounts of the disease were

recorded, but it is too early to draw any firm

conclusions about regional differences. Varietal susceptibility to blight infection of the

tubers, and soil conditions are important and

need further analysis.

Blight infection of the potatoes at lifting, by

contact with diseased haulm, is potentially a

greater source of loss in Scotland than in

England and Wales. This follows from the

later growing season in Scotland combined with

a shorter harvest period. Particularly in seed

crops, however, this problem has beentackled

by chemical and mechanical haulm destruction. In 1956, for example, haulm destruction

was carried out in at least 50 percent of all

Scottish potato crops. Recent field and laboratory experiments carried out in the east of

Scotland and reported by Boyd and Henderson

(1953) have indicated that as potato tubers

mature, their resistance to blight infection

increases. From this, the authors conclude

that '*very early lifting for seed or for the

early ware market, with blight present on the

haulm, will have much more serious consequences than later lifting, when the haulm is

only slightly green."

BLIGHT AND WEATHER DATA

NORTH REGION: Long-term average temperature and rainfall ñgures for Craibstone,

the North of Scotland College farm near Aberdeen, are shown in figure 15, together with

planting and lifting times. The low summer

temperature, which in the two hottest months,

July and August, does not exceed a mean of

56® F., is one of the principal factors in the

generally small importance of blight at this

center, despite a relatively high rainfall. The

temperature figures for Craibstone are fairly

typical for potato growing throughout the region, but rainfall and humidity conditions vary

widely. Blight Is usually worst near the coast,

where humidities are Increased by coastal

fogs.

EAST REGION: Conditions here, although

very variable, are generally drier than In the

other two regions. At North Berwick (fig. 15),

for exannple, average annual rainfall Is only

24 Inches, and the normally low summer rainfall, here and throughout East Lothian, Is one

of the factors associated withthe low Incidence

of blight. In the east region, blight Is most

serious along the north and south banks of the

river Tay, and this may be partly ascribed to

the frequency of sea mists (**haar") leading to

high humidity conditions during the potatogrowing season. Long-term weather records

SCOTLAND

North Reqion

CRAIBSTONE

RAINFALL

Inches

f, East Region

NORTH BERWICK

RAINFALL

Inches

. West Region

AUCHINCRUIVE

RAINFALL

Inches

FIGURE 15.—Long-term average monthly rainfall and mean temperatures in relation to growing periods for three centers in

Scotland.

for Perth, at the head of the Tay estuary, show

that mean temperature conditions there arc

very similar to those at North Berwick (I.e.,

BS^to 60° F. In July and August), but average

rainfall Is higher (2.9 Inches In July and 3.4

Inches In August).

WEST REGION: Figure 15 shows long-term

weather data for Auchinerulveinrelationto the

growing seasonfor early and malncroppotatoes

In the southwest of Scotland. The most Important early-growing district Is centered

around Glrvan, which Is about 20 miles south

of Auchlncrulve. Average temperature and

rainfall conditions at Glrvan are similar to

those shown In the diagram. That potatoes can

be produced during such a cold growing season

(end of February to Jime) Is due to two main

factors: (l)Only a Umlted number of fields,

very near to the seashore and practically frost

free, are used for these early crops; and (2)

the potato variety grown Is Epicure, whlchhas

a remarkable capacity to recover after being

damaged by frost. This early ware crop

virtually escapes any damage from blight, a

fact that Is, no doubt, closed linked with the

low-temperature conditions.

40
Despite the relatively high siimmer rainfall at Auchincruive, blight on the haulm is

of only minor importance in maincrop potato

varieties. This may be, in part, a temperature

effect.

CONTROL MEASURES

Copper compounds, of which copper 03cychloride is the most important, are used in

protective spraying against blight. A high

proportion of this work is carried out by

contractors, who normally spray at highvolume rates. Low-volume spraying is not

practiced on any scale. The close spacing of

the seed crops (about 12 in. x 26in.), combined

with the generally moist growing conditions

leads to a luxuriant growth ofthe potato haulm,

and considerable damage may be done by the

passage of spraying equipment through the

crop. This problem has been referred to by

Gray, Clouston, and Biggar (1952), in writing

about the north region. They say that it is

noteworthy that there is probably more enthusiasm for spraying in the island counties

than anywhere else in the north. The reason

for this is that on the island crofts spraying

is done by hand ( chiefly withknapsack sprayers)

without accompanying mechanical damage to

the potato tops.

The amount of protective spraying carried

out varies in different parts of the country.

Most is done in the important seed-growing

areas near the Firth of Tay. There canbe little

doubt that the object of some of this spraying

combined with later haulm destruction, is to

keep the tubers free from blight infection,

rather than to prolong the growth of the potato

crop. In some of the counties of southwest

Scotland, notably Ayr, Wigtown, Kirkcudbright,

and Lanark, there is little or no protective

spraying.

Data from the "crop-check surveys'* (table

24) show the approsdmate percentages of the

maincrops on which protective spraying and

chemical or mechanical haulm destruction

have been practiced during recent years for

the whole of Scotland.

Boyd (1952) has reviewed the various

methods of haulm destruction usedinScotland.

At the present time, spraying with sulfuric

acid is the nciethod most commonly employed.

The main aim of haulm killing is not to protect

the tubers from blight infection^ although this

end is served. In seed crops, it is to stop

growth when the maximum quantity of seedsized tubers have been formed. The practice

is not compulsory, as in other countries (e.g.,

Holland and France), where the haulms of seed

crops have to be destroyed by a certain date

to prevent the late spread of virus diseases.

Little or no critical experimental work has

been done in Scotland on the economics of this

premature killing in the seed crop. Whether

it pays or not will depend very much on the

variety and on the relative value of seed and

ware potatoes. Up till now, the practice seems

to have been accepted as a good one without

question, but experimental evidence is required

against which to test it.

BLIGHT FORECASTING

Grainger (1950) has discussed the results

of forecasting potato blight in the west of

Scotland during 1944-48, using Beaumont

periods as the criteria on which warnings

are based. The method was foundto be successful, provided that the conception of a '*zero

date" (in this case, July 1) before which

warnings were considered invalid, was employed. Further evidence that this method is

a success in the west of Scotland has been

provided by Grainger (1955), and in this later

paper, he describes an apparatus for automatically recording the occurrence of Beaumont periods. A limited test of the same

method of forecasting blight in the north of

Scotland during 1943-46 has also been reported as successful by Gray, Clouston, and

Biggar (1952).

Since 1949, the Department of Agriculture

for Scotland, in conjunction with the Meteorological Service, has beeninvestigating the possibilities of a blight forecasting service for

the whole country. Each year, an attempt has

been made to correlate Beaumont periods at

a niunber of weather stations with observed

outbreaks of blight in the field: these latter

reports to be sent in chiefly by the Department's potato seed inspectors. The method

has not operated so successfully in the east

of Scotland, where most of the potatoes are

grown, as it has in the other areas.

The rapid changes in micro-climatic conditions over short distances make it very difficult to apply a forecasting system to the whole

of Scotland. Fortunately, with the present

TABLE 24. —Number of potato crops in Scotland survey and percentage of crops sprayed and

haulm destroyed, 1953-56

Item

Potato crops in survey number. •

Protective spraying I)ercent. •

Haulm destinlction percent. •

1953

600

27

52

195^

810

32

49

1955

782

25

47

1956

732

30

52

41
emphasis on seed-potato production, coupled

with a generally late attack of blight, the need

for such a service is not so great as in many

other countries.

POTATO BREEDING

W. Black of the Scottish Society for Research in Plant Breeding, now at Pentlandfield, Roslin, Midlothian, has bred a number

of blight-resistant potato varieties, which

have come on to the market since 1948. In

all cases, the resistance to blight was derived from Solanum demissum. The best known

of these varieties has been Pentland Ace, containing the R3 gene; but none of the varieties

have been accepted commercially on a large

scale. All have been attacked by specialized

races of the blight fungus soon after their

introduction. (See also England and Wales:

Resistant Varieties.)

Black (1952), in parallel with workers in

other countries, has been responsible for much

of the pioneer research on the genetics of

blight resistance. The genetics of so-called

'•major gene" resistance has been most fully

explored. In this type of resistance, which is

shown by S. demis sum derivatives, the plants

exhibit a hyper sensitivity reaction to Phytophthora infestans, which is controlled by a

¿umber of single major genes, either alone or

in combination. The presence of four such

genes has been demonstrated. Recently, Black,

Mastenbroek, Mills, and Peterson (1953) have

proposed that an international nomenclature

should be adopted in designating these genes,

and for this purpose have reconamended the

adoption of Black's system. In this, the four

genes have been named Ri, R2» R3» ^^^ ^4«

The 15 possible genotypes of potatoes containing these genes, together withthe recessive (r)

are shown in figure 16 alongside the equivalent

genotype designations previously usedby Mills

and Peterson inthe U.S. A. and by Mastenbroek

in Holland.

The same workers have also suggested the

adoption of an international system for naming

blight races, to be based on the reaction of

particular races to the previously named

genotypes. Races are named according to

the genotypes that they attack. Thus, race 0

attacks none of the resistant genotypes; race 1

attacks the recessive and genotype Ri; race

1,2 (pronounced; "one, two" andnot ** twelve*')

attacks the recessive, and genotypes Ri, R2,

and Rl R2f but all other genotypes are resistant

to it. No genotype is resistant to blight race

1,2,3,4. This interrelationship between host

and parasite are shown in figure 16 with the

GENOTYPE RACES OF Phytophthora infestans

- Scotland

U.S.A.

Holland

International

A El H J D G E B2 - c 1 _ - F -

A D C B _ BD BC - - - - -

NI N2 N5 N4 _ _ N7 N6 _ _ N8 - N9 -

0 1 2 3 4 1,2 1,3 1,4 2,3 2,4 3,4 1,2,3 1,2,4 1,3,4 2,3,4

s r r r r

Rj D R9 R|

5 S S s s S S S S S S s S S S

- S - - S S S - - S s s - s

R2 C R? R2 - - s - S S S - s s - S s

R3 - RQ R3 - - - s S S - s S - S s s

R4 ^ »^2,5 h

R|R2 CD - R|R2

- - - s - S s

c

- s S s s

- - - - S - - - s s - - s

RjRg - - R,R3 - - - - S - - - s - S - s

R1R4 BD - R,R4 - - - - - S - - - s S - s

R2R3 - - R2R3 - - - - - S - - s - - s s

R2R4 BC - R2R4 - - - - - - s - - s - s s

R3R4 - - R3R4

R1R2R3 - - R1R2R3

- - - - - - - s - - S s s

- - - - - - - - s - - - s

RjRjl^ BCD - R,R2R4 - - - - - - - - - s - - s

'^l'^3'^4 " " ^\^3^A - - - - - - - - - - s - s

- - - - - - - - - - - s s

- - - s

- = Resistant S = Susceptible

FIGURE 16.--International system of designating interrelationships of genes and races of Phytophthora infestans.

42
names previously given to the blight races by

the different workers. It is claimed that *'The

advantages in designating genes and races in

this manner are (a) it probably allows for

any extension that may be necessary In the

future; and (b) it illustrates immediately the

inter-relationships between races and between

genes and races without referring to diagrams

or tables."

This system of naming genes and races

has now been adoptedby all the leading workers

throughout the world. Black's work, which has

included the identification of blight collections

from many countries, has brought to light the

fact that the common race of blight throughout

the world is now race 4.

The chief emphasis in Black's search for

blight-resistant potato varieties is now placed

on ''field resistance," as "major gene "resistance in a variety is of short duration,

owing to specialization of the blight fungus.

A recent review of blight resistance work

in Scotland has been given by Black

(1954).

SUMMARY: SCOTLAND

A predominant partis playedby seed production in the potato industry, and defoliation

losses caused by blight are not usually considered important in Scottish seed crops,

owing to the almost universal practice of

prematurely destroying the haulm in these

crops chemically or mechanically. Present

indications are that blight does not normally

cause serious yield losses through premature

defoliation in any of the major potato-growing

areas. More detailed survey information, particularly for maincrop ware varieties, is

needed to be sure of this, but climatic differences throughout the country make it difficult to carry out systematic surveys. Data

from surveys of tuber blight Infection suggest

that there has probably been a tendency to

overestimate the losses from this cause. Dr.

Black of the Scottish Society for Research In

Plant Breeding has carried out fundamental

work of world w^de importance on the genetics

of blight resistance.

NORTHERN IRELAND

Northern Ireland, wjth a population according to the 1951 census of about 1.4 millions,

has a total area of 2j million acres under

crops and pasture. In 1951 (Northern Ireland,

Ministry of Agriculture, 1956), about 144,000

acres were under potatoes and the total production was 1,197,000 tons, of which 265,000

tons were used for human consumption. Indicating a consumption per head of about 400

lb. per annum. In Northern Ireland, as in

Scotland, a high proportion of the potato crops

are grown for seed. In 1951 the shipments of

seed amounted to 84,000 tons, and 171,000tons

were used for planting in Northern Ireland.

Also, 126,000 tons of ware were shipped, and

In that year, in which the yield was high (8.3

tons per acre), 324,000 tons were used for

stock feed.

Potato growing is fairly equally distributed

among the six counties (fig. 17). The most

Important seed-growing areas are in the

northern parts of Londonderry, Antrim, and

mid-Down; the malncrops for ware are chiefly

in a middle belt; and most of the early and

second-early varieties for ware are in County

Down and parts of County Antrim in the Lagan

Valley.

The principal maincrop varieties are Ar ran

Victory, Kerr's Pink, and Arran Banner. Of

thèse, the purple -skinned late maincrop variety

Arran Victory is by far the most popular for

ware, and it accounts for some 38 percent of

the acreage. Kerr's Pink follows with about

17 percent, and Arran Banner, also much in

demand for seed for export, occupies about

15 percent. The whole range of British varieties Is grown for seed, with fairly large

acreages of Gladstone and Arran Peak (5 to 6

percent), and 1 to 2 percent each of Majestic,

King Edward, Dunbar Standard, and Arran

Consul. Ballydoon Is extensively grown as a

first-early variety for the home trade, and

British Queen as a second-early. In Northern

Ireland, the predominant Arran Victory is

among the varieties least susceptible to blight.

INCIDENCE OF BLIGHT

As would be expected from the distribution

of the rainfall in the months of July, August,

and September (fig. 18), blight conditions in

Northern Ireland are generally severe, and

more so in the north and west thanin the south

and east. The cllmagram for Garvagh In

Londonderry (fig. 19) shows that the average

rainfall there Is higher than that for ^'blight

years" only In the southern zone of England

and Wales (fig. 6). The rainfall figures for

Armagh, perhaps more typical of the waregrowing area, for the months of July and

August in the years 1947-56 are given in

table 25 as against the potato yields for

Northern Ireland taken from the official statistics. It will be seen that, as in England

and Wales, 1947 and 1955 were dry years (and

also years in which the yield was lowest), but

even at Armagh, where the rainfall is lower

than In Londonderry, 8 years out of the 10 had

over 5^ Inches of rain In the 2 months of July

and August.

The necessity for routine spraying Is not In

question In Northern Ireland. Professor

Muskett of Queen's University, who, with R.W.

Wallace of the Ministry of Agriculture, has

43
RELAND

Each dot = IOOO acres

FIGURE 17.--Approximate distribution of potato growing in Northern Ireland and the Irish Republic.

44
GARVACH

SEPTEMBER

FIGURE 18.-Average monthly rainfall and teniperamres in Northern Ireland and Irish Republic, showing weather stations referred to in text.

kindly provided most of the information on

which this section of the review is based, recalls that in his 35 years' experience there

have been very few years with less than a ton

gain from spraying in the maincrops grown

for ware in Northern Ireland, and the usual

gain in Arran Victory is l| to 2 tons per acre.

Outbreaks of the disease on the haulm occur

by mid-July; the attack is almost always under

way on unsprayed maincrops in the second

half of July, and thereafter its progress may

be very rapid, with complete killing of the

haulm by early August in the north. Such

very rapid attacks are exceptional, however;

and attacks from mid-August to late August

are more usual for Northern Ireland as a

whole.

The first spraying, which should perhaps

ideally be given at the end of June or beginning

of July, is in practice generally given in midJuly, with great regularity. The second spraying is given toward the end of July. About 98

percent of the farmers put on the first spray,

and about 85 percent the second. The keenest

growers give a third spraying about midAugust. A mean gain of 1 j to 2 tons per acre

is consistent with an average of about a fortnight's prolongation of growth, due to spraying.

In seed crops the gain inyieldis much less, as

even the maincrop varieties are burned off

before the end of August, but the protective

spraying has the very important function

of minimizing the risk of infection of the

tubers.

SPRAYING MATERIALS AND SPRAYING

TRIALS

Burgundy mixture is the material still

chiefly used for protective spraying; the preferred program is to use the 1 percent mixture

for the first application (to avoid injury to the

young haulm by the stronger mixture) and to

use the 2 percent mixture for the second and

third applications to give as much protection

as possible from mid-August into September.

The leading results of a series of trials of

dusting versus spraying with burgundy mixture, carried out in County Down over the

years 1924-30 (Muskett, 1929; and Muskett

and Cairns, 1931) were as shown in table 26.

In the trials in 1924 and 1925 two applications of 1 percent burgundy mixture were

compared with two of copper dust (15 percent

Cu) at 20 lb. per acre. In 1927 two applications of similar dust at 30 lb. per acre were

compared with two applications of 2 percent

burgundy; and in 1930 three applications of dust

(16 percent Cu) were compared with three

applications of burgundy, the first two at 1

percent and the third at 2 percent. It was concluded that dusting was neither so efficient nor

so economical as spraying with burgundy

mixture.

In the three consecutive years 1924-26, a

series of spraying trials was carried out at

4 or 5 centers in each of the 6 counties,

covering the whole range of maincropvarieties.

The mean results of these trials (hitherto unTABLE 25. --Rainfall In County Armagh, and potato yields for Northern Ireland, 1947-56

Rainfall and potato yields 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956

Rainfall in—

July inches..

August do..

Potato tons per acre •.

3.4

.6

5.6

2.5

3.5

7.9

2.7

3.0

7.6

4.Ö

5.3

7.5

3.3

4.5

8.3

1.7

3.9

7.9

4.1

3.7

8.1

4.6

2.3

7.0

1.8

1.1

6.5

3.8

6.3

7.0

45
IRELAND

Garvaqh

LONDONDERRY

RAINFALL 5

Inches

RAINFALL

Inches

J FMAMJ JASOND

^Markree Castle

SLICO

RAINFALL 5

Inches

JFMAMJJASOND

RAINFALL 5 _

Inches

JFMAMJJASOND

RAINFALL 5 _

Inches

JFMAMJJASOND

J FMAMJ JASOND

FIGURE 19.--Long-term average nxjnthly rainfall and mean temperatures in relation to growing periods for five centers in Ireland.

published), for two sprayings with 1 and 2 percent burgundy mixture, respectively, are shown

in table 27.

The results of such trials carried out at

many centers over consecutive years constitute a survey and leave no doubt about the

level of blight losses. On the basis of the

bulking curve for Majestic in England (fig. 8)

the mean yield for the unsprayed (9.3 tons, or

74 percent of a potential yield of about 12^ tons)

would indicate a 75 percent date for the unsprayed haulm about mid-August; with corresponding dates for the haulm twice sprayed

with 1 ajid 2 percent burgundy mixture, about

September 7 and September 15, respectively.

This agrees very well with general observations on the course of the disease on

sprayed and unsprayed haulm in Northern

Ireland.

Numerous other spraying trials were carried

out over the years 1936-40 at Hillsborough

Agricultural Research Institute in County

Down; at Greenmount Agricultural College,

Muckamore, in County Antrim; and again at

20 to 30 centers throughout Northern Ireland,

in connection with pioneer work on the testing

of the then-new copper oxychloride and cuprous

oxide sprays. The main indication from these

many trials was that 2 percent burgundy mixture was perhaps unnecessarily strong, at any

rate for the first application, and that both

cuprous oxide and copper oxychloride preparations, when used at the sanne copper dosage

as 1 percent burgundy mixture (0.25 percent

Cu) would have about the same protective effect.

A series of trials, again hitherto unpublished, was made at Hillsborough in each of

the 5 years, 1936-40 on the second-early

variety British Queen. The trial crops were

generally planted rather late (in May) éuidthey

ripened off aboutthe secondweekinSeptember.

The leading results for the controls and plots

twice sprayed with 2 percent burgundy mixture (8.10.40) are given in table 28. It has

been possible to estimate the 0.1 and 75 percent dates for the unsprayedhaulm very closely

from the notes made on the condition of the

haulm at various dates, although no blight

key was then used. The 75 percent dates on

the sprayed haulm were indeterminate as

blight and ripening-off effects were concurrent in September.

The blight attack was evidently sonncwhat

later than usual in 1940, but on the average

over the 5 years the spraying appears to

have given a useful prolongation of growth

from about mid-August until the end of August

or just into September each year, with an

average gain in total crop of 2^ tons per

acre.

46
TABLE 26. "Results of dusting versus spraying trials on potatoes in County Down, Ireland,

1924-25, 1927, 1930

Year

1924.

1925,

1927.

1930.

Trials

Number

2

3

1

1

Potato variety

Lochar

..do

Up to date....

British Queen.

Mean total yield

Unsprayed

Tons/acre

12.6

15.4

12.4

Dusted

Tons/acre

I2T3

15.9

9.3

12.9

Sprayed

Tons,

^

acre

3

17.2

10.7

14.4

TABLE 27.--Mean results of spraying with burgundy mixture on maincrop varieties throughout

Northern Ireland, 1924-26

Trials

Mean total crop Blighted tubers

Year

'Unsprayed

Twice sprayed

Unsprayed

Twice sprayed

1 percent

mixture

2 percent

mixture

1 percent

mixture

2 percent

Tn1 xtiire

1924.

Number

27

28

21

Tons/acre

9.0

9.1

9.8

Tons/acre

10.7

11.4

12.3

Tons/acre

11.1

11.9

13.2

Cwt./acre

15

10

5

Cwt./acre

12

5

5

Cwt./acre

12

1925 5

1926 3

Mean........... — 9.3 11.5 12.1 10 7 7

TABLE 28.--Results of spraying trials on British Queen potato in County Down, Ireland, 1936-40

Year

Date on which blight

on unsprayed haulm

reached—

Total yield Blighted tubers

0.1

percent

75

percent

Unsprayed Sprayed Unsprayed Sprayed

1936 July 14

July 22

July 22

Aug. 3

Aug. 5

Aug. 10

Aug. 15

Aug. 10

Aug. 21

Aug. 24

Tons/acre

6.Ö

9.1

11.3

12.3

12.7

Tons/acre

0.3

11.8

15.7

15.0

13.6

Cwt./acre

7

4

11

8

3

Cwt./acre

2

1937 1

1930 6

1939 20

1940 2

Mp Q-n ......•> >••• Aug. 16 10.4 12.9 6 6

47
HAULM DESTRUCTION

Haulm destruction to prevent both virus and

blight infection of the tubers is obligatory in

all seed crops in Northern Ireland entered for

Stock Seed Certificates. The early varieties

are burned off about the last week in July,

and the maincrops about the last week in

August--a little earlier or later in each case,

according to the conditions of the season.

Sodium chlorate is chiefly used, the rate of

application being about 28 lb. per 100 gallons

of water per acre. Sodium chlorate is preferred to sulfuric acid, as it does not require

acidproof machines, and it acts more slowly,

giving opportunity for the tubers to ripen off.

Little burning off is done on the ware crops.

A good indication of the losses due to

blighted tubers in ware crops protectively

sprayed in the usual way is given in a report

on variety trials (Northern Ireland, Ministry

of Agriculture, 1954). In trials at 13 centers

in 1954 and 99 centers in previous years the

mean percentage of blighted tubers in Arran

Béinner was 4.2 percent of the total crop (14

cwt. per acre) and in Majestic 2.3 percent

(7 cwt. per acre). These figures are of the

same order as those shown in the spraying

trials, and indicate a general loss of 2 to 4

percent by infection before lifting. Tuber

infection tends to be less on the close soils

of North Derry than on the poorer open soils

of mid-Antrim.

POTATO BREEDING

Many of the principal new British potato

varieties in recent years have been raised

in Northern Ireland by John Clarke on his

small farm at Mosside, near Ballycastle in

the north of Antrim. His introductions include

Ulster Chieftain, Ulster Premier, and Ulster

Prince as flrst-early varieties; Ulster Ensign

and Ulster Emblem as second-early potatoes;

Ulster Torch, Ulster Beacon, Ulster Tarn,

and Ulster Knight as maincrop varieties, and

Ulster Supreme, a late maincrop. This wealth

of new varieties can be compared only with

the famous Arran series raised by Donald

Mackelvie on the island of Arran in Scotland

earlier in this century.

Much work is done by the Ministry's Inspectors and by Professor Muskett's staff

at Queen's University to assist potato breeding in connection with the important Northern

Ireland seed trade; and the National Institute

of Agricultural Botany have potato centers

for raising virus-tested stocks in North

Antrim, and for multiplying new stocks near

Coleraine for trials in England. Blight-resistant seedlings and new blight-resistant

varieties are tested at these centers under the

prevailing severe blight conditions; but the

commercial cultivation of R-gene resistant

varieties in Northern Ireland is so far negligible. The choice of varieties for ware crops

is dictated by the consumers' demand for

quality rather than by any consideration of

blight susceptibility.

Doling (1957)has recently surveyed Northern

Ireland for races of Phytophthora infestans.

In the commercial varieties he found only race

4 to be widely distributed. In 73 isolates, 70

were race 4 and only 3 were race 0.

SUMMARY: NORTHERN IRELAND

Potato blight is severe almost every year

in Northern Ireland. Routine protective spraying is imiversal, and the need for this has

been well established by a great number of

spraying trials carried out systematically

throughout the country. Any unsprayed haulm

is killed by blight between the end of July

and the end of August, according to locality,

variety, and season, with mid-August as the

mean 75 percent date. The mean gain from 2,

or rarely 3 sprayings is l| to 2 tons per

acre or about two-thirds of the total defoliation

loss caused by blight. There is a constant loss

of 2 to 4 percent in ware crops due to blight

in tubers. Northern Ireland has an important

seed potato trade. In the seed crops, some

of the yield of ware-sized tubers is lost by

burning off the haulm in August, but the blight

losses are slight.

REPUBLIC OF IRELAND

The Republic of Ireland (Eire), with a population, according to the 1951 census, of about

3 millions, has a total land area of about 17.4

nülUon acres. In 1954, some 292,000 acres

were under potatoes, 1,516,000 under corn

and other crops, and 7,929,000 under pasture.

The potato production in that year was about

24 million tons, or about 7.7 tons per acre

(Ireland (Eire) Central Statistics Office, 1955).

The consumption of potatoes per head In 1947-

48 (F.A.O. 1948) was 420 lb. per annum,

or about twice that in England and Wales.

The acreage under potatoes, apart from

temporary rises to 500,000 and 430,000acres,

respectively, in the first and second World

Wars, has fallen steadily from about 458,000

acres at the beginning of the century to 292,000

acres In 1954. But, against this, the yield

per acre has risen from 4.2 tons in 1900-

1909 to 7.8 in 1940-49, and it was over 8Í

tons per acre In the years 1950-53.

The rise In yield during this century reflects

the work done by the Department of Agriculture

In the promotion of good cultivation and

48
manuring, varietal improvement, the suppression of virus diseases, and campaigning

for spraying against blight.

Sugar beet has played an important part

in the recent history of the root crop in

Ireland. This crop, introduced in 1926, was

occupying 74,000 acres in 1954, with a production of 674,000 tons. Thus: In 1900-1909,

458,000 acres produced 1,853,000 tons of

potatoes for a population of 3.2 million; in

1954, 366,000 acres produced 2,248,000 tons

of potatoes plus 674,000 tons sugar beet for

a population of 3.0 million.

Also, to the account for 1954, there must

be added an important export trade in seed

potatoes of the highest quality and freedom

from virus diseases, built up since the 1920's

through the devoted labors of Davidson,

Delaney, and their coworkers in the Potato

Inspectorate, aided by the pioneering investigations of Pethybridge, Murphy, and McKay.

Davidson (1937) describes the foundation of

the industry, and McKay (1955) provides a

practical account of all the work on potato

diseases that has contributed to the present

well-being of the potato in Ireland.

DISTRIBUTION AND VARIETIES

As the map (fig. 17) clearly shows, potato

growing is fairly evenly distributed over the

whole of Eire. The chief seed-producing districts are in Donegal, Mayo, and around

Athlone; early varieties are grown most extensively in the south--in Cork, Waterford,

and Wexford--and also in the Dublin area;

but maincrops for war« are also grown in all

districts. The cultivation is mostly on small

farms.

The principal maincrop varieties are now

Kerr's Pink and Arran Banner. Kerr*s Pink,

a late maincrop variety, popular for its good

yield and quality, is grown in all parts of the

country, and probably accounts for about half

the present acreage. Arran Banner, the other

most extensively grown variety, is notably

earlier in maturity. It is grown not only for

ware but also for seed--large quantities of

which are produced for export, principally

to Mediterranean countries. Majestic, King

Edward, and the whole range of British

varieties are also produced for seed. Culinary

quality in potato varieties is traditionally

appreciated in Ireland, and small acreages

of the superb Golden Wonder, and also some

of the best varieties of the past, even Champion,

arc still grown. Up-to-Date, despite its present

high susceptibility to blight, is still in demand

for its quality in many parts of the world,

notably South Africa, and it is still grown

in Ireland both for seed and ware.

The second-early variety, British Queen,

is also widely grown, again because of its

quality and despite its high susceptibility

to blight. Epicure is still one of the most

popular flrSt-early potatoes.

Small acreages of Alpha and Voran are

grown here and there, but the production of

potatoes for industrial purposes is very small.

However, the production of potatoes for industrial purposes (alcohol, starch, etc.) in

Ireland has been carefully investigated by

Drew (1941) and Drew and Deasy (1939), who

concluded that continental varieties had little

superiority in point of starch production over

Arran Banner ajid other high-yielding varieties

already grown in Ireland.

CLIMATIC CONDITIONS

Ireland has a temperate climate with abundant rainfall, particularly well suited to the

potato crop, of which it is the traditional

European home. Conditions are not so cold

in winter or so warm in sumnner as in eastern

England or in continental Europe. In configuration the country is a **saucer," or large

central plain, comprising the drainage area of

the river Shannon with much bog land, surrounded by broken mountainous country to

the north and west and by hills to the south

and east. The country is swept by maritime

air from the southwest, and humidity conditions tend to be fairly uniform over most of

the country as the mountains arc too broken

to afford much shelter in their lee. In the

absence of surveys for the 75 to 90 percent

stages of blight, such as have been made in

England and Wales in recent years, it is not

possible to zone Ireland for severity of blight

as has been done for England and Wales in

figures 4 and 5. On the whole the worst

districts for blight are those with the highest

rainfall in the months of July, August, and

September; and the long-term average rainfall

distribution maps, given in figure 18, serve

to indicate the regional differences. The blight

epidemics tend to be severe in the west, and

perhaps most severe in Galway, Roscommon,

and parts of Offaly and Westmeath. They

tend to be least severe in the east, notably

near Dublin, which is in the driest part of

the country.

The climagrams (fig. 19) enable the mean

rainfall and temperature conditions in various parts of Ireland to be compared with

those in England and Wale s--and other parts

of the world. At Markree Castle, County

Sligo, which, with Garvagh in Londonderry

in Northern Ireland, fairly well represents

the mountainous and wind-swept seed-growing

areas of Mayo, Sligo, and Donegal, the mean

temperatures in July and August are relatively low, reaching only about 58° F., and the

rainfall in the months of July to September

is very high. At Cork, in the south, given as

fairly typical of the southern coastal zone

from Cork to Wexford where many early

49
varieties are grown, the diagram is very

similar to that for Pcnzance (fig. 6). At

Birr, Offaly, in the central area, the longterm average rainfall and temperatures over

all years are similar to those of the **blight

years" only in the southern aind Fen zones

of England. At Dublin, although this represents the driest part of Ireland, the average

rainfall follows the same pattern as at Birr,

and it is not very much less. Even in this

area blight epideniics tend to be more frequent and severe than in the southern zone

of England. Although there are regional differences in the incidence of blight in Ireland,

these differences are much less than in

England and Wales. For the purpose of estimating average blight losses it nnay not be

greatly misleading to treat Ireland as a

whole--as a single blight zone--with a higher

frequency of severe epidemics than any other

such zone in the British Isles.

As the climagrams would suggest, planting

and lifting dates for the potato crops in

Ireland (Eire and Northern Ireland) are similar to those in the south of England and

Wales. Early varieties in the southern coastal

districts are planted in February and March

for lifting at the end of May and in June. The

nnaincrops are generally planted in April,

and lifted in October (Kerr's Pink) or during

September (Arran Banner).

RECENT BLIGHT EPIDEMICS

In the famine year 1846, Father Matthew

recorded that the potatoes from Dublin to

Cork were blooming in all the luxuriance of

an abundant harvest on July 27, while on

August 3 there was one wide waste of putrefying

vegetation. In terms of our present (B.M.S.)

scale for blight assessment on the foliage

this would suggest that the attack, which had

probably started in early July, was stillinconspicuous from the roadside, though perhaps

up to 25 percent by July 27, but that it was at

50 to 75 percent a week later. Such very early

defoliation, which would now be expected to

result in a loss of half the crop, still occurs in

some years in unsprayed crops of susceptible

varieties in county Galway, but 75 to 90 percent

dates before mid-August appear to be exceptional, and in many years the bulk of the

defoliation loss occurs in September.

The dates of flrst outbreaks of blight have

been well determined in Ireland, where the

Department's instructors and potato inspectors

have been systematically reporting on first

occurrences since 1907. A retrospect of the

results of this work for 50 years (1907-56)

has now been published by McKay (1957).

This unique record shows clearly that the first

outbreaks (often on clamp sites or waste

heaps) usually occur in May or early June

in the early-potato districts of Cork, Waterford, and Wexford; and at about the same

time, or at most a little later, in Galway

and Mayo. In only 9 years out of the 50 was

the appearance of blight delayed beyond midJune, and in most years it became more or

less prevalent throughout the country from

the end of June onward.

These early outbreaks were by no means

always followed by severe epidemics, as the

progress of the disease was frequently checked

by subsequent warm dry weather--rarely until

the end of the season but quite often until

late August or early September.

The general courses of the blight epidemics

after the outbreaks each year, for nnany

years past, have been described by McKay

in appendices to the Annual Reports of the

Minister for Agriculture. The accounts of

the epidemics for the 10 years from. 1947 to

1956 (McKay, 1948-57) enable these years

to be grouped into **blight years,*' when the

disease was more or less severe, and "other

years," when its incidence was relatively

slight. What constitutes a '*blight year" in

Eire nnay be seen from the following brief

extracts from McKay's notes, which refer

to the general incidence of blight. As protective spraying is the rule in Eire, the

progress of the disease, as described, must

be taken as referring in the main to sprayed

crops and not to the unrestricted epidemic

were the crops left unsprayed.

Blight years Blight conditions

1948 Appeared early, but not general

until near nnid-September, then

rapid. Tuber infection low.

1950 General from beginning of August,

but not viriilent until midSeptember. Tuber infection low.

1951 Prevalent from beginning of

August, then a severe epidemic,

during which the superiority of

burgundy mixture was shown up.

Tuber infection high in some

parts.

1952 ComjTLon from, mid-July onward,

rapid kill of tops in September.

Tuber infection low.

1953 General from mid-July onward,

long-drawn-out attack, reaching

its peak in September. Some

heavy tuber infection from lifting

while tops not completely dead.

1954 Widespread from mid-June onward, but development somewhat erratic up to harvest.

1956 General by the end of June, little

severe killing of the foliage until

the end of August, then rapid.

Long-drawn-out attack with some

heavy tuber infection.

50
other years

1947

Blight conditions

-Prevalent from end of June, but

spraying and very warm weather

in August stopped spread on maincrops. Became active again in

September. Tuber infection negligible in late varieties.

1949 Only of minor importance

throughout the season, owing to

the exceptionally dry, warm

weather.

1955 Warm weather in July halted the

attack, but sporing continued on

lower leaves and revived on unsprayed crops in September.

Some serious tuber infectionfrom

lifting when tops not completely

dead.

Table 29 shows the mean rainfall, for Eire

as a whole, in the months of June, July,

August, and September in each of the years

1947-56, and also the mean temperatures in

each month in terms of degrees Fahrenheit

deviation from the long-term averages shown

in figure 18. It woiild have been correct to

express these temperature deviations as a

range; e.g., as "-1 to + l" for July 1950 instead of **0," but for simplicity the mean of

the two extrennes of the range is given, and

this serves well enough to differentiate between the warm and the cool seasons. The

mean potato yields for Eire, each year, as

given by the official statistics, are also entered

in the table.

^^ The distinction between •*blight years" and

**other years" cannot be made very precisely

for Eire, as indeed there was only 1 of the

10 years (1949) in which the blight attack was

negligible, but the data in table 29, read in

conjunction with McKay's notes, would suggest that the frequency of '»blight years'* is

7 or 8 in 10, and that in these "blight years,"

despite all the spraying that is done, the

growth of such maincrops as Kerr*s Pink is

generally stopped by blight in September--

probably by about mid-September; with the

unsprayed crops going down some weeks

earlier.

Table 29 shows clearly the checking effect

of low rainfall and higher mean tenriperatures

during July and August on the blight epidemics.

It also shows that, as in England and Wales,

the years of low average national yield are

not usually the "blight years," but the years

with relatively warm dry summers in which

blight is least severe. This would not be so

in Ireland but for the protective spraying,

which enables the crop to reap the benefit

of the good growing conditions in the blight

years.

PROTECTIVE SPRAYING

After half a century of use and many

challenges from other materials, burgundy

mixture is still the material officially recommended and most generally used for potato

spraying in Ireland. The main reason for this

lies in the high resistance of the deposits

TABLE 29,--Mean rainfall, mean temperature deviation from long-term averages, mean potato

yields for Eire, 1947-56

Year and average

Rainfall Mean temperature deviations-*- Mean

Jime July i^. Sept. June July Aug. Sept.

potato

yield

In.

2.8

3.9

2.6

2.4

2.5

1.3

2.7

3.1

4.7

1.2

5.1

In.

3.7

3.1

5.0

3.2

1.4

4.2

5.3

4.0

3.2

.9

In.

4.2

3.9

6.3

5.2

3.6

4.3

3.2

3.6

1.2

4.0

2.1

In.

3.2

3.9

6.4

5.^

2.4

4.7

5.7

4.9

3.7

2.6

3.5

-1

-1

+1

0

0

-1

0

+1

+3

0

-1

0

+1

+1

0

-2

0

-1

+2

+3

0

+1

-1

+1

+1

-1

-2

+5

+2

+4

-1

-1

+1

-3

+2

-2

+2

+2

-f4

-1

Tons/acre

Long-term average

Blight years:

1948

1950

^.5

S.5

1951 8.6

1952 8.6

1953 8.8

1954

1956

7.7

9.2

Other years:

1947 6.8

1949 7.7

1944 7.4

^ See fig\ire 18 for average temperatures.

51
to washing off by rain and the consequent

relatively long period of protection given,

which, as may be seen from McKay's notes,

is important not only for prolonging the

growth of the haulm to increase yield, but

also for keeping blight off the haulm as long

as possible to reduce the opportunity for the

infection of the tubers.

One percent burgundy mixture (freshly made

up with 4 lb. copper sulfate and 5 lb. washing

soda per 40 gallons of water) is now the

standard spray, but 2 percent burgundy is

still preferred in the west, and is resorted

to elsewhere in wet seasons. The mixture is

applied at the rate of 120 gallons per acre,

still often by knapsack sprayers, but by horseor tractor-drawn machines on the larger

farms. The first application is generally

recommended toward the end of June, and

the second about 3 weeks later, but the

application dates vary according to the prevalence of blight, forwardness of the crop, and

weather conditions. A third application is

occasionally given; hence, again the need for

a spray material that, applied for the last

time towards the end of July, will give protection for the longest possible time in August.

Copper oxychloride and cuprous oxide sprays,

at the same copper dosage as 1 percent

burgundy mixture, are used to some extent

in Eire, but despite their convenience in use,

they have not made the same headway as in

England and Wales or other parts of Europe.

The predominant variety, Kerr's Pink, would

normally grow on to the end of September in

the absence of blight or with very efficient

spraying. In Galway blight candevastate Kerr*s

Pink during July, and in the worst years the

unsprayed crops would be dead by the first

week in August, with a loss of about half

the crop. Although this is rare, even with

three sprayings most of the haulm is usually

killed by mid-September. In Monaghan and

the north midlands Kerr's Pink is sometimes

sprayed only once in the drier years. In

Wexford and Waterford, where the unsprayed

haulm is sometimes killed by mid-August,

and almost always by the end of August, two

sprayings are usual, with a third in wet

seasons. The variety Up-to-Date, where it is

still grown, siiffers rather more severe attack

than Kerr's Pink, but when sprayed as for

Kerr's Pink the differences in loss of yield

are not great. Arran Banner, which occupies

a considerable part of the potato acreage in

most districts, often escapes much reduction

of yield by blight, as it reaches maturity about

the end of August. It is, however, very susceptible to blight in the tubers. Where grown

for seed, as in Donegal, it is often sprayed

only once, about the first week in July, and

then the haulm is burned off with chlorate

about mid-August.

Relatively little haulm destruction is done

in Eire, except on the seed crops. No doubt

the rapid destruction of the haulm by blight

in September commonly renders the treatment unnecessary. Where haulm, destruction

is done, the material chiefly used is sodium

chlorate. No arsenites are employed.

SPRAYING TRIALS

A series of spraying trials on the variety

Up-to-Date was carried out to a unifornn

plan over the years 1927-32 at Glasnevin,

Dublin, by Murphy and McKay (1933). These

trials were made over a run of years in which

slight, average, and severe attacks appear

to have been very fairly represented, in comparison with the general accounts of the

epidemics in more recent years. The spraying was done by knapsack sprayer, using

generous amounts of 2 percent burgundy mixture, with 2 applications in 1927 and 1930,

and with 3 applications in the other years. The

leading results are given in table 30.

Fromi these results it will be seen that even

at Dublin the haulm was completely killed by

blight at the end of August or in early September 5 years in 6. The mean gain from

spraying over the 6 years was 2.5 tons per

acre. Additional trials with 3 sprayings were

also made on British Queen and Arran Victory in 1931, and on Golden Wonder in 1932,

and the mean increase of marketable crop

from all 7 trials in which 3 sprayings were

given was 3^ tons per acre.

In discussing the losses due to blight and

the econonnics of spraying Murphy and McKay

pointed out that in most of the many field

experiments on spraying in Ireland the increase of sound yield was from. l|^ to 2 tons

per acre. Even with this gain, the sprayed

plots always suffered to a greater or less

extent despite the spraying. The authors considered that 3-2 tons per acre, the gainachieved

with fairly complete protection by the 3 sprayings in their trials on all varieties would not

be an overestimate of the total average loss

from, blight in Ireland as a whole. In other

words, the normal practical spraying in Ireland saves about half the total loss from

blight; an estimiate in close accordance with

the findings, in this study, on the efficiency

of spraying in blight years in England and

Wales. ^

Some phytotoxic checking of the growth of

the haulm by the very strong 2 percent

burgundy mixture was noted by Murphy and

McKay in their trials. In the very severe

blight year 1932, in which 1 percent burgundy

was included for comparison, it gave as good

results as the 2 percent mixture. A wealth

of trial data on the comparison of 1 percent

and 2 percent burgundy mixtures is to be

found in the historic reports by Pcthybridgc

(1910-19) on potato diseases in Ireland. The

average results of a series of trials carried

out in Galway over the years 1911-18 were

as follows:

52
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53
Crop:

HealthyBlighted

TotalTons per acre from

crops sprayed with

burgundy mixture of--

1 percent 2 percent

10.8

.5

10.8

.4

11.3 11.2

Thus, it would appear that the expensive

and very strong 2 percent burgundy mixture

has little advantage over 1 percent burgundy,

except where very heavy deposits are required

to persist as long as possible from the last

spraying.

Copper dusting was compared with spraying

in the trials of Murphy and McKay, and even

at very heavy rates of application dusting

was inferior to the burgundy mixture. B ouisol,

the first of the colloidal copper fungicides,

was included in the trials in 1931 and 1932;

not until its rate of application (in terms of

Cu dosage) was brought up to half that of

1 percent burgundy mixture (namely to 1,251b.

Cu per 100 gallons) did the results approach

those of burgundy mixture.

SPRAYING IN RELATION TO THE DEVELOPMENT OF THE POTATO PLANT

Following up clues that became apparent

in the course of the trials made with

McKay, Murphy (1939) went on in the years

1934-37 to evolve a new kind of potato

spraying trial that was to reveal the

essential relationship between increase of

yield and protection of foliage, and provide the yardstick by which blight epidemics could be measured, not only in

Ireland, but throughout the world. No longer

content with determining yields from the

sprayed and unsprayed crops, he studied

the whole course of growth and decline

of the haulm, and of increase in weight

of the tubers (1) in plots kept completely

free from blight by frequent applications of a weak copper fungicide having

no phytotoxic effect, (2) in plots commercially sprayed, and (3) In plots left

unsprayed*

It was not until 1937 that the method

was perfected, and in this year, which

was one in which the blight attack (starting on July 24) was average at Glasnevin

(with 2.0 inches of rain in July, 2.7 in

August, and 1.4 in September), Murphy

at last had plots of Up-to-Date in which

the haulm was kept completely free from

blight, with no phytotoxic effects and

no premature dying off owing to dry weather--

a more or less perfect crop growing on

to the end of its growing period.

Fortnightly liftings on this protected crop

showed the proportion of the ñnal or potential

crop that was present on various dates from

July 5 onward. These figures are givenintable

31, and to this table are added the corresponding figures later determined for

Majestic in England and Wales (see fig.

8).

There is remarkably close agreement

between Murphy's first figures for Upto-Date and the mean figures obtained for

Majestic in England; which indicates that,

in 1937 at any rate, the late maincrop

Up-to-Date was bulking in Ireland at about

the same average rate as Majestic in

England.

From periodic weighings of the leaves

and total haulm, which he also made in

the course of his trials. Murphy concluded

that when the weight of the haulm had been

reduced to about 55 percent of its

maximum and the weight of the leaves to

between 30 and 40 percent of their niaximum, tuber production ceased. At this

point of stoppage of tuber formation he

described the field as looking gray

green at a distance, with stalks still green,

but with all the leaves dead except for

tufts of 6 or more at the apices of the

shoots. This stage, corresponding to 75

to 90 percent on the later evolved B.M.S.

scale (B.M.S., 1947) occurred about August 28 on the unsprayed plots at Glasnevin in 1937. In effect, as Murphy pointed

out, the final yield from the unsprayed

plots should be approximately the same

as the yield reached by August 28 on

the plots kept entirely free from blight

by frequent spraying.

TABLE 31.--Progress of tuber development of

Up-to-Date potato in Glasnevin Ireland, 1937,

and of Majestic in England

Up-to-Date

Majestic,

Date

lifted Yield

Proportion

of total

yield

proportion

of total

mean yield

July 5

July 19

Aug. 2

Aug. 16

Aug. 30

Sept. 13

Sept. 27

Tons/

acre

1.74

3.90

6.87

9.90

12.24

13.93

1A.64

Percent

12

27

47

68

84

96

100

Percent

34

32

73

86

96

100

From a plotting of the tuber growth curve

from the data in table 31, the total yield under

the perfectly protected plants on August 28

54
was 11.9 tons per acre. The actual yield

as determined by lifting was 11.7 tons per

acre, in almost exact accordance with the

theory. The total yields present by various

dates in the plots tiiat received each of the 3

treatments are shown in table 32.

TABLE 32.—Total yields of Up-to-Date potatoes at Glasnevin, Ireland, with 2 treatments and

no treatment, 1937

Treatment

Yield

July 5 July 19 Aug. 2 -Aug. 16 Aug. 30 Sept. 13 Sept. 27

Tons/acre

1.7

2.1

1.9

Tons/acre

3.9

4.3

4.6

Tons/acre

6.9

7.3

7.5

Tons/acre

9.9

9.6

10.0

Tons/acre Tbns/acre Tons/acre

Coinplete control

2 percent burgundy

(3 sprayings)

TInsDraved

12.2

11.3

11.5

13.9

12.9

11.7

14.6

13.6

11.7

The date of stoppage of growth in the plots

sprayed with 2 percent burgundy mixture (on

June 30, July 16, and August 9), as estimated

by Murphy's weighing method, was September

24, very little different from that in the plots

with complete control (Sept. 26). But the

curves given in his paper for the progress

of decline of the foliage show a wider separation in late August and early September,

so that the 75 percent point by the English

method wouldhave been perhaps a week earlier,

in better accordance with the difference of

yield. However, there was a lower maximum

weight of haulm in the plots sprayed 3 times

with the strong 2 percent burgundy mixture,

which indicates some checking effect on growth

that may also have affected the yield.

ESTIMATION OF BLIGHT LOSSES IN IRELAND

The method that Murphy's work suggested

of estimating blight losses, from a mean

growth curve for the tubers and survey information on the dates at which growth is stopped

by blight on the sprayed and unsprayed crops,

has not yet been applied and developed in

Ireland as it has been in England and Wales.

There are not the necessary data for any close

analysis of the blight losses in Ireland region

by region or year by year, and it may well be

that Kerr's Pink often puts on rather more of

its tuber weight in September than is indicated

by Murphy's data for Up-to-Date in the single

season, 1937. It is of interest, however, to take

Murphy's figures as a basis and then see what

measure of defoliation loss they wouldindicate

when some near estimates are made of mean

dates of stoppage of growth by blight.

Perhaps it would be fair to say that in 7 or

8 years out of 10 in Eire the average date at

which blight would stop growth on the lïiaincrops, if they were left unsprayed, would be

about the end of the third week in August

(earlier in Galway, later in Dublin, but August

23 on average). Spraying with 3 applications

sometimes defers the stoppage of growth by

blight until mid-September or even until the

end of the season, but it is probable that with

the usual 2 sprayings, and sometimes only 1,

the average gain from spraying can be put at

about half a month's full growth at the end of

August and beginning of September, equivalent

to stoppage of growth on September 8. The

total defoliation loss from blight and the gain

from spraying, on a 127-ton potential total

crop, on the basis of Murphy's figures would

then be as shown in table 33.

It is difficult to form any estimate of the

mean loss due to blighted tubers in Ireland.

As in England and Wales, the losses in

individual crops may range from negligible

am^ounts to several tons per acre. Perhaps

the loss of 4 or 5 percent as found by Pethybridge over his long series of trials is fairly

representative for sprayed ware crops, and

the loss could be expected to be twice as great

in the unsprayed. Estimates of 4 and 8 percent, respectively, for the losses due to blighted

tubers are entered in table 33. The mean total

loss on a 12|-ton potential crop due both to

defoliation and to blight intubers thenbecome s

3.8 tons per acre; of this loss, 2.3 tons per

acre is recovered by the usual commercial

spraying.

These figures are in reasonable agreement

with the estimate of 3j tons per acre total loss

made by Murphy and McKay on the basis of

their trials; and they serve to illustrate that

the method of estimation based on Murphy s

growth curve gives results of at leastthe right

order. It should be borne in mind, however,

that table 33 is for ware maincrops and not

for the seed crops that are burned off prematurely, and in which the percentage of

blighted tubers is much less. It is also based

on the growth curve for a late maincrop, and

55
TABLE 33.—Loss estimations for maincrops in Eire on basis of Murphy's findings

Item

Date of stoppage of growth

Percentage of potential crop

(from Murphy*s growth curve)

Total crop tons per acre..

Gain from spraying tons per acre..

Total defoliation loss tons per acre..

Percentage of blighted tubers

Healthy crop tons per acre..

Gain from spraying tons per acre..

Total loss tons per acre..

Unsprayed

Aug. 23

76

9.5

3.0

B

8.7

3.^

Sprayed

Sept. 8

92

11.5

2.0

4

11.0

2.3

Potential crop

(no blight)

Sept. 27

100

12.5

0

12.5

it may not hold for Arran Banner. In the

occasional dry, warm seasons, such as 1949

and 1955, the blightlosses are undoubtedly less,

but the lack of rain reduces the crop.

FORECASTING

With severe or moderately severe blight

epidemics occurring in 7 or 8 years of 10,

routine spraying every year is well justified

in Ireland. The requirements of a forecasting

system for Ireland are different from those in

England and Wales, where over much of the

country spraying is rarely needed at all, and

even in the Fens only about every other year

is a blight year. In England one of the main

objects of forecasting is to discourage spraying in the ••no-blight'* years when it can do

positive harm. In Ireland spraying is needed

almost every year, but the fact remains that

the Irish farmers are attempting to combat

severe epidemics with a very small number

of spray applications--gene rally only two, and

at most a reluctant three. The highly retentive

burgundy mixture is used, and this helps, but

still the mean gain from spraying is only about

half the total loss from blight, and there is

every inducement for trying to raise the overall efficiency of the spraying by the best

possible timing of the spray applications in

relation to the weather conditions.

In 1952, a new approach to forecasting was

made by Austin Bourke of the Irish Meteorological Service. In this system, as it has since

been developed and put into use in Ireland

(Bourke, 1953a, 1953b, 1955), the stress is on

detecting every progressive spell of weather

favorable for the development of blight and on

advising farmers whenever the more important

of such spells occur, from June through August,

so that not only their first sprayings but also

their second and, where necessary, their third

sprayings can be applied knowledgeably and to

the best advantage. The very early spells of

blight weather, corresponding more or less

to the first few outbreaks, which, as McKay

(1957) has so clearly shown, oftenoccur in May

or even in April, are not made the subject of

press notices, as this would confuse the

growers and might leadto premature spraying.

It is not until the early blight weather spells

have been such as to permit 3 to 5 generations

of the fungus that an initial warning of the onset

of the disease is issued. This is almost

invariably in the latter half of June, when it has

the effect of strongly reinforcing the Department's long-standing advice that the first spray

should be put on about that time, according to

weather conditions.

A minimum humid period favoring the spread

of potato blight consists, according to Bourke's

"Irish Rules," of

Twelve consecutive hours with relative

humidity not below 90 per cent and temperature not below 50° F, followed immediately by a further four hours with the

plants wet, either due to rain, drizzle or

high hximidity (fog, mist or dew).

Such a minimum **blight weather spell"

means one having an "effective duration" of

1 hour; any longer spell has an effective duration of 1 hour plus the excess over the required minimum.

This is a tempe rature-humidity rule, similar

to the Beaumont rule as employed in England

and Wales in that it has the same minimum

temperature requirement and that it demands

a continuous spell of high humidity; but the

rule differs from the Beaumont rule in that it

was derived from a theoretical consideration

of the findings of Crosier (1934) on the conditions required for the sporulation and infection

processes of the fungus and in that a shorter

minimum period at higher relative humidity is

taken as critical.

56
In the operation of the system, routine

hourly weather reports from a network of 9 or

10 weather stations are used, and the "effective durations'* of all **blight weather spells"

(humidity conditions measured in standard

screens 4 ft. above ground level) are entered

on maps from day to day at the Central Forecast Office in Dublin. The probable geographical limits of each spell are then determinedby

examination of the synoptic weather charts for

the period in question.

IRELAND

CHART IJ CHART 14

June 19th

CHART IS

June 2/st -22nd

CHART 16

CHART 17

June 26th

CHART 18

.2dth

June 30th -July 2nd July 5th-6th

FIGURE 20."Part of a series of charts showing areas of Ireland

affected by successive spells of blight weather in 1953. Effective

durations of the spells at the several stations are given in hours.

(Bourke, 1953b.)

A part of the sequence of these maps for

1953, from Bourke (1953b) is here reproduced

in figure 20 to illustrate the method. After the

spell shown on June 21-22, a first spray

warning was broadcast on June 22 as follows:

Weather favorable for the spread of potato

blight occurred over most of Ireland

during the past week-end, and is likely

to recur in the west during the next few

days. Blight may be noticeable aboutmidweek in many areas. Weather will be

suitable for spraying during most of the

week.

Other warnings were issued following the

spell shown for July 5-6, and later spells on

Jiüy 19-20, August 4-9, and August 29 (for

burning off). In effect, these warnings gave

very good advice for the season, which was one

in which blight became more or less general

from mid-July onward.

It has not been possible to test the Bourke

system of forecasting in Ireland so thoroughly

as the Beaumiont system (with operations

charts) has been tested in England and Wales,

owing to the lack in Ireland of any systematic

blight surveying and assessment by standard

key. The forecasts have nevertheless been in

reasonably good accordance with the general

observations on blight made by the Department's inspectors who have participated in the

work, and there is no doubt that Bourke's

method is among the most sensitive yet proposed for the measurement of "blight weather." The application of the method in Chile

is illustrated in figure 79 of this study.

The advance made by Bourke does not lie so

much in the choice of the humidity criteria,

for Smith's (1956b) work on forecasting by 90

percent humidity criteria in England and

Wales has indicated that there is usually a

fairly close correspondence between Bourke

and Beaumont periods when the principal spells

of blight weatíier occur. Both methods give

good results in the hands of those experienced

in their use. The important step forwardisthe

correlation of critical humidity periods (however defined) with specific weather situations

as revealed by synoptic maps and with air ma ss

analysis (Bourke, 1957). This is a procedure

that naay find an application in many parts of

the world, especially where forecasting has to

be attempted for a wide area with no very close

network of weather stations. To illustrate the

possibilities, Bourke's chart for mid-July

1956 is here reproduced (fig. 21). Working in

Dublin and from a study of the synoptic maps

alone, he was able to see that a severe blight

weather spell which occurred in Ireland over

the period July 14-17 must adso have occurred

in England and Wales, and indeed over much

of western Europe. This was fully substantiated

for England and Wales, and as far as could

later be ascertained, it was also true for the

areas indicated in other countries--from the

Baltic countries to northern France.

57
IRELAND

HIQH

SURFACe WEATHER MAP

0«00 GMT. JULY 14. 1956.

FIGUEŒ 21.—Blight weather in Ireland and parts of western Europe, July 13-18, 1956 (Bourke, 1957): a, Surface weather map for July 14,

showing continental slow-moving, or "puddle," low; b, effective duration of blight weather in Ireland, July 15-18; and c, approximate limits

of blight weather arising from the synoptic situation (cross hatching shows the area most seriously affected).

58
SUMMARY: REPUBLIC OF IRELAND

Potato spraying is now so universal in Eire

that it is difficult to estimate what the loss

from blight would be in its absence. No blight

surveys or critical series of spraying trials

have been carried out recently. But from consideration of the weather conditions, general

accounts of blight in each year, and the results

of trials xinder similar conditions in the past,

it is estimated that in Ireland the haulm of

unsprayed maincrops would be killed by blight

between mid-August and the end of August in

7 or 8 years out of 10, with a miean loss through

premature defoliation and blight in tubers of

3f to 4 tons per acre. Of this loss about Zf

tons per acre can be saved by 2 or 3 applications of burgundy mixture, which prolongs the

useful growth of the haulm by 2 or 3 weeks and

also substantially reduces tuber infection. Eire

has an important seed-potato trade. Burning off

is practiced on the seed crops, and blight

losses in them are small. Many fundamental

contributions to knowledge of potato blight,

including its relation to the growth rhythm of

the potato plant and more recently to forecasting methods, have come from Ireland--the

traditional home of the potato in Europe.

59
CONTINENTAL EUROPE

FRANC E

France grows annually about 2,500,000 acres

of potatoes. Half the crop is used for human

consumption, about 284 lb. per head each year

(Russell (E. J.), 1954). Large quantities are

fed to livestock, principally pigs; small

amounts are manufactured into starch; and the

rest is used for seed. Certified seed production is on a sznall scale. In 1954 it accounted

for slightly under 3 percent of the total potato

acreage. But the seed growers have been

responsible for much of the technical progress

made in French potato production during recent

years. An account of the seed-potato industry

and its future trends has beengivenby Demesmay (1954), The output of French certified

seed plus imports of such seed from other

countries, mainly Holland, is sufficient to plant

only about one-fifth of the total acreage. This

may, in part, account for the rather low

average yield of potatoes in France, which for

the period 1948-50 was 5 tons per acre

(F.A.O., 1955).

POTATO DISTRIBUTION AND VARIETIES

The distribution map (fig. 22) shows a

scatter of potatoes throughout the whole country. There is a marked concentration in

Brittany (19 percent of the total potato acreage

was in this province in 1954), and smaller

concentrations in north-central France (the

Paris basin through to Nord), eastern France,

and across the center of the country (northern

part of the Central Massif). The main early

potato-growing areas are in Brittany and in

the departments surrounding the mouth of the

river Rhone in the southof France. In the latter

area, 56 percent of the potato acreage is

planted witii early varieties. Early varieties

are also grown in the vicinity of the large

towns; e.g., in the Paris basin and near Bordeaux. Brittany is also the most important

seed-growing area, accounting for approximately 70 percent of the total seed acreage«

The only other important seed district is in

northern France, concentrated in the five departments, Nord, Pas de Calais, Somme, Oise,

and Aisne.

Some idea of the relative innportance of the

different potc.to varieties grown in France can

be gaged from figures published by the Seed

Growers* Association: They have also given

a description of these varieties (Federation

Nationale, 1955a, 1955b). Table 34 shows the

acreages of the 10 leading French potato

varieties grown for seed in 1955.

TABLE 34.--Acreagetí of leading potato

varieties grown for seed in France, 1955

Variety

Bint je '

Ackersegen -

Institut de Beauvais.

Etoile du Léon

Saskia

ArxEjn. BaJiner

B. F. 15

Ker Pondy

Krasava

Urgenta

Certified

seed

Acres

43,171

5,920

2,438

2,354

2,240

2,191

2,065

1,529

1,294

1,065

The outstanding point is the preponderance

of Bintje, which is widely grown throughout

France. It is very susceptible to blight, but,

as it is relatively early maturing, Bintje often

escapes the full severity of epidemics. Ackerse gen is the most popular late variety, and

Etoile du Leon and Saskia the most common

early varieties.

SOIL, CLIMATE, AND CULTIVATION

In Brittany and the Central Massif (fig. 23),

the potato soils are generally light, and are

detived from underlying granite and schist

rock formations, in northern France, including

the Paris basin, and in eastern France, the

soils are heavier, but usually overlie a subsoil that is predominantly sandy.

France exhibits the three main European

ciin:iatic types--maritime, continental, and

Mediterranean. In the potato-growing areas

of Brittany, the climate is predominantly

maritime, although it is nriodified In the departments farthest from the coast. This maritime

climate is characterized by a marked autumn

and winter rainfall peak, relatively mild winters, and cool summers. Total annual rainfall

is around 30 inches. In northern Franee (Nord,

Pas de Calais), there is a modified maritime

climate, whereas farther south in the Paris

area continental influences are nciore important. The potato-growing districts of eastern

France are under the influence of a f\illy continental type of climate. Sumimer and winter

temperatures are more extreme than in the

60
^—^ . •,•• .A. :•.v. • . : • •

• • • ; • .•.•••••••. •• •••.?. ■■■. ..•••.•••.•• •.••'•/ • • '. • r^

• ....• • •

Each dor = IOOOacres

FIGURE 22.--Distribution of Dotato growing in France, 1954. (After Klatzmann, 1955.)

west, and the rainfall peak occurs in the summer months. Total annual rainfall, approximately 24 in., however, is lower than in the

west. The true Mediterranean climate with its

hot summers and characteristic periods of

drought is confined to the coastal areas of

southern France. Generally speaking, this is

not an important potato-growing area, apart

from the early crops grown around the mouth

of the river Rhone.

The main planting takes place in mid-April.

Planting of early varieties in Brittany is

carried out at the end of February to early

March. Seed crops in Brittany are burned off

compulsorily in mid-July. The maturity of

mid-season and late varieties varies from

approximately mid-August to mid-September.

One-third of the total manpower in France

is still engaged directly in agriculture, and

apart from the north-central area of the

country, mechanization is not very far advanced. Potatoes are still largely planted by

hand. The seed is not usually cut. Potato rows

are about 28 in. wide, and mid-season andlate

61
FRANCE

V. N O RD;

•. ;A I S NE. ÁRDENNES;.':.^

t'Af-tigny.; -,

Reims ":

BMTTANY

Ploudalmézeau« _ _ ...

'/'^ /Í •CÔTEs DU NORD

f^v^ i"' »Rennes

Brest^

Choteoulin

jr^Ppris ;•

••.Versoi ¡les

.Orleans .'

•.CREUSE •.; •.•••.., _

'HAUTE.'.- .PUYDEDÔME

VIENNE .. :••••.,•;•; Clemont'-..

'•: ." -Fernand

Metz.••....-.^^

.MOSELLE-'-" ■■ "

:.. '•'•.BAS

••'•RHIN

•HAUT.«

'•..•'.RHIN'.

Ar^

'Slrasbourq

C ENTRA L >.M A S'S I F":

AVEYRON

.'• Vjóulouse 'Montpellier»^

FIGURE 23."Key map of France.

varieties are planted from 18 to 22 in, apart

in the rows. The potatoes are almost always

ridged up# Storage is in farm buildings or,

where the winters are mild, in field clamps.

A description of ware potato production

methods has been given by Malmont^ (1954).

FREQUENCY OF BLIGHT EPIDEMICS

The importance and incidence of blight

vary considerably between the different areas.

Broadly speaking. If a line is drawn across

France from Strasbourg to Bordeaux, the

disease is important north of the line. South

of the line, blight is considerably less important, although inthe intensively cultivated river

valley areas (e.g., valleys of the Garonne and

Durance) where some potatoes are grownunder

irrigation, heavy losses fromblightmay occur.

In the north of France, the four mainpotatogrowing districts differ from each other in

climatic and soil conditions and blight Incidence. (1) In Brittany the disease occurs every

year. (2) In north-central France, which Includes the Paris basin, blightIncidence appears

to be fairly uniform, although there are climatic differences within this area. Severe

attacks occur about 1 year In 3. (3) In eastern

France blight Is somewhat less Importantthan

In north-central France. (4) In the northern

part of the Central Massif the disease Is only

occasionally severe.

62
Very little published «urvey data on the incidence of potato blight ona year-by-year basis

exist for France. Oucomet and Foex (1939)

have simimarized verybriefly the attacks of the

disease from 1919 to 1937. In 1953, shortnotes

were given anonymously in the Annales dea

Epiphyties on attacks in 1951, and Barbotin

(1953) has given an account of the progress of

blight in 1952. Information on other recent

years has been given to the authors by H. Darpoux and M. Guntz of the SUtion Centrale de

Pathologie Végétale at VersaUles, and by P.

Dumas of the Service de la Protection des

Végétaux. From this, it was possible to divide

the 10 years 1947-56 into 3 categories; i.e.,

years in which the incidence of the disease

was slight, moderate, or severe. Table 35

shows the results of this division:

TABLE 35.~Incidence of potato blight in

France, 1947-56

Slight

1947

1949

1952

It is not very satisfactory to have to treat

France as a single unit in this way, but it is

not possible to give concise data for the

separate growing regions, as the information

is too fragmentary. In broad terms, the three

categories used in table 35 can be defined as

follows:

Slight: Blight attacks are either absent or very late throughout

northern France.

Moderate: Blight attacks are important

in only some parts ofnorthern

France, or the disease is late

in appearance throughout the

whole area.

Severe: Blight attacks occur early and

are severe throughout northern France.

BLIGHT ON HAULM

The incidence of blight is usually considered

to follow a fairly regular pattern from year to

year. The disease is present every year in

Brittany, where it starts about the middle of

June; the tops of unsprayed potatoes are

usually dead by mid-July. It is normally first

reported in the Paris area about 2 weeks after

its appearance in Brittany, but the course of

the disease both there and in eastern France is

much less certain than in Brittany, and its

incidence is not so severe. In the Central

Massif, blight is usually late and is rarely

severe.

Since 1955, systematic observations on the

development of blight on haulm in relation to

weather conditions have been made at a number of centers throughout France by the Plant

Protection Service in collaboration with M.

Guntz. The aim is to use these data to improve

the existing blight warning services. Progress

curves supplied by M. Guntz for selected

centers in the maingrowing regions of northern

France during 1955 and 1956 are shown in

figure 24.

FRANCE

lOO

BLIGHT

ON

HAULM

per cent 50 -

O

loo

I North Centrol

1955

-

7 JyJ

1956

June July Auqusf Sept

FIGURE 24.—Progress of blight on unsprayed haulm in principal

potato growing regions of northern France in 1955 and 1956;

variety. Bintje.

The year 1955 was exceptional for blight. It

occurred first for a distance of about 50 km.

round the coast of Brittany, but made slow

progress in dry weather except on the coast.

In north-central France, dry weather prevented the development of the disease, but

there were heavy rains in eastern France,

where blight is not usually bad, and these rains

resulted in severe blight attacks. In 1956,

blight appeared late, but made very rapid

progress on the tops of potato plants throughout northern France.

LOSSES DUE TO PREMATURE DEFOLIATION

BRITTANY: Although the first generalized

field outbreaks of blight in this region do not

normally occur before mid-June, first reports

in the area usually come in much earlier,

sometimes by mid-April. The evidence so far

available suggests that there are considerable

variations in the incidence of blight between

the coastal regions and inland Brittany. This

is shown up in figure 25, which presents blight

progress curves for unsprayed Bintje at three

centers. Ploudalmézau is on the coast;

Chateaulin is about 15 km. from the coast; and

Rennes, which is a good deal farther east than

the other centers, is at least 60 km. from the

sea.

63
loo

BLICHT

ON

HAULM

per cent 50

O

loo

1954

'0/

y^-^

0/ \/

><K/

"Dl 0/ .^!^ U^ X

^ 1 0/ cyy

Q./ ^

1955

100

0/ /

Qj /

5Q 0/ 0/

0/ /

or/

Q. / U/ J

1956

June July August" Sept

FIGURE 25.—Progress of blight on ünsprayed haulm at centers in

Brittany, France, 1954-56; variety, Bintje.

There are insufficient data on which to base

yield loss estimates for this area» but some

general statements can be made. The 75 percent blight stage on unsprayedcrops is reached

very early along the coast. Figure 25 shows

that at Ploudéilmézau, this stage was reached

at or before mid-July during 1954-56. However,

this early defoliation is unlikely to have a

serious effect on potato yields, as the coastal

strip is the early growing region and immature potato crops will be dug there from the

end of May onward. Brittany is the principal

seed-growing area, but blight is unlikely to

affect the yields of these crops seriously, as,

in any case, they have to be burnedoff in midJuly under compulsory regulations designed

to prevent the spread of virus diseases. The

seed crop, also, receives the greatest amount

of protective spraying.

Despite the importance of early varieties

and of seed-potato growing in the region,most

of the potato acreage is planted with midseason varieties (in particular, Bintje) for the

ware market. It seems likely that relatively

heavy yield losses due to prem.ature defoliation following blight attacks will occur very

frequently in crops of these ware varieties if

they are unsprayed. In 1956, blight attacks

were late, but figure 25 shows that at Chateaulin the 75 percent stage on unsprayed crops

was reached before the end of July, and at

Rennes the same stage was reached before the

middle of August. The time of naaturity of

Bintje is roughly the middle to end of August,

so that unsprayed crops of Bintje will have lost

up to 4 weeks of growth.

NORTHERN FRANCE, outside Brittany: Differences in the incidence of blight between the

remaining potato-growing areas of northern

France occur, but precise data for compara64

tivc purposes arc lacking. The disease is less

importent in these areas than It is in Brittany,

and as a result there may be a tendency to

minimize it.

The only information on which tentetiveloss

estimates can be made for northern France,

outside Brittany, comes from a series of

spraying trials against blight carried out at

Attigny (Ardennes), during the period 1948-55.

The results of the 1948 trial have been reported by Raucourt, Lansade, and Ventura

(1949), andfigure s for the remaining years have

been given to us by E. Ventura. These trials

were not designed to estimate losses but to

compare the efficiency of anumber of different

spray materials by measuring their protective

effect on the haulm, and to compare sprayed

plots to an unsprayed control. In only 2 years

out of 8 were the spray plots liftedand weighed.

Table 36 shows the dates on which the 75

percent blight stege was reached on the unsprayed controls each year, andthe increase in

yield from spraying in the 2 years that the

crop was weighed.

TABLE 36.--Date when 75 percentblight

occurred on unsprayed Bintje potatoes and

increase in yield 2 years when crop was

sprayed, northern France, 1948-55

Year

.. . Date 75 percent

"blight occurred on

unsprayed plots

Increase in

yield from

spraying

1.948

1949

1950

1951

1952

1953

1954

1955

Aug. 6

9

11

16

(^)

July 17

Aug. 14

19

Percent

33

20

^ No blight.

The variety grown was Bintje. Planting took

place around the first week of May, which is

rather late, except in 1953, when it was at the

end of March. In only 1 year was blight absent.

Of the 6 years when the 75 percent blight stage

was reached in August, the latest date (Aug. 19)

occurred in 1955, when there was a 20percent

net gain from spraying.

At this stage, the results presented in table

36 can only be used as an indicationthat blight

losses due to premature defoliationinthis part

of France m.ay be fairly consistently heavy.

Weather conditions at Attigny are very similar

to those in the potato-growing districts of

eastern France, which is not looked upon as a

bad blight area. The inconsistencies between

the Attigny results andthe general opinionabout
blight in the area can only be cleared up by

more detailed trial and survey work aimed at

estimating losses.

BLIGHT IN TUBERS

H. Darpoux has told ud that '*the damage

caused by blight is nearly always the result

of premature destruction of the potato foliage,

leading to a fall in crop yield. Tuber infection

is usually of little importance except in summers with a particularly heavy rainfall." Both

in 1954 and 1956, however, the rainfall in the

latter part of the summer was high, and heavy

losses from infection, while the tubers were

still in the ground, were reported. Tuber infection is in general regarded as of small importance in view of the widespread growing of

Bintje, which is a very susceptible variety. In

14 years of potato experiments carried out at

high altitudes inthe Central Pyrénées, Chouard

(1949) reported that in no year did blight affect

the keeping quality of the crop, despite the fact

that no control measures were takenagainst the

disease.

It is generally considered that the light soils

in Brittany and the Central Massif are more

favorable to tuber infection with blight thanthe

more loamy soils in the north-central and

eastern growing regions of northern France.

The problem of tubers being infected at lifting

time, by coming into contact with diseased

haulm, does not usually arise in France, owing

to the concentration on early-maturing varieties.

BLIGHT AND WEATHER DATA

The three main types of climate--maritime

(Brittany); continental (easternFrance); Mediterranean (southern Franee)--under which

potatoes are grown are illustratedinfigure 26.

The climagrams give long-term average temperature and rainfall figures for three representative centers, together with planting and

lifting times.

Brittany, which is the worst blight area in

France, has a relatively low summer rainfall.

Compared with eastern France the average

rainfall is lower in Brittany in May, June,

July, and August, although annual rainfall is

considerably higher, due to the high autunrm

and winter precipitation. This points the fact

that humidity and not rainfall is the more

important factor affecting blight attacks. The

importance of humidity is borne out from a

consideration of the two blight curves for

Brittany in 1955 shown in figure 25 above.

In that year, rainfall during the growing season

was greater at ChateaulinthanatPloudalmézau

on the coast. But blight progress was very

much quicker at Ploudalmézau, due to higher

humidity conditions. Rainfall in the center of

Brittany is higher than on the coast, since it

is not until the clouds driven by a prevailing

FRANCE

6 r-Brittany

BREST

RAINFALL 5

Inches

J F M Jk M J

6 r- Eastern France

METZ

s o N D

RAINFALL 5

Inches

J FMAMJ JASOND

. Southern France

AVIGNON

RAINFALL 5

Inches

FIGURE 26.—Long-term average monthly rainfall and mean temperatures in relation to growing periods for three regions in France.

westerly wind reach the higher ground that

the bulk of the rainis precipitated. Nontheless,

the almost invariable story is for blight to be

worse on the coast«

The high summer temperatures and generally dry conditions account for the minor

importance of blight in southern France, and

at the same time point the reasonfor the smaill

interest in midseason and late varieties in the

area.

CONTROL MEASURES

Most of the protective spraying against

blight carried out in France goes on in the

Brittany peninsula, and there, spraying is

concentrated on the seed crop. Little or no

»praying is done on the early crop. In the

potato areas of north-central France, where

the farms are large and highly mechanized,

there is somie protective spraying. In other

areas, where much of the crop is grown on

65
small farms by peasant proprietors, little

action is taken against the disease.

Spraying is most usually carried out at

high-volume rates (about 1,000 liters per

hectare, which is equivalent to 89 gallons

per acre). Generally, 2 or 3 spray treatments

are given to mid-season and late varieties.

In Brittany and other areas where spraying

against the Colorado beetle is compulsory,

a combined treatment against blight and the

beetle is often applied. DDT is the material

most commonly used against the Colorado

beetle, and the usual practice is to combine

the second treatment against this pest with

the first antiblight spray. Experiments with

low-volume spraying against blight have been

reported by Jouin, Hascoet, and Ventura (1954).

E. Ventura has told us that in 1951, when blight

was severe, application rates below 300 liters

per hectare (27 gallons per acre) did not give

an adequate control of the disease.

Sprays are more efficient and economical

than dusts, and are far more widely used.

Most protective spraying is still carried out

with copper compounds, the most important

of these being bordeaux mixture, copper oxychloride, and copper oxide. Experiments have

been carried out by Raucourt, Lansade, and

Ventura (1949) to compare the relative efficiency of bordeaux mixture containing 1 percent and 2 percent copper sulfate. In a bad

blight year (1948), the 2 percent bordeaux

mixture was found to be more efficient under

conditions where only 2 or 3 sprayings were

given. In another series of trials in northern

France in 1954 reported by Guntz, Hascoet,

and Ventura (1954), a depressive effect of copper on potato yields was noted when blight

was absent or attacked the crop late in the

growing season. Reduction in yield from the

use of 2 percent bordeaux mixture in these

trials was about 10 percent.

The organic fungicides are not used on any

scale. It has been found that they do not reduce

the crop in dry seasons, but when blight has

been bad the organic materials have had to be

applied more frequently than the copper compounds to give a comparable protection to the

potato foliage* This finding does not accord

well with one of the French farmer's main

aims, which is to keep his number of sprays

down to a minimum. Some recent spraying

trials in France have compared the relative

efficiency of copper and organic fungicides

(Guntz and Ponchet, 1952; Ventura and

Raucourt, 1953).

Haulm killing is not practiced to any extent

outside the seed-growing districts, in particular Brittany. The tops of certified seed

crops have to be destroyed within a certain

period, as laid down by the seed inspectors,

to prevent the late spread of virus diseases,

and this practice also serves to prevent tuber

blight infection. Materials used are sulfuric

acid and sodium chlorate. Monot (1956) has

described some of the different methods of

haulm destruction practiced on seed crops,

and at the same timie he has compared the

efficiency of a number of different chemicals

used for this purpose.

One method that the French hope to use in

order to reduce blight losses is to grow quickly

maturing potato varieties that will escape most

if not all the effects of blight attacks. To

some extent, they already look upon the widespread growing of Bintje as a control method.

Although very susceptible to blight, Bintje

matures early (mid-August to end of August,

when planted about the middle of April),

and, except in Brittany, it usually escapes

serious yield losses as a result of premature

defoliation even when unsprayed. The Dutch

variety Sirtema, which was put on the market

in 1952, meets this need for very rapid

maturity, but, unfortunately, it is a poor

quality potato and is unlikely to have any

commercial future. Work at La Station

Génétique in Brittany aims to produce a good

quality, quick maturing variety basedon breeding with the old German variety. Erste von

Nassengrunde. Some work on blight races is

also being done at this station, but hope of

producing blight-resistant varieties from

Solanum demissum hybrids has been given up,

owing to the ability of the fungus to produce

new races.

POTATO BLIGHT FORECASTING

The whole of France is covered by a network

of 14 regional Stations d'Avertissements (Agricultural Warning Stations) under the administration of the Plant Protection Service. A

recent account of the working of these stations

has been given by Journet (1955). In brief,

advice on the correct timing of spray and dust

treatments against a wide range of crop pests

and diseases is sent out at regular intervals

from the stations to cooperating growers in

their regions.

For potato blight, growers are advised

individually by post card when it is time to

spray. The warnings are based on field observations of blight sent in by observers

(agricultural advisers, farmers, etc.) plus a

general consideration of past and present

weather conditions in the region. No specific

weather rules are used, and the stations do

not provide a forecasting service in the strict

sense of the term. Advice is also broadcast

over the radio and put in the local newspapers.

Table 37, based on information given to the

authors by P. Dumas, shows the diates on

which advice was given to farmers to spray

against blight by the stations during 1955.

France is thus well ahead of most other

countries in offering a blight warning si^rvicc

to all growers. Full advantage of this service

is not being taken at the present time, but

interest in this type of advice is shown by

66
TABLE 37.--Dates of spray warnings in France, 1955

Station

Angers

Avignon

Beaime

Bordeaux

Clermont-Ferrand.

I^on

Montpellier

Orleans

Paris

Reims

Rennes

Strasbourg

Toulouse

May June

— 9, 21

^ 27 6, 13, 2Ö

— 10, 2Ö

— B, 22

— 11

^ 24 7, 21

23 6, 20

— 11, 29

— 1Á, 30

— 15

^ 18 3, 16

— 20

-- 8, 22

July

20

2 18

6

21

1, 2 26

4, 19

19

21

4, 27

6

9, 2 21

18

August

17

2 2

^ Spray warning for early crops.

^ Advice to kill the haulm of severely infected crops.

the progress of the scheme as a whole. According to Journet, membership is increasing

by 25 to 30 percent each year.

SUMMARY: FRANCE

Blight is most serious in northern France,

particularly in the Brittany area, where the

climate is maritime • Information on defoliation

and tuber blight losses is generally lacking,

though surveys of blight on the haulm are now

being made and there have been numerous

spraying trials. Chemical control is concentrated on the valuable seed crop. The quicknciaturing Bintje is the most widely grown

variety and largely escapes the worst effects

of blight epidennics. French breeders arc trying to produce an even more rapidly maturing

ware potato suitable for winter storage. There

is a well organized plant disease and pest

warning service.

BELGIUM

In Belgium potato yields are ajnong the

highest in the World. Some 250,000 acres of

potatoes are grown each year, with an average

yield of about 10 tons per acre (F.A.O., 1956).

The greater part of the crop isusedfor human

consumption, and only a small proportion is fed

to livestock or used for manufacture. The

estim^ated annual consumption of potatoes per

head in 1947-48 was 315 lb. (F.A.O., 1948).

The breakup of the potato acreage by province

and also into early, nmidseason, and late varieties, together with the acreage grown for seed

is shown in table 38. The figures refer to the

1950 growing season (Belgium, Ministère des

TABLE 38.—Acreages of early, midseason, late, and seed potatoes

in each province, Belgium, 1950

Province

Antwerp

brabant

West Flanders....

East Flanders....

Hainaut

Liège

Ilmbourg

Luxembourg

Namur

Belgium (total)

Early

potatoes

1.000 acres

9.3

3.2

2.7

2.0

.9

.2

.3

.2

.4

19.2

Midseason

potatoes

1^000^ acres

7.3

14.3

48.4

41.6

10.3

1.1

3.0

3.0

1.7

130.7

Late

potatoes

1,000 acres

13.7

12.8

9.7

19.2

7.6

5.3

7.9

11.6

3.4

93.2

Seed

potatoes

1,000 acres

0.1

.6

.7

2.1

Total

1,000 acres

30.4

30.3

61.4

63.5

18.8

6.6

11.2

15.5

7.5

245.2

67
Affaires Economiques, 1953). Belgium provinces and places named in the text are shown

in the map (fig. 27),

The principal midseason and late wareproducing areas are in the belt of sandy

soils running across the north of Belgium,

with the heaviest concentration in West

Flanders and East Flanders. Early potatoes

are grown in the region of Duffel and Mechlin,

which lie between Antwerp and Brus sels^-There

is an early potato district near the coast,

around Lombardsijde in West Flanders; elsewhere some early potatoes arc grown in the

region of large towns. Home seed production

is concentrated in the polder region of West

Flanders and East Flanders, and in the higher

land in the province of Luxembourg. The

varieties grown in Belgium are the same as

those in the Netherlands. Andries (1953) has

reviewed the importation and home production

of seed potatoes. He shows that Bintje is

the variety most widely grown and Eersteling

is the most popular early potato. Of the midseason varieties, after Bintje, come Record,

Eigenheimer, and IJsselster. The principal

late varieties are Alpha and Voran.

Most of the potatoes are grown on light,

sandy soils. These soils are inherently very

poor, but with the favorable rainfall and a

high level of management and manuring, they

produce excellent crops. In the main potatogrowing areas of northern Belgium, the annual

rainfall is about 28 to 32 inches per annum,

with a slight peak in the summer and autumn

months. Winters are relatively mild and the

sumnner is cool. (See climagram, figure 28.)

The growing period for Bintje and other

midseason varieties is from the middle or

end of April to early September; late varieties

planted in the middle of April are mature at

the end of September.

Potatoes are grown chiefly on small farms.

In Flanders, farms are very small, about 5

hectare (12^ acres) in size, and they suffer

s t A

BELGIUM

'•Lombardsijde

SI'N

*. . . • * ^« Antwerp

ANTWERP

• Ghent • Duffel

'..... ^Mechlin.

ST FLANDERS :

• Brussels

BRABANT

H A 1 N A U T

'': Cembloux ;

' . • • :* N A M U R

srN

LIMBURC

LIEGE

50 miles

LUXEM B OURC

Li brament '.

FIGURE 27.--Key map of Belgium.

68
further from the disadvantage of being carved

up into small strips, very often widely scattered, which seriously hinders the efficiency

of working. Despite their small size, they are

farmed on intensive Unes; i.e., the only way

the farmers can get a living is by producing

big crops. Their main crops are rye, oats,

barley, and potatoes. Most of the work is done

by hand and horse labor.

BLIGHT EPIDEMICS: FREQUENCY AND

DISTRIBUTION

Since 1941, annual accounts of the blight

epidemics on potatoes in Belgium have been

given in the bulletins of the Agricultural

Institute at Gembloux by R. Vanderwalle. These

accounts, which are in very general terms,

have been included in reports on the annual

incidence of a large number of plant diseases

(see Vanderwalle (1942)). From them, it is

possible to group the different years into

three categories: severe, moderate, or slight,

according to the incidence of blight (table 39).

In no year was the disease absent.

TABLE 39. —Incidence of blight in Belgium,

1941-55

Severe Moderate Slight

1941 1942 1944

1946 1943 1947

1951 1945 1948

1953 1950 1949

1954 1952 1955

No report was made for 1951, but indications are that this was a *'severe* ' year.

The reports for 1954 and 1955 have not yet

been published, but W. Welvaert has provided

information from which the authors have completed the table.

The relevance oíthe terms ''severe," "moderate,•' and "slight" is best illustrated by

quoting from Vanderwalle's accounts for three

typical years. These are 1953, a "severe"

year; 1945, a "moderate year"; and 1948, a

"slight" year.

1953: Blight severe. "The disease caused

heavy losses to potato growers,

notably in the variety Bintje. It

appeared early (July 1) and a largescale outbreak developed during the

next 8 days. On late varieties, the

disease was less important and

less damaging. Protective treatments were carried out frequently

in July to counter the effect of the

rains. Where this was done a good

control of blight was obtained.

There were also heavy tuber rot

losses in this year."

1954: Blight moderate. "Climatic conditions did not favour the appearance

of the disease until late in the growing season. The result was that

early varieties, which are very susceptible to the disease, and frequently attacked, escaped infection.

The disease appeared on midseason and late varieties in an

irregular fashion, according to

area. In the south of the country,

losses were small. In Flanders,

Brabant and Antwerp, the potato

fields were more heavily attacked.

Crop yields were reduced, and

losses were increased by storage

rots, which in some cases reached

20-40 per cent of the crop."

1948: Blight "slight. "In general, the disease appeared rather late, and as

a result, it caused little damage,

at any rate to the potato haulm.

There was a certain amount of tuber

blight in susceptible varieties."

From table 39 it would appear that the

frequency of severe blight years is about 1

year in 3. Belgium is a small country (11,755

sq. miles), with fairly uniform weather conditions. Nevertheless, there is a markedtendency

for blight to start earlier and be more severe

in the northwest, which is the principal potatogrowing area, than in the south and east.

It seems likely that in the Flanders area the

frequency ot severe blight years is somewhat

greater than 1 year in 3.

LOSSES FROM BLIGHT

There is no existing survey material in

Belgium on which estinnates of blight losses

can be based. Rigot (1953) has suggested that

the total of losses caused by potato blight is

somewhere between 150 to 200 million francs

per annum. This is equivsilent to a loss of

about half a ton of potatoes per acre over the

whole potato acreage each year, and represents approximately 4 percent of the potential

crop.

It is evident from the high average yield of

potatoes that blight losses are to a large extent

controlled. Even so, severe losses do occur.

This is illustrated by what happened in Flanders

in 1954, which was a bad blight year. The tops

of Bintje, despite the fact that practically all

crops of this variety are protectively sprayed

in the region, were dead by mid-August. This

means that from 2 to 3 weeks of growth were

lost by these crops, representing between

10 to 15 percent loss of crop due to premature

defoliation.

According to W. Welvaert, tuber infection

is not a serious source of loss in the main

potato-growing areas of northwestern Belgium.

Iiifection before lifting is generally only slight,

mainly because of the sandy soils. Infection

69
at harvest is avoided by delaying lifting until

the potato tops are dead. This is made easier

by the concentration on growing midseason

varieties in the area and by the relatively great

labor force (due to the small size of the

holdings) available to deal with the harvest.

Vanderwalle in his annual accounts of blight

reported a number of years in which cases

of severe tuber blight infection of Bintje

occurred. It is not possible to tell from these

accounts how widespread the trouble was, but

it seems fair to infer that severe cases are

relatively isolated. Tuber blight infectionis not

a serious problem for the country as a whole.

BLIGHT AND WEATHER DATA

Figure 28 shows the long-term average temperature and rainfetll data, together with the

main planting and lifting times for potatoes in

Flanders. Tcmperatiire and rainfall conditions

in 1954, when blight was severe, and in 1955,

when it was slight, are compared. In 1954,

the first fieldoutbreaks of the disease occurred

about naid-June, and the tops of midseason

varieties were dead by mid-August. In 1955,

blight appeared only at the extreme end of

the summer.

The high July rainfall in 1954 appears to

have been one of the main causes of the severe

attack; August and September rainfalls were

also above average, the mean temperatures

were generally below average in thatsiimmer.

By contrast, 1955 was a hot dry svimmer.

June, July, and August rainfalls were all

below average; July being a particularly dry

month (0.9 inches of rain). July mean temperatures were 2° F. above average, and

August temperatures 3^ above average.

Vandenplas (1955) has explained the cooperation that exists between the meteorological

service in Belgium and the agriciütural advisory and research services. This has not

so far included any detailed work on potato

blight. Jaivenois and Beck (1956), however,

have described a pilot experiment carried out

in 1955 in Hainaut, in which protective spray

applications were made to potatoes following

Beaumont periods.

CONTROL MEASURES

In Flanders all the farnners apply a number

of protective spray treatments each year.

These are given to the midseason and late

varieties; it is mostunusualfor early varieties

to be sprayed. Normally 2 applications are

given: The first, just before the crop meets

in the rows, and the second 2 to4 weeks later.

Welvaert has said that many of the farm.ers

do not spray sufficiently often in bad blight

years, when amininaumof 3 applications should

be given. Most of the spraying is done by the

farmers themselves, very often with knapsack

sprayers. There are also a number of spraying contractors at work, many of whom are

local farmers who possess a tractor and

sprayer and are only too glad to supplement

their income in this way. Outside Flanders,

less spraying is done.

The majority of the farmers use copper

preparations, chiefly copper oxychloride but

sometimes cuprous oxide or bordeaux mixture.

A few farmers (not more than 5 percent)

are using organic fungicides (zineb). To some

extent, the antiblight treatments are combined

with DDT or leadar senate against the Colorado

beetle. The contractors' charge for a course

of 3 protective sprayings in 1956 was 1,200

to 1,500 francs per hectare--equivalent to

£3 10s. to £4 10s. per acre. Haulm destruction

is practiced only in seed crops. Bintje remains the most popiilar variety, despite its

extreme susceptibility to blight. In 1941, Vanderwalle wrote: ''Undoubtedly, Bintje possesses very great assets, both from a culinary

point of view and also on account of its outstanding yield, but because of its great susceptibility to blight, it shoiildbediscouraged.*'

Up to the present, however, no other variety

has been capable of dislodging Bintje.

Some work on breeding blight resistant potato

varieties has been carried out at the Station

pour l'Amélioration de la Culture delà Pomme

de Terre, at Libramont in southern Belgium

(Rigot, 1953). So far, no commercial varieties

have come from this station. Roland (1946)

examined the foliage susceptibility to blight

of a number of commercial varieties, which

he classifiedas resistant (e.g., Aquila), slightly

susceptible (Ackersegen), susceptible (Industrie), and very susceptible (Bintje).

Little or no research or experimental work

is being done in Belgium at the present time

on potato blight control. The last paper on

this subject was that published by Vanderwalle

and Roland (1948), which dealt mainly with a

series of experiments covering the influence

of certain cultural treatments. Including

^Flanders

RAINFALL

inches

RAINFALL

Inches

FIGURE 28.—Montbly rainfaU and mean tempcraiure for Flandew,

Bclglurn: Above, bng-tei-m average; below, ••blight'' and ••no

blight" years.

70
manuring and date of planting, on the incidence

and severity of the disease.

SUMMARY: BELGIUM

Most of the potatoes are grown on small

farms, which produce excellent crops. Little

precise data are available on the importance of

blight or the losses that the disease causes,

but in Flanders severe blight conditions appear

to occur somewhat more frequently than 1 year

in 3. The small size and awkward layout of

the holdings has not prevented growers from

following an annual spray program against

blight. The system of farming is achieved

only at the expense of a low standard of living

to the growers.

THE NETHERLANDS

During the period 1951-55, approximately

400,000 acres of potatoes were grown each

year in the Netherlands. The average yield of

10 tons per acre is among the highest of any

country in the world (F.A.O., 1955). Only 30

percent of the crop is used for home ware

consumption, which in Holland is about 240 lb.

per head per annum. Of the rest of the crop,

25 percent is usedfor manufacture, principally

into farina; 20 percent is fed to livestock; and

the remainder is exported as ware potatoes or

as seed, or used for seed at home. The export

of seed toFrance, Belgium, and other European

countries, as well as to South America and

North Africa, is a most important branch of

the potato industry (Netherlands Ministry of

Agriculture, 1953).

POTATO DISTRIBUTION AND SOIL CONDITIONS

Potatoes are grown in all parts of the country,

with a particularly heavy concentration in the

*'peat colonies" of Groningen and Drenthe in

the northeast (fig. 29). The acreages of potatoes

in the several provinces in 1955, as given in

Nederlandse Rassenlijst (1956), were:

Province:

Thousand

acres

Groningen 48.4

Friesland 22.0

Drenthe 70.9

Overijssel 51.4

Gelderland -- 30.6

Utrecht - 3.0

North Holland 28.7

South Holland 29.4

Zeeland 26.2

North Brabant 45.9

Limburg 20.0

Total (Netherlands) 376.5

The soils of Holland can conveniently be

divided into four main classes, each with its

own well-defined type of potato production.

SEA CLAY soils occur in the north and west

of the country. These soils are derived from

the sea, and in texture they are fine sandy

and silty loams. The land is low lying; a

large proportion of it is below sea level, and

the soil water table is controlled. The ample

soil moisture and favorable temperatures

induce rapid growth of the potato plants; the

variety Bintje planted in mid-April is meeting

in the drills by mid-June. The principal ware

and seed potato areas of the Netherlands are

on the sea clay soils. De Streek, the most

important early district, is a sea clay area in

the northeast part of the province of North

Holland. Seed-potato production comes under

the control of the N.A.K. (Nederlandse Algemene Keuringsdienst) with headquarters at

Wageningen. The spread of virus diseases in

the seed crops is controlled by the compulsory

destruction of the potato haulm on a certain

date each year, the so-called **E'* date. This

varies slightly from year to year and according to the grade of the seed, but destruction is

about the middle of July.

RIVER CLAY soils are much less extensive

than the sea clay soils and are generally less

fertile. The Betuwe region in the south of

Gelderland is the most important river clay

area, and is a ware-producing district.

SAND soils are the principal soils of the

south and east of the Netherlands. Rainfall is

very similar to that of the sea clay areas,

but late frosts during the last weeks of April

and first part of May are more likely to

occur, so that planting is delayed. Growth,

too, is slower as the sands are not so well

supplied with soil moisture. The sand soil

regions are areas of mixed farms, and the

potatoes grown are chiefly for fodder, but

are also for ware. The demand for ware

potatoes from the sand soils increases in

years of low potato production, so that dualpurpose varieties suitable for both ware and

fodder are widely grown.

RECLAIMED PEAT ('•DALGRONDEN")soils

are in the northeastern part of the country. The

top layer of the soil in the **peat colonies"

is manmade. Instead of leaving a sterile subsoil after the peat had been dug inthese areas,

the top layer of peat was used to cover the

subsoil and this in turn was covered with sand

excavated from canals and ditches and brought

to the site by barge. These soils are devoted

very largely to the production of potatoes for

manufacture.

71
NETHERLANDS

sa-N

à^'

NORTH

HOLLANDS

^-^^ ** Rottumerooq

GRONINGEN

• De But

SOUTH ;' UTRECHT

HOLLAND..*

• Groningen \

•• <

' • . .* ' '• /

FRIESLAND \

/

•. /

• /

>•••...•• ••

D R E N T H E

Ï

• Emmeloord

1

1

-E.Polder '. • • ' \

. -/

r^

/^^

u

y'y^ OVE R YSS E L

\

/

V

/-

•• .^ ^

C E L D E R L A N D /

\

•Waqeninqen • Doetinchem y

.-^->^.s-''^

Flushin

•. )

NORTH BRABANT

• Gemert * • y^

^ ^. 1

{

\

\ '—>^^Terneuzen/ "^X,

FLANDERS

\ y

* • ^»- LIMBURG ^

,'rv/

lOO miles

r /

HGURE 29.-Key map of the Netherlands.

72
TABLE 40. --Potato varieties listed for resistance to blight in the Netherlands, 1955

Variety

Voran ,

Bintje ,

Eigenheimer....,

Eersteling ,

Noordeling

IJsselster

Other varieties

Percentage of

Dutch potato

acreage

Percent

30

16

12

5.5

5

5

26.5

Maturity group

Late

Midseason.

. .do

Early

Late

Midseason,

Blight resistance *•

Foliage

7

3

5

3

7.

6

Tuber

6.5

3

3

3

8

8

" Higher numbers indicate greater resistance to blight.

The distribution of the 1955 potato crop ac- WEATHER AND CLIMATIC CONDITIONS

cording to soil region was--

Soil region: Thousand acres Although the types of soil and amounts of

p. . standing water in canals and ditches are very

pt^ rM lía different in the sea clay areas to the west and Kiver v^iay ib.ö ^^^ ^^^^ ^^^^^ ^^ ^^^ ^^^^ ^^ ^^^^ Netherlands,

öand ------- oi*f there is no part of the country more than about

Reclaimed Peat _84^ ^QQ ^^^^ ^^^^ ^^^^ ^^^^^^ ^^^ ^^ ^^^^ ^^^^

Total 376.5 to break the sweep of the prevailing southPOTATO VARTFTTFS westerly and northwesterly winds coming in

I^UIAIU VAKliLlliLb ^^^^ ^^^ North Sea. Even the highest parts

A description of Dutch potato varieties is of the sand areas are not more than 300 ft.

given by Hogen Esch, Nijdam, and Siebeneick above sea level. The summer rainfall and

(1955) and in the Nederlandse Rassenlijst temperature conditions over the whole country

(1956). This variety list, a new edition of which are very uniform. This point is demonstrated

is published each year, covers all agricultural clearly by the climagrams given in figure 30

crops and includes much statistical informa- for Groningen in the north; De Bilt on the

tion. Potato varieties are divided into four border of the sand and clay areas in the middle;

main groups--for ware, fodder, manufacture, Flushing in the extreme south of the sea clay

and export. Many of the varieties fall into two lands in Zeeland; and Gemert on the sand lands

or more of these groups. Thus Voran, which of eastern North Brabant, near the German

occupies the greatest acreage of any single border. At all these stations the long-term

variety, is grown for manufacture, fodder, average temperatures are about 63^ F. in

and export as seed, although its chief use is j^iy and August, and the corresponding mean

for manufacture. Bintje, the next most im- rainfall for these months is between 2 and 3^

portant t^ariety, is grown principally for ex- inches.

port as seed and ware. The most popular Although the winter temperatures in the

variety for home ware consumption is Eigen- Netherlands are muchlower, the mean summer

heimer, and the most important early potato temperature and rainfall figures are very

is Eersteling (synonymous with Duke of York). similar to those for Penzance in the southwest

The Rassenlijst includes a table in which all of England (compare fig. 6). With a macropotato varieties are given a value on a scale climate like that of Penzance and a topogfrom 0 to 10 for a number of economic raphy like that of the English Fens, blight

factors, including resistance to diseases. In conditions in the low-lying parts of the Nethertable 40 the most important Dutch varieties lands would certainly be expected to be severe,

are listed, together with their foliage and ^n important consideration affecting the

tuber resistance to blight, as assessed by this yields of potatoes and the incidence of blight

scale. The higher numbers represent the ^n the Netherlands is that in the low-lying

greater degree of resistance. regions the crops are grown with a controlled

Early varieties are normally planted toward water table, so that except in a few areas,

the end of March, and lifting starts at the notably on some of the sand lands to the east,

end of May. The growing period of midseason the effects of drought do not so greatly affect

potatoes (Bintje) is from mid-April to the end the influence the annual level of potato yields

of August. Late varieties (Voran) are planted as in England and Wales. Table 41 shows the

at the end of April or beginning of May for mean monthly rainfall and tempe rature figures

lifting at the end of September. at De Bilt for each of the years 1950-55. The

^3
TABLE 41.—Potato yields, with rainfall and temperatu res at De Bilt , Netherlands, 1950-56

Year

Rainfall

Mean teniperature

déviation

and

Yield

Mean 75

percent

blight date

June July Aug. Sept. June July Aug. Sept. (Bintje)

1911-50

In.

2.4

2.6

1.8

2.6

3.2

3.6

2.0

2.7

In.

2.9

4.2

2.0

3.9

3.0

5.7

1.4

5.7

In.

3.2

7.6

4.7

3.6

4.5

4.4

2.4

4.9

In.

1.7

4.8

2.2

2.2

1.5

3.5

2.6

1.9

2F.

60

+3

0

-1

0

-1

-2

-5

63

0

-1

0

0

-5

0

-1

63

0

-1

0

-1

-2

+2

-5

OF.

58

-2

+2

-5

-1

-1

0

+1

Tons/

acre

9.6

9.7

10.9

9.8

9.4

10.7

9.5

1950 Aug. 1

Aug. 8

Aug. 18

July 25

Aug. 8

Aug. 18

Aug. 5

1951

1952

1953

1954

1955

1956

average potato yields for the Netherlands are

also given, together with the estimated mean

75 percent date for unsprayed Bintje*

In the Netherlands, the years of highest

average national yield (1952 and 1955) were

not the years in which the rainfall was highest.

In 1954 and 1950, when the rainfall was

nnarkedly high, the yield was below average.

Also, the high yields in 1952 and 1955 were

in the years in which the blight attack was

latest* Thus, it would appear that, with the

relatively constant supplies of the water needed

by the crops, blight has a far more direct

effect on the annual level of potato yields in the

Netherlands than in England and Wales, and

is of greater importance in the national

economy.

Although mean rainfall and temperature conditions are very similar throughout the Netherlands in the summer months, the humidity

conditions, which chiefly affect the incidence

of blight, are variable from area to area.

A study of the criticad weather periods shown

in figures 31 and 32 reveals that the variation

even between low-lying clay areas and the

obviously drier sand areas is not so great as

might be expected.

POTATO BLIGHT FORECASTING

The Netherland workers have been the pioneers in potato blight forecastine. Continuing

the investigations of Li^hnis (1924), Van Everdingen (1926) found that outbreaks of blight

occurred after a period in which the weather

satisfied the following conditions:

1. At least 4 hours of dew during the

night.

2.

3.

4.

Minimum temperature during the night:

50^ F.

Mean cloudiness of the following day

at least 0.8.

Precipitation on the same following day

at least 0.1 mm.

At least 4 hours of dew during the night

was assumed if the mininnum night temperature was at least l^C (1.4^ F.) lower than the

dewpoint at 1900 hours on the preceding evening or 0800 hours on the following morning.

With the aid of these *'Van Everdingen

Rules" and readings from instruments placed

16 inches above ground level in potato fields,

the issuing of blight warning reports was

started in the Netherlands in 1927. The

validity of the Van Everdingen rules under

English conditions was studied by Wiltshire

(1931) and Beaumont (1947). As a resxilt of

his studies year after year under the severe

blight conditions of southwest England, Beaumont was able to propose a simplification of

Van Everdingen rules. Beaumont's rule requires two meteorological measurements only,

i.e. minimum temperature and relative humidity. The Beaumont rule (see section on England

and Wales) was then very extensively tested in

the Netherlands, as described by Post and

Richel (1951), and it has been adopted since

1948 with the following modifications:

For a critical period there must be two

consecutive days on which the mean relative humidity does not fall below 82 per

cent on one day and 79 per cent on the

other. The mean relative humidity is taken

as the average of readings made in standard screens at 2.2m (about 7 ft.) above

ground level, at 0800, 1400 and 19'00 hours.

The minimum temperature on one of the

two days must be at least 50^ F.

Observations from a network of weather

stations over the whole country are employed

by the K.N.M.I. (Royal Netherlands Meteorological Institute) at De Bilt for determining the

critical periods, and notifications of their

occurrence in the several regions are issued

to growers over the wireless. The use of

synoptic maps sometimes enables the critical

periods to be forecast aday or two before their

occurrence, and this earlier notice gives the

74
NETHERLANDS

RAINFALL

Inches

¿ _ Groninqen

RAINFALL 4

Inches

J FMAMJ J ASOND

RAINFALL 4

Inches

RAINFALL 4

Inches

. North Braban!-

CEMERT

FIGURE 30.--Long-term average monthly rainfall and mean tempera-

^tures in relation to potato cropping for four centers in the Netherlands.

growers a little extra time between the warning

and the outbreak in which to do their spraying.

In figures 31 and 32 the principal critical

periods given for the years 1951-55 are shown

in relation to the blight progress curves for

two representative potato varieties. In general,

a very good correspondence is shown between

the first critical periods and the dates of

blight outbreaks and between subsequent periods of blight weather and the rapid progress

of the disease. This agreement is reached

despite the fact that the Netherlands criteria,

with humidity measurements at only 3 times

during the day, arc far less exacting than those

employed for the forecasting in England and

Wales, where the measurements are made

every hour during both day and night and a

break of only 2 hours at any time breaks the

period. With outbreaks commonly occurring in

the second half of June, critical periods in

June, and indeed in late May, are certainly

significant in the Netherlands, and, as in the

southwest of England and Wales, there is no

need for the postulation of a **zero date."

Phenological observations on the state of

forwardness of the crops are, however, taken

into account in issuing spray warnings.

INITIATION OF BLIGHT EPIDEMICS IN THE

SPRING

In his classical paper on the overwintering

and epidemiology of phytophthora infe stans,

Van der Zaag (1956) describes a very thorough

search for the first appearance of infected

plants in the spring. The district chosen for

this work was De Streek, a region in the north

of the province of North Holland where potatoes,

chiefly of the early Variety Eersteling, occupy

about 80 percent of the total acreage and where

potato blight is usually found earlier than

anywhere else in the Netherlands. In 1953, when

the most extensive search was made, 27 foci

were found to be established in the Eersteling

crops in De Streek between May 26 and June 18.

Of these, 15 were scattered over an area of

about 16 sq. km. (4,000 acres) and could be

recognized with reasonable certainty as having

arisen from diseased tubers planted. In one

case, in which careful measurements of spread

were made, it was found that where only about

40 plants in the immediate vicinity of the focal

plant were infected by June 4; by June 23 ten

percent of the plants at a distance of 100 yards

to the northeast and 1 percent of those at a

distance of 300 yards, were infected. From

this and many other observations it was concluded that a single infected plant arising from

an infected tuber planted could infect an area

of at least 1 sq. km. (240 acres) within a

month under weather conditions normal in

De Streek, and that the attacks each year

almost certainly originate in this way. Infection from volunteer plants on refuse heaps

play little part in the story. Van der Zaag

estimated that about 100 diseased tubers would

have to be planted to obtain a single focus,

and that these would be contained in about

1,600 kg. (li tons) of the Eersteling seed

potatoe s.

75
NETHERLANDS

August September

o

lOO

50

o

lOO

50 -

lOO

50 -

lOO

50

• • • • • • • • ••• whole country

• • •••••••••• •• whole country

<^'°V_^/.-^ clay_^y

./y' sand r / ,

/ ^*— c /<•—sprayed

sand areas

clay areas

:nd / /clay- sand/ clay

S Y /NtS/^-S ANeS

••

sand areas

clay areas

1951

-

^

/ /

/ areas •

1952

1953

1954

100 unsprayed—^ J^^^

clay // /

50 - sand / / '1 ^ -

sy-/ /

// /

// /

0 ' -//.^^^--XclayNeS

1955

• • ••• ••

• • •• •

sand areas

clay areas

• • •

• • •

1956

sand areas

clay areas

Critical weather period for whole area.

• Critical weather period for parts of area.

HGURE 31."Blifliit progress curves for Bintje variety in the Netherlands in 1951-56, with the critical weather periods for each year.

76
NETHERLANDS

June Auqusr September

1953

1954

1955

1956

• • •

^ Critical weather period for whole area.

• Critical weather period for parts of area.

FIGURE 32.—Mean hUglit progress curves for Voran variety in the sand areas of the Netherlands, 1953-66, with the critical weather periods for

each year.

In Van der Zaá.g*s view, the development of

an epidemic in any given district must depend

upon (1) the number of primary foci, (2) the

percentage of the land area occupied by

potatoes. (3) the susceptibility of the varieties

grown, and (4) the macro- and micro-climate.

The effect of the degree of tuber susceptibility of the variety on the number of primary

foci could be expected to be very great. In an

experiment in which 200 to 300 inoculated

tubers of several varieties were planted in

soil at a mean temperature of 50® F. the percentage of emerged plants that produced shoots

infected from the tuber was 10 in Eersteling,

4 in Eigenheimer, and none in Voran. Taking

into account the factor of varietal susceptibility and the percentage of land occupied by

potatoes. Van der Zaag surmised that the

probable number of primary foci operative in

starting the blight epidemics in various parts

of the Netherlands would be about 1 per square

kilometer in De Streek (as found by observation), 1 per 20 sq. km. in North Friesland

and Zeeland, 1 per 70 sq. km. in North Brabant,

and as few as 1 per 600 sq. km. in Utrecht--

with an average of 1 per 60 sq. km. (about

15,000 acres) in the Netherlands as a whole.

An extremely interesting observation was

made on the island of Rottumeroog, 11 km.

north of the Groningen coast, in 1954. Here

an area of a quarter of an acre was planted

with tubers that were entirely healthy, and

there were no groundkeepers. On a close

inspection of the island on July 25 no diseased

plants were found. When blight had spread

throughout the province of Groningen, blight

also appeared on the test crop on the island,

indicating that the sporangia were carried by

the wind over a distance of at least 11 km.

fnearly 7 miles) without losing their viability.

77
NETHERLAND BLIGHT SURVEYS SINCE 1951

Since 1951 a countrywide survey of the incidence of blight has been conducted each year

in the Netherlands by the Plant Protection

Service in collaboration with advisory officers

and farmers. The accounts of this work are

given in the Yearbooks of the Service (Netherlands Plantenziektenkundige Dienst, Wageningen, 1953-55; Lint and Meijers, 1956 and 1957

(in press)). Some 400 to 600 observationfields

have been included in the surveys each year,

and a wealth of infornnation has been obtained

on the extent to which spraying is practiced,

the dates of outbreak and progress of blight

on the haulm, and the percentages of blighted

tubers at lifting. Some information has also

been obtained on the effect of the blight attack

on yield, but the figures derived in various

ways from sampling small areas have been

inconmplete and not always very reliable.However a limited number of spraying trials have

been conducted in several districts each year

for which good yield figures are available.

Both the surveys and the trials have been

carried out in relation to the warning service operated by the K.N.M.I. at De Bilt.

Throughout the work a standard key has

been used for the assessment of blight on the

haulm. Most of the definitions are the same

as in the B.M.S, key (see section on England

and Wales), but categories intermediate between 0.1 to 1.0 percent have been added,

and the stages are designated by figures on an

arbitrary 0 to 10 scale. Progress curves obtained by plotting according to this s cade are

reversed: The scale figures being in order, not

to attack, but of freedom from attack (10 = no

attack, 0 = completely dead). Atypical curve is

shown in figure 33, which also gives the B.M.S.

equivalents of the scale. In figures 31 and 32

progress curves for the Netherlands are plotted

according to the English method, using these

equivalents, to facilitate comparison of the

curves with those for other countries.

For the purposes of the survey, observations have been confined to the three principal

maincrop varieties, Bintje, Eigenheimer, and

Voran, and the results analyzed for these

varieties. Seed crops, which are burned off

prematurely, have been excluded. In 1951 and

1952 mean results were given for the Netherlands as a whole, but in 1953 and subsequent years the means have been broken

down according to regions--definedas follows:

''South --Zeeland,western parts

of North Brabant, and

the South Holland Islands.

SEA CLAY ^ Middle-The rest of South Holland, North Holland,

and the North East

Polder.

North--Friesland and Groningen.

RIVER CLAY

SAND {

Certain districts

along the Maas,

Waal, Ryn, and

Ijssel.

South North Brabant and

Llmburg.

Middle --Gcldcrland and

Oveiijssel.

North Frlesland, Groningen, and Drenthe,

including the villages on reclaimed

peat land«

BLIGHT ON THE HAULM

In the review of thé very detailed results

of the Netherlands surveys over the 6years, it

is first necessary to consider how many

crops of each variety were included in the

survey for each region and what percentage

of these crops were sprayed. This informaticm,

for 1953, is given in table 42.

In the sea clay areas the number of unsprayed crops has been too small for the

extraction of mean figures for the unsprayed,

so that for these important areas the survey

provides information about the incidence of

blight on the sprayed crops only. In the sand

areas, where less than half the crops were

sprayed, comparative figures are available

for both sprayed and unsprayed potatoes.. On

average the sprayed Bintje and Eigenheimer

crops received 4 to 5 spray applications each

year in the sea clay areas, and 2 to 3 applications in the sand land area. The Voran crop,

on the sand lands, received two applications.

This matter of number of applications and

of materials used is considered later in the

section on spraying practice.

The information provided by the surveys

concerning the variety Bintje is here chosen

for the fullest study because of the importance

of this variety, not only in the Netherlands

Durch

Scale

IÜ

9 - ^

o

Ol

03

B.M.S.

Scale

for Ö - 05 Per cent

Bliqhr 7 - l-O

on

Haulm

6

5

4

3

2

1

:

1 1 1 1 \,.-

50

25

50

75

95

99

lOO

8 15 22 2<) 6 13 20 27 3 lO 17 24 3 1

June July Auqusr

FIGURE 33.--Comparison of the Dutch scale for tie assessment of

hlight on potato haulm with the B. M. S. scale for unsprayed Bintje

variety on sand land, 1953.

78
TABLE 42.—Number of crops and percentage sprayed of Bintje, Eigenheimer, and Voran potatoes

according to soil areas, Netherlands, 1953

Soil areas

Bintje Eigenheimer Voran

Crops Sprayed Crops Sprayed Crops Sprayed

i^pfl clav- ••...••.••••••••

Number

76

13

85

Percent

94

46

50

Number

44

9

47

Percent

93

7Ö

23

Number

0

0

79

Percent

0

River clay

Sand lands•••••••••••••••

0

30

but throughout western Europe. The data for

blight on the Bintje haulm in each of the

5 years are sunuiiarized graphically by the

progress curves given in figure 31. The

curves for 1951 and 1952 are for the country

as a whole, but the *'unsprayed" curves refer

chiefly to the southern sand areas, where

most of the unsprayed Bintje crops are

situated, while the *'sprayed" curves are the

means for the sprayed crops in both the sand

and the clay areas. For 1953 and subsequent

years, when the survey results were analyzed

according to region, '*sprayed'* and *'unsprayed" curves are given for the southern

sand regions, with "sprayed" curves for sea

clay regions. The surveys did not provide

data for the few unsprayed crops in the sea

clay regions, but to complete the picture

as far as possible, approximate "unsprayed"

curves for these regions are added from a

study of the blight assessments on the controls in a number of spraying trials.

The curves show that there was a good deal

of variation in the course of the blight attack

according to district in the 5 years (and

especially in 1953), but the curves also give

some indication (sufficient for this study) of

the relative severity of blight on Bintje In the

several years for the Netherlands as a whole.

In all 5 years widespread outbreaks (0.1 percent stage) started on the unsprayed in the

second half of June or in early July. The 7 5

percent stage (*'3" on the Netherlands scale)

was reached by the third week of July over a

wide area In 1953 (the worst blight year);

by about the end of the first week In August

In 1951, 1954, and 1956; by mid-August In

1952; and In the second half of August In 1955.

There are not sufficient data to establish a

frequency of "blight years" over a long-term

period, but It would perhaps be reasonable

to say that the curves for 1951 strike a fair

average for Bintje In the Netherlands over

the 5 years. That Is to say, the mean 75 percent date for the unsprayed crops was about

August 7 and for the sprayed crops about

August 24.

ESTIMATION OF LOSSES FROM PREMATURE

DEFOLIATION

The method of estimating average losses

from an average growth curve, as employed

In England and Wales, has not yet been Investigated In the Netherlands. From figures

obtained In periodic liftings on Bintje In the

North West Polder In 1952 and In the North

East Polder In 1955 on crops free from

blight or at most very little affected. It Is,

however, possible to draw up an approximate

mean growth curve for Bintje In the Netherlands (fig. 34). Assuming, as In England and

Wales, that further Increase In the crop of

tubers Is arrested when blight reaches the

7 5 percent (or "3" stage) on the haulm, the

mean percentages of the full potential crop

obtained, as shown by this approximate curve

and In the absence of additional damage from

the passage of sprayer wheels, will be as

shown In table 43.

So far these figures are very tentative,

but they may now be tested by reference to

the actual 75 percent dates and recorded yields

In spray trials. This Is done In table 44 for

trials carried out In each of the 3 years.

PERCENTAGE

OF

TOTAL

CROP

50

/

^

^'/

/

/

<f /

/

,/ 1 Á' 1 1 1

18 25

July

7 14 21

Auqust

28 S 12 19 26

Sepfember

FIGURE 34.—Approximate mean growth curves for Bintje and Voran

varieties in the Netherlands, showing percentage of the total crop

of tubers produced by various dates.

79
TABLE 43.—Estima ted percentage of the

potential crop obtained when the 75 percent

stage of blight on the haulm is reached by

given dates, on Bintje, Netherlands

Date when blight on haulm

reaches 75 percent stage

July 18-

25,

Aug. 1.

7.

U.

21.

28.

Sept. 5.

Estimated percentages of potential

crop

55

64

72

80

87

93

97

100

1953, 1954 and 1955, at three centers, respectively, at Emmeloord on sandy clay in the

North East Polder, at Terneuzen on tl^e sea

clay in Zeeland, and at Doetinchenn on the

sand in the east of Gelderland, The Plant

Protection Service at Wageningen supplied

the data on these trials. Where the original

(total) yields were given inkilograms per acre

they have been converted to equivalent tons

per acre; where they were given in kilograms

per 100 plants they have been leftinthat form.

As the fiül potential yield (with complete control of blight) was not known, it has been

assumed that the percentage of the potential

crop obtained in the sprayed plots was in

accordance with table 43, and the actual and

estimated percentages in the unsprayed crops

have then been compared.

Throughout these 9 trials, imder widely differing conditions, there was a good practical

TABLE 44. —Checks of estimated against actual yields in 9 spraying trials on Bintje,

Netherlands, 1953-55

Place and data

1953 1954 1955

unsprayed Sprayed Unsprayed Sprayed Unsprayed Sprayed

Emmeloord

Date of 15 percent

blight stage on haiilm

Estimated yield percent. •

Actual yield...tons per acre,.

Actual yield percent..

Terneuzen

Date of 15 percent

blight stage on haiilm

Estimated yield percent..

Actual yield...tons per acre..

Actual yield percent..

Doetinchem

Date of 15 percent

blight stage on haulm

Estimated yield percent..

Actual yield...tons per acre..

Actual yield percent..

July 15

50

69.2

53

Aug. 9

Ö2

87.7

86

July 26

65

45.0

59

Aug. 20

92

^ 1P1.8

92

Aug. 28

97

^ 99.0

97

Aug. 8

81

^ 62.3

80

Aug. 3

75

67.2

74

Aug. 6

78

17.0

82

Aug. 8

81

96

Z5

Aug. 10

83

^ 79.6

83

Aug. 24

94

19.4

94

Aug. 17

89

^ 101

89

Aug. 16

87

17.8

^5

Aug. 30

97

12.7

96

Aug. 4

74

10.5

69

Aug. 29

97

20.5

97

Sept. 9

100

13.1

100

Aug. 27

96

14.7

96

Kilograms per 100 plants.

80
agreement between the percentage yield of the

unsprayed crops as estimated from the 75

percent dates and as determined by actual

lifting. Further investigation of the mean rate

of bulking of Bintje in the Netherlands is

obviously needed. It would appear that the

tentative curve of table 43 and figure 34

fits the available evidence fairly well, and

loss estimates based upon it are probably

the best that can be given at the present

stage.

The curves for 1951 (fig. 31), suggested

as representing a general average for Bintje

over the 6 years, indicate an average loss

of 20 percent of the potential total crop in

the unsprayed (75 percent date, Aug. 7) and

5 percent in the sprayed (7 5 percent date,

Aug. 24). In comnnercial crops, however,

where the spraying is done by tractor-drawn

machines (and not by hand as in the trials)

there is wheel damage also to be taken into

account. The magnitude of this danmage has

been investigated by trials in the Netherlands

(Plantenziektenkundige Dienst, Wageningen,

1955, pp. 48-49) and estimated at 3 percent

of the total crop. This figure is in close agreement with that obtained in England and Wales

with similar machines. Accepting this figure,

the loss in the sprayed crops with 75 percent

date of August 24 would be not 5 but 5 + 3, or

8 percent.

Thus, in ware crops of Bintje, with a potential yield (were there no blight) of 12^ tons

per acre, the average yield of the unsprayed

would be 10 tons per acre and of the sprayed

ll| tons per acre--a difference of I7 tons

per acre or 3.75 tqnnes per hectare. In 1954,

which according to the progress curves was

about an average year and similar to 1951,

the mean gain from spraying as determined

on 31 crops of Bintje in various parts of the

Netherlands in the blight survey was 3.9

tonnes per hectare--again invery close agreement with the estimate figure. It must be

stressed that all these estimates so far are

of loss of total yield due to defoliation; they

do not include losses due to tuber infection,

which will be considered later.

The survey data and trial figures available

do not permit a similar computation of average losses in the variety Eigcnheimer, but

it would appear that in general the 75 percent

stage on both sprayed and imsprayed crops

of this variety can be expected to be reached

about a week later than in Bintje. The date

of maturity is also somewhat later. This may

counterbalance the slower progress of blight,

so that in practice there may be no great

difference in the defoliation losses with the

two varieties.

For Voran, the variety predominantly grown

In the sand areas, the surveys provided good

data for both unsprayed and sprayed crops

In each of the years 1953, 1954, 1955, and

1956. For this late variety, producing a full

crop by about the end of September, the growth

curve for the tubers is probably similar to

that for Majestic in England and Wales, and

indeed the results of one periodic lifting trial

available for Voran in North East Polder in

1955 agreed almost point by point with the

English Majestic curve.

Figure 32 shows that blight on the unsprayed Voran crops in the sand areas reached

the 75 percent stage about the end of August

in 1953 and 1954 and in the second or third

week of September, according to district, in

the dry year 1955; blight reached only the 25

percent stage by the end of September in 1956.

The defoliation losses as determined by sample lifting of sprayed and unsprayed portions

of a number of crops in the surveys were in

good accordance with those to be expected

from the growth curve for Voran (shown in

figure 34). In 1954 with the 75 percent stage

on the unsprayed occurring at the end of

August and on the sprayed crop at midSeptember, the gain from spraying was a

little over 1 ton per acre; while in 1953,

when the spraying appears to have been less

effective and the 75 percent date occurred

about September 7, the gain from spraying

was only a little over 7 ton per acre. In 1956,

in five spraying trials, the mean 75 percent

date for the unsprayed was the end of August,

and for the sprayed, mid-September. The

mean total yields were 13.6 tons per acre

for the unsprayed and 15.2 for the sprayed,

giving an increase of 1.6 tons for the sprayed

plots--in exact accordance with the curve. ^

BLIGHT IN THE TUBERS

The surveys provided good data on the mean

percentages of blighted tubers at lifting in the

3 years 1953-55, as shown in table 45. Each

figure in this table is a nciean for a good number of crops, in all cases at least 10, and

sometimes 30 or more.

The figures indicate a high level of tuber

infection in Bintje and Eigenheimer, particularly in the sea clay areas, in the wet

seasons of 1954 and 1956, and fairly high

infection in the sand areas in 1953, while in

the dry season of 1955 the level of infection

was low. Tuber infection in Voran in all 3

seasons was relatively low, though the infection is still considerably higher than for

Majestic in England and Wales. Protective

spraying gave some reduction of tuber infection in all 3 varieties.

CONTROL: MATERIALS AND METHODS

The amount of protective spraying against

blight varies according to area and the potato

variety grown. In general, the majority of

growers in the sea clay regions spray, whereas

in the sand regions growers spray occasionally.

This is in part a reflection of the differing

81
TABLE 45. —Blighted tubers at lifting, as percentage of total crop, for Bintje. Eigenheimer, and

Voran in 2 soil areas, Netherlands, 1953-56

Year and soil area

1953;

Sea clay.

Sand.....

1954:

Sea clay.

Saad

1955:

Sea clay.

Sand

1956:

Sea clay.

Sand

Bintje

Unsprayed

Percent

8.1

9.4

3.9

7.2

Sprayed

Percent

4.8

7.4

11.4

^.5

.5

8.4

7.9

Blighted tubers

Eigenheimer

Unsprayed

Percent

6.2

4.3

2.4

3.9

Sprayed

Percent

2.0

5.0

9.4

3.4

1.4

1.8

17.6

3.4

Voran

Unsprayed

Percent

2.5

2.7

2.3

2.8

Sprayed

Percent

1.4

1.0

.8

2.0

susceptibility to blight of the predominant

varieties in the two areas: Bintje on the sea

clay and Voran on the sand. Figures fron:i

the Netherlands surveys showing the proportion of potato crops sprayed illustrate this

variation and are given in table 46.

Over 95 percent of the Bintje crops on sea

clay have been sprayed during 1953-56. There

has been a considerable difference from year

to year in the amount of spraying done on

Voran in the sand regions, but so far as can

be seen this variation does not bear any clear

relationship to the severity of the blight

attack in the particular years. It would appear

that under one-third of the Voran crops on

sand arc sprayed against blight. The number

of spray applications given also varies. In

the sea clay regions, Bintje for ware normally receives 4 or 5 applications, although

seed crops of the same variety, which are

killed prematurely in mid-July, are given

only 2 or 3 sprayings. Voran on sand is

usually given 2 applications.

The surveys provide very full information

concerning the materials used in the Netherlands for protective spraying over the period

1951-56. Table 47 sets out these materials

and shows their relative importance in the

several years.

Table 47 brings out a number of important

points. The use of bordeaux and burgundy

mixture has been virtually abandoned in recent

years. Copper oxychloride applied alone has

steadily decreased in importance, although

it is still the connmonest fungicide. There

has been a nmarked increase in the use of the

organic fungicide zineb, which is applied

principally in the form of a split treatment

followed by copper. The most recent trend is

in the use of stickers to increase the efficiency of application. The results of numerous

spraying trials in which the use of zineb and

copper compounds are compared have been

reported in recent years (Ormel, 1954; Lint

and Meijers, 1955). In the earlier trials,

zineb was disappointing, but its performance

was greatly improved by increasing the application rate from 2.7 to 4.5 lb. per acre (3 to

5 kg. per ha.). The general experience has

been that with an equal number of spray

treatments, zineb has given a greater increase

in total yield than copper, but that on soils

where tuber blight infection is a problem (the

TABLE 46. —Percentage of Bintje and Voran

crops sprayed, Netherlands, 1953-56

Variety and

soil area 1953 1954 1955 1956

Bintje on sea

clay

Percent

94

30

Percent

98

45

Percent

95

22

Percent

98

17 Voran on sand....

82
TABLE 47.—Spraying materials and percentage of total sprayed area on which they were used,

Netherlands, 1951-56

Material used 1951 1952 1953 1954 1955 1956

Bordeaux or burgundy mixture.,

Copper oxychloride

Cuprous oxide

Colloidal copper preparations

Zineb

Zineb followed by copper..r..

Zineb followed by copper plus

sticker

Copper plus sticker

Other materials

Percent

8

Percent

81

11

2

2

Percent

1

79

A.

12

2

2

Percent

69

4

6

7

14

Percent

52

2

5

9

26

Percent

1

42

<1

3

2

31

8

12

1

sea clay areas) a greater number of diseased

potatoes has offset this effect. In other words»

zineb has proved considerably less effective

than copper In reducing tuber infection. This

can be illustrated by the results of 5 trials on

sea clay and 5 trials on sand in 1954, a year

when tuber infection was very bad in the sea

clay regions. Average results for these trials

are given in table 48. The variety under trial

was Bintje.

Table 48 shows that on both soil types the

total yield of potatoes was highest with the

zineb treatments. On the sand soils, net yield

(total yield less diseased tubers) was 1.5 tons

per acre greater with zineb than with copper,

but on the sea clay, due to the very high percentage of blighted tubers in the zineb-treated

plots, this result was almost exactly reversed. In the light of their experience the

Dutch reconunend that copper alone, or a

split treatment (2 sprayings with zineb, followed by copper) should be used for protective spraying against blight in the sea clay

areas. Zineb, alone, is considered satisfactory in the sand regions.

A high proportion of the spraying is carried

out by contractors, and most of it is done by

ground machines. Fixed-wing aircraft were

used to spray 13,000 ha. of potatoes against

blight in 1955, but expansion of air spraying

is- limited, owing to the small size of the

majority of the potato fields. Recent developments in aircraft spraying have been reviewed

by Maan (1954). He particularly mentions the

research work in spraying against potato

blight of Fransen and Kerssen of the Instituut

voor Plantenziektenkundlg Onderzoek, WagenIngen. Ground spraying is carried out from

the top only without the use of droplances.

In recent years there has been a considerable

development in spraying at low- and mediumvolume rates. Figures, based on a sample

of 293 sprayed fields, show that in 1956, 22

percent of the growers were spraying at less

than 18 gallons per acre; 41 percent were

spraying at 18 to 53 gallons; and the remaining 37 percent at over 53 gallons. To some

extent spraying against blight is combined

with DDT treatments for the control of the

Colorado beetle.

ECONOMICS OF SPRAYING

Leeuwenburgh (1955) has considered whether

blight control is justified on economic grounds.

He states that the cost of one spraying is

approximately 30 guilders per hectare (£1 4s.

per acre), of which, 20 to 22 guilders is the

cost of the material and 8 to 10 guilders the

cost of application. The question of justification depends very much on the price obtained

for the potatoes, and this varies considerably

among the different sorts. For example,

Leeuwenburgh quotes prices of 0.15 guilder

per kilo for seed potatoes, 0.1 guilder per

kilo for sea clay ware potatoes, and 0.06

TABLE 48. —Yield of tubers (over 1-3/8 in. diameter) from spraying trials with copper and

zineb, Netherlands, 1954

Total Sound tubers

-.

Area

Unsprayed Copper Zineb Unsprayed Copper Zineb

Tons/acre

14.7

12.1

Tons/acre

15.1

11.3

Tons/acre Tons/acre Tons/acre

12.9

11.2

Tons/acre

Spn P1 nv ....•••.••••••••• 16.2

13.2

10.4

11.8

11.6

12.7

Sand

83
guilder per kilo for ware and manufacturing

potatoes from the sand regions. (These prices

are equivalent to £15, £10, and £6 per ton,

respectively.) Thus, the actual Increase In

potato yield required to meet the cost of one

spraying Is approximately 200 kg. per ha. for

seed potatoes; 300 kg.per ha. for ware potatoes

from sea clay areas; and 500 kg. per ha. for

potatoes from sand areas. In terms of tons

per acre, these yield Increases become 0.08,

0.12, and 0.20 ton, respectively.

These figures, taken In conjunction with the

greater resistance to blight of the sand land

potato varieties, and also the smaller importance of tuber Infection In the sand regions,

go a long way toward explaining the markedly

snnaller amount of spraying that Is done In

these areas as opposed to that done In the

sea clay areas.

FURTHER CONTROL MEASURES

A considerable amount of research workhas

been done in the Netherlands onthe mechanical

and chemical destruction of potato haulm.

Most of this has been In connection withhaulm

destruction In seed crops (Lint and Crucq,

1956), where roto-beatlng, followed by one or

two sprayings with a chemical such as sodium

arsenlte, has been found necessary to effect

the complete killing of the vigorously growing

haulm. In ware crops attacked by blight,

destructive spraying of the potato tops Is

also recommended under certain circumstances. One spraying is sufficient. Chemicals

used Include sodium arsenlte, the di-nitro

compounds, and sodium chlorate. Growers,

however, have been advised not to use sodium

arsenlte on sand soils, owing to the risk to

succeeding crops of toxic residues remaining

In the soil. Lint and Meljers (1955), reporting

on trials In 1954, concluded that on sea clay

soils It was an econonnlc proposition to burn

off the haulm of a blight-Infected susceptible

variety (Bintje) even when this was carried

out at an early date, due to decreased tuber

blight. On sand soils, where tuber infection

Is normally slight even in susceptible varieties. Lint and Meljers stated that haulm destruction In ware crops was unwarranted.

Further trials in 1955 confirmed these conclusions.

Fundamental work on the genetics of blight

resistance has been carried out by Mastenbroek

(1953), and this worker has cooperated in the

setting up of an International nomenclature for

blight races (Black, Mastenbroek, Mills, and

Peterson, 1953). Mastenbroek and Bruin (1955)

have demonstrated that the present common

field race in the Netherlands Is race 4. This

research on blight races follows earlier work

by Bruyn (1951), which showed the alleged

plasticity of the fungus. Since 1951, H. J.

Toxopeus at the Instituât voor Veredeling van

Landbouwgewassen at Wageningen has been

concerned with the breeding ofblight-re sistant

potatoes and the genetical analysis of immunity. In a recent paper (Toxopeus, 1956)

he has discussed the present possibilities In

breeding for blight resistance in potatoes. He

notes the apparent breakdown of field resistance to blight of the Irishvariety Champion

and suggests that the same thing is happening

to Voran In the Netherlands. He associates.

this breakdown with the ability of the fungus

to adapt Itself even to field-resistant varieties, particularly when these predominate

over other varieties In the makeup of the

country's potato acreage. Toxopeus considers

that the Incorporation of R-gene resistance

Into new varieties, where this is combined

with a high degree of field resistance, may

be very useful. This Is because the effect of

one group of genes Is likely to prevent the

establishment of races ofthe fungus pathogenic

to the other genes. Up till now, no R-gene

blight-resistant varieties of potato have been

grown on a commercial scale in the Netherlands.

Van der Zaag (1956) (in connection with his

work in De Streck) has discussed the possibility of a number of phytosanitary measure s

for blight control. Including the destruction

of primary foci. The main difficulty here lies

In the detection of the foci at a sufficiently

early stage to make destruction worthwhile.

Some consideration has also been given in the

Netherlands to the effect of storage conditions

on the development of blight in tubers. Work

of this kind Is the responsibility ofthe Stichting

voor Aardappelbewarlng (Institute for Potato

Storage) at Wageningen.

SUMMARY: NETHERLANDS

Countrywide blight surveys have been carried out In the Netherlands each year since

1951, using a modlñcatlon of the B.M.S. key

for the assessment of the disease onthe haulm.

Severe epidemics occur on Bintje and other

mldseason varieties In most years, particularly In the sea clay areas, where, ifthe ware

crops were left unsprayed, there would be an

average defoliation loss of about 20 percent of

the potential crop, with a further loss of 4 to

10 percent for Infection of the tubers. Losses

in Voran, the principal variety grown on the

sand lands, are similar to those in Majestic

In the southern zone of England and Wales. The

valuable seed and ware crops of Bintje and

other susceptible varieties almost all receive

Intensive protective spraying. This spraying

usually reduces defoliation losses to about 8

percent, and also gives some reduction oftuber

Infection. Much less spraying Is done on Voran.

84
A marked increase in the use of organic

fungicides for blight control has occurred in the

Netherlands in recent years. The Dutch are

pioneers in potato blight forecasting and have

made many contributions of fundamental importance to breeding for blight resistance and

to the understanding of the origins of blight

epidemics.

DENMARK

Denmark grows annually about 250,000 acres

of potatoes, which produce an average yield of

about 8 tons per acre (Danish Agricultural

Organizations, 1954). According to Russell

(1954), human consumption of potatoes was 282

lb. per head in 1950. In addition, potatoes are

grown for fodder and industrial use. There is

an important export market for Danish seed

potatoes, and to a lesser extent, ware potatoes.

Thus the structure of the industry bears a

very close relationship to that in the Netherlands.

Figure 35 shows the distribution of potato

growing in Denmark. The main concentration

is in Jutland, particularly the west, and there

is a smaller concentration in north Zealand.

Ware potatoes are grown throughout the country, but the fodder, industrial, and seed potatoes

are produced principally in Jutland. There is

a center for early potatoes on the island of

Amager, immediately southeast of Copenhagen.

The most important variety is Bintje. This

is a midseason potato, grown for the home

ware market and also for export as ware

DENMARK

Each dot= 680acres

FIGURE 35.—Distribution of potato growing in Denmark, 1951.

85
and seed. Other varieties grown on a fairlylarge acreage, all of which can be classified

as late-maturing, are Tylstrup Odin (industrial). Alpha (fodder and export), Dianella

(industrial), and Up-to-Date (ware). The crop

is grown principally on the lighter soils. The

main planting takes place in early May and

lifting is carried outin September and October.

Summers are cool, and rainfall is generally

adequate, with a peak in August. This is

illustrated in figure 36.

BLIGHT EPIDEMICS: DISTRIBUTION AND

FREQUENCY

Ernst Gram, who has contributed the infornnation on which this section is based, finds

that blight occurs throughout Denmark,

wherever potatoes are grown, and there is no

evidence to show that the disease is worse in

one part of the country than in another. Attacks

are most severe where susceptible varieties,

in particular Bintje, are grown. The late varieties for feed and industry, with the exception of

Dianella, show a high degree of blight resistance.

Annual reports on plant diseases, including

potato blight*, based on the observations of

plant pathologists and also of agricultural

and horticultural advisers, have been published

since 1884. The present title of these reports,

which include a sunnmary in English, is

Plante sygdomme I Danmark. An attempt has

been made to establish a fixed terminology in

describing the incidence of the various diseases, so that observations from all over the

country can be easily correlated and an overall picture obtained. The following simple

scale is used by all reporters:

^West Jutland

RAINFALL

inches

Incidence of disease Description

J FMAMJ JASONO

BLICHT YEAR 1951

RAINFALL

inches

M J J

NO

r

BLICHT YEAR 947

60V 5

/^^"^^ ^

3

2

i

^//i1 Ï

60 F

SO" F

1 Negligible

2 - Slight

3 Moderate

4 Severe

5 Devastating

In table 49 blight attacks in Denmark are

assessed according to this scale for the period

1945-55. The average yield of potatoes in the

country for each of the years 1945-54 (F.A.O.

statistics) are Included in the table.

The average yield in the 6 •'blight years"

was 7.0 tons per acre, and in the 5 other years

about 7.7 tons per acre. The slight evidence

from this analysis tends to indicate that in

Denmark, unlike England and Wales, the increase in potato yield resulting fromincreased

rainfall in the blight years is not balancing

or exceeding the losses caused by blight.

The number of **blight years" (scale 4-5)

in the 11 years 1945-55 was 6, indicating a

frequency of about 1 blight year in 2. This

agrees very well with an analysis made by

Gram (1935) of the years 1911-34. In 12 years

out of this 24-year period, blight incidence

was described as negligible to moderate (scale

1-3), and in the remaining 12 years, the disease was described as severe or devastating

(scale 4-5).

Blight is reported first on early crops,

usually in June. In 1949 (a *'severe ** year),

the first observation of the disease in the

field was made on June 9, although this was

said to be unusually early. In 1947 (a "slight"

year), first attacks were not reported until

the middle of July. Little exact information

on the progress of blight, after the initial

outbreaks, is given in the annual reports of

TABLE 49. —Yield of potatoes and blight

incidence, Denmark, 1945-55

FIGURE 36.—Mean monthly rainfall and temperature for WestJutland,

Denmark: Above, loiig-term average; below, "blight*' and **no

blight*' years.

Year

Incidence

of blight

Average

yield

Blight years:

1945

Rating

4-4.5

4-4.5

4-4.5

4-4.5

4

4

3.5

2

2.5

3.5

1

Tons/acre

6.0

6.6

7.0

7.4

7.0

8.0

7.0

6.9

8.5

8.5

1949

1950

1951.*

1953

1954

Other years:

1946

1947

1948 •.

1952

1955

86
Plantesygdomme I Danmark. It would appear,

however, that in a severe blight year, the tops

of midseason and early maincroppotato varieties are completely killed between the middle

to the end of August; and those of late maincrop varieties between early to mid-September.

In 1945 {*'severe"), it was stated that the tops

of midseason and early maincrop varieties

were practically dead from blight as early

as the end of July, but this seems to have been

an exceptional season.

BLIGHT LOSSES

A very large number of spraying trials

against blight have been carried out in all

the potato-growing areas of Denmark during

the present century. Figure 37, which has been

adapted from data suppliedby Dr. Gram, shows

the average increase in yield (cwt. per acre)

in the sprayed plots as compared-with the

untreated for each of the years 1934-53. These

results are based on figures from 287 separate

spraying trials carried out with bordeaux

mixture.

If figure 37 is compared with table 49 it

will be seen that there is a very close correspondence between the increased yield from

spraying in any particular year and the intensity of the blight attacks in that year. Thus,

in 1950 and 1953, attacks were severe and

yield increases high; in 1947 and 1948, attacks

were slight and yield increases from spraying

small. Despite the factthatthis correspondence

may not hold exactly for every year, e.g.,

yield increases were greater in 1946 than in

1945, although 1945 was certainly the worse

blight year. Gram (1935) has demonstrated

that over a long-term period the spraying

trials give a very good measure of blight

incidence. Dealing with the period 1911-34,

during which time 1,350 spraying trials were

carried out, he expressed blight incidence

(as measured by the cooperative plant disease

recording scheme) against yield increase resulting from the spraying trials. This analysis

for the 24 years is shown in table 50.

DENMARK

CWT

PER

ACRE

TABLE 50. --Increase in yield of potatoes by

spraying as related to blight incidence,

Denmark, 1911-34

Blight incidence

(rating)

Years

Increase in

yield by

spraying

I..Negligible

2..Slight

3•.Moderate

Number

2

6

4-

4

8

Percent

1.5

10

12

4••Severe••..•• 16

5..Devastating 21.5

Total years 24- —

Stapel (1953) has used the results of spraying trials from. 1910 to estimate the average

loss in Denmark caused by blight through

premature defoliation of the potato plants. He

concludes that the average loss, inthe absence

of spraying, is 15 percent of the crop (Ij tons

on a 10-ton crop). This is just about double

the average yield increase achieved in the

spraying trials illustrated in figure 37. However, it seems a fair assumption that only 50

percent of the total loss due to blight was recovered in these trials, particularly as in the

miajority of them only one spraying was given.

There has been a marked tendency for yield

increase in the spraying trials to be less

during recent years, or to put it another way,

for blight losses to be less severe. This point

can be seen from figure 37, but it is further

illustrated by data supplied by the Plant

Production Office at Skanderborg in Jutland,

and quoted by Stapel. These data show the

average increase in yield for the four 10-year

periods from 1910-50, and are given in table

51.

It is possible that the chemical situation in

the war and postwar years influencedthe position in 1940-50, but a change in the potato

varieties grown is also, no doubt, having an

effect. Acreages of Bintje have increased

during recent years; and although this variety

TABLE 51.--Average yield increase for

potatoes by spraying, in 10-year periods,

Denmark, 1910-50

1934 5 6 7 B 9 I940 I 2 3 4 5 6 7 B 9 I950 I Z 3

FIGURE 37.—Average annual increase in yield from spraying with

bordeaux mixture, Denmark, 1934-53.

1910-20

1920-30

1930-40

1940-50

87
Is very susceptible to blight, it is very early

maturing, and may often escape the full

severity of the attacks«

Returning to Stapel's estimate that, on

average, 15 percent of the potato crop will

be lost in the absence of spraying, he develops

his argument further to estimate an actual

loss of 7 or 8 percent, basing this on the

assumption that half the potato acreage in

Denmark is sprayed. Clearly, this assumes

that the spraying that is carried out is 100-

percent efficient, which it cannot be, so that

this final figure is probably low. In addition,

it appears that 35 percent and not 50 percent

of the acreage is sprayed. Assuming, however,

that spraying is concentrated on the potatoes

that need it most, the probable annual average

loss caused through premature defoliation in

Denmark is around 10 percent.

BLIGHT IN TUBERS

In view of this high loss figure, it is interesting to learn from Dr. Gram that ''our

main problem [in Denmark] is the tuber attack,

owing to its influence on quality, on keeping

in pits or houses, atnd on export values." This

does not imply that the losses caused by tuber

blight are greater than the losses brought

about by premature defoliation. It would be

most surprising if tuber blight losses were

anything like so high as an average of 10

percent, though no figures exist on which to

base an estimate. The statement does imply

that tuber blight is a very real source of loss

in Denmark, and that it is all the more important because of the export market in

potatoes. The blight losses through premature

defoliation are a more intangible quantity,

and there is always a tendency for them to be

balanced by a higher potential yield in the

blight years.

BLIGHT AND WEATHER DATA

The climate of Denmark is under both

maritinne and continental influences. Throughout the country, the main rainfall comes in

summer and autunui. Figure 36 shows the

long-term average rainfall and temperature

conditions for west Jutland, together with the

main planting and lifting times for potatoes.

In Jutland, the average annual rainfall is

between 25 to 30 inches, and mean temperatures in July and August are around 61^ F. In

the Danish Islands, annual rainfallis somewhat

lower, between 20 to 25 inches, and summer

temperatures are a little higher than in

Jutland. The whole country is low lying and

exposed to strong winds blowing in from the

west.

The relative frequency of severe blight

years in Denmark (1 year in 2) would appear

to be connected with the generally high August

rainfall. In west Jutland, the long-term average rainfall for August is 3.5 inches. Weather

conditions in a **no blight" year (1947) and a

**blighf year (1951) are compared in figure

36. Summer temperatures in 1947 were exceptionally high, and this, with the very low

August rainfall, will have prevented the development of blight. In 1951, the very high

August rainfall, 5.8 inches, will have been

one of the main causes of the severe attacks.

CONTROL MEASURES

Spraying is much more commonly practiced

than dusting. The amount of spraying varies

in different parts of the country. Experts

judge that about 35 percent of the total potato

area is sprayed. Copper compoiinds are principally used, of which copper oxychloride is

the most popular. Experiments at Lyngby in

1941 and 1942, comparing the efficiency of a

number of spray materials, including different

formulations of bordeaux mixture, have been

described by Stapel and Peter sen (1943).

Under commercial conditions, it is normal

to apply one or, at the most, two protective

sprays, although Hammarlund( 1956)has shown

that in certain circumstances up to four canbe

applied economically. The need for correct

tinning of the spray application to obtain

maximum protection of the potato haulm has

long been realized in Denmark. An experiment

to show how important it is to apply a protective spray as soon as the disease appears, and

not to delay this application, was made at

Lyngby in 1918, and is described by Gram

(1924). This work appears to have been an

anticipation of the re cent use of blight progress

curves. Because of its historic value. Gram's

original diagram illustrating this experiment

is reproduced in figure 38.

The diagram shows the development of

blight on the sprayed plots and on the unsprayed plots (Ikke spr^jtet) according to a

scale (0-10), where 10 equals complete haulm

destruction. Blight first appeared in the plots

on July 19, and the first set of plots was

sprayed the next day, July 20. The heavy line

shows the blight developrntent on the haulm in

plots sprayed on that date. Other sets of plots

received a single spraying at intervals after

this. The dates of spraying are all shown by

arrows, and a blight progress curve is given

for each set of plots. The columns at the top

of the diagram give the date of spraying, and

the yield per treatment in 100 kilograms per

hectare. The plots which were sprayed on

July 20 (the optimum time) gave a yield of 20

tonnes per ha. (7.9 tons per acre). The unsprayed plots gave 16.2 tonnes per ha. (6.4

88
FIGURE 38.--Effect of date of spraying on yield, Lyngby, Denmark,

1918: Progress curves for blight on foliage are given on a scale

of 0 to 10; yields in 100 kg. per hectare for dates of spraying

(shown by arrows) are given in the columns above (Gram, 1924).

tons per acre). Thus, with optimum timing, a

single spraying gave 14 days' prolongation of

growth in early August, and increased the

yields from 6.4 to 7.9 tons per acre. (Compare

with figure 8 for England and Wales.)

Haulm destruction Is not much used In Denmark. About 10 percent of the potato area Is

treated, mostly with sodium chlorate, which

Is not too satisfactory. Most of the killing Is

done on the late varieties, which have a very

luxuriant haulm, but this Is mainly to facilitate

lifting rather than to protect the crop from

blight. Roto-beaters are also In use.

SUMMARY: DENMARK

In Denmark, the results of numerous spraying trials carried out In all parts of the country have been used In assessing blight losses.

These numerous spraying trials give a very

good Idea of the technical Interest taken by

Danish farmers In blight control over a long

period of years. The average annual loss

from blight through prennature defoliation

has been assessed at 15 percent. In the

absence of spraying. Even so, only about onethlrd of the Danish potato growers take protective action against the disease. To a considerable extent, the growers appear to Ignore

this large Invisible loss. The visible losses

caused by tuber blight seem of more Importance

to thenn. Denmark has an Inmportant export

trade In seed potatoes.

ICELAND

The occurrence of potato blight In Iceland

Is of special Interest, as the temperature conditions are here at the lower limit for any

epidemic development of the disease. The

northern coast of Iceland touches the Arctic

circle (fig. 39), and by far the greater part

of the land area (some 40,500 sq. miles) Is

an uninhabited high tableland with vast snowflelds and glaciers. Potatoes are grown on

the lowlands to the southwest of the Island

about Reykjavik, and also on the coastal

lowlands and In the glens to the south and east.

The potato has been one of the main vegetable

crops since It was Introduced In 1759- About

2,000 acres are planted each year, and the

yields, varying considerably with the severity

of the season, " are In general fairly high,

giving a total annual production of 12,000 tons

for a population of 154,000. All the potatoes

are used for human consumption, and are

all the more Important In the food supplies

because the climate Is unsuitedfor grain crops.

In addition to the Iceland production some

potatoes are Imported from Denmark, Holland, and Ireland.

ICELAND

ARCTIC_ÇIKÇLE^

Snoefell

Myrdalssondur

FIGURE 39.—Key map of Iceland. Shading indicates bwland agricultural areas.

89
An account of potato blight in Iceland has

been published by Davidsson (1951), Sturla

Fridriksson has carried out potato variety

trials at the University Research Institute

at Reykjavik over a number of years (Fridriksson, 1954) and has provided much supplementary infomnation.

CLIMATIC AND CULTURAL CONDITIONS

Blight is known in Iceland wherever potatoes

are grown, but it causes considerable losses

only in the southwestern district from the

southern part of the Snaefellsnes Peninsula

south and east to Myrdalssandur. In figure 40

a climagram is therefore given for Reykjavik,

as the center of this area. It will be seen that

on a long-term average the mean temperatures

in the summer are above 50 F. only in the

months of July and August, and the growing

period is short--at most, 120 days. But this

growing season is increased, in effect, by

pre-sprouting of the seed tubers, which are

kept at temperatures of 50 to 57 , with some

exposure to light, for 4 to 6 weeks before

planting in mid-May. The lower temperatures

over the growing period may also be offset

to some extent by the long-day conditions,

with over 20 hours of daylight in June and

July. Sandy soils are preferred, and the planting is done by hand, with a rather close spacing (12 in. X 24 in.). The crop is lifted in

mid-September to escape the severe autumn

frosts. Kerr's Pink is perhaps the most commonly grown variety, but there are many

others in cultivation, including the indigenous

RAINFALL

Inches

^ Iceland

REYKJAVIK

64°09'N

«in» F ^

-

-

^^ N

^

^ 'y/

/

LIFT

-INC

X

- PLANT /.

-INC y

7/,

so

40

30

20

. BUCHT SEASON

RAINFALL

Inches

^'NO BLICH r'SEASON I940

1V/1r

60»F

50'F

40"F

FIGURE 40."Monthly rainfall and mean temperatures for Reykjavik,

Iceland, in relation to potato cropping: Above, long-term average;

below, *'blight** and "no blight" seasons.

Icelandic varieties (the Red Potato, the Blue,

and the Akranes), the Gullauga (an old Swedish

variety). Early Rose, Great Scot, Up-to-Date,

Eigenheimer, Alpha, and Ackersegen. Ability

of the haulm to withstand some frost injury

toward the end of the growing period is of

great importance under Icelandic conditions.

In trials of varieties from nnany parts of the

world Fridriksson (1954) found only one indigenous variety, as well as Furore and the

American variety. Sequoia, to equal Kerr's

Pink in this respect.

INCIDENCE OF BLIGHT

Blight appears so late in the season that

the foliage does not suffer until the beginning

of September. There is therefore little loss

of total yield due to premature defoliation,

but ill some seasons there is considerable loss

from infection of the tubers and consequent

wastage in store (Davidsson, 1951). '*Blight

years'* must here be taken to mean, therefore,

years in which there is heavy infection in the

tubers. Davidsson reported that severe losses

(amounting to 20 to 40 percent of the crop)

from this cause occurred in 11 of the 41 years

from 1918 to 1950, and Fridriksson informs

us that over the past 21 years the losses

from tuber infection have been severe in 6 years

(table 52).

TABLE 52. --Blight infections of tubers in

Iceland, 1936-56

Severe Slight Negligible

1939 1948 1936

1941 1949 1937

1944 1950 1938

1945 1954 1940

1947 1955 1942

1953 — 1943

— — 1946 — — 1951

— — 1952

""" — 1956

Climagrams for seasons in which blight

was severe (1939) and negligible (1940) are

given in figure 40. The severe blight season

^^o ^^^^cterized by mean temperatures of

55 , 54 , and 53 F. in the months of July,

August, and September, respectively; while

in the **no blight" season the corresponding

temperatures were 52°, 50°, and 45°. With an

average diurnal range from 5° above to 5°

below the mean, it would appear that in such

years as 1939 in Iceland, numerous spells

may occur during these 3 months when the

Beaumont minimum temperature requirement

90
of 50 is satisfied. The rainfall is abundant,

and, occasionally, as in August 1939, veryheavy. The southwest of Iceland is warmed by

the Gulf stream, and humid conditions frequently occur. The long-term average mid-day

(1400 hr.) relative humidity at Reykjavik over

the months of June to September is about

72 percent--a figure rather less than that for

Garvagh in Northern Ireland (78 percent) but

much the same as that for Aberdeen. Low

temperatures in July and August appear to

be the factor checking the development of

blight in the '*no blight" seasons.

In trials at Reykjavik on the susceptible

Gullauga variety, blight was first noted on

the foliage during the third week of August,

both in the '•blight year" 1939 and in 1940.

In 1939 it progressed to about 95 percent by

September 20, just before lifting, while in

1940 the haulm at that time was almost free,

hi both years the total yield was about 10 tons

per acre, but in 1939 4 tons of the potatoes

were diseased.

CONTROL

it would appear that the infection of the

tubers occurs in part before lifting and in

part at lifting. The rather close spacing (24

inches between drills on maincrop varieties)

may make it difficult to provide adequate

soil cover in earthing up; and the necessity

to lift before the frosts must result in many

crops being harvested while the blightedhaulm

is still green. This would afford great opportunity for infection at lifting and probably

accounts for most of the loss from blight

developing in store. With a growing season

already so short, haulm destruction at the

beginning of September to reduce blight in

tubers would involve too great a sacrifice of

total crop.

Protective spraying of the haulm with

burgundy mixture has been found to give

good control of blight in the tubers, as indicated by the results of trials at Reykjavik in

1939, 1940, and 1941 (table 53).

TABLE 53. --Diseased tubers as a percentage of total yield, in sprayed and

unsprayed plots, Iceland, 1939-41

Year and variety Unsprayed

Sprayed with burgimdy mixture

Once

July 18-27

Once

August 1-7

Twice

July 18-25;

iUigust 7-l¿

1939:

Alpha....

Gullajuga.

194Ö:

Alpha....

Gullaxiga.

1941:

Alpha.. •.

-Gullauga.

Percent

7

40

3

20

Percent

5

25

2

11

Percent

2

5

1

3

Percent

2

4

0

0

0

3/2

It is evident that spraying between July 18

and July 27 was too early for outbreaks not

occurring until late in August; but that spraying in early August must have been veryeffective in keeping the disease off the haulm,

so that opportunity for tuber infection both in

the ground and at lifting was greatly reduced.

The relatively low percentage of blighted tubers

in the variety Alpha even without spraying

points to the possibility of obtaining good

practical control by the choice of varieties

resistant to blight in the tubers.

Among the varieties found to be most

susceptible to blight under Icelandic conditions

are Early Rose, Arran Pilot, Gullauga, and

Red Icelandic.

SUMMARY: ICELAND

Despite the low summer temperatures and

short growing period, potatoes are an important

crop in Iceland, and good yields are obtained.

Blight occurs in the lowland areas to the

southwest of the island, where it causes

91
severe losses by infection of the tubers from

late attacks on the haulm in about 3 years out

of 10. The blight attacks, under favorable

humidity conditions, occur when the mean

daily temperatures in July and August reach

54° to 55^ F. but appear to be inhibited when

these temperatures range about 50 to 52 ,

with a mean diurnal rangeof about 10 between

the maxima and minima. Protective spraying

of the haulm with burgundy mixture has been

found to be effective in preventing blight in

the tubers«

NORWAY

Norway forms the western part of the

Scandinavian Peninsula, stretching from

latitude 58° to 71° North, so that a large part

of the country lies above the Arctic Circle

(fig. 41). It is-a mountainous country, and

only in the southeastern parts and in the

region of Trondheim Fjord are relatively large

areas of level land to be found. The maritime

climate of the west coast is associated with

a high rainfall, in many places up to 80 inches

per annum and more. In the southeast (Oslo)

and in central Norway, the continentalinfluence

on the climate is felt, and this is associated

with a summer rainfall peak. Annual rainfall

is lower than in the west, and in parts of the

interior may drop to an average of 12 inches.

The modifying effect of the Gulf Stream on

the temperature conditions of the country

insures a reasonably long growing season in

southern districts, but in the norththe summer

is necessarily much contracted, although the

days are long. Climatic conditions at three

centers in Norway are illustrated in figure 42.

About 140,000 acres of potatoes are grown

annually and produce an average yield of 8

tons per acre (F.A.O., 1956). Human consumption of potatoes is about 224 lb. per

person each year, and there are large quantities of potatoes used for stock feed, with

small amounts for industrial purposes. The

most commonly grown maincroppotato variety

in Norway is As, which shows a fair degree

of resistance to blight, but it is of poor quality

for the ware market. Kerr's Pink, Prestkvern,

Jj^ssing, and Up-to-Date (Grahm) are maincrop varieties widely grown for ware, all of

which are susceptible or highly susceptible to

blight. The growing season of maincrop

varieties in southeast Norway is from May

to September, with lifting continuing into

October, Arran Pilot and Irish Cobbler are

two of the principal early varieties. Early

potatoes are grown mainly in southeastern

Norway and in Rogaland County.

Four principal potato-growing regions can

be distinguished, which, together, account for

70 percent of the total potato acreage. These

are:

Region:

Southeast Norway

(Çistfold, Vestfold, Akershus,

Buskerud

Thousand acres (1955)

35

Region:

Central Norway

(Hedland, Opland)

Rogaland

Trpndelag

Other counties - -

Total - -

Thousand acres (1955)

32

12

20

41

140

These figures are from Erling F^rsundof

the Norges Landbruksh^gskoles AkervekstioTSpk, at Vollebekk near As, who has also

provided the informationabout blight epidemic s

in Norway on which this section is based.

DISTRIBUTION AND FREQUENCY OF

BLIGHT EPIDEMICS

The importance of blight varies very much

from district to district» but all farms in

southern Norway, except those at the highest

altitude, suffer from attacks at some time.

The disease has been found as far north as

Lyngen in southern Troms at latitude 69 °

30' N.--probably the most northerly blight

record in the world. It has also been recorded

550 m. above sea level at Kvikne.

Ffirsund has gathered together the available

evidence on the incidence of blight over the

period 1901 to 1951. The source of most of

his information has been the annual reports

of the Norwegian Ministry of Agriculture,

together with data from the reports of some

of the coiintry's experimental farms. In table

54 the percentage of years during the 51-year

period when blight attacks have been severe

or medium, slight, and absent, are shown for

each of the four main potato-growing regions.

Not a single year has been quite free from

blight in all parts of the country. The variation

between different years and different districts

within a year has been considerable. The high

percentage of years in which blight was

absent in central Norway (1 in 4) and the low

percentage of such years in Rogaland (1 in 50)

is noteworthy. These two regions represent

differing climatic conditions; Central Norway

has a continental, and Rogaland a maritime

(Atlantic) climate (see fig. 42 for Bergen,

which lies just north of Rogalandcounty).

Since 1952, F/4rsund has been in charge of

an investigation into the connection between

weather conditions and blight epidemics to

92
NORWAY

70'N

Potato Bliqht

Sept. 15, 1956

None

Medium

mtttt Severe

250 miles

FIGURE 41.-rKey map of Norway, and incidence of blight on September 15, 1956.

serve as a basis for a blight forecasting

service in Norway. Somewhat more detailed

records of blight have been kept over the

country from that year, and the B.M.S, scale

for the measurement of blight on haulm has

been used since 1954. Maps to show the

93
NORWAY

6

5

RAINFALL 4

Inches

3

2

I

,Oslo

LATITUDE 59°55'N

/

y \

N

-

50°F

/ //. \

- /

ii

^

\

\

\ ^

X" V/

PLANT

-INC / / YA

-

- /LIFTING

V/1

j F M A M j J A s O N D

50

40

JO

20

. Berqen

LATITUDE 60''25*N

RAINFALL a

Inches

JFMAMJJASOND

. Trondheim

LATITUDE 63°26'N

RAINFALL 4

Inches

M A M J J A S O N D

FIGURE 42.--Long-term average monthly rainfall and mean temperatures in relation to potato cropping at three centers in Norway.

TABLE 54.--Percentage of years when potato

blight has been severe, slight, or absent in

4 regions, Norway, 1901-1951

Region Severe or

medium Slight Absent

Percent

43

51

45

Percent

35

31

47

47

Percent

Southeast Norway.

Central Norway,..

10

26

2

Tr^ndelag 8

incidence of blight at different times throughout the several years have been drawn, and

one of these (for Sept. 15, 1956) is reproduced

in figure 41. Blight attacks in Norway over

the period 1952-56 are shown in table 55.

TABLE 55. —Incidence of blight in Norway,

1952-56

Region

Southeast Norway.

Central Norway..

Severe or

medium

Rogaland.

Tr)2$ndelag.

Slight

1956

1952

1956

1955

1956

Absent

1955

1955

No great degree of precision is claimed

for the terms used to describe the epidemics.

Although a severe attack can generally be

taken to mean a severe attack on the haulm

with a resulting yield reduction due to premature defoliation, there has been no marked

attempt to separate defoliation losses from

tuber blight losses. The problem of tuber

infection at harvest increases in the more

northerly potato-growing areas where the

growing seasons and harvest periods are

short. F^rsund gives as an example of a

severe attack the data relating to a crop of

Kerr*s Pink under^^observation at the College

of Agriculture at As in southeast Norway in

1953. Blight was first noted on July 21, and

the crop was completely deadonSeptember 10.

Total yield was 7.4 tons per acre, of which

14 percent was blighted.

In general, blight is important in all the

main potato-growing regions of Norway, and

particularly severe in the southwestern part

of the country. The biggest losses are caused

in the varieties grown for ware, as most of

these are highly susceptible to the disease.

CONTROL EXPERIMENTS

Roer (1957) has described a series of

spraying trials made in southeastern Norway

during the period 1945-54« The variety used

during the first 9 years was Up-to-Date,

which is very susceptible to blight. In 1954,

this variety was replaced by Kerr's Pink,

which is much more commonly grown in the

area and is also less susceptible than Up-toOate, particularly in the haulm. Fungicides

were applied twice with a 2-week interval

betweenapplications. The percentage increases

in total yield from this spraying were:

94
Year:

Increase in yield

from spraying

Percent f

1945 23

1946 17

1947 1

1948 - 5

1949 - 8

1950 19

1951 -- 22

1952 7

1953 7

1954 -- 8

Average 12

The average yield of the unsprayed potatoes

over the 10 years was 9.6 tons per acre, and

of the sprayed, 10.8 tons per acre. This

represents an average increase of 12 percent

from 2 sprayings, or just over 1- ton per

acre. Protective spraying had little effect on

tuber blight infection, which was high, averaging

22 percent on the unsprayedplots. Aneconomic

analysis of these results showed,that a routine

control program against blight on Up-to-Date

was profitable.

Similar results with Up-to-Date were

obtained by Lunden (1947), who carried out

trials in the same area during 1939-44, Again,

two protective sprays were given. In these

trials, however, the field-resistant variety

As was also included. Average results for the

8-year period are shown in table 56. Bordeaux

mixture was used throughout.

^ The difference between the field-resistant

As and the blight-susceptible Up-to-Date is

most striking. Protective spraying of As on a

routine basis was not profitable. The much

smaller percentage of blighted tubers in As

should also be noted.

SUMMARY: NORWAY

Blight is important inall the main Norwegian

potato-growing areas. Losses are principally

in varieties grown for the ware market. The

relatively small effect of the disease on As

points the need for a field-resistant ware

variety.

TABLE 56. --Results of spraying tests with bordeaux on yields of As and Up-to-Date

potatoes, Norway, 1939-44

o

As Up-to-Date

Test

Total yield Blighted tubers Total yield Blighted tubers

UnsBraved...•..•••...••••

Tons per acre

11.6

12.0

Percent

1.6

.7

Tons per acre

S.S

10. Ö

Percent

11.0

Süravpd..........••.••••• 13.0

SWEDEN

Sweden extends in a narrow strip about

1,000 miles long between latitudes 55^ and

69® N. Due mainly to the influence of the Gulf

stream, climatic conditions in the country are

not so severe as might be expected fronn its

northerly position. In the south, mean summer

temperatures exceed 50® F. for about 4 1/2

months, butinthenorthfor only about 2 months.

Potatoes are grown throughout the country

(fig. 43), principally on sandy soils, with the

main concentration in the south: The three

most southerly counties--Blekinge, Kristianstads, and Malmphus --have, between them,

a third of the total potato acreage. About

300,000 acres of potatoes are grown eachyear

and produce an average yield of 5 1/2 tons

per acre (F.A.O., 1955). Curiously enough,

the potato yields in the most northerly provinces are as high, if not higher, than in the

south, despite the shorter growing season.

This must, in part, be a reflection of more

intensive cultivation methods, but it wouldalso

appear that the long-day conditions ofthe nearArctic summer suit the potato crop, provided

that the right varieties are grown.

Approximately 40 percent of the crop is

used for human consumption, giving anaverage

annual intake of somewhat over 200 lb. per head

of the population: Half the potatoes are used

in animal feeding and the rest for manufacture

(farina and alcohol). A very full account of

the distribution of potato varieties in Sweden

has been given by Hagberth (1951), from the

results of a nationwide survey carried out

between 1937 and 1943. The leading variety

was then Magnum Bonum, and other maincrop

varieties widely grown were Wohltmann, Upto-Date, President, Birgitta, and Gloria. The

principal early varieties were Early Puritan

and Early Rose. In the south of Sweden the

potato-growing season is from the beginning

of May to early October. Northwardthe season

is contracted; planting is later, and lifting

earlier. (See climagrams, fig. 44.)

95
SWEDEN

Arctic Circle

MALMOHUS

,, . .VAkarpVKRiSTIANSTADS

Molmo« "

250 miles Each dot = lOOO acres

FIGURE 4S.—Key map of Sweden, and distribution of potato growing, 1955. (After Jonasson. Höljer, and Björkman, 1938, and Sweden. Statistiska

Centralbyran, 1956.)

96
BLIGHT EPIDEMICS: DISTRIBUTION

AND FREQUENCY

Potato blight occurs throughout Sweden,

but the disease is most important in the

southern half of the country, particularly in

the western coastal area. It is there that the

first outbreaks of the season are usually

recorded. In north Sweden, epidemics, when

they occur, are usually late. No special survey

work is in progress, but Dr. Lihnell of thç

Statens Vaxtskyddsanstalt, Stockholm, has provided his own appraisal of blight incidence in

Sweden for the years 1946-55 (table 57), and

other information for this section.

The classification of the epidemics shown

in table 57 covers the losses caused both by

premature defoliation and by tuber infection.

Dr. Lihnell points out the difficulty of concentrating in a few words the truth about

potato blight for a country the size of Sweden,

where conditions vary considerably between

the north and south. In some years, as in

1947, practically no blight occurred in any

part of the country. But in 1955, for example,

the northern parts of Sweden were almost

entirely free from blight, while in the south

one of the worst attacks of the whole 10-year

period occurred, with heavy losses from tuber

infection.

BLIGHT LOSSES

In the absence of survey data, the results

of spraying triéds give the best indication of

blight losses. Figures have been published by

Andren and Pettersson (1956) and Andren

(1946-55) from trials held at two centers in

Sweden during the period 1946-55. These

centers were at Akarp, near Malmö, in the

extreme south of Sweden, and at Nyckelby,

near Stockholm, in central Sweden. In addition

to the yields of the unsprayed and sprayed

plots, details of the progress of blight defoliation on the haulm have also been presented,

measured by a 0 to 10 scale. Fronn this

information it has been possible for most years

to estimate the time the 75 percent blight

defoliation stage was reached on the unsprayed and sprayed plots. These estimates

together with the plot yields and the gain

TABLE 57. —Incidence of blight in Sweden,

1946-55

Severe Slight Negligible

1950

1951

1953

1954

1955 (South)

1946

1948

1949

1952

1947

1955 (North)

from spraying (in tons per acre) are shown

in table 58. In each year a number of different

spray materials were used, and the results

for the sprayed plots in the table refer to

the most successful treatments. The variety

was Up-to-Date in most of the trials, although

in the earlier years it was Magnum Bonum

at both centers.

0 In the 7 years that trials were held at

Akarp, blight incidence for Sweden has been

described by Lihnell (table 57) as severe in

3 years, slight in 3 years, and negligible to

severe in 1 year. There does not therefore

appear to be any particular bias toward

'•blight" or **no-blight'* years in the series

shown in table 58. Where no 75 percent blight

date has been given for the sprayed plots in

this table, it is because the 75 percent stage

was not reached before lifting.

The figures in table 58 can be used, with

some reservations, to estimate blight losses

at the two centers. The losses canbe estimated

as the difference between the yields of the

sprayed (S) and unsprayed (U) plots (the gain

from spraying) expressed as a percentage of

the "sprayed" yield. This is on the assumption

that the ''sprayed" yield is approximately

equivalent to the full potential yield of the

crop, and it is here that reservations need

to be made. In those years when the 75 percent

stage on the sprayed plots was reached before

the „end of September--1949, 1950, and 1954

at Akarp--the "sprayed" yield will be lower

than the potential yield: In all years, the yield

on the sprayed plots will be somewhat lower

than the potential yield due to damage caused

while spraying. With this in mind, the estimated

blight losses for the several years at the

two centers, presented in table 59, can now

be considered.

Losses due to premature defoliation are

high and have occurred every year. This is

partially explained by the evident late bulking

of the crop at these two centers. For e¡xainple,

at Akarp in 1955, the 75 percent blight stage

was reached on the unsprayed potatoes on

September 21, but there was still a gain of

2.5 tons per acre on the sprayed plots; at

Nyckelby in 1948, the unsprayed plots were

75 percent defoliated on September 20, but

the sprayed gained 2.8 tons per acre (table 58).

Particularly in the area represented by this

latter center, where the harvest period is

relatively late and short, it seems likely that

many crops will be lifted before they are

fully mature, so that the trial figures tend to

give a somewhat exaggerated picture of the

benefits to the commercial grower of spraying

against blight. Nonetheless, the disease is

obviously very serious in Sweden, and this

is borne out by farmers taking protective

spraying as a matter of course.

Tuber blight figures have also been given

for all the years of the trials at Nyckelby,

and for some of the years at Akarp. The

97
TABLE 58. —Yield, date when 75 percent blight stage occurred, and gain from sprajring potatoes

at 2 centers in Sweden, 1946-55

Location and year

Date of 75 percent

blight stage

Unsprayed Sprayed

Total yield

Unsprayed Sprayed

Gain from

spraying

AKARP:

1946...

1948...

1949...

1950...

1953...

1954...

1955...

NYCKELBY:

1946...

1948...

1949...

1950...

1953...

1954...

Sept. 14

Sept. 10

Aug. 21

Aug. 10

Sept. 10

Aug. 22

Sept. 21

Sept. 15

Sept. 20

(^)

Sept. 11

Aug. 30

Sept. 18

Sept. 20

Sept. 30

Sept. 20

Tons/acre

6.4

11

9

11

15

11

14

6.2

8.2

11.0

10.1

S.2

S.S

Tons/acre

7.9

12.

11.

16.

21.

14,

16,

8.1

11.0

13.3

14.2

12.3

12.8

Tons/acre

1.5

1.9

2.S

2.3

4.1

4.1

4.0

No data for date of 75 percent blight stage.

percentage of infected tubers in the unsprayed

plots at both centers is shown in table 60. In

all years the variety under trial was Up-toDate, apart from 1946 at Nyckelby, when it

was Magnum Bonum.

The average loss through tuber blight infection for the 6 years of trials at Nyckelby was

just under 8 percent. At Akarp the chief point

of interest is the very heavy loss in 1955 (nearly

60 percent). It is not possible to say how acTABLE 59. —Percentage loss of crop due to

blight at 2 centers in Sweden, 1946-55^

Year

0

Akarp Nyckelby

1946

Percent

19

8

20

31

28

25

15

Percent

23

25

17

29

33

31

1948

1949

1950

1953

1954

1955

Average 21 26

0_TT

* Formula used to estimate loss: (-g- x 100)^

where S is yield of sprayed and U is yield of '

unsprayed plots.

2 No trial.

curately these figures reflect the position

throughout Sweden, but it should be remembered

that they are solely for tuber blight before

lifting, and as such they are high. Tuber infection at harvesttime is also a problem in

Sweden, which becomes more acute in the north,

owing to the shorter growing season and shorter

harvest period. The use of haulm killers should

greatly reduce this source of loss.

BLIGHT AND WEATHER DATA

The climagrams (fig. 44) for three centers

in Sweden show the strong continental influence

affecting the country's climate. There is a

TABLE 60.—Percentage of blighted tubers at

2 centers in Sweden, 1946-55

Year

0

Akarp Nyckelby

1946

Percent

10.0

2.5

57.8

Percent

0.1

16.1

8.3

6.8

12.7

2.3

1948

1949

1950

1953

1954

1955

No trial.

98
SWEDEN

-Malmö

RAINFALL 4

Inches

JFMAMJJASOND

RAINFALL

6

5

4

3

2

1

^ Srockh

LATITUDE

50°

olm

59°0'N

.-X

N

"

Inches

/

11

V/

\

. -

-^

^ LIFTING

//x

: PLANT y

-INC /^

-

J FMAMJ JASOND

RAINFALL

r- Härnös

LATITUDE

50° F

and

M°3S'N

/'

.-^^

N,

-

Inches

/

i//Á

\

\,

/ 1JFTINC

s

1

X

:^~~ =^r^ PLANT / X

-INC /

1/ / j

JFMAMJJASOND

FIGURE 44.--Long-term average monthly rainfall and mean temperatures in relation to potato cropping at three centers in Sweden,

showing shortening of growing period at higher latitudes.

marked summer rainfall peak and a wide

difference between summer and winter temperatures. The wettest month of the year is

August. Continental conditions are modified in

the south by warm winds blowing across the

land from the Atlantic, and in the north by the

presence of the Baltic Sea and the Gulf of

Bothnia. These various sea influences, resulting in high summer humidity conditions, appear

ta be one of the main factors leading to the

relative frequency of severe blight years--

approximately 1 year in 2. Combined with the

high humidities in the southern parts of

Sweden are summer temperatures generally

favorable for blight. For example, long-term

mean July and August temperatures at Malmb

are 62° and 60° F., and at Stockholm 63° and

60°. The climagram for Harnbsand shows the

shortness of the growing season for potatoes

(approximately 110 days), but a fairly large

acreage of the crop is grown even farther north

than this. Under these conditions, tuber infection at harvesttime has proved to be the major

source of loss caused by blight.

CONTROL MEASURES

Dr. Lihnell has told the authors that practically all growers who produce potatoes on a

commercial scale spray their crop once or

twice, and growers that use better farming

methods spray 3 or 4 times in the season.

Haulm destruction has become popular in

recent years. Investigations into the biology of

Phytophthora infestans and on spraying techniques are carried out at the Royal Agricultural College, Uppsala. A recent paper from

there by Bjorling and Sellgren (1955) has given

an account of field experimental work on blight

infections through the upper and lower surfaces

of potato leaves. Although these workers

found that by far the greater proportion of the

infection took place through the upper surfaces, there was sufficient infection throughthe

lower surfaces for them to state that both

surfaces need to be sprayed to give efficient

protection against blight. Work on protective

spraying and potato haulm destruction is also

being carried out at the Institutet for Vaxtforskning och Kyllagring, at Nynasham, by B.

Emilsson and his colleagues.

SUMMARY: SWEDEN

Although survey informiation on blight incidence in Sweden is not available, the results

of spraying trials lead to the conclusion that

losses from the disease both by premature

defoliation and tuber infection are potentially

very heavy. Losses are, however, reduced by

a universally applied annual spray program.

Tuber infection at lifting--a direct result of the

short growing and short harvest periods--is

the main problem in northern Sweden. In the

south, yield losses brought about by premature

defoliation is the more important aspect ofthe

disease.

WESTERN GERMANY

About 3 million acres of potatoes are

grown annually in Western Germany,

producing an average yield of just over

8 tons per acre (F.A.O., 1955). Total potato

production is about 26 million tonnes

per annum, and Van Hamelen (1955)

states that this is used as follows:

Production for--

Seed

Ware

Fodder

Starch manufacture • Alcohol manufacture

Shrinkage loss •

Total

Million tonnes

2.9

8.2

12.0

.2

.2

2.5

26.0

99
Human consumption accounts for less than

half of the crop. Even so, consumption is very

high, and in 1953 was 372 lb. per head per

annum. Enough certified seed is produced to

plant about one-third of the potato acreage,

and in addition some seed is exported, principally to Switzerland, Italy, France, and Brazil.

Fodder potatoes, grown mainly for pigs, account for about 50 percent of the total production.

POTATO DISTRIBUTION AND VARIETIES

Potato acreages, by province (fig. 45) for

1954, are shown in table 61. These figures were

supplied by the economist Intelligence Unit.

The principal areas producing potatoes for

the ware market are in Lower Saxony, the

south of Schleswig-Holstein, and the north of

North Rhine. This region contains a high proportion of medium-sized farms, ranging from

50 to 300 acres. A very large acreage of

potatoes is in Bavaria. The acreage is chiefly

in small or very small farms, and the potatoes

are for home consumption. Early potatoes are

grown throughout the country, with the most

important district centered on Dollbergen,

between Hanover and Brunswick (Braunschweig). Seed potatoes, also, are grown inall

areas, with a concentration of crops in Lower

Saxony, which produces about half of all the

certified potato seed grown in Western Germany. Much of this seed production goes on in

the north of Lower Saxony in the area adjoining

the North Sea. Stade is one of the main seedproducing centers.

GERMANY

NORTH SEA

S4°N

Sfade •^Hamburg \ MECKLENBURG

'/ ••.• ../• •• I

[•Ebersdorf A

denburg • ^>* • ' ' ' • •

/ • Bremen ;.

^^ LOWER SAXONY • Uelzen '• BRANDENBURG '

NETHER- ^V• ••• ••••Honove?.°»'''.T" ; Manöver« «Brunswick • . ' :" <^

»Berlin^

POLAND

LANDS .^SanSruV" i.i \ _ W /^^ NORTH RHINE /SAXONY-ANHALTBELGIUM- ,-,HiNE.:

f ^PALATINATE."

SAXON Y _ ,

^^j'wESTPHALIA .-'HESSE /

^\ •Bonn'--' /

/THURINGIA

' ^ Darmstadt • • ^°^ Kissingen N ^

-NSAAR- BAVARI A \

•" BADEN • Nuremberg ^ >^

£ C Ay O

-^ ^ '

¿ o

FRANCE /

I • Freudenstadt

/ -

^ A K

^^ y

\ A

iàlH

^f -

\ WURTEMBURG •

/

^^y

\

^^

200 miles

ncURE 45.--Key map of western and eastern Germany.

100
TABLE 61.--Potato acreage, by province, Western Germany, 1954

Province

Schleswig-Holstein

Hamburg

Lower Saxony

Bremen

North Rhine Westphalia.

Hesse

Rhineland Palatinate...

Baden-Wurttemherg

Bavaria

Total

Early

1,000 acres

9.5

U5

>1

.9

.6

38.9

11.8

U.l

13.3

30.4

165.2

Variety

Mid-season

and maincrop

1,000 acres

128.0

3.5

665.7

2.1

389.6

255.2

244.5

322.1

762.2

2,773.0

Total

1,000 acres

137

4.2

711.7

2.8

428.7

267.0

258.6

335.4

792.6

2,938.2

Most of the varieties grown originated in

Germany. In a recent review, Hix (1955) has

compared the importance of the different midseason and maincrop potato varieties from the

results of a nationwide survey. His figures,

showing the makeup of the Western German

potato acreage in 1954, are shown in table

62.

Hix did not include first-early varieties in

his survey. Erstling is the most important

early potato, and most of the seed of this

variety has to be imported. German

early varieties grown on a fair-sized scale

are Oberambacher Frühe, Sieglinde, and

Vera.

Table 62 shows the preponderant part played

by maincrop (late) varieties as opposed to

midseason potatoes. This is a somewhat different state of affairs from that in France and

Belgium, where a muchhigher.percentage of the

potato acreage is planted with midseason varieties. The three most important varieties are

Ackersegen, Heida, and Bona. The part played

by these potatoes varies in different parts of

the country. Ackersegen is the most widely

grown variety throughout almost the whole of

Germany and is particularly important in south

Germany. In Bavaria, it makes up half of the

acreage; and in Hesse, about two -thirds. Heida

is hardly grown at all in Bavaria and Hesse,

but in the other provinces it occupies a tenth

to a half of the potato acreage. The midseason

variety Bona is concentratedinnorth Germany,

particularly in Westphalia, where it is even

ahead of Ackersegen and makes up almost a

third of the potato crop.

The two main blight-resistant varieties derived from Solanum demissum hybrids, now

grown in Germany, are Aquila and Maritta.

Aquila does not occupy a large acreage, but

Maritta is important, particularly in Bavaria.

Recent descriptions of Germian potato varieties

have been given by Snell (1952).

SOIL AND CLIMATIC CONDITIONS:

VATIONS

CULTIMost of the potatoes are grown on light,

sandy soils. The climate throughout the greater

part of Germany is continental in type, with

relatively severe winters and the rainfall peak

coming in July and August. Summer temperatures are not high; in most places the mean

temperatures in July and August do not exceed

65^ F. In the north, true continental climatic

conditions are modified by the presence of the

North Sea and the Baltic; whereas in the south,

the fact that most of the land is high leads to

cool summer conditions.

There is a difference of 1 to 2 weeks in the

tinae of planting potatoes between the north and

south of Germany. The earlier plantings take

place in the north of the country, from the

beginning of April onward. Digging of early

ware varieties starts from the middle to the

end of June; and midseason and late varieties

are lifted from mid-August to the end of

September. Except in Schleswig-Holstein and

parts of Lower Saxony, individual holdings are

mostly small and subdivided into numerous

strips, which may be widely scattered. This

is a great hindrance to mechanization and,

incidentally, to the blight control measures.

Opitz (1949) describes potato cultivations in

Germiany in detail.

POTATO BLIGHT INVESTIGATIONS

Before the second World War, research and

investigational work on the potato crop in

Germany was concentrated in what is now

the Eastern Zone, where the larger part of the

crop was grown. Workers on potato diseases

in Western Germany hada considerable amount

of leeway to make up after the war, and are

only now getting fully into their stride.

101
TABLE 62.--Potato varieties, use, and

percentage of acreage in Western

Germany, 1954

Varieties and use

Midseason

For ware:

Bona

Concordia

Flava

Olympia

Others

For ware, fodder, or industry:

Augusta

Mittelfrulie

Total

Mainerop

For ware:

i^gnes

Heida

Virginia

Others

For ware, fodder, or industry:

Ackersegen

Aquila ,

Cape 11a ,

Magna ,

Maritta

Voran

Others ,

For industry:

Robusta

Total

Acreage

Percent

9.4.

1.0

.2

2.5

1.2

1.8

16.9

3.6

12.6

1.2

1.9

44.0

1.1

1.0

1.0

6.4

4.2

5.3

83.1

In the late 1920's and 1930's K. O. Müller

(1928), with his W-races, and Schick, working

at the Kaiser-Wilhelm Institute, were among

the pioneers in the search for blight-resistant

potato varieties. Since 1947, Rudorf and his

colleagues have continued this line of research

at the Max-Planck Institute, Voldagsen i^ber

Elze, in Hanover. Also, since the war, fundamental studies in connection with schemes for

forecasting blight have been carried out in

different parts of Western Germany, notably

by Johannes (1953a), and Uhlig (1955).

The Plant Protection Service (Pflanzenschutzdienst) is organized on a provincial

basis, and has been responsible for carrying

out the majority of the trials on blight control

in recent years. Individual Plant Protection

Offices are also responsible for collecting information on the incidence of plant diseases

from local observers, and datgi on potato blight

from this source, presented on a national basis,

have been published.

BLIGHT EPIDEMICS: DISTRIBUTION AND

FREQUENCY

An attempt has been made by Voelkel (1954)

to delimit those areas in Germany where

potato blight is most important. His method

embodies a conception of **pest or disease

zones'' (Schadgebiete), which he defines as

**areas in which a pest or disease in most of

the years in a particular period, is always

more or less severe." Figure 46 is from

Voelkel's map of the **Schadgebiete" for potato

blight. The darkly shaded areas, which are

most concentrated in Lower Saxony, are the

real blight zones.

Voelkel based his zoning upon a combination

of the total number and the frequency of

**severe" blight reports received by the various Plant Protection Offices from the different districts during the period. The bad

blight areas in Western Germany are thus

shown as (1) the south of Schleswig-Holstein,

(2) Lower Saxony (Oldenburg and Hanover),

(3) the western half of North Rhine-Westphalia,

and (4) a small area in southeastern Hesse.

Areas 1 to 3 form a continuous zone, comprising practically the whole part of Western

Germany on the great sandy plain that stretches

across the north of Europe. In this northern

blight zone, the disease is most serious in the

districts near the coast.

Annual accounts of potato blight in Germany

were published from 1920 to 1931 in the

Mitteilungen aus der Biologischen Reichsanstalt für Land und Forstwirtschaft. FromLl925

to 1931, these accounts included maps showing

the distribution of the disease. Since 1948,

general accounts of blight for most years

have been compiled by Dr. Harle from the

records of the Plant Protection Offices, and

published in Nachrichtenblatt des Deutschen

Pflanzenschutzdienstes, Braunschweig. (See

Härle, 1951.) It is possible from this material,

together with information supplied by Dr.

Johannes, to divide the years 1948-55 into two

categories--*'moderate" and '*severe"--according to blight incidence. This has been done

in table 63, which also shows the average

potato yields (F.A.O. statistics) in particular

years.

In no year was blight absent, and although

the incidence of the disease varied considerably between different areas of Western Germany over the period 1948-55^ in each year

there was at least some part of the country

where blight was stated to be severe. The difference between a moderate and a severe blight

102
Bliqhf severe in more

than Syears in 20.

Bliqht severe in mcxe

fhan Syears in 20,

and over 25 'severe'

reporfs per disfrict.

FIGURE 46.--Blight zones inGermany, 1925-44. (After Voelkel. 1954.)

year lies in the time of destruction of the

potato haulm by the fungus. Generally, in a

**severe" year, the haulm over a large part

of the acreage is killed before the end of

August, with a considerable resulting loss in

yield. In a ** mode rate" year, the blight attack

comes late and defoliationloss es are generally

small* This can, perhaps, best be seen from

Harle's account for 1950, whichappears to have

been the worst year in the 8-year period; and

for 1952, when blight was least damaging:

1950. Blight severe. ** Blight on haulm,

present in the Rhineland in May, was

sporadic in June, and from July

generally distributed and severe in the

whole of Western Germany. . • The

disease was especially severe in

Schleswig-Holstein, Oldenburg and

Westphalia. On some varieties, such

as Bona (midseason), the foliage was

completely killed within a few days.

It appeared that the spraying against

blight was often done too late. In

Schleswig-Holstein and in part also

in Oldenburg, blight intubers, (especially on the lighter soils) was

relatively slight despite the severe

attacks on the haulm."

1952. Blight moderate. The summer in this

year was dry and hot, and, up to the

end of July, blight incidence was

described as very slight. In August

*'although blight was long delayed with

outbreaks only here and there in this

month, the disease on haulm and

tubers was in parts still very severe,

especially in North Rhine Westphalia,

Lower Saxony, Hamburg and Schleswig-Holstein. South Germany remained almost free of the disease."

The attempt that has been made in table 63

to define the incidence of blight in certain

years for the whole of Germany, is, at best,

unsatisfactory. Although the disease may be

severe throughout the whole country in a

particular year (as in 1950), the more general

pattern is one of variation between different

regions. The only satisfactory way of defining

epidemics is on the basis of a nunnber of

relatively homogeneous potato-growing areas,

and, unfortunately, the data are not available

to do this. The highest yield (9.8 tons per acre)

during the period 1948-54 was in 1950--the

worst blight year. Without a clearer method of

defining the epidemic in that year, it is impossible to explore fully the reasons for this,

but it indicates that a high loss from the

disease in a blight year may be amply offset

by an actual increase over normal production.

BLIGHT ON HAULM AND LOSSES

Very few of the accounts of spraying trials

in Western Germany include a description of

the progress of the disease on the haulm. An

exception is some recent work by Johannes

(1953b), who used a scale from 0 to 5 to estimate the amount of defoliation, but this scale

is rather lacking in precision. It thus becomes

very difficult from published information to

make statements on the timing of blight

epidemics in the different regions of the

country. However, Dr. Scheibe of the Pflanzenschutzamt at Hanover has given us a verbal

account of blight progress on potato haulm in

his area, which covers the eastern and larger

half of Lower Saxony. From this account it is

possible to get an idea of the effect of blight

in one of the principal "Schadgebiete" of

Western Germany. In this part of Lower Saxony

blight occurs every year, most commonly

appearing in July. In a bad blight year, it

appears at the end of June and the tops of

unsprayed maincrop (or late) varieties are

dead by the middle to the end of August. The

crop would normally ripen off by the middle

to the end of September. The frequency of such

blight years is about one in three.

Thus, in a bad blight year there may be a

loss of 6 or 7 weeks' growth on unsprayed

maincrops. By analogy with England and Wales,

where the length of the growing season is very

similar, this will inyolve a loss of about 30

percent of the potential yield. But this is putting

the picture at its blacicest. Even in the bad

blight year, the yieldloss is unlikely to average

30 percent on the unsprayed maincrops, and it

will be reduced in the area as a whole by

protective spraying. Moreover, the earlier

maturing potato varieties areunlikely to siiffer

so severely from the effects of premature defoliation.

The results of large numbers of spraying

trials in many of the potato-growing regions of

Western Germiany have been reported in the

literature since 1946. These trials have varied

from properly replicated experiments to

simple demonstrations with one sprayed and

one unsprayed plot. It might be possible, as

103
has been done in Denmark, to use the results

of these trials, expressed as yield increases

from spraying, to estimate blight losses on a

countrywide basis. To the outsider, however,

they present many difficulties of interpretation. Some of the most important of these difficulties are: (1) Very often the results for a

number of years and/or a number of varieties

of differing maturity groups have been averaged; (2) far too little precise information on

the progress of blight on the haulm is given,

and even general notes on the severity of blight

in the year of the trial nnay be lacking; and (3)

the results of the trials are often exprèssed as

percentage increases in yield from spraying,

without quoting the actual yields of the sprayed

and control plots.

An idea of the scale of the yield increases in

some recent trials can be gained by quoting

representative figures. Johannes (1953b) has

given the results of experiments carried out

in 1949 and 1950 at a number of centers in

Western Germany, where the miain object was

to test the relative efficiency of different rates

of copper application and amounts of water in

low-volume spraying. In 1950, which was a

severe blight year, trial centers were at

Uelzen and Ebersdorf in Lower Saxony, at

Sandrup and Bonn in North Rhine Westphalia,

and at Darmstadt in Hesse. The average

increase in yield(all varieties) over the 2 years

for the optimum treatment was 26 percent.

This figure corresponds very closely to an

average increase of 23 percent of the unsprayed

yield, which was the result of trials by Ext

(1950) during 1950 in Schleswig-Holstein. Two

sprayings with a copper compound were given

to a number of different varieties. The variation in yield increase between some of the

varieties in Ext*s trials are shown in table

64, and the figures illustrate some of the

difficulties of interpreting results of this kind.

In southern Germany, Malmus (1954) has

reported on 412 spraying trials in different

parts of Bavaria during 1949, when blight was

severe in the province. Of these trials 72

percent gave a yield increase as a result of

spraying, and this increase averaged over 13

percent. To what extent the yield increases

quoted above for the three sets of trials reflect

the level of losses caused by blight during

severe blight years in parts of Germany is a

matter of conjecture. The efficiency of the

spraying is a most important factor, and it is

most unlikely that full blight loss was prevented.

Malmus (1949) has made some overall blight

loss estimates for the German potato crop.He

estimated that 33 percent of the crop was lost

by blight in 1916, and 25 percent in 1926. In

years of less severe attack thanthese, Malmus

suggests that the loss is about 10 percent. It

seems most unlikely that the average annual

loss by premature defoliation in Western Germany will exceed 10 percent of the potato crop.

This is on the basis of the careful analysis by

Stapel (1953) of blight losses in Denmark,

where losses were estimatedto be of this order.

The weather and other growing conditions inthe

blight zones of Western Germany are very

similar to the general conditions in Denmark.

Outside the blight zones, where half the total

crop is grown, blight losses are likely to be

less than in Denmark. It is important to see this

loss of potential crop in its proper perspective.

An analysis of West German potato yields in

moderate and severe blight years (table 63)

shows that in the years of moderate attack, the

average potato yield was 8.2 tons per acre; in

the years of severe attack it was 8.8 tons per

acre.

Little data exist on the extent of tuber infection, either in the soil before lifting or at

harvesttime. Dr. Johannes has said that in

Western Germany, as a whole, tuber blight is

not a very important problem, and this he

associates with the generally sandy soils on

which potatoes are grown and the relatively

long growing season. Speadcing of Lower Saxony,

Dr. Scheibe stated that although trouble may be

caused in a wet harvest year, tuber blight is

not usually a problem.

TABLE 63.—Incidence of potato blight and average yield of potatoes inWestern Germany, 1948-55

"Moderate" blight years Average yield "Severe" "blight years Average yield

Tons/a

1949 ,

1952

1954

1955

Average.

7.4

Ö.3

9.0

8.2

1948

1950

1951

1953

Average

Tons/acre

§^2

9.8

8.6

8.4

B.S

104
TABLE 64. —Increases in yield of certain potato varieties from copper sprasring,

Schleswig-Holstein, Western Germany, 1950

Variety Mattirity

Average increase

in yield from

spraying

Range of

increase

Flava Midseason

- .fin

Percent

17

36

27

Percent

5 to 29

-2 to 66

S to 43

Bona

Heida Maincrop

BLIGHT AND WEATHER DATA

Uhlig (1955) has pointed out that the main

blight zones in Western Germany (Voelkel's

•'Schadgebiete") lie predominantly in those

parts of the country where the mean relative

humidities in July and August are over 75

percent. But so far no detailed studies of blight

conditions in relation to macro-climate have

been made. The long-term rainfall and mean

temperature data shown in figure 47 for three

centers in different potato growing areas

reveal a very similar pattern, and no very

WESTERN GERMANY

. Hamburq

82 ft.

RAINFALL

inches

RAINFALL

Inches

, ^ Hanover -,

*• 187 ft.

5 - y""^ ^\^^

J -

2 ^"*~"

1 -

^

/

-^

PLANT /

-INC /

/ 1/ / /111

V/

\

;; LIFTING

//A

V

-

•F

great difference in the average monthly rainfall and temperature figures between the

centers occurs. A greater annual and summer

rainfall is found in the higher parts of Germany; at Freudenstadt (2,395 ft.), for example,

the average yearly rainfall is 58 inches, but

these wetter districts lie outside the main

potato-growing regions. The higher summer

humidities due to sea influences, which occur

in the north, are not correlated with a higher

rainfall in these regions; and there is no

simiple relationship between blight and rainfall.

Summer temperatures throughout the country

are generally favorable to the disease.

A study of blight incidence in relation to

weather at Braunschweig (Brunswick) over a

period of 3 years (1952-54) has been made and

is given in table 65. Climate at this center is

very similar to that at Hanover, and may be

considered as reasonably typical of weather

conditions in the great sandy plain of northern

Germany. In this table the descriptions of the

blight attack in the several years have been

given by Dr. Johannes.

During both the *'slight" and the '*severe"

attacks in 1952 and 1953, respectively, rainfall

was below average, although low rainfall was

most marked in the "slight" year. The most

unusual year of the 3 from the point of view of

relationships between blight and weather was

1954. Despite extremely wet conditions in the

summer (July rainfall: 7-8 inches), the very

low temperatures in July (5® F. below average)

delayed the onset of blight, amd the attack was

late.

CONTROL. MEASURES

RAINFALL

inches

.Nuremberq

l,OI4ft.

J F M A M J

FIGURE 47.—Long-term average monthly rainfall and mean temperatures in relation to potato cropping for three centers in Western

Germany.

Dr. Johannes has given a general account

of chemical control measures against blight

in Western Germany. In the north of the

country, protective spraying is commonly

practiced. Seed potato growers (export and

home consumption) are obliged by their contracts to use protective sprays on their crops;

also, there are a number of organizations of

large growers that insist on protective spraying as a necessary operation ifthe organization

is to buy the member's potatoes. Inmost cases

the work is done by the farmer himself; for

the small farmers, communal machines are

105
available. Spraying against blight is muchless

common in the south of Germany.

An estimate of the amount of spraying done

in the area covered by the Pflanzenschutzamt

at Hanover has been supplied by Dr. Scheibe,

and his figures illustrate some of the factors

at work. The area falls into three main divisions. In the seed and ware-growing district

in the northwest, about 50 percent of all

growers spray each year; the growers of early

varieties for seed usually spray three times,

and the growers of maincrop varieties for

seed usually only once. In the central waregrowing district, 20 to 30 percent of the

growers spray once or twice. In the early

ware-growing district centered onDollbergen,

no spraying is done.

The most commonly used spray materials

in Western Germany are, in order of importance: Copper oxychloride (45 percent Cu);

copper oxides (35 or 70 percent Cu); andorganic

fungicides, in particular, zineb. Dr. Scheibe

has said that in Lower Saxony about 90 percent of the spraying is with copper materials,

and about 10 percent with organic fungicides.

Most of the spraying is done at high volume,

but the low-volume rate is increasing in

popularity. From the results of trials in 1949

and 1950 at centers in different parts of

northern and central Germany, Johannes

(1953b) concluded that the best low-volunne

results were obtained by an application of 4

to 6 lb. of 50 percent Cu in 40 gallons of

water per acre. Sommer (1951) has given

details of the spraying techniques employed

in the important seed-growing region around

Stade in Lower Saxony. Throughout Germany,

spraying against blight is to some extent

combined with spraying against the Colorado

beetle, but in most years the first two applications against the beetle are given too early

for it to be worthwhile to combine them with

an antiblight treatment.

Haulm destruction is not practiced on any

scale, even in seed crops. Where it is used,

the chemicals most, commonly employed are

D. N. O. C. and D. N. B. P.

Over 80 percent of the West German potato

acreage is planted with maincrop or late

varieties (table 61), most of which, andnotably

Ackersegen, show a high degree offield resistance to blight. Schaper (1950) has described

the blight susceptibility of most of the commonly grown German varieties. An interesting

conclusion that he came to was that there is

no correlation between the foliage and the tuber

resistance of potato varieties. The same

worker at the Max-Planck Institute is engaged

on research into the biotypes of the fungus.

Western Germany is one of the few countries

in the World where named ^. demissum x S.

tuberosum hybrids, in this case Maritta and

Aquila, have become accepted into commercial practice on an appreciable scale, although

these two varieties have not retained their

original blight resistance. Recent West German work on the resistance of numerous wild

Solanum species and on the incorporation of

this resistance into commercially acceptable

potatoes, has been reviewed by Rudorf,

Schaper, Ross, Baerecke, and Torka (1950) and

Rudorf (1950).

Much attention has been paid in recent years

to the development of weather rules by whichto

forecast the appearance of blight epidemics.'

The operation of a blight forecasting service to

growers is the responsibility of the individual

provincial plant protection offices, and it is

unlikely that a centralized system of issuing

warnings will be developed. At the present

time, the plant protection office at Oldenburg

(W. Holz) is the only center in Western Germany issuing regular blight forecasts based

on meteorological data, although other offices

send out advice on spraying against blight when

first reports of the disease intheir areas comte

in. The main centers for research into forecasting methods are at Braunschweig and Bad

Kissingen. Johannes (1953a) and Uhlig (1955)

have both tested the Beaumont criteria and also

the vapor pressure rule of Thran (1952), which

was developed in Schleswig-Holstein. They

considered that both these methods give insufficiently accurate forecasts under West

German conditions, and have formulated more

complicated rules based on crop humidity

5 At an international cxjnference on forecasting (Ullrich, 1957)

held at Bninswick in December 1956, Uhlig explained that according

to the "field-humidity rules" employed in a trial warning service in

1954-56, a critical period was one in which the relative humidity,

measured in screens 2 m. (6| ft.) above ground level, was over 88

percent for 15 hours in the absence of rain, and for 12 hours following

rain, with minimum night temperatures falling during the period to

12°-16° C. (53°-61° F.) but not below, and the maximumday temperamre rising to 20° C.(68° F.)orover, either during the period or within the following 12 hours. The rule was used in conjunction with 3

hourly weather telegrams. The critical periods from a network of

stations were plotted on an operations chart to reveal the areas

affected; and epidemic spread of blight was not anticipated until three

critical periods had occurred.

At the same conference Thran explained that according to the

•'air-mass and accumulated temperature rule" employed in SchleswigHolstein, a critical period is one of 2 consecutive days with vapor

pressure 12 mm. or more, minimum night temperature 10° -17° C.

(50°-6aP p.), cloud five-tenths or more, and maximum temperature

on at least one of the days reaching 23° C. (73° F.) or more, the

readings being taken in screens at 2 m. above ground level. It was

found that potatoes were attacked by blight following such a critical

period only when certain accumulated temperatures had been reached.

Figures for these (which ranged between 600 for chitted first-early

varieties and 1,650 for very late varieties)could be obtained approximately by multiplying the number of days from April 1 by the mean

day temperatures in °C. (With long-term mean temperatures for

Hamburg, this would indicate early June as a zero date for first earlies

and early August for the very late varieties.)

In his very detailed microclimatic investigations, Johannes (1953a)

found that a critical period for the spread of the fungus occurred when

the relative humidity, measured in the upper third of the crop, exceeded 90 percent for a period ofat least 33 hours, with temperatures

between 10° and 20° C. (50° -68° F). A fall in hunüdity to not less than

82 percent for 4^ hours did not break the period; but 2\ hours at a

minimum of 72 percent did. Two critical periods were found necessary

for epidemic spread. Johannes' critical period refers to conditions in

the crop; its relation to conditions in screens has not yet been determined.

106
TABLE 65.--Blight incidence in relation to weather at Braunschweig, Western Germany, 1952-54

Average and

year

Long-term average...

Difference from

average for—

1952 ,

1953 ,

1954 ,

Rainfall

June

In.

2.4

-.2

+.2

-1.0

July

In.

3.3

-2.3

-1.1

+4.5

hig.

In.

2.9

-1.4

-1.0

+1.7

Sept.

In.

2.2

+ .2

-.7

+1.1

Temperature

June

61

-2

+2

0

July

ix64

+1

+1

-5

Aug.

:2-

63

+2

0

-1

Sept.

57

-4

0

+1

Blight

attack

Slight.

Severe.

Late.

periods. One of the main troubles with these

new rules, however, is that they are very

difficult to apply at the practical level of a

large-scale warning service to farmers. Dr.

Ullrich is continuing with the work" on this

problem that was started by Johannes at

Braunschweig. The need for blight forecasts

varies in the different parts of the country.

In the north of Germany (Schleswig-Holstein

and the north of Lower Saxony) a routine annual

spray program is the general rule, so that there

is little need for a blight forecasting service.

In mid-Germany, it is usually a problem to

know whether to spray or not, and blight forecasts would be most valuable. Blight occurs

infrequently in southern Gernaany, and a service is less necessary.

SUMMARY: WESTERN GERMANY

It is possible to define fairly well the bad

blight areas in Western Germany. These lie

principally in the north of the country, where

air humidities in July and August are highest.

But the present system of blight surveying is

not sufficiently precise to define epidemics in

particular years satisfactorily. It is also very

difficult on the basis of present data to estimate blight losses, but there is st:»n)ngevidence

that in recent years these losses have been outbalanced by increased potato yields due to the

additional rainfall in the blight years. Chemical

methods of control are generally employed in

the north, but to a muchless extent in southern

Germany. Apart from the smaller need to spray

in the south, the mechanization of spraying is

greatly hindered by the small size and awkward

layout of the holdings. Potato varieties showing

a high degree of field resistance to blight occupy a large part of the West German potato

acreage. The acceptance of Solanum demissum

hybrids into commercial practice reflects

pioneering German work in the search for

blight-resistant potato varieties.

AUSTRIA

The greater part of Austria is mountainous Average yield is about 6 tons per acre

(fig. 48) and includes the eastern extension of (F.A.O., 1955). Hunrian consumption at somethe Alps. Arable farming is carried on mainly what over 200 lb. per head each year accounts

in the lower land of the east of the country, for less than half the total potato production,

which is also the driest area (fig. 49). Climatic and there is a large usage for fodder and the

conditions vary considerably over short dis- manufacturing industry.

tances, according to altitude, but in the east the Short notes on the occurrence of blight are

climate is typically continental. About 400,000 included in an annual account of plant diseases

acres of potatoes are grown annually. Acreage and pests given in Pflanzenschutz Berichte,

figures by province in 1952 (Austria, Statisches For the year 1951 and those following, a shortZentralamt, 1953) were: hand notation of describing blight epidemics

according to their severity and distribution

Province: 1,000 acres ^^^ ^^^^ adopted (see Schreier, 1957). There

Vienna 12.4 is a 1 to 4 scale for severity (1, negligible;

Lower Austria 198.3 2, medium; 3, severe; 4, very severe) and a

Upper Austria 82.3 1 to 3 scale for distribution (1, local; 2,

Burgenland 29.9 fairly extensive; 3, general). Table 66 shows

Styria 48.4 the description by this method of blight epiCarinthia 28.4 demies in Austria during the period 1951-56.

Salzburg 7.2 Thus, the epidemic in 1956 was severe and

Tirol 9.6 fairly extensive. No measure of the epidemics

Vorarlberg 3.5 in 1952 and 1954 was given. Dr. Beran, to

whom we are indebted for the following notes,

Xotal 420.0 has told us that no special blight investigations

107
AUSTRIA

I

/\.

I

/ \'

UPPER

r

MU'

•Ünz

í

/

\

f I- /

\ .'AUSTRIA

, /

LOWER

-BERG/ TIROL • Innsbruck / SALZBURG

^i.^- v^.

.--^N.^ ( TIROL •.

S T Y R I A

C A R I N T H I A

J^Vicnna ^

AUSTRIA ...<) f

• ^ (

' <o

.V

10 o miles ^ ^ f

4yN

FIGURE 48,--Key map of Austria and altitudes.

are in progress at the present time. The

special problems of the disease under alpine

conditions have, however, been extensively investigated in the neighboring country of

Switzerland, notably by Hänni (1949) and by

Aebi (1956), who has employed progress curves

and used Beaumont criteria in fore casting.

Blight is of economic importance in the whole

of Austria, but in the eastern parts, which are

the principal potato-growing areas, it is only

serious in years of above-average rainfall.

The attack varies greatly, according to variety.

Early and midseason varieties are most severely attacked, those commonly grown being

Erstling, Sieglinde, Saskia, Oberambacher

Frühe, Bintje, and Bona. American varieties

introduced after 1946 proved to be very susceptible, and this was one of the main reasons

why they quickly went out of cultivation. Under

normal weather conditions, loss in yield in the

108
. Vienno

656 ft

RAINFALL

Inches

TABLE 66. --Blight incidence in Austria,

1951-56

6,- Innsbruck

1,968 fr.

RAINFALL

Inches

Year Blight epidemic^

1951 3/3

1952

1953 3/3

1954

1955 3/3

1956 3/2

FIGURE 49.—Long-term average monthly rainfall and mean temperatures in relation to potato cropping for two centers in Austria.

blight areas ranges from 5 to 20 percent,

according to variety.

The proportion of growers spraying against

the disease is greater in the west than the east.

First figure indicates severity and second

figure distribution of disease. (See p. 107.)

In the west, it has been necessary for some

years to spray against the Colorado beetle,

and between 10 to 20 percent of the potato area

is also sprayed against blight. Spraying against

the beetle-has only recently become necessary

in the east; there, not more than 5 percent

of the growers take action against blight. It

is principally the earlier varieties and valuable

seed crops that are sprayed. Haulm killing is

not practiced as a control measure against

tuber infection.

POLAND

Although relatively small in total area,

Poland comes third among the countries of the

world in order of potato production. (The

U.S.S.R. comes first, and Germany second.)

Nearly 7-1/2 million acres of potatoes were

grown each year over the period 1948-52, and

these produced a yield of about 5 tons per

acre (F.A.O., 1955). Potatoes are grown

throughout the country; they form a major

item of human diet and are also grown extensively for industrial purposes and animal

fodder.

About 70 percent of the potato area is

planted with varieties showing a high degree

of field resistance to blight. Examples of these

are Ackersegen, which occupies about half of

the total potato acreage, Wohltmann, Switez

Dolkowski, Silesia, and Lenino. About 10

percent of the area is made up of varieties

that are highly susceptible to blight, such as

Early Rose, Alma, and Flava, but these are

principally early or midseason varieties and

so normally escape the worst effects of blight

epidemics. The remaining 20 percent of the

potato acreage is occupied by varieties showing

an intermediate degree of resistance, e.g..

Mittelfrühe. The potato-growing season is

generally from early May to the endof September.

The climate of Poland is predominantly

continental in type, though the maritime influence of the Baltic Sea is felt in the north

of the country. Average annual rainfall throughout Poland is relatively low; at Warsaw, it

is 22.3 inches, but there is a marked summer

rainfall peak, and the wettest months of the

year are July and August. The mean temperature at Warsaw in July, the hottest month,

is 65 OF, (fig. 50). According to an analysis of

survey data on blight incidence dating back to

1926, sent to us by H. Dubniak of the Polish

Ministry of Agriculture, the areas of Poland

where blight attacks are most frequent and

severe are those where the average August

rainfall is 80 mm. (3.2 inches) or over.

These **blight" areas can be divided into three

main regions: (1) A mountainous region inthe

south of the country; (2) the center of Poland,

where blight occurs along the river valleys;

and (3) the northern region, which is subdivided into an eastern and western zone. The

map (fig. 51) shows the approximate boundaries

of these different regions. The losses caused

by blight vary in the three regions. The greatest losses occur in the north, where blight

attacks are earliest and most frequent; in the

southern mountainous region, the attacks are

generally late.

During recent years, the worst attacks have

been in 1930, 1931, 1934, 1942, 1949, 1952,

and 1954. This gives a frequency of *'severe"

blight years of about 1 year in 4. In such

years, the disease is present throughout the

whole country. From the point of view of

blight epidemiics, 1955 was an exceptional year,

owing to an abnormal distributionof rainfall. In

that year, the attack was relatively slight in

the north, but severe damage was caused

109
POLAND

RAINFALL

Inches

FIGURE 50.--Long-term average monthly rainfall and mean temperatures in relation to potato cropping for Warsaw, Poland.

POLAND

BALTIC SEA

^//A^V^

j^-r--, 7'%\

most o\fectcd yy-^'.^y ç-^'' j

by bliqht ,r

}/

, \ »Warsaw

>^-i

iAFIGURE 51.--aight areas of Poland, according to survey data for

1926-55.

through the center of the country, in areas

where it normally has little effect. Figure

52 shows the zoning for blight damage in 1955.

The damage was measured by the amount of

defoliation caused by blight on the variety

Ackersegen toward the end of the growing

season (the exact time is not specified). It is

of particular interest that the survey information is available to enable the Polish Ministry

of Agriculture to produce a map of the type

shown for 1955, which can form the basic

material for the assessment of countrywide

blight losses.

The normal time for the development of

epidemics is during the second half of August.

Widespread attacks of the disease earlier or

later than this are out of the ordinary. A very

rough comparison of the amount of defoliation

caused by the disease on Ackersegen in a

'*blight" year (1954) and in a year when the

disease was slight (1956) can be made from

figures provided by H. Dubniak. These are

shown in table 67.

In 1954, the 50 percent blight defoliation

stage was reached in general at the end of

August. The normal time of maturity for this

,1 """ i"'n''•1'' ' '.i\

"' ",.'.I..!."„: ,,',ir.

iin;!""',::^, """:-'li".!'^

DefolioMon

per cent

0-20

20-50

'••'.'"iMl

: II i'",' 'i'

II

".I

I"'! ',:'^-'"i''""i I -

•I ..'.'-. I" ' ''-I, , 'if

'-!]•••' •" '• 111.'":

FIGURE 52.--Extent of defoliation by blight at end of growing period

on Ackersegen, Poland, 1955.

TABLE 67. --Defoliation of Ackersegen at end of

August, 1954 and 1956, Poland

Percentage of

defoliation by blight

at end of August

Ackersegen crop

None.,

0-10..

10-40,

50...,

variety is around the end of September, so that

approximately 3 to 4 weeks growth will have

been lost. On the other hand, in 1956, over

three-quarters of all Ackersegen potatoes were

still unaffected by blight at the end of August.

The relationship between premature defoliation

and loss of crop yield has not yet been worked

out, but it has been estimated that in "blight"

years, crop loss from this cause falls between

the limits of 10 to 30 percent. Dubniak points

out the difficulty of using average potato growth

figures to estimate blight losses in particular

years. Thus, in 1955, when planting was late

because of a cold wet spring, losses due to

blight would tend to be overestimated. Blight

seldom causes widespread or heavy losses in

Poland due to tuber infection.

Research on Phytophthora infe stans is

carried out at the Laboratory ofthe Institute of

Cultivation and Acclimatisation of Plants at

Gdansk Wrzeszcz. This includes work on the

biology of infection, the aggressiveness of particular biotypes, the length of the incubation

period, and the relation of time of infection by

the fungus to the chemical composition of

potato leaves. Breeding of new potato varieties

resistant to blight is also in progress at this

Institute.

110
UNION OF SOVIET SOCIALIST REPUBUCS

With an area of over 8-1/2 million square

miles, the Soviet Union is the largest connected

land mass under one central government inthe

world. The estimated population in 1954 was

about 215 million (F.A.O., 1956). Potatoes are

one of the country's principal crops and over

the period 1948-50, 31 percent of the total

world production of potatoes was in the U.S.S.R.

This compares to 5 percent of the total world

production in the U.S.A.

Apart from ware and animal feed, potatoes

are grown extensively as an industrial crop,

particularly for the manufacture of alcohol,

much of which is used in the synthetic rubber

industry. Figures relating to human consumption of potatoes are not published, but the crop

forms a large part of the diet. From 1934

to 1938, the average area under potatoes was

about 7 million hectares (17-1/2 million acres)

with an annual average production of about 74

million tons. This means that the average yield

of potatoes was around 4 tons per acre and it

has continued to remain at about this figure.

The potato-growing area has been considerably

enlarged since the Second World War, owing to

the incorporation of the Baltic Republics into

the Soviet Union, and also the eastward and

northward extension of potato growing. Under

the fifth Five-Year Plan, the potato area was

to reach 10 million hectares by 1955 (Oxford

Economic Atlas, 1956).

DISTRIBUTION OF POTATO-GROWING AND

CLIMATIC CONDITIONS

The principal potato-growing areas are in

European Russia, west of the Urals. Large

acreages are grown in the Baltic Republics,

White Russia (Belorussia), and the Moscow

Province. The northern Ukraine is an important potato region. Roughly speaking, the 70°F.

isotherm for July (fig. 53), which divides the

Ukraine and continues in a northeasterly direction across European Russia, marks the

southern limit of intensive potato production.

In the north, latitude 60^ N., which passes

through Leningrad, fixes the approximate

northern limit of intensive potato culture, although small quantities of potatoes are grown

above this, even as far north as the Arctic

Circle (fig. 54).

East of the Urals, the most important potato

areas are in western Siberia, and a scattered

acreage of the crop is grown through the Siberian Plain, and eastwards as far as the

vicinity of Lake Baykal. Relatively small

acreages of the crop are grown in the far east

of the Soviet Union, with some potatoes in the

area north of Vladivostock, and in the island

of Sakhalin.

Rainfall is generally low in Russia. In few

parts of the country does it exceed 25 inches

per annum, and over vast areas it is below 10

POLAND : WHITE

.' RUSS I A

FIGURE 53.—Key map for European Russia, showing July isotherms.

inches. The wettest parts of the country are in

Transcaucasia, adjoining the eastern seaboard

of the Black Sea (at Batum, average rainfall

is 93 inches per annum), and parts of far

eastern Russia, where areas are influenced by

the summer monsoon winds. Over most of the

country winters are severe and the length of

the winter in the more northerly axeas sets a

limit to successful potato cultivation.

In the main potato-growing areas of European

Russia, annual rainfall is between 18 to 25

inches per annum. At the upper limits, rainfall

is generally adequate, as there is a marked

summer rainfall peak and the months of maximum rainfall are July and August. This is shown

in the climagrams for Leningrad and Moscow

(fig. 55). Long droughts however, do occur. At

the lower rainfall limits, the crop may frequently suffer from drought. The lower rainfall

areas are toward the south and have higher

summer temperatures, so that the rate of evaporation of soil water is increased. Russell

(1942) says **In the black soil region (of the

Ukraine) potatoes are grown on the flat, not on

ridges; this would cause too muchloss of water

by evaporation, and also loss of soil by wind

erosion.'* Despite the trend for rainfall to decrease and temperature to increase in a

southerly direction, there is a remarkable

general uniformity in the weather conditions of European Russia, due to the topography, which is that of a great undulating

plain with no mountains except at the

edges.

Ill
U.S.S.R.

^Vladivostok

y ;

2000 miles

Each dot represents 100,000 tons.

Heavy lines indicate potato qrowinq areas.

FIGURE 54.—Potato production in the U. S. S. R. Data are based on the

(By special permission, Oxford

Lorkh (1945) has given data for the lengthof

the potato-growing season (May to September)

in the Moscow Province, over the 8-year

period, 1934-41. This averaged 147 days from

the time of planting till the first killing frost in

the autumn, with a range in time of 126 to 176

days. Thus, there is normally ample time for

the maximum crop of potatoes to be formed.

In areas northof Moscow, the decreasing length

of the growing season begins to be a limiting

factor to potato yields. Outside European

Russia, the generally low rainfall and the

usually short growing season for potatoes are

unfavorable to the crop. In all regions, potatoes

are grown for early summer, autumn, and

winter use. In the main regions of cultivation,

they are used for ware, animal feed, and industrial purposes; in the remaining areas,

they are used mainly for ware. Seed potatoes

are grown in all potato-producing regions.

POTATO VARIETIES,SOIL CONDITIONS, AND

CULTIVATIONS

Both Russian and foreign potato varieties

are grown. Common early varieties are Epron,

Epicure, and Early Rose. Berlichingen is a

widely grown midseas on potato, and Lorkh is

one of the most important maincrop (or

Oxford Regional Economic Atlas: U.S.S.R.and Eastern Europe. 1956.

University Press.)

medium-late) varieties; Wohltmann (late) is

grown for industrial use and animal fodder.

While the early varieties are normally highly

susceptible to blight, the late varieties generally show a fair degree of fieldresistance. A

number of Solanumdemissum hybrids have been

put on the market. These include Kameraz,

Hybrid 42, andPhytophthora-Resistant. Details

of the principal potato varieties grown in the

U.S.S.R. have been given by Zaitsev (1950 and

1953). Most of the potatoes in the more northerly

parts of European Russia are grown on sandy

soils, while in the Ukraine, they are grown on

black organic soils. A feature of Russian farming is the large amount of hand labor that is

still employed, and this applies to potato

growing, including planting and lifting. A recent

development has been the planting of potato

crops on the square, with from 24 to 28 inches

between rows and plants. Before this, most of

the crop was grown in 24 inch ridges, with

approximately 18 inches between plants.

DISTRIBUTION, FREQUENCY, AND TIMING

OF BLIGHT EPIDEMICS

Potato blight is most damaging in those

parts of European Russia where conditions arc

most favorable to potato growing, i.e., in the

112
U.S.S.R.

. Leninqrod

RAINFALL 5

Inches

TABLE 68. --Incidence of blight in the

Leningrad Province, 1946-55

_ Saratov

RAINFALL

Inches

FIGURE 55.--Long-term average monthly rainfall and mean temperatures in relation to potato cropping for three centers in the U. S. S. R.

Baltic Republics, White Russia, the Moscow

and Leningrad Provinces. In a southerly and

easterly direction from this region, the importance of the disease diminishes, although

outbreaks continue to occur as far east as

western Siberia« In the southern parts of the

U.S.S.R., attacks are infrequent, and in the

Central Asian republics and southern

Kazéikhstan they do not occur at all. In some of

the potato-growing areas in the far east of the

Soviet Union, as on the Island of Sakahlin,

blight occurs regularly every year.

Some idea of the frequency of severe blight

years in the Moscow Province can be obtained

from data given by the Research Institute of

Potato Growing (Kartofelnoe Khozyaistvo, 1946)

for the 17-year period 1918-34, during which

time there were 7 *'blight'* years. This is a

frequency of 1 in 2-1/2 years. In this area,

blight appears about the end of July to early

August in **severe" years, but information on

the subsequent developmient of epidemics is

lacking. Table 68 sets out data on the incidence of blight in the Leningrad Province during

the 10 years 1946-55. This and much other

information for the U.S.S.R. has been provided

by I. Polykov of the All-Union Research Institute of Plant Protection at Leningrad.

Severe Slight Negligible

1946^

19482

1949^

1953^

1954^

1950

1955

1947

1951

1952

^ First appearance of disease: Early August.

First appearajLce of disease: End of July.

In the severe blight years (1 year out of 2)

in the Leningrad area, the disease normally

appears at the end of July or early August,

although it may sometimes occur earlier than

this. Dr. Polykov mentions that in 1954 (a

severe blight year), early varieties were

completely defoliated by the disease at the

end of August, and midseason varieties were

dead by the middle of September. This is unlikely to have resulted in a large loss of yield

through premature defoliation, as the times

mentioned coincide fairly closely with the

normal ripening times of the two classes of

potato in the area. But there is evidence that

destruction of the potato haulms by blight does

occur earlier than this. Kameraz (1954) has

stated that in 1953, on many farmis in the

Leningrad Province the tops of maincrop as

well as early varieties had been destroyed by

mid-August. This will have resulted in about

4 to 6 weeks* loss of growth inmaincrops with

a consequent heavy fall in crop yield. Data on

the progress of blight, other than in 1953 and

1954, are not available.

BLIGHT LOSSES

Rozhestvenski (1934) estimated the average

annual loss of potatoes in the Soviet Union as

10 percent of the crop, due to attack by blight.

This figure included losses from premature

defoliation, together with tuber blight losses.

The same worker also gave estimates of the

variation in losses from blight between different potato-growing regions, and his miap,

setting out these variations, is reproducedhere

as figure 56. Although Rozhestvenski*s estimates should be treated with reserve, as they

were not based on systematic survey information, the estimated varying losses between

regions appear to reflect, with reasonable

accuracy, the differing importance of blight in

the main potato-growing areas of Russia. Thus

the disease is most damaging in the regions

immediately south and southeast of Leningrad.

The Baltic Republics were not then part of the

Soviet Union, so are not zoned on the map.

It is not possible to state with any accuracy

the relative importance of premature defoliation and tuber blight losses, although Dr.

Polykov has told us that, for the Soviet Union

113
SVERDLOVSK

ODESSA

l2-l5«»/o

7-IO*/o

am 2-äX

r J'^r^-^

FIGURE 56.—Average annual losses from potato blight in U.S.S.R.

(After Rozhestvenski, 1934.)

as a whole, the two problems are of about

equal importance. That heavy losses brought

about by early defoliation of the potato plants

do occur has been shown for the Leningrad

Province in 1953, but how frequently this

happens is not known. The available literature

also gives little help in deciding to what extent

tuber infection with blight prior to lifting is a

a problem; but the indications are that infection at lifting is a more serious source of loss.

In most of the potato-growing regions of the

U.S.S.R the harvest period is short. Over

large areas, e.g., Leningrad region and western

Siberia, the short harvest period will be combined with a relatively short growing period,

so that in many years there will be a considerable likelihood of tuber blight infection

occurring at harvesttime, unless steps are

taken to deal with blighted haulms.

BLIGHT AND WEATHER DATA

The climagrams for Leningrad, Moscow, and

Saratov (fig. 55) should be considered in relation to figure 56. The general similarity in

the weather conditions of the several areas of

European Russia is brought out by the three

climagrams. Summer temperatures increase

from north to south; in the south low summer

rainfall and a high rate of evaporation often

limit potato yields, and the crop very frequently sxxffers from drought. Even in the

higher rainfall potato regions of European

Russia, drought is a problem in some years,

and the average rainfall figures conceal a

considerable variation in rainfall from year to

year.

It has been estimated that losses from blight

are highest in the Leningrad region. This does

not have the highest summer rainfall, but summer humidity conditions are higher there than

in the Moscow region, and summer temperatures are lower. Also, tuber blight infection

at harvesttime is a greater potential threat

in the Leningrad region than in areas farther

south, owing to the shorter growing period for

potatoes. Although the long-term average temperature and rainfall figures for Moscow

might lead to the suppositionthat blight attacks

are unlikely to be serious very often in this

area, such an interpretation is too superficial.

Apart from other considerations, it ignores

the variation in rainfall from year to year,

which is illustrated by figures for the Moscow

province during 1935 and 1938, shown in table

69. These data have been given by Lorkh

(1945).

The survey information is not available to

link the progress of blight epidemics in particular years with weather conditions in the

Moscow Province. However, it seems likely

from the rainfall figures alone, that in 1935

early defoliation by blight in the area will have

caused heavy yield losses; whereas in 1938,

the potatoes will have suffered severely from

drought. How frequently a year like 1935 occurs

is not known. In the potato-growing regions to

the south of Moscow, represented by the climagram for Saratov, blight losses are estimated

as being relatively small. Apart from the low

rainfall of these regions, mean summer temperatures are high, with a large diurnal range,

and the long growing season minimizes the

risk of tuber infection at harvest. In general

it would appear that, throughout European

Russia, the problem of drought to the potato

grower is very much greater than the problem

of blight.

CONTROL MEASURES

It is characteristic of the Russian approach

to agricultural problems that in the control of

blight strong emphasis is placed onthe sprouting of seed tubers before planting. A main aim

TABLE 69.--Average rainfall for Moscow Province, 1935 and 1938

Year and average July August September Total

1935

Inches

5.6

.6

2.8

Inches

3.9

.3

2.9

Inches

4.8

1.0

2.2

Inches

14.3

1.9 1938

Lone"-term average • 7.9

114
behind this is to speed up the growth cycle of

the potato crop so as to avoid or minimize the

effects of blight attacks. The process is

usually referred to as **vernalization/* and

normally requires the provision of heated and

lighted buildings. This is a major obstacle to

the widespread adoption of vernalization, particularly when regarded in relation to the

severe Russian winter. Arnautov and Novikov

(1946) have described a method of sprouting

potatoes in open trenches, which has been successfully employed at the Research Institute

for Potato Growing at Korenevo in the Moscow

Province. The potatoes were placed in the

trenches during the second half of April. At

night and during daytime frosts, the potatoes

were covered with straw or mats. Whether

this method is at all widespread in commercial practice is not known. It obviously requires a large amount of hand labor, but this

is not likely to be a limiting factor to its

adoption in the U.S.S.R. at present. The advantages of drying tubers immediately after

harvest as a prophylactic measure in the

control of blight has been discussed by Bordukova (1947). She places considerable emphasis on the large quantities of potatoes that

could be saved by adopting this measure, and

this suggested practice indicates that a good

deal of tuber infection must occur at lifting.

The development of chemical or mechanical

methods of haulm destruction might be a valuable alternative approach to the problem. At

the present time the chief method is hand

cutting.

According to Dr. Polykov, spraying and

dusting against blight as protective measures

are not widely practiced in the U.S.S.R. The

potato crops most commonly sprayed are early

and midseason varieties grown for seed.

Bordeaux mixture is mainly used, although

some copper oxychloride is used. Dusting is

done with '*AB," a powder containing chalk,

gypsum, and approximately 36 percent copper

carbonate.

The Russians were the pioneers in sending

expeditions to Central America and South

America to collect potato species possessing

disease resistance and other valuable economic

properties for use in their breeding work. The

principle applied in this case was the

geographical method of N. I. Vavilov, then

director of the Institute of Plant Industry at

Leningrad. This method is based on the fact

that the number, range, and variety of forms of

a particular plant increase toward the locality

in which it has originated. Between 1925 to 1932,

Vavilov, Bukasov (1933), and Juzepczuk made

extensive collections of wild Solanum species

and, as early as 1931, the variety, Phytophthora-Resistant, a Solanum demissum hybrid

that is still being grown, had been produced. In

their breeding for blight resistance, the Russian

workers up to the present time have relied

mainly on S. demissumí x S. tuberosumhybrids.

To what extent they are still hopeful of tackling

blight control through breeding is not clear.

A recent account of potato breeding, including

blight resistance work in the U.S.S.R., has been

given by Hawkes (1957).

Little is known of methods used in the

U.S.S.R. for the forecasting of blight and the

issue of blight warnings to growers. Reference,

however, should be made to a paper by Naumova (1935), in which the author states that

field observations and controlled experiments

carried out during 1933 and 1934 in the Leningrad area confirmed the rules of Van Everdingen (1926) relating to the appearance of

blight epidemics. Further experiments were

conducted by Naumova to ascertain the effect

of temperature on the incubation period of the

blight fungus. These experiments took into

account diurnal variation of temperature. The

results were used to construct a nomogram

from which the incubation period could be obtained for various minimum, mean, and maximum temperatures during the days following

infection.

SUMMARY: U.S.S.R.

Potato blight is most serious in those parts

of European Russia where summer rainfall is

highest. Even in these areas, however, drought

is a more serious actual and potential menace

to the potato crop. There is an almost complete

lack of evidence on which to base reliable loss

estimates, but the shortness of the growing

season for potatoes in some areas has made

tuber infection a serious problem. In control,

emphasis has been on an ** es cape*' method

(vernalization) as opposed to chemical methods. The use of chemicals is largely confined

to the valuable seed crop. The Russians were

the first to realize the potentialities of wild

Solanum species in breeding for blight resistance.

CYPRUS

Potato growing in Cyprus exemplifies conditions under a Mediterranean climate. This is

characterized by a summer drought period

with high temperatures. The pattern of potato

growing inthe countries bordering the northern,

southern, and eastern shores of the Mediterranean, whichinclude Greece, Israel, Egypt,

Morocco, parts of Spain, and southern Italy, is

a very similar one. Two crops of potatoes are

grown in the year, both under irrigation. The

first crop is grown during the spring and early

summer months; the second during the late

summer, autumn, and winter. The period during

the naiddle of the summer is unsuitable for

115
potatoes owing to the high temperatures, and

in most of the countries, particularly in the

north of the area, there is a period in the

winter when conditions are too cold for potatoes.

Cyprus is the third largest island in the

Mediterranean Sea and has an area of 3,572

square miles (fig. 57). About 13,000 acres of

potatoes are grown each year, and the greater

part of this crop (about 60 percent) is exported,

CYPRUS

Famaqusfa

FIGURE 57.--Cyprus, in relation to eastern Mediterranean,

principally to the United Kingdom. The average

yield per acre is approximately 4 tons (F.A.O.,

1955). The most important potato-growing

areas are in the plains (up to 750 ft.) in the

Famagusta and Nicosia districts, where there

is a light red soil (terra rossa). Figure 58

shows the two cropping periods during the

year, and these are of about equal importance.

The first crop is planted in February andlifted

principally in May and June; this is the spring

or summer crop. The second--the winter

crop--is planted in August and matures about

the end of November. Larger yields are generally obtained from the spring crop, as more

irrigation water is available at this time. About

50 percent of the spring crop is planted with

certified Irish seed, whereas the winter

crop is planted with once-grown seed

from the spring crop of the same year.

Dormancy troubles do not appear to arise at

this time, no doubt due to the high temperaRAINFALL

Inches

SPRING CROP

S O

WINTER CROP

FIGUEIE 58.-Long-term average nionthly rainfaU and mean temperatures in relation to potato cropping at Famagusta, Cyprus.

tures. Only two potato varieties are grown

commercially: Up-to-Date and Arran Banner,

of which Up-to-Date is the more important.

Both these varieties are very susceptible to

blight. Accounts of potato growing inthe island

are available (Cyprus, Department of Agriculture, 1939; and Littlejohn, 1947). A feature

of farming in Cyprus is the small size and

fragmentation of the holdings. According to

Allan (1956), the 1946 census of the island

showed that the average farm consisted of 13

plots of an average size of 1.4 acres. These

plots are often widely scattered and are commonly many miles apart. This is a great

hindrance to improved farming methods, including blight control measures.

IMPORTANCE OF POTATO BLIGHT

Nattrass (1937) records that blight was

first found in Cyprus on a potato crop near

Famagusta in April 1931. The disease was

believed to have been introduced in Irish seed.

G. P. Georghiou has informed us that blight

appears more frequently and is usually more

severe in the main potato-growing areas in

the southeast of the island. He says that it

is worse on the spring crop of potatoes. Over

the 7-year period 1950-56, blight was recorded as **severe'* on the spring crops in

1952, 1955, and 1956, and on the winter crop

in 1951. Heavy yield losses sometimes occur

in the spring crop, which may be reduced by

up to 25 percent. In blight years the disease

usually appears when the plants are about 8

inches high, and there may be 75 to 90 percent defoliation about 1 month before the

normal lifting time. Tuber infection is not

much of a problem and is believed to occur

mainly at lifting. An analysis of recent severe

blight years in relation to average annual

potato yields in Cyprus appears to show up

a connection between low yields and blight

years (table 70). Without further analysis,

however, any such relationship should be

116
TABLE 70. --Average yield and blight

occurrence in Cyprus, 1950-56

Year

1950.

1951.

1952.

1953.

195^.

1955.

1956.

Average yield

Tons/acre

4.1

3.6

3.5

5.4

5.4

4.3

3.7

Blight

Severe^ winter.

Severe, spring.

Severe, spring.

Severe, spring.

viewed with extreme caution. Both potato

crops in Cyprus are grown under irrigation,

and the amount of irrigation water available

is likely to have a far greater effect on yield

than the presence or absence of blight. In addition, time of lifting has a pronounced effect

on yield, and in Cyprus this is very much

under the influence of a varying export demand for potatoes.

A study of the long-term average weather

data in relation to the growing periods of the

two potato crops at Famagusta (fig. 58) explains the greater importance of blight in

the spring crop. Temperature appears to be

an important controlling factor. In the spring

crop, temperatures are favorable to the disease until about the end of April; after this,

conditions begin to get too hot. Thus premature defoliation is the main problem, with

tuber infection of little importance. In the

winter crop, temperatures are generally too

high for blight until the last month of growth

(November). Chambers (1952) records that

in 1951 an attack of late blight occurred at

the end of the autumn growing season and

caused tuber infections, which resulted in

severe losses.

No work has been done on the humidity

conditions associated with blight attacks on

the spring crop. There is no evidence that

the channel-type irrigation practiced has any

effect on blight. Table 71 shows blight incidence

in relation to spring rainfall at Famagusta

during 1950-55. The figures have been obtained

from the island's Annual Reports, which unfortunately do not include temperature data.

In 1952 and 1955, the blight years, March

rainfall was about twice the average for that

month, but in 1950, when blight was of little

or no importance, March rainfall was even

higher. The high rainfall in April 1953 did

not lead to severe blight. On the basis of

table 71, there does not appear to be any

very close connection between total rainfall

during the spring and blight attacks for this

important potato growing center in Cyprus.

TABLE 71.--Monthly spring rainfall and occurrence of blight at Famagusta, Cyprus, 1950-55

Year

Rainfall

Blight

January February March April

1950

In.

2.4

3.7

4.4

1.8

1.3

.7

In.

0.3

3.2

3.1

1.6

2.2

.2

In.

3.0

.4

2.7

1.8

1.3

2.7

In.

0.5

1.9

.2

3.3

1.0

.3

1951

1952 Severe.

1953

1954

1955 Severe.

Lonß^-term average 3.8 2.6 1.3 .8

G. P. Georghiou has told us that control

measures against blight are rather limited.

Copper and dithane fungicides are used, but

only by about 5 percent of the commercial

growers* Applications are made by knapsack

sprayers. In the spring potato crop, control

appears to resolve itself mainly into protecting the crop over a relatively limited

period, after which temperature conditions

are likely to become unfavorable to the fungus.

The problem in the winter crop seems to be

tuber infection at lifting, which could be prevented by removal of the potato haulms before

harvest. The growing of varieties showing a

high degree of field resistance instead of

the blight-susceptible Up-to-Date and Arran

Banner might go a long way toward control.

However, finding the right variety is not easy.

Commenting on the performance of Majestic

(the principal maincrop variety in England

and Wales) in variety trials held in Cyprus,

Littlejohn (1947) says, **Majestic, which gave

consistently high yields, always developed a

serious fault in that the tubers cracked badly

and tended to deteriorate very rapidly in

storage."

117
NORTH AMERICA

UNITED STATES OF AMERICA

The potato acreage in the United States was

just short of 1^ million acres in 1953. This is

less than half the area that was grown in 1930,

Although there has been this marked decline in

the acreage, total potato production has remained at about the same level, owing to

increased efficiency in cultural practices.

Production has not kept pace with increasing

population, and a marked feature of the American potato industry is the falling demand for

potatoes in competition with other vegetables.

At the present time the total production of

f»otatoes is between 350 to 400 million bushels

approximately 10 million tons) per annum,

and consumption is around 345 million bushels.

Demand for potatoes is relatively inelastic,

and almost all the crop is sold for human consumption. According to Smith (1956), the annual consumption of potatoes per head in the

United States is about 104 lb.

The average yield for the whole country

over the period 1942-51 is given as 190 bushels

(5.1 tons) per acre. (1 U. S. bushel of potatoes

weighs approximately 60 lb.). There are considerable differences between the average

yields for the different States; the highest

being around 360 bushels (10 tons) per acre

in Maine (United States Department of Agriculture, 1954).

POTATO DISTRIBUTION

Figure 59 shows the distribution of potato

growing in the United States. A relatively few

States produce the bulk of the crop, and of

these, the most important, in order, are Maine,

Idaho, California, and New York. The greater

part of the early crop comes from the Southern

States and California. Some certified seed

potatoes are produced in over half the States,

but the chief seed-growing States are Maine,

North Dakota, and Minnesota. Maine is by far

the most important of these; half the total

U. S. seed production comes from this State

alone. The acreages of potatoes grownin some

of the leading States in 1953 are shown in

table 72.

Each dor = lOOO acres

HGURE 59.--Distribution of potato growing in the U. S. A,, 1953. (Based on Neg. U. S. D. A. 46360 and U. S. D.A. Agricultural Statistics, 1954.)

118
TABLE 72.—Acreages of potatoes in leading

States, U.S.A., 1953

State

Lame

Idaho

California

New York

North Dakota...

'vlinnesota

Pennsylvania...

Wisconsin

ivächigan

Colorado

North Carolina.

Florida

Potatoes

1,000 acres

156

153

124

106

94

78

62

61

53

54

46

42

Despite the very large area over which

potatoes are grown, production within States

tends to be concentrated in relatively small

growing centers. Examples of these are the

eastern part of Aroostook County, Maine; Long

Island in New York State; the Red River Valley

in parts of Minnesota and North Dakota; and

Kern County, Calif.

POTATO VARIETIES

American potato varieties are usually classified as early or late. The early varieties,

such as Irish Cobbler, correspond to the longday types of temperate regions; the late varieties are day-neutral or short-day types.

Stevenson (1949) stated that 13 varieties (Told

and 6 new) constitute about 96 percent of the

total U. S. potato acreage. These varieties

are shown in table 73.

Since 1949, there has been a decline in the

popularity of some of the older varieties,

notably in Green Mountain. The acreage of

Red Pontiac has increased, and Kennebec, a

late, blight-re sistant variety, has become

considerably more important.

Some varieties are grown in relatively restricted areas. An example of this is Red

McClure, which is grown principally in the

San Luis Valley of Colorado. Again, there

may be a preference for white-skinned or

red-skinned varieties. Thus, in Maine, where

white-skinned varieties are favored, the acreage is made up largely of Katahdin; but in

North Dakota the predominant varieties are

the red-skinned Red Pontiac and Triumph.

A full description of American potato varieties has been given by Clark and Lombard

(1951).

SOIL AND CLIMATE

Potatoes are grown principally on mineral

soils and to some extent on muck soils (reclaimed peats) and other organic soils, such

as the prairie soils in the Red River Valley.

Climatic conditions vary considerably in the

different growing areas. In the South the

winters are relatively mild. In Florida, for

example, the main planting of potatoes takes

place in December and January. These crops

are lifted by the end of May; during the height

of the summer, temperatures are too high

for potato growing. Moving northward up the

eastern side of the United States, winters

become progressively more severe and planting of the potato crop correspondingly later.

In the Northeastern States, average winter

temperatures are well below freezing, and

planting cannot take place before the middle

of May. Summer tennpe rature s are relatively

low and rainfall is high, providing ideal growing conditions for potatoes, but the growing

season is short, as frosts start to occur from

about the middle of September onward. In the

North Central States, winters are severe, and

the growing season is about the same length

as in the Northeastern States. Summer temiperatures are considerably higher, however,

and rainfall is generally adequate. In the

Western States, lack of rainfall is the principal factor limiting potato production, and

most crops are grown under irrigation. In

parts of California, potatoes can be grown

all the year round, but in the States farther

inland low winter temperatures occur, and

it is possible to produce a summer crop

only.

An account of potato growing conditions and

methods in the major growing areas of the

United States has been given by Dykstra and

Reid (1956) for the South; by Lombard, Brown,

and Dykstra (1948) for the Northeastern and

TABLE 73. --Most important potato varieties and percentage of total U. S. potato acreage, 1949

Early varieties Percent Late varieties Percent Late varieties Percent

Irish Cobbler

Triumph

White Rose

Bed Warba

15

13

9

1

Katahdin

Rasset Burbank...

Chippevm

Green Mountain...

Sebago

28

7

7

7

3

Red McClure

Pontiac

2

2

Red Pontiac

Russet Rural

1

1

119
North Central States; and by Edmundson,

Schaal, and Landis (1951) for the Western

States. An important feature of potato growing

is the short growing season, which in most

districts does not exceed 120 days*

POTATO CULTIVATIONS

Generally speaking, potato growers in the

U. S. A. tend to be specialists, and as such

are well equipped technically and mechanically to deal with the crop. Much greater

emphasis is placed on the mechanization of

every job to do with potato growing than in

European countries. For example, the table

stock (ware) crop is universally planted by

machine •

The potato seed is cut prior to planting.

The rows are wide (36 inches), and the seed

pieces are planted closely in them (10 to

12 inches), although there is variation from

these figures, depending on variety, soil, or

climatic conditions. Almost all crops are

grown on the ridge. The wide rows enable

interrow cultivations and spraying to be carried

out with less damage to the plants. Particularly

is this so in the case of spraying; also guards

are fitted to the tractor and sprayer wheels to

lift the haulm. A wide range of potato pests,

including flea beetle, leaf hopper, and Colorado

beetle, have to be dealt with by spraying or

dusting in most potato areas. Farmers are

much more aware of the need for spraying

and spray very much more frequently than in

Europe.

Large storage houses, very often built partially into the ground, are used over a great

part of the United States to keep the potatoes

during the winter months.

POTATO BLIGHT: REGIONS CHOSEN

FOR STUDY

A great deal of work on many aspects of

the epidemiology and control of potato blight

has been carried out in the United State s under

a wide range of growing conditions for the

potato crop. An intensive study has been made

of the effects of the disease in four widely

separated potato growing areas, among which

there are marked variations in the incidence

of blight and in the problems that the disease

poses. The areas chosen were, Aroostook

County, Me; Hastings Section, Fla.; Red River

Valley, Minn, and N. Dak.; and South Fork,

Long Island, N. Y.

Following the discussion on these particular

areas, an account is given for the UnitedStates

as a whole. The location of places named in

the text is shown on the key map (fig. 60).

U.S.A.

FIGURE 60.—Key map of United States of America.

120
Aroostook County, Maine

According to Simpson and Shands (1949), 12

to 15 percent of the total potato production of

the United States is grown in Maine, and of

this 85 to 90 percent comes from Aroostook

County. The farming land in Aroostook County

lies in a narrow belt along the eastern and

northeastern part of the State, adjoining the

Canadian border. For the past 30 years,

Aroostook has led the United States in potato

production. About 130,000 acres of potatoes

are grown annually, producing average yields

of around 10 tons per acre.

Aroostook is principally a late ware-producing area, and only a few early varieties

(5 to 10 percent) are grown, some of these for

seed. About 25 percent of the total acreage is

planted for certified seedproduction. Schrumpf

(1953) states that the chief variety grown is

Katahdin, which over the last 15 years has

largely replaced Green Mountain. Katahdin

shows some resistance to blight, whereas

Green Mountain is highly susceptible, both

in the foliage and the tubers. The variety

Kennebec, which has the Ri gene for blight

resistance, is also widely grown in the area.

The potato soils are loams. Of the cultivated land 60 percent is Caribou loam, which

contains a high percentage of sand. Potatoes

are also grown on Washburn loam, which is

a heavier soil and less well drained.

Climatic data for Caribou in central Aroostook are shown in figure 61, which may be

taken as representative of the whole area.

Annual rainfall is between 30 to 35 inches.

Summer temperatures are cool and rain is

usually abundant and well distributed, naaking

for ideal potato-growing conditions. Winter

temperatures are well below freezing. The

growing season is about 120 days. Planting

takes place from mid-May to early June, and

lifting from the second week in September to

the second week of October. After the middle

of September, killing frosts are likely to occur.

pAroosfook County

CARIBOU

RAINFALL

Inches

RAINFALL

inches

FIGURE 61.--Monthly rainfall and temperatures for Aroostook County

Maine, U. S. A,: Above, long-term averages; below, ''blight*' and

**no blight" years,

and from the second week of October, there

is always the likelihood of severe damage

from freezing to tubers left in the ground.

Thus there is an annual race against time

at harvest, and end-of-season growth of many

crops has to be sacrificed to insure successful harvesting of the whole acreage.

Frequency of Blight Epidemics

Hyre and Bonde (1955) have described the

severity of blight epidemics in Aroostook

County over the period 1943-55. Their descriptions are set out in table 74, together

with the average yields of potatoes in Maine

from 1943 to 1953.

TABLE 74. --Severity of blight epidemics and average yields of potatoes

in Aroostook County, Maine, 1943-55

Severe Moderate Slight

Year Yield Year Yield Year Yield

1943

1945

1947

1950

1951

1954

Bu./acre

355

261

350

4Ö0

445

1948

Bu./acre

3S5 1944

1946

1949

1952

1953

1955

Bii./acre

268

358

465

360

370

121
These blight assessments have been based

on the results of specific surveys plus general

observation. They refer to the total effect of

the disease in a particular season and not to

defoliation alone.

During the period 1943-53, potato acreages

have fallen, and there has been a general rise

in yields due to improved cultural practices.

The average yield over the years in this

period, when blight has been described as

*'slight" has been 359 bushels per acre, and

in the 5 years when the disease has been

*'severe," the average yield has been 361

bushels. Thus, for all practical purposes, the

yield of potatoes has been the same in the two

sets of years.

Blight on Haulm

Blight seldom occurs to any extent in Maine

before the end of July. The months in which

the first recorded field outbreaks have been

made in Aroostook County from 1943-55 are

shown in table 75. This has been compiled

from material appearing in the Plant Disease

Reporter and supplied to us by R. Bonde. The

years in which the disease has been assessed

as *'severe" are indicated.

TABLE 75. --Time when blight was first recorded and years when severe,

Aroostook County, Maine, 1943-55

July

outbreaks

August

outbreaks

September

outbreaks Blight

generally

Year Severe Year Severe Year Severe

absent

1944

1945

1954

X

X

1943

1946

1950

1951

X

X

X

1947

1949

X 1948

1952

1953

1955

In 7 years out of 13 the disease was either

generally absent or its effect was slight. In 2

years (1943, 1947), when the disease was described as ''severe," Cassell (1944a) and

Bonde and Schultz (1949) have said that the

main damage was caused by tuber infection.

In 4 years out of 13 (1945, 1950, 1951, 1954)

there were fairly heavy losses due to premature defoliation. In the same years tuber

blight infection was also heavy. The 2 years

(1945, 1954) that damage to potato tops by

blight was the worst were both years of July

outbreaks. An early outbreak date, however,

is not necessarily followed by severe defoliation losses, and this is shown in 1944.

In that year, a field outbreak was recorded

on July 8, but according to Cassell (1944b) it

did not develop, owing to the onset of hot dry

weather in early August.

Very full information on the amount of defoliation during the 1954 season is available

from a survey carried out in late August and

early September 1954 and reported by Bonde

(1955b). Defoliation in this year was probably

the heaviest of any year in the 1943-55 period.

The results of the survey of the whole potato

area are summarized in table 76.

Generally, the fungus appears relatively

late in Aroostook, and rarely causes complete

defoliation of potato plants on a large scale.

TABLE 76.--Amount of defoliation and percentage of total fields (2,160) affected,

Aroostook County, Maine 1954

Amount of defoliation

(percent)

Percentage of

total fields

0

Percent

39 9

< 1-5 17 0

5-25 18 8

25-75 10.5

75-100 13.8

This is intimately bound up with the very

intensive spray program against the disease,

which is followed by practically all growers

in the area.

Blight Losses Due to Premature Defoliation

The growing season is short (120 days),

and the growth curve (fig. 62), adapted from

Akeley, Stevenson, and Cunningham (1955),

of potatoes in Aroostook County is correspondingly steeper than in areas where potatoes

have longer to grow. These workers have

122
summarized the results of 3 years of vinekilling trials at Aroostook Farm, Presque

Isle, held between 1950 and 1952, in which

they have given details of potato yield increases during the later stages of growth.

An extract from these results shows that in

the 4 late varieties (Sebaso, Katahdin, Kennebec, and Green Mountain), approximately 45

percent of the final yield was formed after

mid-August, and approximately 15 percent

after early September.

U.S.A.

ÜO

80

60 /

40

/

20 1 „ ..

When these figures are related to the growth

curve of potatoes in the area, they indicate no

yield losses due to premature defoliation on

the sprayed plots. In 3 years out of 6 (1952,

1953, 1955), losses on the unsprayed plots

due to premature defoliation will have been

negligible. In 1950 and 1951, there will have

been about a 15 percent yield loss, and in 1954,

almost a 40 percent loss on the unsprayed

plots. It may be concluded that premature

defoliation caused by blight at Aroostook Farm,

which may be taken as representative of the

whole area, is not a serious problem on the

sprayed crop, but blight will cause heavy

losses in unsprayed crops very frequently.

In the area as a whole, the only data on

which an approximate yield loss due to prennature defoliation can be based are the survey

figures for the bad blight year of 1954, already

referred to in table 76. This survey tookplace

over a period of 2 weeks, August 26 to Septennber 10, and during this time the blight

position in individual fields will have changed

very radically. If it is assumed, however,

that very much the sanne state of affairs would

have been revealed if the whole survey had

been carried out on one day, Septennber 2,

which is the mean of the two end-dates, the

following loss from blight would occur:

lO 20 30 9 19 29

AUGUST I SEPTEMBER

FIGURE 62.—Mean rate of bulking curve for late varieties at Aroostook Farm, Presque Isle, Maine, 1950-52.

Figures have been supplied by Dr. Bonde

that give the dates at which the 75-90 percent

blight defoliation stage was reached on the

unsprayed potato plots at Aroostook Farm

over the period 1950-55 (table 77).

TABLE 77. —Percentage defoliation due to blight on unsprayed and sprayed potato plots,

Aroostook Farm, Maine, 1950-55

Percentage of

total number

of crops

Defoliation of

haulm by blight

on Sept. 2

(percent)

Yield loss

(percent)

9

9

25

57

90 - 100

50 - 90

5 - 50

0 - 5

30

15

10

0

Year Date of 75-90 percent defoliation

on unsprayed plots

Defoliation on sprayed plots

at lifting

1950

1951

1952

1953

195^

1955

Aug. 31-Sept. 2

Sept. 3

Sept. 8^

Sept. 10^

Aug. 20

Sept. 14

Percent

(^)

1 1/2

5-10

^ Trace.

^ Only 20 percent defoliation at lifting.

Over the whole acreage, this works out as

an average loss of yield, due to premature

defoliation, of 6.5 percent. Since this is an

approximation, it would be better to state it

as a loss of 5 to 10 percent, with a reasonable

certainty that average losses did not exceed

10 percent.

The effect of premature defoliation, in addition to lowering yield, is to reduce the percentage of salable tubers in the sample. This

is particularly so, if defoliation occurs very

early. Stevenson, Akeley, and Webb (1955)

give figures for a trial with artificial inoculation held in 1954 at Aroostook Farm. An extract from their results is shown in table 78.

A crop, maturing normally, produces about

95 percent United States No. 1 tubers. In a

highly competitive market, such as that in

123
TABLE 78. --Date when 90 to 95 percent defoliation from blight was reached in a potato trial

and yield of U. S. No. 1 tubers, Aroostook Farm, Maine, 1954

Variety Date when 90-95 percent

defoliation reached

U.S. No. 1 grade tubers

in sample

/ Aug. 13

Percent

35

Green Mountain \ Aug. 20

/ Aug. 13

\ Aug. 20 Katahdin ,

69

51

Ö0

the United States, it is usually only the No. 1

tubers that the farmer is able to sell. Early

defoliation also reduces the percentage of

solids in the tubers. This, however, is not

a simple cause-and-effect relationship, as

the percentage of solids is also intimately

bound up with seasonal weather effects.

Blight in Tubers

The largest losses in Aroostook County

from blight appear to be the result of rot

following tuber infection with the fungus.

Cassel (1944a) estimated an overall loss of

10 percent of the 1943 crop in storage due

to blight. Large losses from the same cause

were observed in the 1944 crop by Cassel

(1944c) and in the 1945 and 1947 crops by

Bonde and Schultz (1949). Bonde and Robinson

(1950) reported that considerable loss from

blight tuber rot occurred in some fields in

Aroostook in 1949.

Tuber infection may take place in the soil

before lifting, and this occurs to a greater

extent in the Washburn loam soils, which are

usually less well drained than in the Caribou

loam. Bonde (1955a) reported that blight infection in Washburn loam soils may be accentuated by primary infection of the tubers

with powdery scab, Spongospora subterránea.

According to Bonde, there is generally less

than 1 percent tuber blight at lifting time in

the unsprayed plots at Aroostook Farm, which

is on Caribou loam. Cassel (1944c) attributed

some of the loss in the 1944 crop to the heavy

rain preceding and during a hurricane of September 14, which washed fungus spores into

the soil and into contact with the tubers.

However, Bonde and Schultz believe that

most of the rot found in Aroostook is from

infection that occurs while the crop is being

harvested. The combination of a short growing season and a generally late attack of

blight makes the Aroostook farmer particularly vulnerable to this. The advent of haulm

killers during recent years has not entirely

removed the danger, as heavy rains over the

short harvest period may wash off the materials and thus to some extent nullify their

effect.

124

Blight and Weather Data

The mean air tennperature and rainfall

figures for Caribou in Central Aroostook,

together with the approximate planting and

lifting periods of potatoes in Aroostook County

are shown in figure 61. The low winter temI>eratures insure that in most years '*groundkeepers" are killed, and therefore cannot

act as a possible source of blight inoculum.

Occasionally, however, heavy falls of snow

before severe weather conditions set in enable

some **self-sets" or **groundkeepers" to survive. Sumnner temperatures are cool. Neither

low nor high temperatures during the growing

season are factors likely to hinder the development of blight. Hyre and Bonde (1955),

in a survey of weather data for a series of

nonblight and blight years over the period

1902-53, concluded that at no time did high

temperatures limit the effect of blight. Sevenday mean temperatures were below 75 ^F.

throughout the years.

Although there is a broad correlation between rainfall and blight epidemics in the

area (e.g., 1954, a wet year, was a bad blight

year; and 1953, a dry year, was generally

free from blight), many other factors, including source of inoculum, need to be considered. In 1956, rainfall in June, July, and

early August was high, but blight appeared

late in the area, and this was ascribed to

lack of inoculum following a year (1955)

largely free from blight. Hyre and Bonde

in drawing a **critical rainfall line" took a

cumulated rainfall of 11.25 inches over the

period June 2 to September 15 as the base

above which blight was likely to be severe.

The rainfall and temperature figures for 1954

(a blight year) and for 1953 (a no-blight year)

are compared in figure 61.

It is impossible to study true weather-blight

relationships in this area, because, apart from

unsprayed control plots at Aroostook Farm,

all potatoes are sprayed. Only in years when

rainfall is so heavy that growers cannot get

through their crops with sprayers is a severe

blight epidemic likely to develop. Such a state

of affairs occurred to some extent in 1954,

when there were 15 rainy days in June, 22 in

July, and 19 in August.
Control Measures

It has been considered for some time that

potato cull (or dump) piles are a very important source of primary blight infection in

Aroostook County. Bonde and Schultz (1943),

as the result of extensive work carried out

between 1935 and 1942, concluded that "the

cull pile is the chief source of ... infection.'*

These cull piles occur beside potato storage

houses and merchants' warehouses. Over the

last 10 to 15 years an intensive program has

been conducted to get farmers to clear them

up, but a survey carried out by Webb and

Bonde (1956) shows that this has not been

entirely successful. One of the chief methods

of dealing with the cull pile is to spray it with

a haulm killer early in the growing season.

Some form of protective spraying against

blight is followed by all growers in the area

every year. The first sprays are put on in

early- to mid-July (before the crops start to

meet in the drill), and spraying is continued

at weekly to fortnightly Intervals, depending

on weather, up to lifting tinne. On ave rage, 6 to

7 protective sprays are put on, but under prolonged conditions of blight weather, up to 12

sprays are given.

The use of organic fungicides (nabam plus

zinc sulfate, for instance) is universal. There

are two nnain reasons for this: (1) They are

cheaper than the copper spray materials, and

as practically all growers do their own spraying, the cost of materials is one of the principal items in the total cost; and (2) there are

no phytotoxic effects from the use of organic

fungicides. The effect of copper in a **no

blight" year can depress yield as a result of

scorch injury. In a spraying trial reported

by Stevenson, Akeley, and Webb ( 1955), carried

out at Aroostook Farm in 1953, the yield of

U.S. No. 1 tubers for 6 varieties was depressed

an average of 8 percent by spraying with basic

copper.

The main disadvantage of using organic

fungicides lies in their poorer adherence to

the potato foliage; as a consequence they

need to be applied more frequently than the

copper sprays.

In general, potato crops are sprayed from

the top only; droplances are not used to any

extent. Since 1950 the use of low-volume

spraying has very much increased. Peikert

and Bonde (1954) have reviewed trials with

low-volume machines carried out during the

seasons, 1950-53. The normal rate of application is about 40 U. S. gallons per acre.

At the present time approximately 7 0 percent

of the farmers in Aroostook County are using

low-volume spraying. About half the antiblight

sprays are combined with treatments, principally DDT, against pests. The effect of these

antipest sprays is to prolong the growth of the

potato tops.

A spray warning service for Aroostook

County was started in 1931, and reviews of

possible methods on which to base a forecasting service for potato blight in the area

have been made by Hyre and Bonde (1955) and

Hyre (1955). It is now proposed to introduce a

forecasting service to growers based on moving graphs derived from rainfall and temperature data. A certain amount of circumspection

has to be followed in issuing spray warnings

and blight forecasts, as there is always the

danger of frightening off the buyers of the

seed crop, which forms a very important

part of the total potato acreage.

It was at first thought that the growing of

blight-resistant varieties would provide an

answer to the problem of the disease in this

area. The results of promising trials with the

variety Kennebec in 1949 and 1950 have been

published by Akeley, Lombard, and Stevenson

(1952). It was soon found, however, that specialized races of the blight fungus were present and that these were capable of attacking

Kennebec and other blight-resistant varieties.

The presence of 11 races of blight in Aroostook County has recently been shown by Webb

and Bonde (1956). Kennebec, which is widely

grown now receives a series of protective

sprays, similar to that givento other varieties.

Potato haulm killing, to protect the tubers

at lifting, is now general. Farmers start

killing the haulm, beginning with the seed

crops, in the first week of September. Some

growers still rely on frosts to kill the haulm,

while others use roto-beaters. But the tops

are chiefly killed chemically with sodium

arsenite, and little trouble from vascular

discoloration of the tubers follows the use

of this nnaterial. Some crops are killed me>-

chanically by roto-beating.

Summary: Aroostook County

Blight does not cause heavy yield losses

due to premature defoliation of the potato

plants, except occasionally, as in 1954. Even

in 1954, little or no blight occurred in the

majority of fields toward the end of the growing season; where large losses occurred,

they fell on the individual grower rather than

on the area as a whole. That this is the general

state of affairs in severe blight years is

illustrated by a comparison of average yields in

severe blight years (361 bushels per acre)

with yields in slight years (359 bushels).

Tuber infection in the soil is not likely to be

heavy, except on some of the poorly drained

fields. Tuber infection at lifting time, however, has in the past been a serious problem,

due to the short growing season and short

harvest period. The use of vine killers should

now greatly reduce this source of loss.

The universal spray program with organic

fungicides has an undoubted effect in reducing

125
yield losses as a result of early defoliation.

But the fact that severe blight years only

occur once in 2 years and the results from

the unsprayed plots at Aroostook Farm must

lead one to question the need for such an intensive program. One of the aims behind

spraying has been to keep the foliage as completely as possible free from blight, to prevent

tuber infection at lifting. As a result, heavy

tuber losses would not occur when potatoes

were lifted. Protective spraying continues up

to lifting, but the need for the last one or two

sprays has now gone. At the present time,

the spray program does not appear to have

adjusted itself to the changed conditions brought

about by the use of baulnn-killing methods.

Hastings Section, Fla.

The Hastings section, situated in northeastern Florida, is the oldest and largest

potato-growing area in the State. In 1952,

potato plantings totaled about 16,000 acres.

Yields vary considerably from year to year,

but on average, are about 190 bushels (5 tons)

per acre. *'New crop" (or early) potatoes are

produced, chiefly for sale outside Florida,

from the end of March to early June. During

this tinne, the crop has to meet connpetition

from the stored potatoes of the Northeastern

and Midwestern States and the early varieties

coming from other Southern States. The bulk

of the crop is planted with seed from the

Northern and Eastern States. The variety

Sebago accounts for 95 percent of the acreage.

Details of potato growing in the area have

been given by Brooke and Spurlock (1950)

and Eddins, McCulbin, Ruprecht, and Stevenson (1950).

The soil is principally Bayboro-Bladen fine

sand and fine sandy loam, and many of the fields

are rather poorly drained, s o that waterlogging

of the soil is not uncommon. Drought conditions

are also likely to occur during the growing

season; most farms have artesian wells, which

can be used for irrigation. The climate is subtropical, and potatoes are grown under shortday conditions. There are considerable differences in rainfall from year to year. Thus,

in March 1945 the rainfall was 0.23 inches,

and in March 1948, 9.47 inches; the April

rainfall in 1942 was 0.28 inches, and in April

1947, 5,97 inches. Very heavy rainstorms

may occur: In 1944, 3 inches of rainfell within

2 hours on April 3. The crop is also liable to

frost damage, but very high temperatures do

not usually occur, except occasionally at lifting time when there may be sunscald of the

tubers. Average rainfall and temperature

figures for the area are set out in figure 63.

The growing season is 90 to 100 days, and

crops are dug before they are mature. Planting takes place from December 20 to February

1, and lifting from April 1 to May 15.

U.S.A.

RAINFALL

Inches

.Hostings Secfion

JACKSONVILLE

RAINFALL f,

Inches

FMAMJ J ASOND

. BLICHT YEAR 1944

RAINFALL ^

Inches

NO BLIGHT YEAR 1951

80

- 70°

60

50

Jan F M A M

.Red River Valley

CRAND FORKS

J F M A M

RAINFALL 4

Inches

FMAMJ JASON D^

BLICHT YEAR 1948

Á-nrYZ~/

VÄ'A/MA^

May J J A s

NO BLIGHT YEAR 1955

70V y^^ -

60

I//J^

\-

-'//,A

V/ V/,

-//.V.'//.V/ i^%^^

70 F

60

50

M J J A S

FIGURE 63.--Monthly rainfall and temperatures for Hastings section,

Fla., and Red River Valley, Minn, and N. Dak.: Above, long-term

averages; below. **blight" and "no blight** years.

Blight Epidemics: Frequency and Losses

A.. H. Eddins has summarized the damage

caused by blight, and the dates of the first

126
field outbreaks of the disease, over the period

1931-55. These data are set out in table 79.

**Damage*' in this table refers to the estimated reduction in yield due to premature

defoliation plus loss from tuber infection.

The various categories of damage, together

with their equivalent in percentage loss of

crop and the number of years out of 25 (1931-

TABLE 79. --First field outbreaks of blight and amount of damage to potatoes, Hastines section,

Fla., 1931-55

Year

1931.

1932.

1933.

1934.

1935.

1936.

1937.

1938.

1939.

1940,

194-1.

1942,

1943,

First field report

Apr....,

Jan. 28,

Feb. 5,

Apr. 6,

Mar. 15.

Jan. 2,

Mar. 1,

Mar. 8,

Apr. 15,

Mar. 15,

Apr. 1,

May....,

Damage

Moderate.

Do.

Severe.

Do.

None.

Moderate.

Severe.

Moderate.

Do.

Little.

Moderate.

Do.

Little.

Year

1944.

1945.

1946,

1947.

1948,

1949,

1950,

1951.

1952.

1953.

1954,

1955,

First field report

Mar. 1.

None...,

Mar. 13.

Jan. 25.

Feb. 25.

Feb. 15.

Apr. 20,

None....

Apr. 16.

Mar. 20,

Apr. 9.

May 1.

Damage

Severe.

None.

Moderate.

Do.

Do.

Do.

Little.

None.

Moderate.

Do.

None.

Do.

55 period) that each has been recorded are as

given in table 80.

Estinnated losses caused by blight are seen

to be small. In summarizing losses in potato

crops in Hastings from 1932 to 1943, Eddins

(1943) estimated an average annual loss of 4.4

percent, and since that time losses have

tended to decrease.

There is no point in comparing potato yields

in this district for *'blight" and **no blight'*

years. Large fluctuations in yield occur from

year to year, due to drought, flooding, and

frost damage. Economic considerations play

a big part in deciding when potato crops are

lifted in an early district, and the time of

harvesting will in turn have a marked effect

on yield.

Blight on Haulm

Blight may attack the crop throughout the

growing period. A severe attack, which started

soon after planting in 1937, has been described

by Eddins and Clayton (1943). The attack was

TABLE 80. --Rating for '^damage*' as related

to loss of crop and number of years each

type of damage occurred, Hastings section,

Fla., 1931-55

in part due to a high percentage of blighted

tubers in the seed, which had come from the

Eastern States.

Very early field outbreaks are the exception, however, and the disease is usually first

noted in March or April. This will be seen

from the summary set out in table 81, where

outbreak dates have been arranged, by nnonth,

for the period 1931-55.

TABLE 81. --Months in which outbreak

occurred and frequency during the period

1931-55, Hastings section, Fla.

Outbreak occurring in—

January

Febioiary

March

April

May

No blight ,

No data (1934-),

Years' outbresLk:

reported

Number

3

3

7

7

2

2

1

None....

Little..

Moderate

Severe.•

There is no general association between

the earliness of the first outbreak and the

amount of damage done to the potato haulm.

In 1947, for example, sharp frosts followed

the initial outbreak on January 25; potato tops

were killed, and the disease did not reappear

until mid-March. The course of the attack in

this year has been described by Eddins (1947).

Dr. Eddins has also supplied figures showing the amount of defoliation caused by blight

in his spray plots at Hastings over the period

1949-54 (table 82).

127
TABLE 82. --Amount of blight on haulm at lifting time, Hastings section, Fla., 1949-54

Year

Blight on unsprayed

plots at lifting

Blight on nabam sprayed

plots at lifting

1949

Percent

100

13.8

0

15.0

57.5

Percent

7.5

1950 0

1951 0

1952 (M

1953 (^)

1954 0

^ Trace.

In only 1 year (1949) out of the 6 did the

sprayed plots show a significant increase in

yield over the unsprayed, and serious defoliation is the exception. Sometimes, à prolonged

period of rain after the disease has become

established prevents farmers from taking

protective action, and then widespread severe

defoliation may occur. This happened in 1944.

Blight in Tubers

Overall losses from blight infection of

tubers are not serious. According to Dr.

Eddins, *'Tuber infection during recent years

has caused little loss, as the foliage has

been kept almost free of late blight. If rains

occur during epidemics, and the foliage is

infected, a trace to 10 percent of the tubers

may be infected." Only occasionally, as in

1944 when heavy rains exposed the tubers,

does a serious amount of infection take place

while the potatoes are still in the ground.

Infection at lifting is not usually severe. If

the potato tops are blighted at this time, the

conditions of lifting play an important part.

When the land and potato tops are wet, infection is likely to be heavy, but if the fields

are dug under dry conditions little loss should

occur from tuber blight.

The crop is washed and dried before shipment for sale. However, the effect of this on

the development of tuber blight has not been

determined. No figures have been published

on the amount of infection developing during

transit of potatotes from the Hastings section. That such development in transit can be

a serious problem in early potatoes has been

shown by Ramsey (1942), who found that consignments of Florida potatoes inspected onthe

Chicago market in January 1940 frequently

showed 5 to 10 percent tuber blight.

Blight and Weather Data

Average temperature and rainfall figures

for Jacksonville together with planting and

lifting dates for potatoes, are set out in figure

63. An account of an unusual and very early

attack by blight in 1937, given by Eddins and

Clayton (1943), has already been mentioned.

A week after planting, the disease was found

in seed pieces, and a high proportion of these

rotted in the ground. Many of the sprouts that

were produced were killed by blight before

or shortly after emerging. Stands were reduced 10 to 20 percent in most of the fields

in the Hastings area, and in individual farms

up to 90 percent of the plants were lost. The

disease continued to attack the plants after

emergence and became very destructive in

some fields. Rainfall, although not particularly

heavy in January 1937, was well spread out,

and air temperatures were abnormally high

(13*^ F. above average). In table 83 a comparison of the January figures for 1937 and

the succeeding years up to 1945 brings out

these points.

Rainfall and temperature data for 1944 (a

blight year) and 1951 (a no-blight year) are

compared in figure 63. The course taken by

blight in 1944 has been described by Eddins

(1944) and by Rhoads (1944). The season was

favorable to growth until March 21, at which

date a little blight was general through the

TABLE 83.--Number of rainy days, total

rainfall, and mean temperatures in January

at Jacksonville, Fla. , 1937-45

Year Rainy days Total

rainfall

Ufe an

Temperature

1937

1938

1939

1940

194-1

1942

1943

1944

1945

Number

12

9

5

7

11

6

4

7

7

Inches

2.60

1.17

1.59

1.61

3.18

1.30

1.62

3.16

3.93

Op.

68.1

56.5

58.2

47.4

56.2

55.1

59.4

56.7

57.6

128
fields. During the last 10 days of March,

5.43 inches of rain fell, followed by 3.16

inches in a 2-hour period on April 3. Blight

spread rapidly during this time, and by April

20 the plants in nearly all the potato fields

had been killed by the disease and by water

damage. During and after the rain, it was

impossible to get into the crops to protect

them against blight and the growing season in

most fields was reduced by 2 to 3 weeks. In

1951, the no-blight year, rainfall was below

average for each of the 5 months, January to

May.

The weather conditions associated with

severe blight losses appear to be a fairly

prolonged period of heavy rains, but this

happens only exceptionally. Eddins, Ruehle,

and Townsend (1946) have stated that blight

at Hastings **has caused maximum losses

when the rainfall has ranged between 5.56 and

8.31 inches during the last 30 to 40 days of

the growing period." Temperature conditions

are generally favorable to blight development

during the growing season, though frosts,

which may occur as late as March, have been

known to kill off the haulm and completely

stop an epidemic (as in 1947). It is possible

that the short-day conditions under which the

crop is grown, leading to heavy dew fornnation, play a part in blight development in

this area.

Control Measures

Protective measures are taken every year

by the majority of growers. According to

Brooke and Spurlock (1950), most growers

in 1949 were not equipped to spray against

blight, and were using dusts as a control.

Since then, there has been an increase in

spraying as opposed to dusting, and a change

from copper dusts to organic fungicides. In

1956, the use of nabam sprays with zinc sulfate

was general. The danger of copper dusts

causing scorch injury to potato foliage in dry

seasons with a consequent reduction in crop

yields has been pointed out by Eddins and

Clayton (1943). Spraying starts when the

plants are 6 to 10 inches high, and 3 to 6

sprays are given during the growing season.

This regular annual spray program is also a

feature of the **new crop" potato growing

areas in Florida and Alabama. According to

Dykstra and Reid (1956), control measures

are more haphazard in other Southern

State s.

Haulm killing before lifting is not carried

out. It is difficult to see how this could become

a regular practice in an early area, as the

early grower cannot afford to wait the 2 weeks

between top killing and lifting, which is necessary to prevent tuber infection. The market

for his crop may quite easily disappear while

he is waiting.

Summary: Hastings

Blight does not cause a heavy average loss.

Even in the absence of control measures (as

shown by spraying trials at Hastings), there

was a serious loss of yield due to the disease

in only 1 year in 6. It appears that farnners

in the area consider the expense of a regular

annual control program necessary to insure

themselves against occasional serious loss.

Even so, the control program broke down in

1944 under conditions of prolonged rain.

Red River Valley, Minn, and N. Dak.

The Red River, which is formed by the

juncture of the De Sioux and the Otter Tail

Rivers just North of Wahpeton, N. Dak., is

the boundary between Minnesota and North

Dakota. It flows north for 533 miles and

eventually discharges into Lake Winnipeg in

Manitoba, Canada. The width of the valley

varies considerably along this length and in

places it is 200 miles wide.

There were 78,000 acres of potatoes in

Minnesota in 1953, and 94,000 acres in North

Dakota (U.S.D.A., 1954). Separate acreage

figures are not published for the Red River

Valley, but in both States the valley is the

most important potato area. Average potato

yields are below 190 bushels (5 tons)per acre.

About 40 percent of the potatotes are grown

for seed. Red-skinned varieties are pre^ferred,

and the chief variety is Red Pontiac. The soil

is a black, organic sandy loam; the better

soils are on the land nearest the river. The

whole area is very flat and drainage is a major

problem, particularly as short, very heavy

falls of rain may occur during the growing

season. The mean annual temperature and

rainfall figures for Grand Forks, N. Dak., in

the center of the valley, are shown in figure

63. Planting takes place from the end of

April to the end of May, and lifting is in

September and early October. The full growing season is about 120 days.

Frequency of Blight Epidemics; Extent of

Losses

Over the 10-year period 1946-55, blightwas

serious in only 1 year. This was in 1948, but

even then, damage due to the disease was

confined to the northern part of the valley

and did not occur to any extent south of Grand

Forks. In a series of fungicide trials carried

out during the 6-year period, 1945-50, at 3

centers in the North Dakota half of the valley,

blight was present only in 1948, and then in

no more than trace amounts. The results of

these trials have been summarized by Wallin

and Hoyman (1954). In the Minnesota side of

the valley, Buchholtz (1948 and 1949) has

given reports of spraying trials on Bliss

Triumph in 1947 and 1948. In neither year

129
was there any significant increase in yield

as a result of spraying.

A feature of the disease in this area is

its usually late first appearance. Wallin and

Hoyman (1954) have given outbreak data for

the North Dakota part of the valley. The

earliest occurrence on record was in the

latter part of July in 1952. In 1944 and

1948, blight was reported on August 3 and 2,

respectively. During the period 1949-53, the

disease was first observed in the last 2 weeks

of August or the first weekinSeptember. First

outbreaks usually occur some little time

earlier in the Minnesota part of the valley

than in the North Dakota part.

Loss of yield through premature defoliation

is not a serious problem in the Red River

Valley. There is, however, a danger that

severe infection of the tops in individual fields

may occur under certain weather conditions.

The generally high day and the low night

temperatures during the latter part of the

growing season lead to heavy dew formation,

and if blight inoculum is present in a field

it will usually be able to survive. Hoyman

and Wallin (1957) have been working with

blight isolates from this area, which have

been able to survive in leaves at considerably

higher temperatures (over 100° F.) than those

at one time considered possible. In a field

where blight **smolders'* along in this way,

3 or 4 days of cool, cloudy weather will give

the disease the opportunity to develop rapidly.

Blight in Tubers

Tuber infection is the main problem in the

Red River Valley. Infection in the soil is not

usually important, though individual growers

may have heavy losses. Heavy losses are associated with the generally poor drainage conditions in the valley. Reporting the result of a

survey carried out at harvesttime in 1948 (the

bad blight year), Cookand Lutz ( 1950) recorded

that **tuber rot was worse in the low wet areas

than in the higher better drained parts of the

fields. In the low areas up to 100 percent of

the tubers were affected with blight, and had

rotted. Only 1 percent or less of the potatoes

were affected in the better drained areas."

These low areas, however, form only a small

proportion of the total acreage planted to

potatoes.

Infection of tubers from blighted tops at

lifting is the main source of loss. Even this is

not very important, owing to the general

absence, or negligible amount, of blight at

harvest and the generally dry conditions at

lifting.

Blight and Weather Data

Temperature and rainfall figures for the

growing season in a blight year (1948) and a

no-blight year (1955) at Grand Forks, N.Dak.,

are shown in figure 63. The progress of the

disease in 1948 has been described by Eide

and Brentzel, quoted by Miller and O'Brien

(1948). Eide (Minnesota) stated that **during

July 30-August 3 blight was found scattered

in an area about 7 5 miles long and 25 or less

wide, extending from a short distance south

of East Grand Forks to north of Kennedy. By

August 11-17, it was present practically over

the entire area. By September 1, it had killed

the vines in many fields, being especially

severe around East Grand Forks." Brentzel

(N. Dak.) noted that the weather was favorable

for the spread of blight during the end of July

and early part of August, but after that it was

unfavorable for the spread of the disease. In

1955, blight caused little or no damage. The

main difference between the 2 years was the

considerably higher rainfall in August 1948.

The somewhat higher temperatures in 1955

may have retarded the development of the

disease in this year.

The factors operating against blight in this

area are the high temperatures, low rainfall,

low humidities, and rather short growing

season. These factors also operate against

large potato yields.

Control Measures

Dusting rather than spraying is carried out

in the Red River Valley, owing to a general

shortage of water. In the Minnesota part of

the valley, dusting appears tobe more common

than in North Dakota, where at the present

time little is done. There was a marked reduction in the quantities of fungicides used in the

area following the withdrawal of price supports

for potatoes by the Federal Government after

1950.

Dusting with DDT and Toxaphene is a

routine measure against insect pests, particularly leaf hopper, in the valley. Early blight

(Alternarla solani). although present in most

years, is not considered a serious problem.

Organic dusts are most commonly usedagainst

late blight and a high proportion of the application is done by airplane.

Wallin and Hoyman (1954) consider that a

lengthy control progrann against blight is unwarranted in the North Dakota part of the

valley, and the results of spraying trials both

in North Dakota and Minnesota confirm this.

The same writers, and also Wallin, Eide,

and Thurston (1955), have discussed the value

of a blight warning service and a method of

basing this on an analysis of temperature and

humidity data. An effective warning service

for blight would be of great value in an area

like this, where the disease occurs only

occasionally. However, there are a number of

factors operating against a service: (1) The

large size of the valley and fairly considerable

variations in weather conditions within the

valley; (2) the difficulty in obtaining weather

130
data; (3) the difficulty of getting the forecasts

out quickly enough to the individual growers.

At the present time the forecasting service

obtains its weather data from instruments operated by interested individuals (plant pathologists, growers, etc.) and not from the main

Weather Bureau stations. These stations do

not give sufficiently good coverage of the area

and many do not supply humidity figures. The

problem of notifying growers is one of a small

trained personnel that has to deal with a

potato crop thinly scattered over a wide area.

One factor that operates in favor of spraying

according to a warning service as opposed to

routine spraying is that growers are already

carrying out an annual control program against

insect pests. Thus, the equipment is available

to control blight in the odd year that conditions naake it necessary. Hoynman (1947) has

discussed the value of different haulm killing

methods under Red River Valley conditions.

Most haulm destruction is now carried out by

roto-beating, and it has been found that this

does not cause vascular discoloration of the

potato tubers, unless there have been drought

conditions preceding killing.

Sumnnary: Red River Valley

Blight is not a serious problem. A routine

control program against blight does not appear

justified in the area, and many growers take

no action against the disease. A blight warning

service would be of great value in the Red

River Valley, but there are many difficulties

in the way of this. In its absence, it naight pay

growers to put on 2 to 3 applications of organic

fungicides in August as an insurance against

the odd blight year. Applications in July appear

unnecessary, in view of the generally late

appearance of blight, and September applications would almost certainly be too late to

have any beneficial effect on yield.

South Fork, Long Island, N. Y.

In 1953, 55,000 acres of potatoes were grown

on Long Island, a slightly higher acreage than

that grown in up-State New York. The average

yield was 320 bushels (8-1/2 tons) per acre.

Potato growing on Long Island can be divided

roughly into 3 areas: (1) The western area,

which is rapidly being built up and has never

been a bad area for blight; (2) the North Fork

where almost all the potatoes are grownunder

irrigation; and (3) the South Fork, where the

greater part of the acreage is grown without

irrigation. Most of the work on blight under

Long Island conditions has been carried out in

the South Fork area, which has, therefore,

been selected for more Intensive study.

In the South Fork, potatoes are produced

principally for table stock purposes. The chief

varieties grown are K^.tahdin and Green

Mountain, with some Irish Cobbler along the

coast. Seed comes from Maine and Prince

Edward Island. Soil type is a sandy loam,

which tends to be low in organic matter.

Figure 64 sets out the mean annual temperature and rainfall figures for the area. Planting extends from the end of March up to midApril, and lifting, which starts in the middle

of August, goes on through September into

October.

. Lonq Island

SOUTHAMPTON

RAINFALL 5

Inches

FIGURE 64.--Long-term average monthly rainfall and mean temperature in relation to the potato growing season for South Fork, Long

Island, N. Y.

Frequency of Blight Epidemics

The severity of blight attacks over the 10-

year period 1946-55 in the South Fork area of

Long Island has been summarized in table 84,

The first field outbreaks of blight are not

usually recorded before mid-July. Table 84

shows that there was severe blight dannage

in 1 year out of 10, and that in the remaining

9 years the disease was either absent or

caused very little damage.

Overwintering of the fungus in this area

has been investigated by Peterson (1947). He

concluded that infected seed pieces and cull

piles were the chief sources of inoculum in

the spring. Although diseased groundkeepers

were readily found in grainfields that had

produced a potato crop the year before, none

of them appeared to have been produced from

diseased tubers. It was thus not possible

to incriminate groundkeepers as inoculum

sources.

Blight in Tubers; Blight Losse f

The amount of infection with blight taking

place while the tubers are still in the ground

is usually from none to a trace. Skaptason,

Peterson, and Blodgett (1940) stated that the

high copper content of the soils in the district

was probably afactor ininhibiting the germination of the blight sporangia. The copper has

accumulated under conditions of continuous

spraying with bordeaux mixture over a period

of years, as potatoes were very often grown

on the same fields year after year. Tuber infection at lifting is not a serious problem,

owing to the longer growing season here than

in many other parts of the U.S.A. The potato

131
TABLE 84. --Blight incidence and source of data for South Fork, Long Island, N. Y. , 1946-55

Year Blight incidence Source of data

1946 Blight present but little or no

damage.

Probably worst year for blight

ever recorded in the State.

Blight present but loss practically negligible.

Blight damage slight or nil.

United States Plant Disease

1947

Survey (1946).

Miller and Wood (1947).

1948 Miller and O'Brien (1948).

1949-55 Information supplied by L. C.

Peterson and confirmed for

1951-54 by Hyve (1955).

haulm has usually been dead a week or more,

before lifting.

The only recently published estimate of

losses was given for the bad blight year of

1947. In this year Miller and Wood (1947)

quote an estimate that there was an overall

loss in New York State of 20 percent of the

potato crop. Fifteen percent of the loss was

attributed to reduced yield as a result of

premature defoliation, and 5 percent to tuber

infection.

Results have been given by Buchholtz (1948,

1949) of spraying trials against blight, carried

out at Sagaponack, South Fork, Long Island,

in 1947 and 1948. These have been set out

graphically for 1947 in figure 65. This figure

incorporates some additional material supplied by Lr. C. Peterson, showing the progress

of the blight on the tops of the treated and

the control plots. The yield of the unsprayed

plots in this year was 27 percent below that

of the plots sprayed with bordeaux, and this

figure represents approximately the loss

that blight caused in the trial. There was

no significant increase in yield from spraying

in 1948.

Blight and Weather Data

Long-term average temperature and rainfall

data for Southampton, situated in the South

Fork of Long Island, are shown in figure 64.

Despite the high summer rainfall, blight is

severe only occasionally, and one of the main

reasons for this appears tobe the high summer

temperatures. Mean monthly temperatures at

Southampton in July and August are 70^ and

69 F., respectively. However, this is not the

complete story; in 1947, the only bad blight

year during the 10-year period 1946-55, mean

U.S.A.

BLIGHT

ON

HAULM

per

cent

lOO

50

J o

PLANTED

MARCH

23

M M i M I I i i

sprayings Unsprayed

Bordeaux

May June

Tons/a

Unsprayed 7-7

Nabam+Zn 9'I

Bordeaux 10-5

July

i

Auqust ^\ FTED

SEPT

3

FIGURE 6.5.-Progress of blight on haulni and corresponding yields of sprayed and unsprayed Green Mountain variety at

Sagaponack, Long Island, N, Y.. 1947.

132
temperatures in both July and August were

2 to 3 above normal. The attack in 1947

seems to have been associated with very high

humidity conditions.

Control Measures

All commercial growers follow an intensive

spray program at South Fork. First applications are put on when the potato plants are

4 to 6 inches high. The initial sprays are

combined with an antipest treatment, usually

DDT. The most important insect pests are

the Colorado beetle, flea beetle, and potato

aphid.

Most of the spraying is now done with

organic fungicides. The organic fungicides are

not so efficient as copper when applied at the

same intervals, but there has been no serious

blight year since 1947. There may be danger

of overcompacting the sandy soil in this area

by the large number of spray treatments that

are commonly given. Some growers follow a

split spraying schedule, starting off with an

organic fungicide and finishing with copper

over the latter part of the season, when the

danger of blight is greater. Practically all the

spraying is at high-volume rates, and the

fungicides are applied from the top. If it is

necessary to kill the haulm before lifting, the

most popular method is with mechanical

beaters. Sodium arsenite and the di-nitro

compounds are used for chemical destruction.

An account of control methods against blight

has been given in a publication of the New York

State College of Agriculture (1955).

A blight warning service is operated in the

State by R. S. Dickey, Plant Pathologist of

the New York State College of Agriculture

at Cornell University, Ithaca. This service

works on the basis of first and later confirmed

reports of the disease, which are then sent

to county agents, who in turn inform the

growers. Approximately 95 percent of the

potato growers are informed of the presence

and progress of blight in this way.

Summary: South Fork, Long Island

Only in 1 year out of 10 (1946-55), has

blight caused serious damage in the area. It

is unlikely that this has been mainly due to

the spraying program followed, as experience

in this and other parts of the United States

indicate that a spray program may breakdown

under severe blight conditions, particularly

when organic fungicides are the chief materials

used. In this area, it appears that the amount

of protective spraying could be reduced.

BLIGHT EPIDEMICS IN THE U.S.A.: GENERAL

The preceding sections have shown that there

are considerable variations in the importance

of late blight in the different potato-growing

areas of the United States. The most favorable

areas for the development of the disease are

also those which provide the best conditions

for potato growing, namely a relatively cool

summer with abundant rainfall. These areas

are the New England States, we stern New York

State, and Pennsylvania--in short, the northeastern region.

In the remaining Eastern States, as far south

as North Carolina, potato blight is not so

important. However, a distinction should be

drawn here between the relatively low-lying

land stretching in from the coast and the

mountainous areas of some of these States.

In the mountainous regions of North Carolina

and West Virginia, for example, blight occurs

to some extent every year. The Southern

States, which concentrate on the production of

a spring and early sumnner crop of potatoes

are only occasionally affected to a serious

extent. The two States from which the largest

losses have been reported are Florida and

Alabama. The importance of blight also falls

off in a westward direction across the country.

The disease can be severe in parts of some

of the North Central States; e.g., Wisconsin

and Minnesota, but high summer temperatures

and relatively low rainfall linnit its development. In the arid and semiarid conditions of

the Western States, where potatoes are grown

under irrigation, blight is not a serious

problenn, although it is sometimes reported.

The first report of Phytophthora infestans

in the United States dates back to 1843, when

Stevens (1933) states that it was found near

Philadelphia, but it is of comparatively recent

occurrence in some States. Walker (1950)

writes that it was reported on potato for the

first time in Florida in 1903; in California

in 1904; in Texas in 1931; in Kansas in 1936;

and in Colorado in 1941. The first year that

blight developed so extensively as to become

an econonnic problem in the North Dakota part

of the Red River Valley was 1942, according

to Brentzel (1943).

SEVERITY OF BLIGHT EPIDEMICS

In a country so vast as the United States,

with potatoes being grown under all kinds of

varying climatic conditions, generalized statements about the severity of blight on a year-byyear basis mean very little. Within nnost States,

too, conditions differ widely, and this has a

marked effect on the development of blight.

In Minnesota, for example, which is roughly

the same size as Great Britain, blight was

severe in the Red River Valley in 1948, but

was hardly found anywhere else. In 1952 inthe

same State, there was widespread infection

with blight in south Minnesota, but only traces

of the disease could be found in the Red River

Valley.

Short accounts of blight epidemics from the

different States were included withdata on other

133
plant diseases over the period 1917-39 in

Supplements to the [U.S.] Plant Disease Reporter (Plant Disease Bulletin until 1923).

Supplements giving accounts of the severity

of blight on potatoes and tomatoes, again on a

State-by-State basis, were published for each

of the years from 1946 to 1952. These reports

show the differing importance of blight for

particular areas, which can be expressed as a

frequency of severe blight years. Thus the

frequency of severe blight years in Aroostook

County, Maine, is about 1 in 2. Cook (1949)

has summarized data for a number of States.

He gives the frequency of years in which the

disease has been important as 7 out of 20 in

Vermont; 13 out of 34 in New Jersey; 5 out

of 20 in South Carolina; and 2 out of 31 in

eastern Virginia.

SOURCES OF BLIGHT INOCULUM

It is generally agreed that the most important

sources of blight inoculum are infected seed

pieces and cull piles. In the Southern States,

practically all the potato acreage is planted

with northern-grown seed, and the degree to

which this is infected with blight will play an

important part in the development of epidemics

in the South. Peterson (1947) has investigated

the possibility of a number of weed and cultivated solanaceous plants acting as inoculum

sources of the fungus under Long Island conditions. He concluded that **none of the perennial

members of the Solanaceae found growing

wild on Long Island function as overwintering

suscepts for P« infestans.** There is no evidence that solanaceous weeds are important

in other parts of the United States. Similarly

groundkeepers have not been incriminated as

inoculumi sources even where they occur,

although over large parts of the United States

the severe winters make the survival of

groundkeepers exceptional.

Outdoor tomatoes may serve as a source of

infection to potato crops; but, in general, the

transference is the other way round. About

50 percent of the tomato transplants in the

United States are grown near Tiftonin southern

Georgia and from here they are shipped to all

parts of the north. In 1946, these plants were

infected with blight and proved a potent source

of disease inoculum to tomato and potato crops,

in what turned out to be a disastrous blight

year. Tomatoes may serve as an oversummering host for the disease in the Southern

States. In wet summers, a number of outdoor

tomato fields are abandoned by the growers

and blight surviving on these can be carried

over on to the winter potato crop. An account

of the importance of tomato blight in the

United States is included in a report on the

world distribution and present status of the

disease compiled by Miller and O'Brien ( 1955).

METHODS OF BLIGHT ASSESSMENT

In assessing the percentage of defoliation

caused by blight in fungicide trials, most

U. S. workers now use the method described

by Horsfall and Barratt (1945) and Horsfall

(1945), which is based on a probit scale. In

field survey work, however, nnany plant pathologists still prefer to describe the degree of

blight infection in words, i.e., as *'trace,"

"slight," ''moderate," or **severe." In the

summer of 1956, an opportunity was taken of

comparing this latter method of assessment

with the British method, using the B.M.S.

scale. This comparison was naade on numerous

blighted crops in eastern and central United

States. Table 85 shows the approximate equivalents.

TABLE 85. --Descriptive designation of blight

attacks compared to B. M. S. scale for blight

Designation of

blight attack

Equivalent percentage

defoliation

Trace

Slight.

Up to 0.1

0.1 - 1.0

Moderate 1-50

Severe Over 30

LOSSES FROM BLIGHT

Over the period 1917-39, estimated losses to

the potato crop caused by blight for each State

were published in Supplements to the [U.S.]

Plant Disease Bulletin and the [ U.S.I Plant

Disease Reporter. These losses were expressed as the percentage of total crop lost

per State, which was converted into a loss

of so many thousands of bushels. No attempt

was made to differentiate between the losses

due to premature defoliation and the losses

caused by tuber infection. Chester (1950),

writing about these loss estimates, which were

also given for numerous other plant diseases,

states that **the annual disease loss reports

were based on estimates of loss in the various

States made by the principal Survey Collaborators, usually with the assistance of other State

specialists. These loss estimates were sometimes little better than guesses . . . Yet they

represented the only available comprehensive

body of data on losses from plant diseases in

the United States, and they are undoubtedly

more reliable than some other published

disease loss estimates." The map (fig. 66)

shows the sort of use made of these estimates.

Since 1939, no systematic countrywide attempt has been made to estimate losses due

to blight. The survey carried out by Bonde

(1955b) on the amount of defoliation caused by

134
ñ I to 2-4%

[7\| 0.5 to 0.9%

FIGURE 66.—Percentage losses from potato blight for the period, 1920-29. U. S. A. (After Bar rus, Boyd, and Wood, 1931.)

blight in Aroostook potato crops reported

above provides the type ofinformation on which

yield loss estimates can be based. The Aroostook potato-growing area is relatively compact,

however, and it would be very much more

difficult to carry out a survey of this nature in

the Red River Valley.

The intensive study made of four potatogrowing areas in the United States leads

to the conclusion that heavy losses due to

premature defoliation by blight are the exception. Even when losses occur, the overall

picture may be a bigger potato crop in the

**blight" than in the **no-blight'* years. This

fact, however, will be of little consolation to

the individual grower who does suffer a heavy

loss in the bad blight year. The short growing

season in many of the potato areas means that

a loss of a few weeks growth has a big effect

on total yield, and also, most important in a

country where the supply of potatoes almost

always exceeds demand, that it causes a

fall-off in the percentage of salable-sized

tubers.

Tuber infection in the soil does not appear

to be a serious cause of loss, except under

badly drained soil conditions. Infection at

lifting time has been responsible for heavy

losses in the past, and this has been largely

a consequence of a short growing and lifting

season, coupled with wet lifting conditions. It

has, for example, been an important problenn

in Aroostook County (growing season 120

days), but much less important on Long Island

(growing season 150 days). The use ofinsecticides, principally DDT, tends to intensify this

problem, as potato tops remain green longer

where these are used. Efficient haulm killing

methods should largely prevent this type of

blight loss.

BLIGHT AND WEATHER DATA

Among the most innportant factors that

influence the development of epidemics in

particular districts are the weather conditions in the Z months preceding the month of

lifting the crop. Thus, in Aroostook County,

where the main harvest month is September,

the weather in July and August largely decides

the severity of the blight attack. Table 86 sets

out the average temperature and rainfall

figures for the 2 months before the lifting month

as against the frequency of severe blight year s

in 5 potato-growing areas in the United States.

These are the 4 areas that have already been

considered in detail, together with the North

Platte Valley in Nebraska, anirrigateddistrict

in one of the Western States in which severe

blight epidemics do not occur.

135
TABLE 86. --Average monthly temperatures and rainfall before lifting and frequency of blight

years in 5 U. S. areas

Average monthly

temperature--

Average monthly

rainfall— Frequency

of severe

blight

years

Area

2 months

before

lifting

1 month

before

lifting

2 months

before

lifting

1 month

before

lifting

Aroostook County, Kfeine

Hastings, Fla

°F.

64

63

70

68

71

62

69

69

66

62

Inches

4.0

2.9

3.0

3.4

2.4

Inches

3.3

2.4

3.8

2.9

1.3

Percent

50

16

South Fork, Long Island, N. Y

Red River Valley, Minn, and N. Dak..

North Platte Valley, Nebr

10

10

The frequency of severe blight years has

been expressed as a percentage. Thus, a

percentage of 50 means that 1 year out of 2

is a severe blight year. A "severe blight

year" is defined as a year in which there is

a loss of more than 10 percent in yield on

unsprayed potato crops as a result of premature defoliation.

AROOSTOOK COUNTY: The temperature

and rainfall conditions are very favorable to

blight. The diurnal range of temperature

during the potato-growing season is about

20 F. Very hot days are uncommon. Rainfall

is well distributed and relative humidities are

average.

HASTINGS: Temperatures are higher than

in Aroostook, but are not so high as to limit

seriously the developnnent of blight. However,

it should be noted that temperatures are continuing to rise here at the end of the growing

season, whereas in Aroostook they are falling.

Diurnal range of temperature is approximately

20 F. Rainfall is low and badly distributed,

and this together with the fact that it is an

early crop being grown in the area are

probably the chief factors limiting blight

losses.

SOUTH FORK, LONG ISLAND: High temperatures limit blight development. Rainfall

is higher than in Aroostook, and relative

humidities are also high. Conditions here are

similar to those in the potato-growing region

of eastern Virginia, where Cook (1949) has

reported that only 2 years out of 31 were

severe blight years. Average total rainfall for

the 2 months before lifting at Norfolk, ineastern

Virginia, is 8.0 inches, but average monthly

temperatures at 66° and 74° F. limit the

the development of the disease.

RED RIVER VALLEY: The average

temperature and rainfall figures suggest a greater frequency of blight years

than, in fact, occurs. The average temperature figures are obscuring a much

greater diurnal range of temperature

than in Aroostook County. In the Red River

Valley, the diurnal range is from 25 to 30^ F.,

and very hot days are quite frequent, with the

temperature rising above 100 on occasional

days. Rainfall is well distributed, but relative

humidities are low. The area is also subjected

to strong drying winds during the potatogrowing season.

NORTH PLATTE: Temperature conditions

are similar to those in the Red River Valley.

This and the low rainfall account for the absence

of blight.

The general picture for the U.S.A. is that

high temperatures are one of the chief factors

limiting blight development. In the Western

States, high temperatures are normally combined with low rainfall to make conditions even

more unfavorable to the fungus. The coolest

growing areas, which are also wet ones., are

in the New England States, and these are also

the areas in which potato blight is most severe.

CONTROL MEASURES

The two most marked features of the control

of potato blight in the United States are (1) the

intensive control program followed by almost

all growers in those areas where blight occurs

to any extent, and (2) the preponderant use of

organic fungicides (Cox, 1957).

The justification for an intensive control

program must be considered on economic

grounds, but there are a number of basic

reasons why the United States growers are

spray conscious. Many of them, particularly

in the important potato areas, such as

Aroostook County, are potato specialists. They

keep no livestock, and the other crops they

grow are there primarily to give the land

a rest from potatoes or to build up soil

fertility. Another type of specialist is the

man who produces two crops a year. He grows

a summer crop of potatoes in the north and

then moves south with his equipment to grow

a winter crop. The result of this specialization

136
is that most growers have their own spraying

equipment and, concentrating most of their

efforts on one crop, wish to make the best

possible job of it. In addition, there is the need

to deal with a wide range of insect pests, so

that in the potato-growing areas of the United

States, spraying would go on even in the

absence of blight. It thus becomes very difficult to decide what is the true cost to the

grower of spraying against blight, since he

will be using the same equipment as he uses

against insect pests, and in some treatments

antipest and antiblight sprays will be combined.

In addition, if he is a specialist with specialized

equipment and stops spraying against blight,

about the only thing he saves is the cost of the

fungicide, as the productive alternatives for

his men and equipment are probably nil.

Whether a routine program for blight control is needed in any particular area in the

United States can best be decided by considering the effect of the disease on unsprayed

crops. No large-scale surveys of the amount

of defoliation caused by blight on unsprayed

crops have been carried out. Indeed, in areas

like Aroostook County, where all the growers

spray, they cannot be carried out. In the

absence of survey material, the results of

spraying trials must be used, but it is pertinent to ask how true a picture of the uncontrolled disease the unsprayed plots in these

trials give, surrounded as they are by sprayed

potatoes.

With these points in nnind, the yield losses

caused by blight on the unsprayed plots at

Aroostook Farm, Maine, over the period

1950-55 (table 77) can be considered in relation to the cost of a routine control program.

Table 87 sets out these losses, which have

been estimated from the 75 percent blight

defoliation dates in relation to the potato

growth curve (fig. 62). No account has been

TABLE 87. —Percentage loss of yield due to

blight on unsprayedpotatoes Aroostook Farm,

Maine, 1950-55

Year

1950

1951

1952

1953

1954

1955

Average.

Loss

Percent

15

15

40

12

taken of tuber blight losses, which in any case

were very small.

The average yield of potatoes in Maine is

approximately 360 bushels per acre, so that

an annual average loss of 12 percent represents

a loss of 43 bushels per acre. In monetary

terms, putting the value of 1 bushel of potatoes

at $1 (a low figure), this equals an annual loss

of $43 per acre. This loss has been prevented

by spraying. Assuming an annual average

application of 6 sprays at $5 per spray ($2 for

materials), the annual cost of spraying amounts

to $30 per acre. This cost is seen to be welljustified, particularly as one benefit not taken

into account in these calculations is that there

is a higher proportion of Grade 1 tubers from

the sprayed potato crop in blight years.

At the other extreme, the value of a routine

control program in the South Fork of Long

Island or the Red River Valley in Minnesota

and North Dakota appears very questionable

when viewed against the fact that the frequency

of recent blight years has only been 1 year

in 10. The application of 4 antiblight treatments at $5 each for a period of 10 years costs

$200 per acre, which exceeds the average

value of an acre of Minnesota potatoes. Wallin

and Hoyman (1945) have stated that a lengthy

control progranni is not warranted in the North

Dakota part of the Red River Valley.

Cook (1949) wrote that '*the routine application of fungicides that is so often reconamended

is economically sound only for those areas

where disease losses occur with sufficient

regularity so that the benefits over a period

of years will profitably counterbalance the

cost of spraying or dusting every year.*' He

stressed the need for an effective blight

warning service that would tell the growers

the years in which spraying is unnecessary,

and thus save them considerable sums of

money. By advising potato and tomato growers

in eastern Virginia not to spray during 1947,

Cook considered that he saved them up to

$2,000,000.

One criticism of the U. S. control program

that has been developed in Aroostook County

(see p. 126) is that spraying goes on too long

in an endeavor to keep the haulm completely

free from blight up to lifting. With the present

use of haulnn killers, the last 1 or 2 sprays

should be unnecessary.

The advantages or disadvantages of using

organic fungicides have been discussed under

Aroostook County. One of the most important

advantages of these materials is that in hot

dry summier s when blight is absent, they do

not have a phytotoxic effect with a consequent

lowering of the yield of the sprayed crops.

Stevenson, Akeley, and Webb (1955) and Eddins

and Clayton ( 1943), among others, have referred

to this phytotoxic effect where copper materials are used. However, copper is still

generally recognized as being a more effective

control of blight in bad blight years, and when

these years occur there is a marked tendency

for growers to switch back to copper sprays

during the bad season.

137
METHODS OF SPRAYING

The U, S. grower appears to have very

effectively cut down the losses normally associated with spraying. In addition to relying

chiefly upon organic fungicides, he grows his

potatoes in wide rows (36 inches) and fits

wheel guards to his tractor and sprayer

wheels. If there is a tendency for yield per

acre to fall to some extent as a result of these

wide rows, it should be remembered that

with its vast land resources, output per man

and not output per acre is the primary consideration in North America. Another factor

minimizing wheel damage losses is the growing of upstanding potato varieties,like Katahdin

and Red Pontiac, which have replaced sonme of

the older more sprawling varieties, such as

Green Mountain. Against this, however, it

should be remembered that the very large

number of spray treatments so often given will

have a tendency to overcompact the soil

between the potato drills with a consequent

adverse effect on the growth of the crop.

In general, high-volume spraying (100 toi25

U.S. gallons per acre) is used, although in

Aroostook County low-volume spraying has

gained very rapidly in popularity during the last

few years, and now 70 percent of the growers

are spraying at low volume. Dusting is carried

out in a few areas where water is in short

supply, but spraying is generally recognized

as being the more efficient technique.

Top spraying without the use of drop lances

is universal. The grower aims to protect the

lower foliage by starting his spray program

early and keeping pace with the growth of the

crop. But in view of the known poor adherence

of the organic fungicides, his outlook on this

point appears sanguine. There seems to be a

tendency for the growers to make rather a

fetish of spraying, without considering too

closely the efficiency of the job they are making.

HAULM, OR VINE, KILLING

The interest in **vine" killing is of comparatively recent origin and nnost of the work

on the subject has been reported since 1945.

Apart from preventing blight infection of the

tubers at lifting time, the practice has a number

of other advantages, including (1) the control of

tuber size in seed crops; (2) prevention of

late-season spread of virus diseases in seed

crops; and (3) easier harvest by destroying

top growth and weeds.

Haulm killing is now widely practiced

throughout the potato-growing regions of the

United States. In an area like Aroostook County,

where it has obvious advantages on a number

of counts, it is more or less universally

carried out every year. Various methods are

used; of these, spraying with sodium arsenite

or with the dinitro compounds, in particular

DNBP, is the most common. Rose and Cook

(1949) have reviewed these and other methods

used in the United States. Tops are also killed

with chemical dusts (calcium cyanamide), by

rotobeaters and flame burners.

A number of writers have reported a

browning of the stem end of tubers in crops

treated in this way, and also a vascular

discoloration of the tubers. Peterson and

Gwinn (1952) have summarized the literature

on this subject, which contains a large amount

of conflicting evidence. In general, however,

workers agree that one of the principal factors

involved in the use of chemical sprays is the

speed with which the haulnn is killed; and the

faster the rate of killing, the more likely is

vascular discoloration to occur. The fact that

this trouble has been widely reported in North

America and not from the British Isles would

indicate that it is associated with the relative

immaturity of the American crops at killing.

At the present time (1957), vascular discoloration brought about by vine killing appears to

be of little innportance in the United States.

This may be associated with a greater use of

sodiunn arsenite, which kills the tops slowly.

POTATO BLIGHT FORECASTINGS

The first attempts in the U.S.A. to forecast blight outbreaks by an analysis of weather

data were made by Moore (1937) for the

Charleston area of South Carolina, and Melhus

(1945), who has reported on a blight forecasting service that issued warnings during 1943

and 1944 to potato growers in the Upper

Mississippi Valley States.

Cook (1949) developed a blight forecasting

method for eastern Virginia based on weekly

nnean temperatures and cumulative rainfall.

The critical temperature above which blight

would not be important was considered to be

7 5 F. Cumulative rainfall data from past

blight and nonblight years was used to fix a

*'critical rainfall line," which by definition

was **a straight median line ... constructed

between the lines for rainfall for the blight and

nonblight years."

Blight forecasts were based on a 2-week

period during which the mean weekly temperatures remained below 75° F., and the actual

cumulative rainfall line remained above the

"critical rainfall line." Growers weretoldnot

to spray when weekly meantemperatures were

above 75 F. and/or the actual cumulative

rainfall line fell below the "critical rainfall

line" for a period of 2 weeks. The method

was said to be 84 percent accurate for eastern

Virginia.

Cook (1947) considered that ''the purpose of

the forecasts is to eliminate needless spraying

or dusting in years whenblight is not imiportant

and to insure prompt and adequate protective

measures in years when it threatens to cause

serious damage." The emphasis here, on

138
preventing needless spraying is interesting,

but is readily explained by the fact that only

2 out of 31 years in eastern Virginia had

been bad blight years. Cook used his method

successfully in 1947, when he issued four

warnings to potato and tomato growers

not to spray. His forecasting charts for 1946

(a **blight" year), and 1947 (a "nonblight"

year), are reproduced as figure 67.

More recently, studies have been made in

the North Central States by Wallin and Waggoner (1949); Wallin and Samson (1953); and

Wallin, Wade, and Darling (1953); and in the

Northeastern States by Hyre and Horsfall

(1951); Hyre and Bonde (1955); and Hyre

(1954 and 1955), to test the efficiency of

Cook's method under differing climatic conditions. Wallin found that in Iowa, Indiana,

and Wisconsin, cumulative rainfall and temperature data alone did not give a sufficiently

reliable basis on which to issue blight forecasts, and this he attributed to the lack of

correlation between periods of high humidity

favorable to the fungus and the amount of

rainfall. In further studies in North Dakota

and Minnesota, Wallin and Hoyman (1954) and

Wallin, Eide, and Thurston (1955) have used

the frequency of 10-hour periods during which

the temperature does not exceed 75 F. and the

relative humidity does not fall below 90 percent with subsequent daily temperature maxima

ren:iaining below 95 to predict the development of blight. Hyre (1955) has reported that

a modification of Cook*s method has been

tested out successfully in a number of the

Northeastern States. The chief difference between this and Cook's original method is the

replacement of a fixed '^critical rainfall line"

by moving graphs based on the total precipitation over a 10-day period.

The fact that cumulative rainfall forms a

reasonable basis for making forecasts in

eastern United States, whereas it is unreliable

in the North Central States, may be associated

with the greater frequency and larger amounts

of rain that occur in the east. There is one

great advantage in the use of simple rainfall

and temperature data for making forecasts in

that this information is readily available

through the United States Weather Bureau,

while humidity data is not generally available.

At the present time (1957), potato blight

forecasting in the United States is still in a

very experimental stage. Some of the difficulties with whichithas to contend have already

been discussed in the section on the Red River

Valley. The collection and the dissemination of

information on blight occurrence, spread, and

severity are accomplished through the U.S.D.A.

Plant Disease Survey, of which Paul R. Miller

is the head. This is centered at Beltsville,

Md. Cooperating State pathologists provide the

information on which the blight situation for the

country as a whole is built up, and they receive

U.S.Â.

1946

' 7-OAY MOVING AVERAGE TEMPERATURE /'

DAILY MEAN TEMPERATURE AND «'

CUMULATIVE RAINFALL

FIGURE 67.--Forecasting charts for a blight year (1946) and a nonblight year (1947), at Norfolk. Va. (From Cook, 1949.)

the full picture back from Beltsville in the

form of twice-weekly newsletters.

BREEDING FOR BLIGHT RESISTANCE

The United States Department of Agriculture

and the State experiment stations cooperating

in the National Potato Breeding Program are

now responsible for the very large majority

of new varieties released to the farming community. The ainm is not to produce a number of

varieties that can be grown throughout the

country but to breed varieties suited to the

particular needs of the various States, which

differ widely. However, the cooperative nature

of this work makes it possible to find varieties

that have a wide adaptability, e.g., Kennebec.

Stevenson (1956), who was in charge of this

work until recently, has described the progress made in producing potato varieties resistant to various diseases, including late blight,

and resistance to insect pests.

139
The story of the search for American

blight-resistant potato varieties has been given

in detail by Peterson and Mills (1953). Pioneer

work in the use of Solanum demis sum as a

source of blight resistance was carried out

by Reddick at Cornell University; and Reddick

and Peterson (1945) have described the characteristics of Empire, the first **blight resistant"

variety to be introduced in the U.S.A. Ennpire

was first grown in 1940 at the Cornell University Agricultural Experiment Station,

Ithaca, N. Y., and placed in the hands of

commercial seed-potato growers in 1945. At

the time of its introduction, Reddick and

Peterson wrote of its blight resistant qualities, *'• . • it must be assumed for the present

that Empire ultimately may break down.*'

This, in fact, proved to be the case; and this

breakdown has been repeated with more recent

introductions, where resistance has beenbased

on major genes, owing to specialization of the

blight fungus. The most successful of the

"blight-resistant" varieties to come onto the

market has been Kennebec, which contains the

R^ gene for blight resistance. This variety has

been described in detail by Akeley, Stevenson,

and Schultz (1948). Although Kennebec was

quickly attacked by blight, both the haulm and

the tubers, the variety possessed other desirable commercial properties, and it is now

widely grown, particularly in Maine. The

position is, however, that where Kennebec is

grown, it now almost always receives a full

antiblight spray schedule.

Breeding for field or partial resistance to

blight is now considered to be the most hopeful

approach to the problem. Some of the newer

American potato varieties, in particular,

Sebago, show a higher degree of field resistance than the old varieties like Green Mountain, which they are replacing. Up to the

present, this trend cannot be said to have had

any effect on the intensity of the farmters*

blight-spraying programs.

Much research into the genetics of blight

resistance and the various physiologic races

of Phytophthora infestans, including the * "plasticity" of the fungus, has been carried out

in recent years. The principal workers in the

United States now dealing with these aspects

of the disease are W. R. Mills of the Pennsylvania State College, L. C. Peterson of Cornell

University, and M. E. Gallegly of the University of West Virginia. Mills and Petersonhave

cooperated with Black of Scotland andMastenbroek of Holland to produce a universally

accepted, international nomenclature for blight

races (Black, Mastenbroek, Mills, and

Peterson, 1953).

SUMMARY: UNITED STATES OF AMERICA

Potatoes are grown under a great range of

conditions, and the incidence of blight varies

considerably between different regions. An

intensive study has been made of blight epidemics in four widely separated areas and

this has illustrated the variation in blight

problenns throughout the country. The disease

is most serious in northeastern United States,

where summers are cool with abundant rainfall, i.e., the best conditions for maximum

potato crops. The overall yield losses associated with severe blight years, however, may

not be great, as has been shown by a comparison of yields in a number of recent slight

and severe blight years in Aroostook County.

Average yields for the two sets of years were

very similar.

Possible loss of crop yield through premature defoliation does not appear to have

played such an important part in shaping the

farmer's control program against blight as

the fear of tuber blight infection. Until recently, heavy tuber losses were common,

and these followed the lifting of crops while

some green, blighted haulm remained alive.

That the haulm was still green reflects the

short growing and harvest periods for the

crop throughout large parts of the United

States. Thus, the farmers' intensive antiblight

schedule aimed and still aims at keeping the

potato tops free from blight to prevent infection at harvest. The present use of haulm

killers should enable farmers to cut down on

the number and frequency of their blightpreventive spray treatments.

CANADA

The area planted with potatoes in Canada is

approximately 300,000 acres per annum. In

1940, just over half a millionacres were grown,

but plantings have fallen steadily since then.

On the other hand, production methods have

shown a consistent improvement, so that total

production of potatoes, which is normally between 60 to 70 million bushels per annum

(1 bushel weighs approximately 60 lb.), has

remained at about the same level over recent

years (Parks, 1955a). Approximately 85 percent of the acreage planted produces table

stock (ware) potatoes, and the remaining 15

percent is devoted to seed-potato production.

There is an important market for Canadiçin

seed potatoes, principally in the United State s,

Argentina, Uruguay, and Cuba.

The average annual consumption of potatoes

is somewhat higher in Canada than inthe United

States. It is now about 150 lb. per head per

year, but there is a marked downward trend

in the face of competition from other vegetables. The annual yield is just under 6 tons per

acre.

140
CANADA

FIGURE 68.--Distribution of potato growing in Canada. 1953. Each cJot equals 1.000 acres.

POTATO DISTRIBUTION AND VARIETIES

Potatoes are grown to some extent in all the

Canadian provinces, but conditions are not

very favorable to the crop in central Canada,

and the largest concentrations are in the east

(see fig. 68, after Hudson, Stutt, Van Vleit,

and Forsyth, 1949). Important growing areas

are the Maritime Provinces, notably Prince

Edward Island and the St. John Valley in New

Brunswick; the Gaspe"" Peninsula and an area

round Montreal in Quebec Province; and the

counties north and west of Toronto in Ontario.

About 85 percent of the total Canadian certified

seed crop is grown in Prince Edward Island

and New Brunswick; each province grows

about 25,000 acres of seedpotatoes per annum.

In western Canada there is an important

potato district in the Lower Fraser Valley

of British Columbia. Table 88, taken from

figures published by the Dominion Bureau of

Statistics, shows the breakup of the Canadian

potato acreage by province in 1953, which was

a year of generally high yields. These illustrate the better growing conditions for potatoes

that are found in the areas nearest the oceans.

Potato statistics are not published for Newfoundland. Wart disease (Synchytrium endobioticum) exists there and potatoes cannot be

exported from Newfoundland to any other of

the Canadian provinces.

The varieties grown in Canada are almost

all the same as those in the U.S.A., the most

important being Katahdin, Sebago, Irish Cobbler (early), and Green Mountain. The only

varieties of importance that do not appear

in the United States list are Canso and Keswick,

which were bred specifically for blight resistance and have been derived from initial

crosses using Solanum demi'ssum as one of

the parents.

SOIL, CLIMATE, AND CULTIVATION

Potatoes are grown over a wide range of

soil types fronn the red sandy loams on Prince

TABLE 88. --Acreage and yield of potatoes

in Canada, 1953

Province Acreage Yield

Prince Edward Island

Nova Scotia

New B27unswick

Quebec

1,000 acres

39

12

49

100

63

19

13

15

11

Bu./acre

273

231

289

166

Ontario

Manitoba

Saskatchewan.

Alberta

British Columbia....

189

170

131

179

300

Total or average.. 321 209

141
Edward Island to the dark organic soils of

the Red River Valley in Manitoba. The length

of the growing season is almost everywhere

short (not more than 120 days). This is mainly

due to a late spring, and it affects not only

central Canada, but also the Maritime Provinces, which are subjected to the cold Labrador

current of the North Atlantic and the cold

winds that sweep down from the interior of

northern Quebec. The longest and most equable

growing season occurs in the limited area of

the Lower Fraser Valley and on Vancouver

Island in British Columbia, and this is reflected

in the high yields of potatoes grown there.

The same degree of specialization does not

exist among potato growers in Canada as in

the U.S.A. Nonetheless, even on so-called

mixed farms where potatoes are grown, the

range of crops is probably smaller than on

European farms producing potatoes. Growers

are usually well equipped mechanically, and

nearly all of them have their own spraying

machines. Potato cultivations are very similar

to those carried out in the U.S.A., and have

been described in detail by Parks (1955b).

POTATO BLIGHT INVESTIGATIONS

Research on the disease in Canada has tended

to concentrate on two main problems. First,

the chemical control of blight: This includes

the testing of different fungicides and haulm

(vine) killers. Most of this work is now done

at the Science Service Laboratory, Charlottetown, P.E.I. Second, the identification of blight

races and the breeding of blight-resistant

varieties: This work has been carried out at

the Science Service Laboratories at Fredericton, New Brunswick, and Ottawa.

The Dominion Laboratory at Charlottetown

is famous as a center of research into potato

diseases. It was founded in 1915, and the first

Plant Pathologist-in-Charge was Paul A.

Murphy, who, both on Prince Edward Island,

and later in Eire, did much fundamental work

on the biology and control of Phytophthora

infestans. A case study treatmient of Prince

Edward Island is now given, followed by a

generalized account of blight, covering the

whole country.

Prince Edward Island

Prince Edward Island has a total area of

about 1,000,000 acres. Between 35,000 and

40,000 acres of potatoes are grown annually,

producing an average yield of approximately

6| tons per acre. About 70 percent of the

total potato acreage is used for the production

of certified seed.

The most important variety grown is Sebago,

most of which is exported as seed. Other

varieties grown are Green Mountain (principally for home consumption), Irish Cobbler,

Katahdin, and the blight-resistant varieties

Kennebec, Keswick, and Canso. There is a

very characteristic red sandstone soil on the

Island. The most intensive farming is carried

on in Prince County in the northwest and

most of the potato crops are grown there.

The potato-growing season extends from

the end of May to the end of September (about

110 to 120 days). The Island is subjected to

cold air currents from the north untü late

into the year, which accounts for the late

planting time. Average temperature and rainfall data for Charlottetown are set out m

figure 69. Summer temperatures are not high,

and it is unusual for the thermometer to go

above 85° F. Annual rainfall is about 42

inches with a slight peak in the autumn and

winter. In general, apart from the shortness

of the season, growing conditions are very

favorable for potatoes.

Frequency of Blight Epidemics

Descriptions of blight epidemics on Prince

Edward Island are included in the Annual

Reports of the Canadian Plant Disease Survey.

In a number of years estimates of yieldlosses

caused by premature defoliation due to blight

and estimates of tuber blight losses have been

made. These, together with the field outbreak

dates and an appraisal of the severity of the

disease supplied by L. C. Callbeck of the

Science Service Laboratory, Charlottetown,

are set out in table 89 for the period 1945-54.

(In those years where no estimates have been

given, yield losses were described as low.)

A clear picture of epidemics on Prince

Edward Island is shown up by this table. In 4

years out of 10 (the **severe" years), blight

occurred early, continued to spread, and

caused heavy over alllosse s throughpremature

defoliation of the potatoes; in 1 of these 4 years,

tuber blight losses were also heavy. In4 years

out of 10 (the ''severe (late)*' years), the

disease was widespread at the end of the season and caused little loss through defoliation,

but in the 2 out of the 4 years it brought

about large tuber blight losses. In 2 years

out of 10, the disease was present in only

trace amounts and losses were negligible.

There is no doubt that blight is a most serious

.Charlorretown -1 70

RE.I. ^^.^^^^

RAINFALL 60

Inches X X

/ N. .

50

/ r// \

-

/

^y/. \

- 40

/ ^Y/,^

/

/

/

/^%ièv/ \

30

20

^

PLANT /

-ING /VA ^

/LIFT

/-INC

°F

FIGURE 69.--Long-term average monthly rainfall and mean temperature for Charlottetown. Prince Edward Island, Canada. (For

climagrams for Fredericton. Lethbridge, and Agassiz, see fig. 71.)

142
TABLE 89. --Outbreaks and losses from blight on Prince Edward Island, Canada, 1945-54

Year Outbreak date Severity Defoliation

losses Tuber blight

1945 July 14

End Aug.

July 22

July 26

End Aug.

Fid-Aug.

July 20

July 21

July 10

July 19

Severe (late)

Trace (late)

do

Percent

0

5-10

20

20

15

Percent

1946 2-5

1947 (") 1948 Severe

Severe (late)

do

1949

8

1950

10-12

10

1951 Severe

Severe (late)

Severe

do

1952 ""

1953 ""

1954 ""

Trace.

problem on the Island, particularly when it is

remembered that the losses described above

occurred despite an intensive control program.

Blight Losses Due to Premature Defoliation

Estimates have been quoted above of yield

losses from premature defoliation. These

estimates referred to the Island as a whole,

and losses on unsprayed crops would have

been considerably more serious. This can be

seen from figure 70, which shows the progress

of blight on potato haulm, and the yield of

sprayed and unsprayed potatoes (over the period 1948-55) from trials held at Charlottetown.

The results of these trials have been published

in the Annual Reports on Potato Fungicides by

L. C. Callbeck for 1949-55, and for 1948 by

Callbeck (1949a). If the difference in yield

between the sprayed and unsprayed plots is

taken as being approximately equal to the yield

loss due to blight (a fair assumption in these

trials), then the average loss from blight over

the 8-year period (1948-55) at this center

equaled 26 percent of the potential crop, and

in no year did it fall below a 10-percent loss

of crop.

The variety each year for these trials was

the susceptible Green Mountain, whereas the

variety most connmonly grown on the Island

is Sebago, which shows a high degree of field

resistance to blight. The results on the sprayed

plots shown in figure 70 refer to the plots

sprayed with bordeaux mixture. In 3 out of

the 8 years (1948, 1953, and 1955), the sprayed

plots were artificially inoculated with

Phytophthora infestans, but these 3 years were

all years of severe blight epidemics on Prince

Edward Island, and the inoculation is unlikely

to have had much effect in increasing blight

losses. The yield loss in 1952 seems very

high in relation to the progress of blight on

the haulm of the unsprayed plots, even though

planting was very late that year. The results

in 1952 must therefore be considered

anomalous. Nevertheless, it will be seen that

blight losses are very heavy, and heavy

nearly every year.

Blight in Tubers

The amount of tuber blight in the sprayed

(bordeaux) and unsprayed potato plots atCharlottetown are set out in table 90.

TABLE 90 --Tuber blight in sprayed and

unsprayed plots, Charlottetown, Prince

Edward Island, 1948-55

Year and

average

Amount of tuber

blight in—

Sprayed

plots

Unsprayed

plots

1940

Percent

0.4

.3

1.3

1.1

.3

.6

.9

1.0

Percent

6.6

194.9 20.4

1950 18.1

1951 9.3

1952 S,S

1953 4.1

1954. 1.6

105^- 7.2

Average. .7 9.5

Here again, the variety in the trials was

Green Mountain, which is highly supceptible

in the tubers; Sebago is resistant inthe tubers.

Thus, the high degree of infection while the

potatoes were still in the ground can to a large

extent be explained by the susceptibility of the

variety under test. Even so, it is remarkable

that tuber rot losses in the unsprayed plots

should be so high almost every year, and this

would appear- to be connected with the generally

high moisture content of the Island soils.

143
CANADA

loo

BLIGHT

ON

HAULM 50 -

per

cent

May June July August September

i i i 1 1 \ \

loo

50

O

Planted

7sprayinqs '^ A 1949

/Burnt

y off

y Sprayed II-8

^^ Unsprayed 10-5

1 1 1 1 i i \ \

lüü

50

O

Pía nted

8 sprayings

y

^ Burnt

off

1950

Sprayed ll'O

Unsprayed 9*4

1 \\ i 1 i i

üü

50

O

Plant

6s

ed

jrayinqs 1/^ Burnt

/ off

•

•

1951

Sproyed 9-0

Unsprayed 7-O

too

50

\ \ ), \ \ \

6 sprayings -^^"^ / 1952

/Burnt

Planted

y off

1 Sprayed 12-1

^. Unsprayed 8-3 i

\ { \ \ \ \ i

loo

7 sprayings

1 r 1953

50 - / Burnt

O

Planted

II

1 otf Sprayed 8*5

Unsprayed 7*1

i 4 i i i J i

)0

7 sprayings X 1954

O

Plañí-ed

/

Burnt

off

I sprayed 9-8

O i| — 1 Unsprayed 4-7

1 i i 1 i i i

loo

so

o

-

7s

Planted

1

prayings

/

Burnt

off

/

1955

Sprayed 8'l

Unsprayed 5'5

May June July August September

FIGURE 70.--Progress of blight onliauîm oí sprayed and unsprayed Green Mountain potatoes, with corresponding total yields in tons per acre.

Charlottetown. Prince Edward Island, 1948-55.

144
The estimates of tuber blight losses in the

commercial crop quoted in table 89 show that

in some years (as in 1948, 1949, 1950) losses

are very heavy. In the past, a high proportion

of this loss will have followed the iniection

of the tubers by blighted haulm at lifting time,

and this is understandable in an area with a

short growing season like Prince Edward Island. Haulm killing has now greatly reduced

tuber blight losses in the commercial crop,

but heavy tuber infection via the soil can still

occur on the Island, if the combination of an

unsprayed crop and a blight-susceptible potato

variety exists.

Blight and Weather Data

Mean air temperature and rainfall figures

for the experimental station at Charlottetown

are shown in figure 69, together with the planting and lifting times of potatoes. Temperatures

are usually favorable to the development of

blight throughout the growing season.

The average total rainfall for the July-toSeptember period at Charlottetown is 10.53

Inches. L. C. Callbeck has set out the July-toSeptember rainfall against the severity of blight

for each year over the 34 years, 1922-55.

The results are shown In table 91-

The precipitation for the period rangedfrom

6.38 inches in 1945, when blight was severe

at the end of the season, to 20.1 inches in 1942,

a year of an early and severe attack. In 1935

there was no blight with 15.7 inches of rain,

but it was severe in 1951 when the precipitation

was almost identical, 15.6 inches. Both 1954

and 1955 were very bad blight years, although

rain was below normal. In 1954 there were

8.85 inches during July to September, and in

1955, 8.78 inches.

Although there is a broad correlation between the amount of rain and the severity

of blight, the number of years that are

exceptions to a general rule indicate that total

rainfall figures alone are insufficient to divide

the '•nonblight'* from the •'blight'* years.

Callbeck has given the main reasons for

this: ''A total July to September precipitation

TABLE 91. --Severity of blight in relation to

rainfall, Charlottetown, Prince Edward

Island, 1922-55

Total rainfall.

Years with--

July to September

(inches )

No

blight

Trace of

blight

Severe

blight

Below 9 5

Niimber

2

1

1

Number

6

2

1

^fumber

5

9.5-11 5

7

Ovp-r 11 5

9

of above average might be made up largely

of infrequent heavy rains, each of which was

quickly followed by conditions favoring rapid

drying, and thus making it impossible for the

fungus to become established. On the other

hand, the summer might be characterized by

numerous light rains and drizzles whose total

fall would be considerably below average, but

which coupled with dews and periods of high

relative humidity, would keep the foliage damp

for many hours at a time."

Since 1951, in an attempt to find a closer

relationship between weather conditions and

the severity of blight epidemics, studies of

weekly weather records, including humidity

data, have been made in relation to the occurrence of the disease. Table 92 shows the

weather summary for 1954.

The first field outbreak of blight in 1954

occurred on July 19, and in unsprayed fields

potato tops were dead by early August. Those

farmers who sprayed got through this period,

and conditions became less favorable to blight

after the middle of August.

From the records kept since 1951, it seems

that blight makes its appearance about a fortnight after the first week following July 1 in

which the average relative humidity remains

at 80 percent or above. A week at this humidity

level once blight has started increases the

disease. This fits in well with what happened

in 1954. Weekly humidity figures are now

being used on Prince Edward Island as one

of the main criteria on which potato blight

forecasts are based.

Control Measures

The importance of cull piles as a source

of blight inoculum has been recognized for a

number of years, and a successful campaign

has been waged against them. In the summer

of 1956, so far as was known, there was only

one potato cull pile in existence on the Island.

During recent years. Green Mountain has

become considerably less important on Prince

Edward Island, and Sebago increasingly important so that it now dominates all other

varieties. The use of Sebago may be looked

upon as a partial control measure against

blight, as this variety exhibits a high degree of

field resistance to the disease. However, the

differing susceptibility of Green Mountain and

Sebago must not be looked upon as the main

reason for the substitution of the one variety

by the other. Green Mountain fell into disrepute

with table stock growers, owing to net nee rosis,

which develops in the tubers following primary

leaf roll infection. The chief advantages from

the point of view of blight control in growing

Sebago are that the progress of the disease can

be kept in check on this variety with fewer

spray applications than are required on Green

Mountain, and that tuber blight infection is

considerably less.

145
TABLE 92. —Weather summary at Charlottetown, Prince Edward Island, 1954

Week

July 1-7

July 8-14

July 15-21

July 22-28

July 29-Aug. 4..

Aug. 5-11

Aug. 12-18

Aug. 19-25

Aug. 26-Sept. 1.

Sept. 2-8

Sept. 9-15

Sept. 16-22

Sept. 23-29

Total

Mean Average

temperature humidity

°F. Percent

64.9 80.0

66.6 70.6

65.8 86.0

66.3 88.4

66.7 84.1

62.6 88.6

62.6 80.1

63.0 78.2

62.1 71.2

62.4 75.3

55.¿. 76.5

56.8 81.8

54.2 82.9

Rain

Inches

0.35

.17

2.25

1.21

1.18

.93

.47

.04

.5S

.44

.60

.18

.45

B.S5

Days with rain

Number

5

3

3

5

3

2

3

1

2

1

4

37

' In addition^ there were 11 days over the period on which rain fell, though the amount was too

small to be recorded.

The majority of potato growers on the Island

follow j an annual protective spray program

against blight. Most of this spraying is carried

out by the farmer, and there is very little contract (custom) spraying. Where dusts are used,

however, they are usually applied by contractors. Practically all spraying is carried out at

high volume (80 to 125 gal. per acre), and

sprays are applied from above the crop only.

The chief materials used are the carbamates

(dithiocarbamates), but a fair proportion of

the growers are still using bordeaux mixture,

owing to its better adherence to the potato

plants. The phytotoxic effect that occurs with

the use of bordeaux in hot dry summers in

other areas in North America has not been

noted on Prince Edward Island. This, together

with the fact that bordeaux is still being used

extensively, despite the obvious disadvantages

in preparing the material, emphasizes the

usually wet and humiid conditions under which

potatoes are grown.

About half the antiblight sprays are combined with treatment against insect pests.

These are the Colorado beetle, potato flea

bettle, and aphids. Early blight (Alternaria

solani) is not a problem. Haulm killing is now

a common practice, and the principal chemical

used is sodium arsenite. A few growers use

calcium cyanamide in dust form.

A survey of the blight control practices of

Prince Edward Island potato seed growers was

carried out in 1953 (a severe blight year).

Questionnaires were sent out by post to 300

fanners and 107 replied, which represents 2.4

percent of all seed growers on the Island. The

results of this survey were presented in the

1953 Report on Potato Fungicides. A brief

summary of the findings follows.

1. 77 percent of growers used a wet spray;

8 percent used a dust; 15 percent applied no fungicide.

2. Of those growers who used a spray, 48

percent used carbamates; 31 percent used

bordeaux; 9 percent used fixed copper;

12 percent used a split schedule (carbamates followed by copper),

3. From 1 to 9 spray treatments were given;

average 3.4 treatments* From 1 to 4 dust

treatments were given, average, 2.8

treatments.

4. The average number of treatments given

according to the type of material used

were: Carbamates, 3.9; bordeaux, 3.2;

split schediole, 3.0; fixed copper 2.1.

5. The number of sprays applied varied with

potato variety: Average figures for

Green Mountain were 4.8; for Canso, 4.3;

for Katahdin, 3.5; for Sebago, 3.4; and on

Irish Cobbler, 3.2.

6. 24 percent of the growers used a haulm

killer.

L. C. Callbeck, commenting on this survey,

stated that the average number of spray

treatments given (3.4) was about half the optimum required to control blight in this year.

A considerable amount of experimental work

with potato fungicides and on haulm killing

methods has been carried out for some years

now at the Science Service Laboratory at

Charlottetown. This includes annual screening

tests of new materais. Callbeck (1950a) has

reported on investigations into the copper/lime

ratios in bordeaux mixture, and the same

146
worker (Callbeck, 1954) has given details of

tne etfect of incorporating zinc salts in potato

fungicides. Haulm killing trials have been

described by Callbeck (1949b). It has been

found that less tuber blight has occurred

m plots sprayed with bordeaux mixture than

in those sprayed with organic fungicides in

each year since 1945, when the organic s were

first tested at Charlottetown. Mean tuber

blight percentages for the period 1945-51

were bordeaux, 0.7; nabam, 5.1; zineb, 6.9Í

unsprayed, 13.1 A possible explanation is the

lethal effect on blight spores of copper residues

accumulated in the surface soil, whereas

dithiocarbamate residues remain toxic for a

much shorter time.

A blight warning service for growers was

started in 1947. Weekly press and radio

reports based on field observations of the

disease and current weather conditions are

issued. Average weekly relative humidities

exceeding 80 percent are considered to favor

the onset or increase of blight. The Island is

well placed to receive up-to-date reports on

the progress of blight, since 70 percent of the

potato crop is for seed, and 30 seed inspectors

are out in the field during the summer months.

A good deal of circumspection has to be exercised in the issue of blight warnings for the

very reason that seed production is so important, and it is essential not to frighten away

the buyer of the seed crop.

Summary: Prince Edward Island

Potato blight is a most serious disease on

the Island. Average annual yield losses due to

premature defoliation following blight attack

on unsprayed plots at Charlottetown over the

period 1948-55 equaled 26 percent of the possible crop. Premature defoliation also causes

heavy losses in the commercial crop despite

the control program. Tuber blight is a serious

problem, particularly where a blight susceptible variety like Green Mountain is grown.

However, now that haulm killers are available

there should be good control of infection at

lifting. This used to be a trouble due to the

short growing season for potatoes. It is surprising that a survey carried out in 1953

revealed that 15 percent of the seed growers

had no control program for blight.

BLIGHT EPIDEMICS: FREQUENCY AND

LOSSES

The first records of potato blight in Canada

go back to the early 1840's, when the disease

was found in the Maritime Provinces. Since

1920, annual accounts of the severity of the

disease in the different Canadian Provinces

have been published in the Reports of the

Canadian Plant Disease Survey.

Blight is most serious in the Maritime

Provinces (Prince Edward Island, New Brunswick, Nova Scotia) and Newfoundland; in

western Quebec and eastern Ontario; and in the

Lower Fraser Valley of British Columbia.

These are also the principal potato-growing

areas in the Dominion and the areas where

potato-growing conditions are most favorable.

There is an obvious link between the blight

areas in eastern Canada, and those in the

Northeastern States of the U.S.A.

Blight has not been recorded to any extent

north of latitude 52°, but only a few potatoes

are grown in the north of the country, owing

to the shortness of the growing season. H. N.

Racicot has estimated that 30 to 40 percent of

the losses due to blight in Canada are caused

by premature defoliation of the potatoes, and

60 to 70 percent of the loss is due to tuber

blight infection. Most of the tuber blight is

caused by contamination with infected haulm

at lifting time, and is therefore largely preventable. Blight infection in the soil is not,

generally speaking, an important problem,

and this is possibly linked with the fact that

most potatoes are grown on light soils.

The Annual Reports on blight have been

examined in detail for the 10-year period

1946-55, and much of the data for individual

areas given below, including quotations on the

course of the disease in blight years has been

obtained fronn this source.

MARITIME PROVINCES. Prince Edward Island is the worst area for blight in Canada, and

L. C. Callbeck has described the frequency

of severe blight years there as 3 years out of

4. The disease is not quite so serious in Nova

Scotia and New Brunswick, and the frequency

in those provinces is about 1 year out of 2.

The principal potato-growing area in New

Brunswick lies in a narrow strip of about 60

miles in the St. John River Valley, along the

western boundary of the province. This area

is opposite the potato-growing district in

Aroostook County, Maine, which has been

considered in detail in the section of this

study covering the U.S.A. The chief difference

between the two areas is the greater acreage

and concentration of potatoes on the U.S.A.

side of the border, but the importance of blight

and the problems that the disease presents

are very much the same. In New Brunswick

1954 was a bad blight year. The folloydng extract shows how serious the disease was.

**Late blight was first reported in N.B. on

16 July . . . The disease developed rapidly

and by mid-August practically every field in

the potato growing areas showed sonne infection. Some fields, particularly Irish Cobbler

were already completely killed . . .** On a

number of occasions, including 1956, the initial

blight outbreaks in New Brunswick took the

form of an attack concentrated in stennlesions

and lesions on the growing points of the potato

plants. No satisfactory explanation has been

put forward to account for this type of attack,

and it does not seenntobe correlated with potato

variety. However, once stem lesions have been

147
established within a crop and the weather becomes favorable for blight development, the

potato field Is then destroyed very rapidly.

The principal losses in New Brunswick

result from yield reduction caused by premature defoliation. Haulm killing has cut out a

large part of the tuber infection that used to

occur. Infection in the soil is not usually a

serious problem due to the large **hills"

and light soils in which the potatoes are

grown. J. L. Howatt has drawn a distinction

between those years in New Brunswick when

the disease has been severe and those years

when it has caused severe losses. The disease

is said to be severe in 1 year out of Z, but in

many of these severe years, farmers who

follow a regular control progrann are able to

keep the disease under control, and overall

potato yields for the province may even be

enhanced by the wetter growing conditions.

Occasionally, the wet weather may be so

continuous that the spraying program breaks

down because the farmer cannot get his machine

through the crop. This happened in 1954, with

the result that most crops were prematurely

defoliated and there were heavy yield losses.

Years like this occur about 1 year in 8. They

are reflected in high potato prices during the

autumn and early winter. In fact, Howatt has

gone so far as to define a bad blight year in

New Brunswick as a year in which the price of

table stock potatoes exceeds $3 per bushel

shortly after harvest!

QUEBEC. In this province, blight was severe

in 5 out of the 10 years from 1946 to 1955. The

importance of the disease varies with district,

and it is worst in the area around Montreal,

where the frequency of severe blight years is

about 1 year in 2. Along the south bank of the

St. Lawrence from St. Anne-de-la-Pocatiere

to Mont Joli, the frequency is about 1 year in

3. The chief climatic differences between the

Montreal and this area are the cooler growing

conditions along the St. Lawrence and the

tendency for dew formation to be less there

than in western Quebec, owing to the more

northerly latitudes with shorter nights.

Although the first field outbreaks of blight

in the province may occasionally be reported

in early- to mid-July (as in 1954), the disease

in general appears later in Quebec than in the

Maritimes; tuber blight losses rather than

yield reduction following premature defoliation constitute the main problem. The course

of events described for 1952 seems to be

typical of a severe blight year in Quebec

Province: **Late blight was first reported

in fields about August 6 . . ., and by August 18

it was known in many districts. Late blight

was slow in developing, but warm weather

accompanied by heavy dews and rain favored

its development in September, Many growers

dug their potatoes while the tops were still

green or partly destroyed by late blight and

stored the tubers in warm storage spaces.

Losses at digging time were as high as 10-

80 percent of the crop ... It Is conservatively

estimated that 10 percent, or 1 1/2 million

bushels of the crop, was lost. Fields dug

after the tops were killed with a herbicide

showed little tuber rot."

ONTARIO. The disease is most importantin

eastern Ontario, but even there the frequency

of severe blight years is only about 1 year in

4 or 5. During 1946-55, the worst outbreak of

blight in eastern Ontario appears to have taken

place in 1951, when it was described as follows: .**... late blight became epidemic by

mid-August. It was estinnated that about 10

percent of the acreage was slightly affected,

25 percent moderately affected, and 65 percent

severely affected. The reduction inyield, from

the premature death of the vines, varied from

0 to 25 percent, and averaged 5 percent. An

additional 5 percent was lost from decay of the

tubers,"

MANITOBA, SASKATCHEWAN, ALBERTA.

Blight is not important in these provinces.

The first record of blight in Saskatchewan was

made in 1946, and in southern Alberta in 1952.

The main potato-growing area in Manitoba lies

in the Red River Valley, which has been described in the U.S.A. section of this study.

BRITISH COLUMBIA. The most important

potato-growing area is the Lower Fraser

Valley, where the frequency of severe blight

years is about 1 year in 3. Over the period

1946-55, serious losses in the potato crop

have occurred in 1948, 1953, and 1954. The

greatest source of loss is due to blight infection of the tubers, and much of this is caused

by contamination with infected haulm at lifting.

BLIGHT AND WEATHER DATA

Figure 71 shows the long-term average air

temperature and rainfall conditions for three

centers across Canada. These are Fredericton, New Brunswick; Lethbridge in southern

Alberta; and Agassiz at the eastern end of the

Lower Fraser Valley in British Columbia. The

average temperature and rainfall at Charlotteto^Am, Prince Edward Island, has already been

shown in figure 69. These climagrams illustrate the very different conditions under

which potatoes are grow^n in Canada.

MARITIMES, QUEBEC, AND EAST ONTARIO

are very similar as to temperature and rainfall

regime in the potato-growing areas. Winters

are severe, so that the survival of potato

groundkeepers is unusual. During the summer

months, rainfall is well distributed, and very

high temperatures are unusual. The diurnal

range of temperature is approximately 20® F.

Summer temperatures are higher in western

Quebec and eastern Ontario than in the Maritimes, but very high temperatures are exceptional. The mean air temperatures for July,

148
CANADA

RAINFALL

Inches

RAINFALL

Inchcx

-Fredericton

NEW BRUNSWICK

RAINFALL 5

Inches

_ Lethbr

ALBERTA

50-F

dqe

/

/

-X

\

/

\

/

1

\

- /

/A

V/

* k it \ -

1 LIFT

-INC

y^ \

i

m\:„'/y s,-

1 -INC y/,

IrriqaHon practised

, Aqassiz

BRITISH COLUMBIA

70

60 „

50

40

30

20

FIGURE 71.—Long-term average monthly rainfall and mean temperanires in relation to growing periods for three centers in

Canada.

August, and September, together with the

average total rainfall for the July-September

period are compared in table 93 for five

centers in the region. The frequency of severe

blight years at each center is also given.

The greater severity of blight on Prince

Edward Island is most probably explained by

the higher humidity conditions on the Island.

This is not shown up by the table. Humidity

data for Prince Edward Island has been discussed on page 145 but, in general, Information on humidity Is scanty or lacking for the

region. However, the folly of dogmatizing about

humidity and blight are Illustrated by an Interesting observation made In New Brunswick

on the Interrelationship of temperature and

humidity with blight attacks. The Science

Service Laboratory at Fredericton maintains

a potato substation at Alma, on the coast about

60 miles west of St. John. Although humidity

conditions at Alma are high, due to frequent

coastal fogs, blight Is almost always worse

some distance Inland where hunaldlty Is lower.

This Is explained by the higher Inland temperatures; the lower temperatures on the

coast slow up the progress of the disease.

The lower rainfall In eastern Ontario partly

explains the lesser importance of blight there

as compared to the other centers (table 93).

The lower summer temperatures at St. Anne,

compared to those at L*Assomption, play a

part In the less serious nature of blight at

St. Anne. Particularly Important In Quebec

are the September temperatures, as the blight

attacks tend to be late In the province and the

main source of loss arises from tuber blight

Infection. The higher September temperatures

In western Quebec, combined with greater dew

formation, would seem to be an Importantfactor

In making blight losses greater than In eastern

Quebec.

MANITOBA, SASKATCHEWAN, ALBERTA.

Weather conditions In central and western

Canada, apart from the coastal areas, are

unfavorable to blight. Generally speaking,

precipitation during the growing season Is low

and evaporation high. Dally temperature

maxima are high and the diurnal range of

temperature Is around 25°to 30°F., as compared to nearer 20°ln eastern Canada. Relative humidities are low during the summer,

and this Is partly the outcome of the dry

warming winds that blow across the region

and the abundant sunshine.

TABLE 93. --Temperature and rainfall during growing season and frequency of blight years,

5 locations, Canada

Centers

Mean tenrperature Total rainfall,

July-Sept.

Frequency of

severe blight

July Aug. Sept. years

Charlottetown. Prince Edward Island

Fredericton, New Brunswick

L'Assomption, W. Qiebec

St. Anne-de-la Pocatiere, W. Quebec

DP!hi F Ontario

66

67

69

65

70

°F.

66

65

67

63

69

^F.

58

57

58

55

61

Inches

10.5

10.3

10.5

10.7

S.5

Percent

75

50

50

33

20

149
The climagram for Lethbridge in figure 7 1

illustrates the type of rainfall regime typical

of thé prairie provinces, i.e., a marked summer

rainfall peak. Even so, rainfall is deficient

and potatoes are grown under irrigation in the

area. Summer temperatures are lower at

Lethbridge than in the potato-growing areas

of Manitoba and Saskatchewan. However, blight

is not a problem.

LOWER FRASER VALLEY, BRITISH COLUMBIA. The climagrann for Agassiz shows

the longer growing season in this area than

in the rest of Canada. The mild winters allow

potato groundkeepers to overwinter and these

may serve as a source of blight inoculum.

Blight is checked by the drier conditions of

July and August, but high rainfall around lifting

time has made tuber blight infection a serious

problem.

CONTROL MEASURES

The blight control program in Canada follows very much the same pattern asthatin the

U.S.A. Recent accounts of methods have been

given by Callbeck (1950b and 1953). As in the

U.S.A., the spray program has to deal, in addition, with a number of insect pests.

The main differences between the two countries appear to be that (1) the control program

is generally less intensive in Canada (in comparing similar areas in the two countries, such

as Aroostook Co. with the Maritime provinces,

a greater number of sprays are applied in

Aroostook than in the Canadian region); (2)

there is a relatively greater use of copper

fungicides in Canada than the U.S.A. (although

organic fungicides are mainly used in Canada,

principally due to their lower cost and the ease

with which they can be applied, copper materials, in particular bordeaux (10:5:100), are

still used by a large number of growers); and

(3) there is a greater relative use of dusts in

Canada. Again, comparing the Maritimes with

Aroostook, dusting is carried out on a greater

scale in the Maritimes. According to Howatt,

up to 20 percent of the growers use dusts in

some years in New Brunswick, but inbad blight

years, many of those who nornnally dust, change

over to spraying.

In general, blight control is good in the

Maritime Provinces and the coastal areas of

British Columbia. In New Brunswick there is a

tendency to change over to low-volume spraying, though this does not seem to be happening

much elsewhere in Canada. The blight control

program is inadequate in many parts of Quebec

Province; while in central Canada few

measures are taken to control blight, as the

disease is not usually a serious problem.

One feature that enmerges is that not enough

growers are using haulm-killing methods.

BLIGHT-RESIST ANT POTATO VARIETIES

AND BLIGHT RACES

Two varieties, Keswick and Canso, containing the Rjgene for blight resistance and derived from Solanum demissum hybrids, have

been grown fairly extensively in Canada.

Shortly after they had been introduced Into

commercial practice they were found attacked

by specialized races of the blight fungus and

they are now sprayed against blight inthe same

way as other varieties. A comparison of the

foliage and tuber resistance to blight of 64

varieties, including some British ones, was

made under severe blight conditions in 1954

at St. Anne-de-la Pocatiere by Campagna

(1955).

Much work on the identification of blight

races from the different Canadian Provinces

has been carried out at Fredericton by Howatt

and his colleagues. The results of surveys

carried out by them, showing the distribution

of blight races throughout Canada, have been

published in the 1954 and 1955 Reports of the

Canadian Plant Disease Survey. The 1955 results are shown in table 94.

The amount of material sent in by the different provinces varied considerably, and the

survey does not pretend to be a complete one.

Both surveys in 1954 and 1955 showed race 4

to be predominant in Canada. Race 0, originally considered to be the comnaon race,

cannot be determined with accuracy in the

presence of other races, so that it is difficult

to show its importance. From the result of

surveys carried out in 1952 and 1953 at

Ottawa, Graham (1955) concluded that in

Canada, **. . . the so-called common race of

P. infestans is actually composed of Races 0

and 4.''

Recent work at Fredericton by Howatt and

Hodgson (1954) and Howatt and Grainger (1955)

has questioned the adequacy of the newly proposed international nomenclature for blight

races (fig. 16) on the grounds of the erratic

behavior of certain blight isolates on the differential hosts.

SUMMARY: CANADA

The Maritime Provinces are the worst area

for blight in the Dominion. A close analysis of

epidemics on Prince Edward Island has been

possible. In other parts of Canada a frequency

of **blight" years has been established from

the accounts given in the Reports of the

Canadian Plant Disease Survey. The major

source of loss from blight has been tuber

infection, particularly infection at harvesttime. This is closely linked with the short

growing and lifting periods for potatoes. The

control program is very similar to that in the

U.S.A.

150
TABLE 94. --Distribution of blight races in Canada, 1955

Race of Phytophthora

infestans

0..,

1..,

2..,

3..,

4..,

1,2..,

1,3..,

1,4..,

2,4..,

3,4..,

1,3,4..

Total,

Prince

Edward

Island

19

59

2

13

94

New

Brunswick

13

50

2

7

21

1

22

117

Nova

Scotia

2

6

12

1

5

3

12

43

Quebec

4

14

2

1

3

5

29

Manitoba Saskatchewan British

Columbia

17

Total

2

2

2

50

147

2

10

28

3

41

17

304

MEXICO

Potatoes are not an important crop in

Mexico. With a populationof nearly 30,000,000,

the total annual production of potatoes is only

about 150,000 tons (F.A.O., 1955). Atthenaost,

this allows an average annual consumption of

5 lb. potatoes per head per annum, but this

figure takes no account of the portion of the

total production which is used for seed. The

main reason for the low production of potatoes--about 80,000 acres are grown annually--

is the low yield per acre. In 1954, this was

1.8 tons per acre. The result is that potatoes

are very expensive and are considered a

luxury crop.

In Mexico (fig. 72), potato production can

be divided into two main types; (1) The cultivation of indigenous varieties at high elevations in the Sierra; and (2) the cultivation of

imported varieties at lower altitudes, usually

under Irrigation.

The Sierra is the principal area of production. Two of the most important native varieties grown in this region are Leona and

Amarilla de Pueblo. Inciported potato varieties

come largely from Denmark and Holland and

include Alpha, Furore, Gineke, Prunamel,

Bintje, and Up-to-Date. The imported varieties

differ from the indigenous potatoes inthat they

have a shorter period of vegetative growth

and are potentially higher yielders.

Potato blight is the chief factor limiting

potato production in Mexico. Potatoes are

grown chiefly where and when blight is not

likely to be a problem, i. e., at high elevations

in the Sierra and under irrigation during the

dry season, which occurs in the winter months

(fig. 73). Although the se conditions are inimical

to blight, they are also very unfavorable to good

potato yields and are the main factors responsible for the present low yields that are obtained. Growers found it impossible to produce

potatoes at lower elevations in the wet season,

owing to the earliness and rapidity with which

they were destroyed by blight.

A program was established in Mexico in

1951 by the Rockefeller Foundation, with the

object of improving potato cultivation methods

and exploiting new areas of production. Among

the areas that show possibilities of becoming

important centers of potato production are

those known as the **high valleys*' of the Mesa

Central (Central Plateau)--principally the

valleys of Mexico and Toluca. It has been

demonstrated during the last few years that

potatoes can be grown with much better results in these areas if use is made of the

rains. To do this it is necessary to grow

potato varieties showing sonne resistance to

blight and to protect them with an intensive

spray schedule.

BLIGHT EPIDEMICS

The greater part of the area in which potatoes are grown in the Sierra is situated above

3,000 meters, i.e., about 10,000 ft. Potato

blight Is not a serious problem at these

altitudes. The story is very different in the

high valleys. At Toluca, where systematic

blight observations began in 1951, the disease

has been devastatingly severe each year since

then, according to information from Dr.

Niederhauser. Its effect at this center can

be illustrated by what happenedto anunsprayed

crop of Alpha in 1956. The crop was planted on

May 15, and blight was first noted on the foliage

on June 20. The tops were completely dead by

July 25, and only a few marble-sized tubers

were produced.

151
MEXICO

FIGURE 72.--Mexico, showing potato growing areas referred to in text.

BLIGHT LOSSES

It can be seen that the effect of blight in the

high valleys Is to cause complete loss of the

unprotected potato crop. In the past, owing

to the low level of technical development in

RAINFALL

Inches

^ Puebla

7000ft.

_

-HTTTV/VZ}1

-

50

40

30

20

J F M A M J

*—CROPPING PERIOD-»

FIGURE 73.—Long-term average naonihly rainfall and mean temperatures for Puebla. Mexico: The rainfall regime is typical for the

potato growing areas in Mexico, but temperatures vary greatly,

according to altitude.

cultural methods, this has meant the virtual

elimination of potato growing from these

valleys during the summer months. Thus, the

cultivation has had to be in areas where,

owing to unfavorable climatic conditions, both

potential and actual potato yields are very low.

How serious this is can be seen from some

figures quoted by Niederhaus er and Cervantes

(1956) in which they compare the national

average yield of potatoes in Mexico withyields

of crops protected against blight and grown at

two centers in the high valleys. The original

figures were giveninterms of tons per hectare,

but have been converted in table 95 into tons

per acre.

The figures in table 95 relating to Chapingo

and Toluca are for small acreages of potatoes

only. Even so, the differences between potato

yields there and the national average yields

are most striking.

According to Dr. Niederhauser blight infection of the tubers is very slight (usually hard

to find) in the Sierra, and it is only a little

more prevalent in the high valleys. There

appears to be no good explanation for this.

152
TABLE 95. --Comparison of national average yields of potatoes with those at 2 centers where

potatoes were protected from blight, Mexico, 1952-55

Average yields of potatoes

Year Mexico

(national average) Chapingo Toluca

1952

Tons/acre

1.8

1.9

1.8

Tons/acre

9

7

11

10

Tons/acre

1953

17

195-4

13

1955

15

U

Thus, the main losses caused by blight in

Mexico result from the premature defoliation

of the potato plants by the disease.

BLIGHT AND WEATHER DATA

Figure 73 shows the average temperature

and rainfall conditions for Puebla, a potatogrowing district lying to the southeast of

Mexico City. The rainfall regime is typical of

that in the potato-producing areas of Mexico,

i.e., a dry season during the winter months

(November to April) and a wet season in the

summer (May to October). Rainfall in the

summer is often very heavy and continuous.

Mills and Niederhauser (1953) state that at

Toluca, in 195Z, rainfall was over 20 inches

in both July and August. Temperature conditions in the potato districts vary, mainly with

differences in altitude.

At Puebla (7,000 ft.) growers plant their

potatoes in late January, during the dry season,

hoping for just enough rain to grow a crop

but not enough to favor blight development. At

Leon (6,000 ft.) the farmer grows his crop in

the dry season under irrigation. Work being

carried on at Toluca (8,000 ft.) is designed to

show how potatoes can be grown during the wet

season (May to September or October), but this

can only be done if a rigorous blight control

program is followed.

In the Sierra (above 10,000 ft.), potatoes

are growninthe summer months. Niederhauser

attributes the lack of blight under these conditions to low temperatures. Unfortunately

weather records are not available, but this

state of affairs accords well with observations

made in parts of the Sierra of Peru, as reported by Bazán de Segura. She tells us that

blight has never been reported from the important potato-growing areas of the Province

of Puno, where temperatures average about

50° F. during the growing season.

In Mexico, potatoes have also been grown

at sea level as a speculative crop in the

states of Veracruz and Tamaulipas, near the

Gulf of Mexico, and in the state of Sonora,

near the Gulf of California. Blight has not been

observed on these plantings, which are grown

under irrigation during the dry season. Mean

temperatures during the summer (which exceed 80°F.) are too high for potato growing.

CONTROL MEASURES

It is apparent that the main way of controlling blight in Mexico up to the present

time has been to escape the disease. The

most successful of these escape methods has

been to grow the potatoes in the dry season,

under irrigation, chiefly at heights above

5,000 ft. where growing conditions are cool.

But irrigation is expensive and not generally

available.

The most hopeful means of obtaining economic yields of potatoes appears to be to make

use of the summer rains, and at the same time

to employ modern methods of blight control.

The workers of the Rockefeller Foundation at

Toluca and elsewhere have shown that this can

be done. Niederhauser and Mills (1953) have

discussed the growing of potato varieties showing a degree of **field" or **partiar'resistance

to blight (notably Alpha and Voran). It has been

shown that these varieties can be satisfactorily protected by ordinary spray schedules,

when more susceptible varieties cannot be

produced commercially even with regular

and thorough spraying.

Recommendations for the spraying program

have been made by Niederhauser and Cervantes

(1956). Generally speaking, the first spraying

should be at about the tinne the first rains are

expected. If the plants emerge from the ground

some time before the rainy season, it should

be given while the plants are still small (4 to

5 inches high). Usually the spraying should be

repeated at intervals of 10 to 15 days. At

Toluca, 6 applications are normally required

over the growing season, and sometimes 10

are needed. Both bordeaux mixture and the

organic fungicides have given satisfactory

blight control in the high valleys.

POTATO BREEDING PROGRAM

Mexico is the site of a most important

potato breeding program, sponsored by the

153
Rockefeller Research Foiindation. This program alms at the production of new varieties

showing complete, or almost complete blight

resistance. A number of factors make the high

valleys near Mexico City an excellent area for

the direct field selection of blight-re sistant

breeding nnaterial« Most, if not all, of the wild

species of Solanum that are highly resistant

to blight are found in central Mexico. These

species have been in close association with

Phytophthora infestans over a long period of

time. Mills and Niederhauser (1953) first

demonstrated that this association has given

rise to a number of naturally occurring biotypes of the fungus in the field. Since then,

Niederhauser, Cervantes, and Servin (1954a

and 1954b) have reported the natural occurrence of 14 of the 16 races of blight named in

the international nomenclature (fig. 16). In

addition, they have found an additional race,

provisionally termed ** 1,2,3,4,5,'* capable of

attacking a clone of S» demissum which apparently possesses a fifth gene for resistance

to blight. Thus, in central Mexico, the highly

specialized races of the blight fungus are

among the prevalent field races, and it is

under these conditions that the search for

blight-re sistant potatoes Is being made.

Field experience In Mexico has shown repeatedly that resistance to blight based only

on major genes, either singly or In combination, whichhave beenderivedfromS.demissum

or other wild Solanum species. Is of little or

no value. While this type of resistance confers

immunity to specific races of the blightfungus,

other races are either already present or

quickly arise to attackthe plants. Niederhauser

says, **We have not yet found a tuber-bearing

Solanum species that Is Immune to JP. Infestans at Toluca." Field or partial resistance

shows much greater promise. Niederhauser,

Cervantes, and Servln (1954a) have described

the Interaction between host and fungus, where

•'partial resistance" Is exhibited: *'Several

species. Including some clones of S. demissum,

show a high resistance to P. Infestans even

though foliage lesions are produced on them.

These resistant-type lesions are slow spreading, often turn ne erotic, and sporulation Is

sparse. Petiole and stem lesions tend to be

superficial and do not kill the distal portions

of the affected leaf or stem. Relatively few

lesions develop on a plant, and these are more

often on the lower older leaves." The genetlcal

basis of **fleld resistance" has not yet been

fully explored; major genes may or may not

be Involved. The Importance of field resistance lies In the apparent ability of this resistance of a variety to remain constant

despite specialization In the blight fungus.

The potato-breeding program In Mexico Is

now working to Incorporate a high degree of

such resistance In commercially acceptable

potato varieties.

A recent event In Mexico, of fundamental

and historic Interest, has been the discovery

of circumstances In which Phytophthora Infestans has aji active sexual phase (Niederhauser, 1956; Gallegly and Gallndo, 1957. Not

only have oospores been obtained readily In

culture, but at Toluca they have been foiind

on potato foliage In the field. Oospores are

formed abundantly by the pairing of two

compatibility groups of the fungus. In the

Initial experiments, these compatibility groups

were represented by sets of isolates from

Mexico, which were paired with isolates from

the United States and Canada. Both compatibility groups are present In the fieldIn Mexico.

Dr. Niederhauser writes: *'Having found

oospores in leaf lesions in the f?.eld, we believe

that the oospores probably do function in the

field, and enable the fungus to survive In the

field through the dry season."He continues:

"The finding of oospores. . . suggests that any

partial resistance, we find may be effective

against any race [of blight], since the pathogen

Is happily breeding as the season progresses

and sooner or later should come up with a race

that would take over. Furthermore, the partial

resistance that we have observed In^.

demissum and other species has apparently

been stable for many hundreds of years and

has enabled these species to survive."

High hopes, therefore, exist for the production In Mexico of commercial potato varieties

showing a high degree of resistance to blight.

To what extent such varieties would be adaptable to the main potato-growing regions of

the world, situated In more northerly latitudes,

remains to be seen.

SUMMARY: MEXICO

Mexico is an example of a country where

blight has largely been escaped. But the effect

of this escape on potato yields has been little

short of disastrous. The use of modernprotective methods against the disease has been

demonstrated at Toluca and elsewhere to be a

more effective method of overcoming blight.

The Imaginative action of the Rockefeller

Foundation, In establishing a potato-breeding

program In this country under conditions of

maximum severity of blight. Is paying handsome dividends.

154
SOUTH AMERICA

PERU

Peru is of particular interest in any study

connected with potatoes. Lake Titicaca, which

is partly in Peru and partly in Bolivia, is

regarded as the World center of origin of the

potato plant (fig. 74). In addition, potatoes

play a larger part in the diet of the population

of Peru than in any other country of South

America. The combined weights of potatoes

and other starchy roots consumed per head

in 1952 was 169 kg. (372 lb.), most of this

being in the form of potatoes. This is a rate

of human consumption comparable to that in

some of the large European potato-producing

countries, such as Germany and France. The

population of Peru is about 9 million persons,

and to produce enough potatoes for thena

requires a large acreage of the crop, as

the average yield per acre is very low. Over

the period 1948-52, 543,000 acres of potatoes

were grown annually, and these produced an

average yield of 2.3 tons per acre (F.A.O.,

1956).

Low yield per acre is in part a reflection

of inferior cultural methods, but a more

important factor is the unfavorable climate

in which the greater part of the potato crop

is grown. Production is carriedout indifferent

areas throughout the length of the country, but

PERU

Puno*^VN['H"ca

•Arequipa

FIGURE 74.—Key map of Peru.

all of these fall into one of two distinct zones.

These are the '*Costa,'* or Pacific Coast zone,

where potatoes are grown in the river valleys

at heights up to 9,000 ft.; and the "Sierra,"

or mountain zone, where they are grown from

9,000 to 13,500 ft. According to figures given

by Hawkes (1941), 30 percent of the total

production comes from the Costa and 70

percent from the Sierra. There is no reason to believe that these proportions have

changed appreciably in recent years. The chief

characteristics of the two zones are as follows:

COSTA: The haciendas (plantations) are

owned by Creoles or Europeans,

and worked on modern lines. Many

of the fields are large, and cultivations, including spraying, are

mechanized. In general, potato

growing is possible all the year

round, owing to the absence of

frosts; but, depending on altitude,

the main growing season is either

in the winter or summer. At Lima

(520 ft.) the growing season is in

the winter (May to November);

summer temperatures are too high

for successful potato production.

At Arequipa (7,600 ft.) the main

f[rowing season is in the summer

January to June). Annual rainfall

is very low throughout the Costa;

at Lima, average annual rainfall

is 1.8 inches, and at Arequipa, 4.2

inches, but sea fogs give rise to

high humidity conditions. It is

necessary to grow all potatoes

with channel irrigation. The majority of the crops are planted

with indigenous varieties, but some

North American varieties are also

gro^jvn.

SIERRA: Most of the country's potato production is in these regions. Yields

are lower than in the Costa. Cultivation is chiefly carried out by

Indians, and methods are primitive. Part of the crop is still

preserved as the traditional "papa

seca*' or chuno. Most crops are

grown during the sunnmer, without

irrigation. Rainfall is generally

adequate, but temperatures are

low (mean monthly temperatures

seldom exceed 55° F. during the

growing season), and this is one of

the main reasons for the low

potato yields. Slow-maturing indigenous potato varieties are

grown.

155
Much of the information on potato blight in

Peru has been provided by Consuelo Bazán de

Segura, of the Central Experiment Station of

La Molina, at Lima.

BLIGHT EPIDEMICS: GENERAL

Potato blight was considered of little or no

economic importance in the Costa until 1947,

when it caused heavy losses. No satisfactory

reason for the first appearance of the disease

in epidemic proportions at this time has so

far been put forward. Since 1947, there have

been a number of other years when severe

attacks of blight have developed, but the incidence of the disease has shown a considerable

variation from year to year.

Bazán de Segura (1952a) states that potato

blight has been troublesome in the Sierra

since 1949- There are variations in the importance of the disease between the different

regions in the Sierra, and up to the end of

1956 there had been no reports of blight

occurring in the important potato-growing

areas in the Department of Puno.

Under the Costa conditions, where seamists

favor the disease, blight remains localized in

the aerial parts of the potato plants and no

case has been observed oftuber infection. This

is due to the small amount of rain during the

growing season. On the other hand, in the

Sierra, tuber infection is a most important

problem. Msiny of the Indian growers in the

Sierra think that the leaf and tuber attacks

are caused by two completely independent and

different diseases. Reasoning in this way they

see nothing to be gained from saving the plants

by spraying or dusting if the tubers are to be

attacked by the '*other" disease. Little protective action against blight is taken in the

Sierra, but since 1947, a high proportion of

the growers in the Costa have been spraying.

BLIGHT ON THE HAULM

COSTA: Details have been suppliedby Bazan

de Segura of blight incidence during the period

1951-56 in the Carabayllo Valley, a potatogrowing district, 16 kilometers to the north

of Lima. These data are shown in table 96.

TABLE 96o--Blight incidence in the Carabayllo Valley, Peru, 1951-56

Year

1951.

1952.

1953.

1954.

1955.

1956.

First appearance

of blight

July 15

July 20

May 17

(M

(^)

June 15

' Blight of little importance.

75 to 90 percent blight

stage on unsprayed

potatoes by—

Sept. 8

Sept. 2

The disease was described as severe in 1952

and 1956. No record of the 75 to 90 percent

blight stage was made in 1951 and 1953, and

in the other 2 years, 1954 and 1955, blight was

of little importance.

The observations given in table 96 show

that even with this very limited number of

records there was considerable variation in

the time of the first appearance of blight--

between May 17 to July 20. The disease may

attack the potato plants from shortly after

emergence onward. Bazán de Segura (1950)

states that during a number of bad blight

years in the Lima area, it was very common

to find blight attacking very small plants

(5 cm. high).

The time at which the 75 percent blight

stage occurs in relation to the stage of

growth of the potato plants decides the scale

of the loss that blight produces through

premature defoliation. Bazán de Segura and

Lamas Carrera (1953) have described the

results of a spraying trial carried out in the

Carabayllo Valley in 1952 (a severe blight

year), when the amount of defoliation caused

by blight was measured at intervals, and the

produce of the sprayed and the unsprayed

plots was weighed. Blight assessments were

made by Horsfall and Barratt's method. The

effect of the disease on the control plots of

the two potato varieties in the trial is shown

in table 97.

156
TABLE 97. --Blight record on untreated plots at Carabayllo Valley, Peru, 1952

Variety Time of

planting

Date when tops

reached 75 percent

blight stage

Yield

Papa Blanca...

Papa Amarilla.

June lÖ-July 2

June lÖ-July 2

Sept. 4

Sept. 6

Tons/acre

1.3

.3

From planting until 75 percent of the tops

had been destroyed by blight was just over

2 months, and this accounts for the very low

potato yields. Blight attacks of this kind,

which occurred in the Carabayllo Valley in

both 1952 and 1956, obviously have a completely devastating effect on some unprotected

potato crops. But the position is complicated

by the wide range of tinne over which crops

are planted in the area. This stretches from

May to July. Without detailed survey information on this and other points, it becomes impossible to assess with any accuracy the

extent of blight losses in the Valley.

There are wide variations in the incidence

of blight among different growing areas in the

Costa within a particular year. This is so,

even where the areas are in close proximity.

For example, 1956 was a severe blight year

in the Carabayllo Valley, but the disease was

only present in trace amounts in the outskirts

of Lima.

SIERRA: Blight attacks are not so severe

on the potato tops in the Sierra as in the Costa.

The disease does not usually appear until 3 or

4 months after planting. There are still large

areas from which it has not been reported.

The course of a typical blight attack at the

Maco hacienda has been describedby Llaveria,

Revilla, and Sanchez (1955). Experimental work

is carried out at this hacienda in conjunction

with the Station of La Molina, and these workers

give the results of a fungicide trial against

blight carried out during th.e 1953-54 growing

season (table 98). Blight on haulm was measured by a simple scale numbering from Oto 4.

The potato variety grown was Maco.

Since the fungicide was applied by hand,

and the applications were repeated at frequent

intervals, it can be fairly assumed that the

yield of the treated plots (11.5 tons per acre)

shown in table 98 represents approximately

the full potential yield of the potato crop.

There was, therefore, a 36 percent loss in

yield on the control plots due to premature

defoliation by blight. This seems rather a

high figure, in view of the relatively long

time (130-135 days) between the time of

planting the controls and the time the 75 to

90 percent blight stage was reached on them.

However, it would appear that the normal

potato growing season in the Sierra is a long

one (around 170 days), and this is due to the

cool conditions and the slow-maturing indigenous potato varieties that are grown.

Bazan de Segura has told us that in a blight

year in the Sierra, the disease appears sometime in January and the 75 to 90percent blight

stage occurs sometime in February. This is

the most precise information that is available.

A marked characteristic of blight in the

Sierra is the way that the foliage attack is

localized almost entirely at the base of the

leaf petioles and on the main stems of the

plants. The leaf petioles become brittle and

are easily broken by the wind. The name

**Palo Negro" ("The Black Stick") given

to the disease by the growers has arisen

from this form of attack.

BLIGHT IN TUBERS

Tuber infection before lifting the crop does

not occur in the Costa. But according to

Llaveria, Revilla, and Sanchez (1955), there

have been many cases in the Sierra where

tuber blight, in conjunction with secondary

rots, has caused up to 30 percent loss of

crop. It is not stated whether losses on this

scale occurred before or after lifting, but

infection at lifting is a major problem. However, infection in the soil also takes place;

in the trial at Maco carried out in 1953-54,

the control plots showed 5 percent infection

with blight at harvest.

Lack of knowledge about the disease is the

main reason for losses occurring after harvest

in the Sierra. Very often the farmer's reaction

to the appearance of blight on the potato tops

has been to lift the crop, and this has resulted

in severe tuber infection.

BLIGHT AND WEATHER DATA

COSTA: Long-term average temperature

and rainfall figures for Lima are shown in

figure 75, together with the main growing

season for potatoes in the area. The crop is

grown under irrigation during the winter

months. Total annual rainfall is extremely

low (average 1.8 inches), and the small amount

of rain that does fall comes during the potatogrowing season. At this time, however, Humboldt current effects give rise to more or less

157
TABLE 98. --Blight record of Maco potatoes in plots at Maco hacienda, Peru, 1953-54

Plots

Time

of

planting

Blight outbreak

Date when tops

reached 75 to 90

percent blight

stage

Yield

Treated^ Oct. 10

Oct. 10

1st week of January

1st week of JanuaryAfter March 1

3d week of FebruaryTons/acre

11.5

Control. 7.4

The figures refer to the treatment which gave the best results; in this case, a copper dust.

RAINFALL

Inches

Costa Region

LIMA (520fr)

- SO'F

y^r¥77^77.

80

70 ,

60

SO

40

30

20

O N Dec I Jon F

LIFTING ,

A M J

, PLANTING

H

FIGURE 75.—Long-term average monthly rainfall and mean temperatures in relation to potato cropping at Lima, in the Costa

region of Peru.

permanent sea fogs, and days with even a few

hours of sunshine are rare.

Bazan de Segura (1950) has studied blight

incidence in the vicinity of Lima in relation

to weather conditions during the period 1945-

50. The main object of this work was to

formulate weather rules on which to base a

blight forecasting service for the area. Relative humidity and temperature data were

recorded at the Central Experiment Station

of La Molina. The study included a detailed

comparison of weather conditions in 1945 (a

no-blight year) and 1949 (a blight year).

In both 1945 and 1949 average relative

humidity figures remained above 75 percent

practically every day (24 hours) during the

growing season. The chief difference between

the two seasons was the higher relative humidities at night in 1949. In 1945, there were very

few occasions when a relative humidity of 96

percent or over was reached during the night

hours; whereas in 1949, relative humidity was

100 percent for many hours on nnost nights.

From these observations, it was considered

that a tentative rule based on nocturnal humidities could be put forward. This was as follows:

A nocturnal humidity of less than 95

percent should not endanger the crop; but

night humidities greater than 95 percent

will indicate the possibility of a severe

blight outbreak.

This rule is stated in very general terms,

and in particular, it lacks precision as to the

number of nights of high humidity that are

likely to give rise to severe blight conditions.

No mention is made in it of temperature conditions, and these are compared for 1945 and

1949 in table 99.

Bazan de Segura considered that the large

diurnal range of tennperature in May and June

1945 probably had an inhibiting effect on the

blight fungus, which in part was responsible

for 1945 being a no-blight year, but her general

conclusion from the study of weather data in

these and other seasons was that relative

humidity and not temperature is the principal

factor in blight outbreaJcs. Temperature conditions, particularly duiing the main part of the

growing season, are generally favorable to

blight. Table 99 illustrates the fact that the

diurnal temperature range is greatest at the

beginning and end of the growing period.

The nocturnal humidity rule given above

was used with success to issue spray warnings

against blight to farmers near Lima in the

winter of 1950. Unfortunately, however, the

personnel has not been available to continue

with this work since that year.

SIERRA: Precise weather data are not generally available for these regions. The winter

(May to Septennber) is a dry, cold season.

Potatoes are grown in the wet season, during

the summer months, from October to April,

but there are considerable variations in the

amount of rainfall at this time in different

parts of the Sierra, and temperatures also

vary, mainly with altitude.

Temperature and rainfall have been recorded

at the Maco hacienda since 1953, and sonne of

this information has been supplied by Bazan

de Segura. In figure 76 rainfall and tempe rature

data for Maco are shown in a '*no-blight"

season (1955-56) and a *'blight'' season(l953-

54). In 1953-54, blight first appeared at the

beginning of January and reached the 75 to 90

percent stage in the third week of February

on unsprayed potatoes. It seems likely that

the low temperatures in 1955-56 played an

important part in checking blight in this season, as the monthly mean figures were below

50° F. throughout the season.

158
TABLE 99. --Average monthly temperatures at Lima, Peru, in 1945 (no-blight year) and in

1949 (bligjit year)

Year and month

1945:

i^ril.....

toy

June

July

August....

September.

October...

1949:

J^ril

toy

June

July

August....

September.

October...

Mean minimum

55

45

46

50

52

53

47

55

53

51

4B

51

4Ö

50

Mean maYinrnm

Op-

§9

84

80

74

74

80

83

87

82

77

74

75

77

77

Diurnal range

34

39

34

24

22

27

36

32

29

26

26

26

29

27

Mean

72

65

63

62

63

67

65

71

68

64

61

63

63

64

Sierra Region

MACO (lM60fr)

BLIGHT SEASON 1953-54

RAINFALL

Inches

60'F 5

NO BUCHT'SEASON 1955-56

-

%

-

- V\ 1

-^^1ii

V,V/,^Y/,^

N Dec Jon F M

60 F

50

40

FIGURE 76.—Temperature and rainfall ponditions in a "blight** and

a **no-blight" year at Maco, in the Sierra region of Peru. The

potato growing season is from October to April.

Blight has not been recorded from the

potato-growing areas in the Department of

Puno, which is one of the colder regions of

Peru, with an annual mean temperature of 42o

F. According to Bazan de Segura, it seems

reasonable to suppose that the disease does

not occur there, because of the low temperatures.

CONTROL MEASURES

In the Costa, many of the growers use

fungicides (copper and dithiocarbamates)

against blight. Most of these fungicides are

applied as sprays with mechanical equipment.

Some growers start spraying when the potato

plants are only 2 to 4 inches high, and continue

at 4- to 5-day intervals, even in the absence

of the disease.

It might be possible to prevent the carryover

of the blight fungus from year to year under

Costa conditions, in view of the fact that tuber

infection in the soil does not occur. But it is

not clear to what extent the disease is perpetuated by the succession of potato crops that

are grown throughout the year in parts of the

Costa. Also it is not known how important a

part infected seed potatoes brought in frona

outside play in the development of epidemics.

Control measures are not nearly so common

in the Sierra, where it has been estimatedthat

not more than 5 percent of the growers take

action against blight. Llaveria, Revilla, and

Sanchez (1955) have mentioned some of the

factors unfavorable to blight control in these

regions. They say that the small peasant

farmers are in the main against spending

money to combat blight, as they are not yet

convinced that it is necessary. The topography

is also unfavorable for control measures. The

application of sprays is very oftenimpossible,

owing to lack of water. The cultivated land is

situated on the slopes of the hills a considerable distance from the streams, which flow

through the bottom of the valleys. The water

has to be carried in drums or cylinders,

either by mule or on trucks, and this is not

always possible, owing to the poor condition

of the hill roads.

In these circumstances, the use of dusts

applied by hand machines has been advocated,

and successful control of blight with such

materials has been demonstrated in the trials

at Maco in 1953-54. The prevention of tuber

infection at lifting is one of the most urgent

control measures required in the Sierra at

the present time. Recommendations to cut and

remove infected haulm 10 to 15 days before

lifting have been made, but it is doubtfulif this

advice is very often followed.

159
A suggestion for limiting blight losses in

the Sierra by "disease escape** hasbeenmade

by Bazan de Segura (1952a). She has proposed

that where it is possible to grow the potatoes

under irrigation, planting should be advanced

fronn the normal time (September-October) to

July-August. On the assumption that blight

will continue to appear at about the same time

(January), the effect on yield through premature defoliation may be much reduced. This

method, however, would appear to have only

a very limited application, owing to the lack

of irrigation facilities in the Sierra, and the

early-planted crops may suffer from frost

damage. Another approach to the problem on

sinnilar lines might be the growing of quicker

maturing potato varieties to replace the present indigenous sorts.

Work on the identification of a number

of different blight races in Peru and the

testing of potato varieties for resistance

to blight has been reported by Bazan de

Segura (1952b).

SUMMARY: PERU

Severe losses from potato blight in Peru

were not reported before 1947. Precise data

on the incidence of the disease from year to

year and between the different parts of the

coimtry are lacking; but available information

shows that heavy yield losses through premature defoliation occur at times both in the

Costa and in the Sierra. The main potatogrowing districts are in the Sierra, where

it would appear that low temperatures result

in a generally slow progress of blight on the

haulm. These low temperatures are also unfavorable to high potato yields. A major

problem in the Sierra is the danger of tuber

infection at lifting time. This arises principally from the general ignorance of blight

shown by the Indian growers.

CHILE

Potato blight was not found in Chile until

1950, but in 1951 the Chilean crops suffered

a first and devastating epidemic, comparable

in some respects to that in Ireland in 1846.

The story of this latter-day catastrophe has

been told by Müller (1952) who went out to

Chile as an F.A.O. expert in 1951 to review

the situation in the light of his European

experience and to assist the Chilean Government in the inauguration of control measures.

The progress of the disease in relation to

weather conditions, from the first outbreaks

until 1956, has been the subject of a special

and very detailed study by P. M. Austin

Bourke, who, also under the United Nations

Technical Assistance Program, but as a representative of the World Meteorological Organization, went out from Ireland in 1955 to assist

and advise on meteorological aspects of the

problem. Before Mr. Bourke left on his mission the authors discussed with him the particular data desired for Chile in the present

study of blight epidemics. The information he

has generously provided, in correspondence,

and in his U.N. (W.M.O.) report (Bourke, 1956),

has enabled us to compile a rather full section

for Chile, tracing the course of establishment

of the disease in a country where it was until

recently unknown, and exemplifying the use of

blight progress curves with the **Irish Rules"

for the definition of blight weather, in world

blight exploration.

POTATO DISTRIBUTION AND CULTIVATION

In Chile, which has a population of about

6 million and a total land area of some 286,000

square miles, potato growing is almost entirely

confined to a region extending from about La

Serena in the province of Coquimbo in the

north, to the islands of Chiloe in the south

(see maps, fig. 77). This region, which extends

over a thousand miles, may be divided into

three zones. In the northerndry zone, including

the provinces of Coquimbo and Aconcagua,

there is scarcely any rain in the summer

months, but there is potato cropping all the

year round with the help of surface irrigation.

In the southern zone, comprising the provinces

of Valdivia, Osorno, Llaiiquihue, and Chiloe,

CHILE

Each dot = lOOO ocrei

FIGURE 77.—Sketch maps of Chile, showing localities referred to in

text and approxünate distribution of potato growing.

160
abundant sumjmer rainfall occurs, and with

much lower temperatures in the winter, the

principal cropping is in the summer, with

early varieties in the late winter and spring,

much as in southern Ireland. In the central

zone, including all the provinces from Santiago

to Cautin, conditions are intermediate between

the two extremesOf the total of 57,724 hectares (145,000

acres) of potatoes grown in Chile in 1954-55,

as shown by the official statistics, 18,665

hectares were in the southern zone. This

zone, in which the greatest losses from blight

have occurred, is of special importance, not

only because of the high degree of local

dependence upon the potato for staple food

supply, but because, up to the present, it is

the only Chilean seed-growing area (Montaldo,

1950). The acreage in the northern zone is

relatively small, only 5,600 hectares. The

annual production of potatoes for Chile as a

whole, in recent years, has been about 6

million quintals (59,000 tons), with a mean

yield of about 4.3 tons per acre. The mean

annual consumption of potatoes per head is

about 17 5 lb., but in the south, where the

potato is the most important food crop, the

consumption is about 400 lb.

Before the appearance of blight in Chile

most of the potatoes grown were of the

**Corahila" type--a varied mixture of indigenous varieties with red skin and firm yellow

ñesh, all very susceptible to blight and rather

slow in bulking« Some European varieties,

such as President and Industrie, were grown

before 1951, and since that time there has

been an extensive substitution of Ackersegen

and Voran for the Corahila. Most of the potato

plots or fields in Chile are small, and manual

cultivation is the rule. Planting distances are

variable, but generally they are about 30 inches

between the rows and 15 inches between sets.

Earthing up is not widely practiced.

The c limagrams in figure 78 show the

long-term monthly average rainfall and mean

temperatures in relation to the cropping

periods for La Serena in the north and Puerto

Montt in the south.

THE BEGINNING OF BLIGHT IN CHILE

The first confirmed record of potato blight

in Chile was on October 17, 1950, when the

fungus was identified in the laboratories ofthe

Ministry of Agriculture on the foliage from a

crop growing in Mallarauco, a shelteredvalley

southwest of Santiago. In December of that

year it was also found on tomato plants in the

same district. It was then soon discovered

that the disease was present to a greater or

less extent throughout all the potato-growing

areas. Evidence later collected by Müller

(1952) showed that this blight or "tizdn" had

already been causing damage in some of the

smaller islands of Chilene in 1949. It does

not appear to have been present in 1937, for

a careful search was made for it in Chiloe

in that year by both Chilean and American

plant pathologists in connection with a seedpotato certification scheme, and no trace of

it was then found. Bourke narrows down the

date of introduction to some time between

1937 and 1948, and considers that it probably

came from the Argentine; not, as has been

suggested, by way of migrant labor returning

to Chilcie from the Argentine but by way of

ships* stores. The most severe blight epidemic

on record in- the Argentine occurred about

Buenos Aires in 1940-41, and it may have

been that diseased tubers were first brought

to Chiloe at that time by ships plying between

Buenos Aires and Chile round the southern

tip of the continent (see map, fig. 77).

Whatever the route of introduction, the

presence of the disease in parts of Chilo'e

by early 1949 has been fairly well established.

From study of the available weather records

Bourke found that conditions were almost

certainly unfavorable for the development of

the disease from then until early in February

1950, after which the symptoms were probably

masked by the dying down of the winter crop.

But conditions at that time, with many humid

periods and much heavy rain, were ideal for

tuber infection both in the ground and at

lifting. Oehrens (1953), in his unpublished

thesis, '*Tizón de la Papa,'* recorded that

many serious cases of rotting of tubers in

storage occurred in the southern provinces

during the autumn months (March to May) in

1950. Thus the disease, first recognized in

October 1950, was distributed throughout the

country in the seed potatoes from the 1949-50

crop in the south. It happened that the weather

conditions were exceptionally favorable for

blight in the following season, and it was then

that the first devastating epidemics occurred.

The old story of alarm, scientific inquiry,

and suffering of a peasant population was

enacted in miniature, but it was enacted

again.

The parallel between the first epidemics

in Chile in the years 1950 and 1951, and those

in Ireland in 1845 and 1846, is quite instructive.

In the first of the Irish Famine years the

attack was also late--not until September in

general; only a part (estimated at about a

quarter) of the crop was lost, but the seed

was universally infected. It also happened

then that weather conditions in the following

year were exceptionally favorable for blight,

and so came the really devastating and tragic

epidemic, with the tops killed, not inSeptember

but early in August, when the tubers were still

very small. It takes at least two blight years

to build up a first epidemic, and in this sense,

blight years are most serious when they come

in pairs.

161
RAINFALL

Inches

Northern Zone

LA SERENA

3 -

IRRIGATION

2 h it X^*^^^^^^^^-

ÍZ1 Awv^-^ J I \t i i\

70

H 60

50

40

30

20

CHILE

J A S O N Dec I Jan F

WINTER CROP ^

M A M J

iJWINTER CROP

SPRING CROP

SUMMER CROP

Southern Zone

PUERTO MONTT

RAINFALL e

Inches

FIGURE 78,—Long-term average montbly rainfall and mean temperatures in relation to cropping in the northern and southern potato growing

zones of Chile.

COURSE OF THE EPIDEMICS IN THE SOUTHERN ZONE

It is in the southern rainy zone of Chile that

blight is of the greatest consequence. Bourke

(1956) has collected and sifted all the available

information on the incidence of blight in this

region for each year since 1949, and he has

presented this in the form of estimated blight

progress curves. From these the probable

mean starting and 75 percent dates are taken

and lined up in table 100, as against the mean

temperature and rainfall figure s for December,

January, and February in each of the years.

There is perhaps a month's range each way in

planting and lifting dates, but typically the

maincrops are planted about the end ofOctober,

are aboveground by the end of November, beginning to form tubers in December, producing

most of their crop in January and February,

and dying down with little growthleft in March.

Thus the months of December, January, and

February in the southern zone of Chile correspond approximately withthe months of June,

July, and August in the south of the British

Isles. (See tables 3 and 29 for England and for

Ireland.)

The mean temperatures over the growing

period in this part of Chile are very similar

to those in southern Ireland, and the rainfall

figures, though perhaps varying more from

year to year, are also very similar.

162
TABLE 100. --Rainfall, mean temperatures, and deviations from mean temperatures in relation

to blight at Puerto Montt, Chile, 1949-56

Year

Rainfall Deviation from mean

temperature

Blight in

southern zone

Dec. Jan. Feb. Dec. Jan. Feb. Started

about--

75 percent

date about--

Average '

In.

4.4

3.7

4.6

3.7

.9

4.6

7.4

8.1

In.

3.5

9.5

7.0

1.1

6.0

2.7

5.4

6.1

In.

4.1

5.6

2.0

1.7

3.9

2.3

5.1

2.6

OF.

57

+1

0

+3

+3

+3

+2

+1

OF.

59

+2

-1

+2

+1

0

+3

-3

OF.

59

+3

-2

+2

+3

+1

+2

+1

Feb. 4

Dec. 8

Nov. 25

Dec. 23

Dec. 27

Dec. 3

Jan. 3

1949-50 --

1950-51 Mar. 31

1951-52 Jan. 31

1952-53 Feb. 28

1953-54

UhT, 26

1954-55

VhT. 31

1955-56

J/feir. 10

mr. 31

Long-term average ten^eratures are for 1911-40; rainfall averages are for 1911-48.

BLIGHT

ON

HAULM

per cent

lO

I950 1951

I950-5I

^

^

o - ^

y"

-

o

No records |<^

X X X X X 1

_ m^^

23

1 I

33

1.

1951 1952

oo 1951-52 /^

50

18 20

^

13

^

25

O !

l..l^^^ 1

1955 1956

lOO

1955-56

50 - ^^-

O ». 1

5 15 16

34

1

NOVEMBER DECEMBER JANUARY FEBRUARY MARCH

FIGURE 79.--Approximate progress of blight attacks in relation to blight weather spells for unsprayed maincrop potatoes in the southern zone

of Chile. The effective duration in hours ofthe longer spells is given; the shorter spells ranged from 1 to 10 hours. (Data from Bourke,

1957.)

163
In each year since 1950 blight attacks in the

southern zone of Chile have started at sometime between the end of Novennber and the end

of December. In 1950-51 the disease made

rapid progress, and by causing 75 percent

destruction of the haulm by the end of January,

it gave rise to a loss of about half the crop.

In all the subsequent years the progress of the

disease has been retarded after the outbreak,

so that it has not reached the 75 percent stage

until sometime in March, thereby causing little

or no defoliation loss, but considerable infection in the tubers.

To some extent the courses of the epidemics in the 6 years were consonant with the

simple mean temiperature and rainfall figures.

In the one severe blight year, 1950-51, the

mean temperatures were below average. In

four of the subsequent years the monthly

mean temperatures were 2° to 3^ F. above

average, and many of the rainfall figures were low. In 1955-56, however, when the

least severe epidemic occurred, the January

mean temperature was well above average, and

the rainfall in December and January was high.

Thus, although in southern Chile as in the

British Isles, high rainfall and nmean monthly

temperatures only slightly below average tend

to favor the development of blight and temperatures 2 to 3 F. above average with lower

rainfall tend to hold it in check, rainfall and

temperature figures alone are inadequate indicators of the occurrence of blight weather.

By study of the available humidity records

for southern Chile and the application of his

**Irish Rules" (see section on Republic of

Ireland), Bourke was able to obtain a fair indication of the **blight weather spells*' that

probably occurred in each of the 6 years. In

figure 79 these spells are here charted in relation to the estimated progress curves for

blight in the years 1950-51, 1951-52, 1955-56.

Bourke stresses the importance of the **effective duration" of the blight weather spells;

the longer spells are therefore distinguished

by taller blocks in the diagrams, and their

effective durations in hours are also given in

figures. Beaumont periods occurred in correspondence with most of these longer spells,

but were rarely associated with the shorter

spells of 1 to 10 hours.

Figure 79 shows clearly that in 1950-51 the

rapid progress of the disease was related to

the numerous spells of blight weather, some of

long duration, which occurred at short intervals throughout December, January, and February. In 1951-52, with several spells of long

duration in November and the beginning of

December, the attack began earlier than in 1950.

At first its progress was rapid, but thereafter,

until the beginning of February, there was only

one short spell and the progress of the disease

on the maincrops was arrested. In 1956, with

only short spells in December despite the high

rainfall, the attack did not start until the beginning of January. It was stimulated by along

spell at that time, but thereafter there was

only one short spell until the end of February,

and the attack did not become serious until the

haulm was dying down in March.

In the southern potato-growing zone of Chile

severe blight weather conditions such as those

accompanying the epidemic of 1950-51 have

not recurred in the subsequent 5 seasons, and

from a general analysis of past weather records Bourke estimates that such exceptionally

severe epidemics are unlikely to occur in tb^s

region more often than 1 year in 8 or 10.

**Blight years," in which the attack is more

moderate and much of the loss is by tuber

infection, will occur more often, but there is as

yet insufficient data to establish their frequency. Although the mean temperature and

rainfall figures during the growing period in

southern Chile are very similar to those in

southern Ireland, and the mean diurnal range

of temperature is also much the same (about

15° F.), blight weather evidently occurs less

frequently than in Ireland, and the general

incidence of blight over the years has so far

been much lower. Bourke attributes this to the

lesser frequency in this part, of Chile of young

atmospheric wave depressions with wide warm

sectors such as are responsible for the longer

and more important periods of blight weather

in Ireland.

EPIDEMICS IN THE NORTHERN ZONE

The extreme north of Chile is one of the

driest regions in the world; and the desert conditions do not change until about latitude 27°

S., where the mean annual rainfall is about

half an inch. From there southward, the rainfall progressively increases to about 5 inches

at La Serena (30^ S.), 10 inches at Valparaiso

(33° S.), and 100 inches at Valdivia (40° S).

The coastal potato cultivations in the northern

potato zone about La Serena are dependent

upon irrigation, but even so, potato growing

would not be possible over much of the year

but for the influence of the cold Humboldt

current, which ^trikes the Chilean coast at

about latitude 40 S., and sends a branch northward up to the coast of Peru. This ocean current cools the air above it, giving rise to much

sea fog and cloud and keeping the temperatures

low and remarkably constant. Thus, although

it i^ at about the same latitude as Brisbane

(30 S.), La^Serena has a mean temperature

of^about 52 F. in midwinter, rising only to

65 F. in midsummer.

A mean temperature of 65° F., though rather

high, is by no means high enough to inhibit

blight. Even so, the climagraun for La Serena

(fig. 78), with its striking dearth of rainfall,

would not suggest much likelihood of blight

attacks. The low temperatures for the latitude

and the proximity to the sea should give rise to

suspicion, but the diagram provides no direct

164
indication of the dense and wetting morning

fogs (camanchacas) which are common along

the coast and sometimes persist for muchof the

day.

In 1950 blight appeared in the winter crop

about La Serena--the disease having been

brought in with seed from the south--and

although the attack on the winter crop was not

then serious as the winter was cold, the

infection appears to have spread over, with

favorable weather in October and November

1950, onto crops of the spring and summer

plantings. According to Müller the attack

reached epidemic proportions from January

1951 onward, dimiinishing in November 1951.

The winter of 1951 was mild and the winter

crop in that year (sown about May) suffered

severely. Montaldo (1953), cited by Bourke, reported that a crop of the imported variety

Sebago, near La Serena during this winter

season, was a complete failure because the

newly emerged plants were heavily infected

from neighboring crops of the Corahila potatoes. Weather conditions were favorable for

the infection of spring-planted crops in November and December 1951, but the disease

disappeared around La Serena by January

1952. In subsequent years the disease has recurred to some extent in the winter and spring

crops, but the attacks have been less and less

severe and the summer crops have been virtually free except in 1955, when Bourke found

blight present on April 14, at the 10 percent

stage by the B.M.S. key, on almost every crop

he inspected in the La Serena region. This was

associated with quite abnormal rainfall in

March.

No survey records or eventhe most tentative

of progress curves are available for the northern zone, but it would appear that over the

summer months, with fairly hightemperatures

and virtually no rain, there is little blight,

despite the sea fogs and despite the surface

irrigation (which Bourke considers to have

no effect on the incidence of the disease). In

the La Serena district, however, blight weather

can occur in the winter months with mean

temperatures about 55° F., and extremely little

rain. Whether they occur or not depends upon

Humboldt current effects and not upon the

movements of atmospheric depressions, for

this part of Chile is beyond the northern limit

of their track.

CONTROL MEASURES

Immediately following the first epidemic in

1950-51, varieties having some marked field

resistance to blight, notably Ackersegen and

Voran, were brought in from Europe to replace

the very susceptible Corahila mixture of varieties. At first these **white" potatoes were not

popular, but the prejudice appears to have died

down, and Bourke reports that the more resistant varieties have now replaced the Corahila to the extent of 60 percent or more in

the areas most subject to blight. Breeding work

on other varieties is in progress.

Some protective spraying with bordeaux mixture and other copper fungicides is done inthe

La Serena and central areas, but the majority

of ^the growers in the south, particularly in

Chiloe, where the yields are small, are not

convinced that it is an economic control

measure. Müller (1952) advocated all the usual

cultural methods of control now practiced in

Europe after a century's experience of the

disease, notably earthing up of the drills to

protect the tubers from infection, precautions

at lifting, and measures to minimiize the inclusion of blighted tubers in seed stocks. He

also advocated the avoidance of winter cropping

in the south, to break the continuity of infection, and recommended the establishment of a

**tizón look-out service." Bourke has carried

this latter suggestion much further and shown

how a forecasting service can be operated.

He has also advocated quantitative blight survey work, on the lines of that carried out in

England and Wales in recent years, to establish the true significance of the disease in

the various regions and to guide control policy.

SUMMARY: CHILE

Potato blight was unknown in Chile until the

late 1940*s, when it was introduced probably

by infected ware tubers in ships' stores from

the Argentine. Outbreaks were identified in

many parts of the potato-growing regions late

in 1950, and in 1951 there was a devastating

epidemic that caused great distress and privation, particularly in the Chiloe region, where

the climatic conditions are rather similar to

those of southern Ireland. Fortunately, and

perhaps owing to less frequent persistence of

moist warm air masses over the area, severe

epidemiic conditions have not recurred since

1951, although the disease has been present

every year and has caused some infection of

tubers. Other severe epidemics are to be

expected in the future, but there is a hope

that their impact may be lessened by the introduction that has taken place of more resistant

varieties than the native Corahila. Severe

epidemics can occur in northern coastal areas,

despite very low rainfall, owing to the prevalence of fogs.

165
AFRICA

UNION OF SOUTH AFRICA

In the Union of South Africa, with a total

population in 1954 of 13-1/2 million, there is a

considerable production of potatoes. The

F.A.O. (1956) figures for 1948-52 show some

140,000 acres each year, with a production of

about 230,000 tons, indicating a mean yield of

under 2 tons per acre. The TransvaalHighveld

to the east of Johannesburg is by far the most

important center of production (see map, fig.

80), In this area, in which there are some

40,000 acres, the average yield is about 4 tons

(60 bags) per acre. There are lesser concentrations of production at similar cdtitudes

(5,000 to 6,000 ft.) to the east of the Orange

Free State, in Natal, and also on the lowland

along the extreme southern seaboard from

Cape Town to Port Elizabeth. The Union is of

particular interest in a world study of potato

blight, (1) owing to the striking contrast between the clinnatic conditions at an altitude of

5,600 feet in the Transvaal and those in the

coastal areas of the south, and (2) because in

the Transvaal the presence or absence of

primary infection appears to play a greater

part in determining the occurrence of blight

epidemics than is the case under European

conditions. There is little in the literature

concerning the incidence of blight inthe Union,

and for information the authors are indebted

to J. E. van der Plank, who has carried out

inquiries into recent epidemics, in part for the

special purposes of this study.

CLIMATIC AND CULTURAL CONDITIONS

Figure 81 shows the long-term average

temperatures and rainfall for Bethal, well

representative of the potato-growing areas

in the Transvaal Highveld; and also for George

and Cape Town in the south. The climagrams

suggest that in the Transvaal, with abundant

summer and low winter rainfall and the mean

temperature line above 50° F. from August to

May, there will be potato cropping only over

this period; whereas at George in the south

there will be cropping all the year round, and

around Cape Town winter cropping will be the

rule--the low rainfall and high temperatures

in the summer months being unfavorable for

the crop.

These indications are borne out in practice,

although at Bethal, with an upland continental

climate the diurnal range of temperature (from

54° to 80° F. in December and January) is

greater than in the maritime regions. Fine

warm days follow sharp cold nights, and the

potato-growing period is limited by the occurrence of frosts. The average date for the

last severe frost is September 20 to 30, and

for the first severe frost of the succeeding

winter about April 20 to 30, so that the growing

period during which the plants may be aboveground is from the end of September to midApril. Planting may be done in September, but

the first heavy rains are generally needed to

moisten the ground, and planting is not generally completed before mid-October. With nearly

optimum temperatures and plenty of sunshine

a 120-day growing period is adequate, and the

first crops will normally ripen in January.

When the crops are nearly mature they are

earthed up well and normally kept inthe ground

until they are to be marketed. This marketing

period extends from January (for the very

earliest crops) through the rest ofthe summer,

autumn, and winter until August. There is little

winter rainfall- The soil does not freeze to any

great depth; there may be surface penetration

of frost before the sun melts it during the day,

but there is no permanent freezing. Thus, there

is no clamping or other intermediate storage

of potatoes, as in Europe. They are left in the

ground and lifted, sorted, and bagged as required for market. Planting is in rows 36 to

45 inches apart, the widest rows (45 inches)

being those of farmers who like to earth up

very high for the storage period. Spacing between plants in the row is 16 to 22 inches. Many

growers start earthing up when the plants are

very small as a precaution against tuber moth.

The climagram for Bethal suggests that with

a 120-day growing period and withthe abundant

rain and favorable temperatures, late plantings

would be made up to mid-January to obtain

crops maturing in April before the frosts.

But, in fact, the second half of the long season

favorable for the growth of potatoes is little

used, owing to the incidence of blight.

Around George, 240 miles east of Cape Town,

rain is well distributedthroughout the year; the

climate is temperate, and near the coast the

area is free from frosts. Potatoes are grown the

year round, and blight is severe, especially in

the autumn and early winter. There isa constant

carryover of infe ctionfrom one cropto another.

Around Cape Town the climate is notably

different from that at George and also from

that in the Transvaal; it closely resembles

that of Western Australia. The summers are

dry, hot, and windy. The cropping period is

fronn March to October or November, utilizing

the winter rains. Severe blight epidemics occur

over the winter and spring and are the limiting

factors in potato production, which has now

fallen to a relatively small amount.

BLIGHT EPIDEMICS IN THE TRANSVAAL

HIGHVELD

In the 1955-56 season the potatoes, all Upto-Date except for a very few Kennebec, were

planted in September and October and were

closing across the drills by about the end of

November. On December 18-20, blight was

166
AFRICA : EAST 6 SOUTH

BE LG IAN

CONGO

FIGURE 80.—Key map of East and South Africa: Shading indicates areas where potatoes are grown.

167
SOUTH AFRICA

.Transvaal Hiqhveld^

BETHAL ,

S,600f» ,

RAIN FALL

Inches

I^CROPPINC PERIOD*|^-CROP IFFT IN CROUNDBLICHT

ON

HAULM

per cent

lOO

50

O

.South Coastal

CEORCE

RAINFALL

Inches

CROPPING ALL THE YEAR ROUNDCape Town

RAINFALL

Inches

JASO

SPRING CROPPING h«-WINTER CROPPING

FIGURE 81.—Long-term average monthly rainfall and mean temperatures in relation to potato cropping for three centers in the Union of

Soutb Africa, with blight curves for the Transvaal.

found scattered over the whole area, and by

January 12, 1956, only a few green leaves

were left* The weather then cleared and further

spread of blight was stopped. On December 20,

the estimated crop for the whole area was

2,300,000 bags of 150 lb. each. After the epidemic the actual yield was 1,850,000 bags. Of

the drop, 350,000 bags, or 15 percent of the

expected yield, was attributed to loss by premature defoliation; and 100,000 bags, or about

4-1/2 percent,, to rotting of the tubers in the

ground from blight infection and other causes.

The loss in individual fields depended on the

age of the crop at the time of the blight attack.

The rainfall and mean temperature figures for

the season were:

Rainfall Mean temperature

Month: (inches) ^*

September 0 57

October 5.5 60

Novemiber 5.3 63

December 8.4 64

January 1-3 65

The 1955-56 season was worse than average

for blight, the attack being earlier than usual;

this may have been associated with the high

rainfall in December, but there were also dull

and overcast conditions in late December, continuing into January. The progress of the disease was stopped by the dry weather later in

January, and the few leaves then remaining

green were not further attacked.

Precise information is not available for

previous years. In general, blight starts toward

the end of December and does not reachthe 75

X>ercent stage until the end of January, when

growth is over in most of the crops. Indications

of the probable mean blight progress curves

for most years, and for the bad blight year

1955-56, are given in the climagram (fig. 81).

The estimated loss of 15 percent of the crop

by defoliation in 1955-56 is fairly consistent,

with some 2 to 3 weeks' loss of growth in

this particular year.

In most years the loss from blight is slight

in crops planted as early as possible (in

September and October), as these are already

dying down (in January) before much defoliation by blight occurs. The major loss arises

from the fact that the incidence of blight renders

it impossible to utilize the second half of an

otherwise excellent season for potato growing.

The market quality of potatoes left in the ground

from January deteriorates by sprouting and

other causes by July; and were it not for blight,

second plantings would be made in December

or January to mature before the first frosts at

the end of April and to provide supplies of potatoes in good condition after relatively short

storage, to supply the winter market. The result is a scarcity of good potatoes onthe market

from July onward. Crops planted in December

or January would be cut down by blight while

the plants were still small, and thus there would

be little or no yield. Experience noW being won

in the high valleys of Mexico (q.v.) seems to

point a way toward overcoming this loss of

the second crop.

SOURCES OF INFECTION

On the Transvaal Highveld it cannot be assumed that primary sources of blight infection

are always present to start an epidemic when

weather conditions become favorable for its

development. As the climagram indicates,

there is almost always abundant rainandfavorable temperatures for blight from October

168
onward. Although many of the earliest planted

crops are well forward and in flower by November, no blight has ever been seen in that

month. In the 1955-56 season, despite

exceptionally heavy rain during October, November, and December (49 inches in 11 weeks

at Harrismith) and squelching fields, no spot

of blight was found in the Highveld up to

December 31.

The seed from the Highveld crops saved for

planting in the following year is riddled off

from the ware, or taken from crops specially

intended for seed. In either case it is lifted

late, and it is often stored with a light straw

covering under the dense canopy of evergreen

trees, which gives adequate protection from

the frosts. It may be that this combination of

circumstance s--the late harvesting of seed

after it has lain during the autumn in warm

soil and the temperate winter climate--is

unfavorable to the survival of Phytophthora

infestans in the local seed, so that in some

years there is little primary infectionfrom this

source.

It is possible thatthe first infection may come

in from other areas. It is knownthat the disease

occurs in winter crops of potatoes and tomatoes in the frostfree. areas near the east coast.

Some support for this idea comes from the

fact that blight epidemics normally occur in

summer crops in the coastal province of Natal

before there are any outbreaks far inland, in

the main production area of the Highveld.

Lastly, there is undoubtedly some blight

infection in imported seed. Much seed is still

imported annually, mainly from Scotland. Because of the opposite seasons, the imported

seed arrives late, and is mostly planted from

the second half of November onward, fully 2

months after the planting of the local seed,

and this might explain the timing of the blight

epidemics, which commonly begin, not in

November as might be expected frominfection

in the local seed, but in December.

CONTROL MEASURES

Spraying plays practically no part inHighveld

farming. At least one farmer with a large

acreage and proper equipment sprays regularly

and says it pays him, but the spraying is

basically with DDT to control tuber moth.

Zineb is added for blight, but a part of the

advantage may come from control of early

blight. Alternarla solani, which is very common, and sometimes more injurious than the

late blight caused by Phytophthora infestans.

Some seed of Kennebec, an Ri resistant

variety, has been introduced, but in 1955-

56 it suffered almost as severely from

blight as the Up-to-Date variety. Two R1R2

types (Onikwa 52 and Onikwa 53) have been

tried in the Cape Province, but are not

promising. Blight races 1; 1.2; 3; and 4 are

certainly present in the Union, and skepticism

is felt about the value of varieties containing

the R-genes, singly or in combination. Anote

on three sorts of resistance to late blight by

Van der Plank (1957) distinguishes between(1)

R-gene resistance; (2) field resistance associated with late maturity (as in Voran and in

the Dutch Robijn now grown in Kenya); and (3)

field resistance not associated with late maturity (as in Sebago).

On the whole, the only important control

measure in the Highveld has been to plant

early for blight escape; but this is effective

only at the cost of a sacrifice of supplies for

the late market, which commands the best

price. Attempts are being made to breed

varieties of about the same maturity as Up-toDate but with tubers having a longer period of

winter dormancy. One variety of this kind is

already under test and shows promise. It is

considered that breeding to this end may in

some ways be an easier task than breeding for

disease resistance, which is also under way.

Dr. Van der Plank thinks the Highveld area

may be so isolated from airborne infection

in other areas and have storage conditions so

unfavorable for the survival of the fungus in

the tubers, that epidemics might be suppressed

by growing only local seedor by heat treatment

on the Jensen principle of imported seedto insure its complete freedom from blight infection.

Against this there is the fact that the humidity

conditions, as apart from rainfall and temperature, of the Highveldhave not yet been investigated. It is possible that records of blightweather spells by Bourke's Irish Rules (see

section on Chile) or of Beaumont periods (see

England and Wales) might reveal that the

relatively late outbreaks (in December) are due

to a general absence of critical blight weather

before that time, rather than to any lackof the

very few and widely scattered primary infectors that are necessary to start an attack over

a wide area (see Van der Zaag, 1956).

SUMMARY: UNION OF SOUTH AFRICA

In the Transvaal Highveld, the principal potato-growing area of the Union, the major

loss from blight is a loss of the whole of the

second half of an otherwise ideal potato-growing season. Late-planted crops could be used

to provide potatoes for the winter market if

the disease were controlled. Even with re striction to early planting for disease escape, losses

are still occasionally severe, as for example

in the 1955-56 season when the loss was about

20 percent of the expected crop. The frostfree

areas of the Southern Cape, the only part of

the Union.with adequate winter rainfall, should

be the natural source of out-of-season crops;

in the past it was usual for Cape potatoes to be

sent in quantity to northern markets in the

winter. For many years, however, blight has

limited potato production in this area, and the

Cape now imports potatoes from the north. In

Natal, crops of boththe January andthe September plantings are severely attacked in some

years, and the damage is of particular consequence in seed crops for the Transvaal.

169
SOUTHERN RHODESIA

Conditions of potato growing in Southern

Rhodesia (fig- 80), where according to F.A.O.

(1955) there were some 5,000 acres in 1952,

with a production of about 16,000 tons, are

in some respects similar to those in the

Transvaal, but higher temperatures in the

winter months permit cropping all the year.

G. R. Bates reports that more than half of

the Southern Rhodesia crop is produced within

a radius of about 35 miles of Salisbury. Since

blight became established in that locality in

1952-53, there has beenan increasing tendency

to explore new areas for potato growing, and

quite substantial amounts are now grown in

the Midlands, around Bulawayo, and on the

high ground between Salisbury and Umtali.

Both seed and ware are also grown throughout

the mountainous eastern districts, where there

are humus-rich friable soils. The tubers have

an excellent keeping quality--a characteristic

rare in other areas of Southern Rhodesia.

CLIMATE AND CULTIVATIONS

The rainfall pattern in the Salisbury area

somewhat resembles that at Bethai (fig. 81).

There are 6 dry months and 6 rainy months.

The rains start with thunderstorms in midNovember, increase to January and February,

which is the wettest period, and end in March,

with a few thunderstorms in April. May is dry,

and the weather then remains dry until midNovember. July is the coldest month, but here

the mean July temperature (56 F.) is well

above the 50^ level, in contrast to that at

Bethal (46° or 47°).

Three potato crops are grown during the

year--the main, or summer crop, which is

usually planted in November with the onset of

the rains, and harvested in February; a firstirrigation crop, planted in March andharvested

in June; and a second-irrigation crop planted

at the end of July and harvested in October.

Frosts normally occur in June and July.

For irrigation crops a typical rotation is

potatoes, followed by maize--for two seasons

at least--during the summer, and wheat or

barley in the succeeding winter, followed by

green cropping or summer fallow. Potatoes

are grown usually every third or fourth year

on the same land, but the rotation is now being

widened. Until recently, the variety grown

has been almost invariably Up-to-Date, which

is very susceptible to blight but has a strong

consumer preference, is well adapted to the

latitude, and has a fair tolerance to leaf roll.

It is also drought resistant.

BLIGHT INCIDENCE: 1946-53

J. C. F. Hopkins, who was at Salisbury from

1926 to 1953, has told the authors that there

were no records of potato blight in Southern

Rhodesia until 1946, when it was found on

Up-to-Date in the Enterprise district of Salisbury, on a first-irrigation crop about May 10.

In the particular crop of 13 acres on which it

was then found, there had been 13 irrigations

before flowering, and with 2 inches of rain in

May, conditions were unusually wet. Although

most of the foliage was killed, little if any

loss of crop occurred, and all the tubers were

well developed. A little blight was seen on

neighboring farms that year. It is not known

how the blight was introduced; the affected

crop was grown from local seed. Potato

supplies for troops during the war may have

brought in the disease.

Nothing further was seen of blight after

1946 until 1951-52. In 1952 there was asevere

attack in the eastern seed-growing area about

Inyanga (fig. 80), and by 1953 the disease had

spread to the main Salisbury area. Dr. Hopkins

conducted a seed-potato growing scheme

(summer crops) onthe mountains about Inyanga,

and between 1940 and 1947 there was no blight

in the area. In 1949-50 the Portuguese started

potato growing with imported seed just over

the Mozambique border, at over 7,000 ft. in

the mist belt near Manika. Blight attacks

becajne very severe, and it is possible that

southeasterly winds during the winter spread

the disease across the country.

EPIDEMICS SINCE 1953

G. R. Bates and W. E. Kerr have reported

that blight has been severe every year in

Southern Rhodesia since the first general

epidemic in 1953. This has been due mainly

to a succession of very wet summers. In the

summer, or main, crop, overall blight losses

have been estimated as never less than 25

percent, and occasionally they have reached

50 percent. Individual summer crops on some

farms have been a complete failure. In each

of the 5 seasons blight was noted in the Salisbury district between December 20 and January 10, except in 1956-57, when, following an

unusually early and prolonged rainy spell in

mid-November 1956, the disease appeared

before the end of that month.

The pattern of infection among the firstirrigation crops has varied considerably. In

1953 and 1954 it was sporadic and losses did

not exceed 10 percent. Subsequently, losses

of up to 30 percent have occurred. This has

been due to very wet seasons with extensions

of the rains into May or even June, and also

to a tendency among the Salisbury growers to

plant the first-irrigation crop earlier and

earlier, with the result that the summer and

first-irrigation crops have overlapped. Irrigation is mainly by the furrow method, although

overhead irrigation is becoming more common.

With furrow irrigation, blight tends to attack

170
the stems and leaf stalks rather than the

leaves; but this may perhaps be a characteristic of the potato variety, for stem lesions are

very common on Up-to-Date, even when it is

grown under natural rainfall in the southwest

of England.

No blight has yet been observed on the

second-irrigation crop, but this matures during

the hottest period of the year and the keeping

quality of the tubers is poor.

CONTROL

All attempts to control blight by spraying

have failed on the sunamer crops, chiefly

because of difficulty in obtaining adequate

protection under conditions of torrential rain.

More promising results have been obtained

on the first-irrigation crops, particularly

with the dithiocarbamates.

A fair amount of work has been done on

resistant varieties. Little prospect of success

is expected until more attention is devoted,

as it has been during the past 2 years, to

cultural practices and the choice of varieties

for the various croppings according to their

several cropping characteristics. R-gene

resistance has proved a failure for summer

cropping conditions in the varieties tested

(Kennebec, Pentland Ace). Better results have

been obtained with fie Id-resistant varieties,

such as Furore, Voran, Robijn, and inparticular. Pimpernel; but all these have their

disadvantages, and search is continuing for

more acceptable field-resistant sorts. Recent

records show that blight races 0; 1,3,4; 3,4;

4; and 1,4 are present in summer crops, and

races 4 and 1,3,4 in irrigation crops.

Disease-escape methods are being used

and explored. The seed crops along the eastern

border, which once were threatened with

extinction because of the devastating blight

attacks, are now planted much earlier to

escape the most dangerous blight-period

(December-January). Elsewhere inthe country

the main (summer) ware crops can be planted

before about mid-October only where supplementary irrigation is available. Field trials

over the past 2 seasons have shown the value

of planting in the third week of September

under such conditions, and heavy yields have

been obtained. Ulster Prince is being tried

for very early summer planting to escape

blight.

EAST AFRICA

The problems of potato growing in East

Africa have been reviewed by Moreau (1944b),

who has given an admirable account of the

particular localities in the mountains of

Northern Rhodesia, Nyasaland, Tanganyika,

Kenya, and Uganda where the crop is grown,

as well as a critical study of rainfall and

temperature conditions. The production of

potatoes over the whole of this vast area is

small and very localized, and with some

exceptions, the '^English*' potato is a minor

crop, providing but a small part of the native

diet. In Uganda in 1955, for example, there

were 500,000 acres of beans, 520,000 of

cassava, 1,585,000 of coffee, 425,000 of groundnuts, 380,000 of maize, 1,218,000 of millet,

220,000 of peas, 1,414,000 of plantains,

582,000 of yams and sweetpotatoes, and only

15,000 acres of the ^'English" potato, or

Solanum tubero sum. Most of the potato cultivations are primitive, and often they are a

luxury vegetable that the natives can sell for

a good price despite the small yield, which

is often less than that of the earliest **new**

potatoes in Europe. Cans of such potatoes

are frequently sold by natives at the roadside

to passing long-distance lorry drivers.

Nevertheless, with good husbandry excellent

crops of S. tubero sum can be raised in suitable areas, and the cultivation of this crop

is becoming more important. Moreau (1944b)

concluded that the short-day conditions were

of no practical consequence, and that there

was no reason to suppose that S. andigenum

potatoes would do any better than suitable

varieties of S. tubero sum. According to F.A.O.

(1955) the potato acreage in recent years has

beenabout 12,500 in Kenya, 2,500 in Tanganyika,

15,000 in Uganda, and 2,500 in Northern

Rhodesia. Mean yield figures are scanty and

are not too reliable, but actual yields probably

range from 1 or 2 tons per acre in native

cultivations interplanted with beans, to 4, or

very exceptionally, even 8 tons per acre with

good European husbandry. Potato blight has

invaded most parts of East Africa only within

the past 20 years. Watts Padwick (1956) has

attempted to assess the approximate losses

now caused by the disease from data received

in response to a questionnaire sent out as

part of a Colonial Plant Disease Survey. For

all parts of East Africa he has put the percentage loss at 10 percent or more. For the

purposes of the present study, Kenya has

been considered at some length, as typifying

the equatorial mountain cultivations. The

sections on Uganda, Tanganyika, and Northern

Rhodesia are therefore shorter.

171
KENYA

KENYA

^^500ft. contour

Site of first

outbreak,1941

^Meru KENYA

'{^'^-MT. KENYA

2°N

EQUATOR 0°

POTATO CROWING AREAS

European

MI. KILIMANJARO

Africon

200n

FIGURE 82.—Key map of Kenya, showing principal potato growing areas and localities referred to in text.

Potatoes have probably been grown in Kenya

for as long as the Highlands have been settled

by Europeans. The acreage, however, is

relatively small and yields are low. According

to F.A.O. (1955) the mean area for the crop

in the years 1948-52 was about 12,500 acres,

with a mean production of about 21,000 tons,

or 1 3/4 tons per acre. With a population of

about 6 millions, the annual consumption

of potatoes, together with other starchy roots,

was estinnated at only about 20 lb. per person.

Blight was first observed in Kenya in 1941

by R. M. Nattrass, to whom the authors are

indebted for a first-hand account ofthe disease

in Kenya from its beginnings up to the present

time.

DISTRIBUTION OF POTATO GROWING

AND CLIMATIC CONDITIONS

As shown by the key map (fig. 82), potatoes

are now grown on both sides of the Rift Valley

at altitudes between 6,000 and 9,000 ft.,

between latitudes 2° N. and 2° S. ofthe Equator.

The cultivation is in both native and ''settled"

areas within this altitude range. In all native

areas the potatoes are produced as a food

and as a cash crop. There is some specializa172
tion in potato growing here and there by

European farmers, but this is an indication

of inclination rather than topography. Almost

all the area between 6,000 and 9,000 ft. is

suitable, but the best parts lie within 7,000

and 8,500 ft.

The climagrams (fig. 83) illustrate the

mean rainfall and tennperature conditions at

two centers, respectively, west and east ofthe

Rift Valley. These figures show the general

pattern of the rainfall, but the actual precipitation in individual months is exceedingly

variable. East of the Rift there are two

plantings. One, the **long rains** crop, is

planted in March and lifted about July, and

the other, a *'short rains** crop, is planted

in October or November and lifted about

February. The **short rains** crop is often

light, as the rainfall is considerably less

than that in the **long rains** crop. West of

KENYA

. Wesr of Rifr Valley

SONCHOR

RAINFALL

Inches

.Easrof Kift Valley

NAIROBI

5,700ft

RAINFALL

Inches

O N D

•^SHORT RAINS CROP —

FIGURE 83.-Long-term average monthly rainfaUand mean temperatures in relation to potato cropping, west and east of the Rift VaUey

in Kenya.

the Rift the rains are heavier and more

evenly distributed over the year. There is

one planting in April or May, with lifting in

August or September.

During the rains at the higher altitudes,

especially east of the Rift, there are periods

of mist and much drifting light rain in addition

to the heavy falls. So far as temperature is

concerned, there cannot be said to be any

regular succession of spring, sununer, autumn,

and winter. The seasons are dependent upon

the advent of rains, and it is only lack of

rain that stops potato cropping at any time

of the year. The mean temperature lies

between 60 and 65 F. the whole year ro\ind,

and the controlling temperature variation in

the highlands is the diurnal range, which

becomes excessive on all days when clouds

are absent. The mean diurnal range at Nairobi,

however, is 18° (52° to 70°) in August. It

rises to 23 or 24^ only in January and February.

THE BEGINNINGS OF BLIGHT IN KENYA

Dr. Nattrass first found the disease in

May 1941, about the middle of the east Rift

on the **long rains'* crop (see climagram).

The focal point appeared to be on high land

at about 8,000 ft. near the eastern escarpment

of the Rift Valley 60 miles north of Nairobi

(see key map). The affected variety was

Kerr*s Pink, in a crop for which imported

seed had been planted. On his return to

Nairobi, Dr. Nattrass found the disease in a

garden at 7,000 ft., about 20 miles west of

the city. Within the following few weeks

reports of the **new disease** began to come

in from practically all the potato-growing

areas east of the Rift, particularly from the

Aberdares Range and Mount Kenya--a large

tract of country including extensive areas

of Kikuyu reserve. By the middle of November,

to quote one agricultural officer, **the disease

had assumed epidemic proportions over the

whole of this area; squatters* shambas were

heavily infected, and the crops in many parts,

almost, if not wholly, a failure.'*

The disease did not appear across the

Rift Valley until a year after the original

outbreak; the first notification of the disease

on the west escarpment was in June 1942.

A month later it had reached the foothills of

Mount Elgon on the Uganda borders. From

this point it spread westward into Uganda

and eventually into the Belgian Congo. Southward, Tanganyika was reached in the same

year (1942). One isolated area inthe Usambaras

in Tanganyika held out until 1943, although

heavy attacks occurred in the closely settled

slopes of Mount Kilimanjaro. In 1944 the

Tanganyika plant pathologist reported that the

disease was present in all the important areas

of the Territory.

173
SOURCE OF THE FIRST INFECTIONS

Until 1939-45, potato growing was almost

entirely confined to African production,

although one or two European farmers had

imported seed and grown commercial crops

from time to time. With the low prices then

prevailing, however, commercial production

of potatoes by Europeans was much restricted

or even nonexistent. Apart from squatters*

production on European farms, which supplied

most of the European market demand, there

was (and still is) some growing of potatoes

at higher altitudes as a native food crop to

replace maize. The native crops were not

pure varieties but mixtures of odd sorts long

since gone out of cultivation by Europeans.

At the time of the blight outbreak a number of

Europeans were growing what had then be come

a lucrative crop on account of Service demands.

Kerr*s Pink was the favorite variety, and a

large consignment of it was imported from

the United Kingdom for the 1941 crop. This

extension of potato growing coinciding with

an exceptionally wet season in 1941 may

possibly have given rise to circumstances

more favorable for the introduction of blight

than there had ever been before. Dr. Nattrass

is convinced that if blight had developed on

any previous occasion in Kenya, it would almost

certainly have become endemic and could

scarcely have escaped observation. Except

at high altitudes, there are no frosts to kill

the haulms; once potatoes have been grown on

any piece of land it is practically innpossible

(with ordinary cultivations) to prevent groundkeepers coming up. They grow like weeds and

may persist for years.

CONTROL MEASURES

The onset of the new disease in 1941 caught

the Europeans unprepared for any direct control measures, and native cultivators were in

any case unable to do anything about it. The

effect was devastating on the European Kerr's

Pink, and some of the miscellaneous native

varieties were completely destroyed. Two of

these native varieties, however, the **Kinongo

Kerai*' (since identified as Northern Star)

and another, known locally as **Kigezi Blue'*

(later identified as Skerry Blue), showed

considerable resistance to the disease. The

first step therefore was to advise that these

two varieties should be collected and multiplied

with all possible speed.

The importation of blight-resistant hybrids

for test under the local conditions began in

1943, and this testing, centered at the Scott

Agricultural Laboratories at Nairobi, has

continued since that time. In the early days

a suggestion from Reddick and others that

Kenya should produce its own short-day potato

was investigated, and some breeding work

was done with Solanum andigenum. Beyond

the raising of some fantastic hybrids with the

ordinary domestic varieties this achieved

nothing. The main work has been with Black's

R-gene resistant hybrids from Scotland, of

which about 60 have been tried up to the

present time. These R-resisters were at first

very successful, being immune from attack

by the blight races 0 and 2,4, then the only

races that appeared to be present. Some of

these varieties were grown on a fairly large

scale, and although they were eventually

attacked, they are still in cultivation. As new

blight races appeared, seedlings withrnore

genes for resistance came from Dr. Black,

and were tested against them. So far the blight

races identified from Kenya are 0; 2,4; 4;

1,2; 2; and 1,3,4, (See fig. 16 for these race

designations.) Race 3,4 has also been identified from Kenya, but on tomato. Some of the

above races were found for the first tinne in

material from Kenya.

In 1950 Phytophthora infestans was found

on the leaves of perennial woody Solanums

(S. indicum, S. panduraeforme, and S, incanumí

in widely separated localities of bush and

forest (Nattrass and Ryan, 1951). The races

of the fungus so far identified on these wild

shrubs are 2,4 and 4. S. incanum is susceptible to some races but resistant to others;

another species, the treelike S, aculeastrum

is susceptible to all races. The importance of

these wild hosts is that they flourish in the

wetter forest regions where blight conditions

are very favorable, and they can act as a

perennial source of infection.

A number of Black's resistant varieties

have been distributed over the country in both

European and African areas. They have proved

to be resistant to the dominant blight races

and are now in considerable demand. Others

are still being tested, and the two most

promising varieties to suit the local conditions

are a RjR^ and R3R4.

The most popular commercial variety in

Kenya at the present time, however, is the

Dutch Robijn, which was introduced from

John Clarke in 1945 and has no R-genes. It

stands up to blight better than most varieties

in Kenya, other, than the R-resisters, and it

has largely replaced Kerr's Pink. Dr. Black

has informed the authors that Robijn was

bred fronn Roode Star x Preferment, and

that it is a variety with a fairly high field

resistance, which seems to suit the Tropics,

as it has also done well in trials in Southern

Rhodesia. On a scale from 1 to 5 for field

resistance in which 1 denotes completely

resistant and 5 completely susceptible.

Dr. Black has placed Robijn, from the results

of his trials, at 3.5, which compares with 3

for Voran and 4 for Kerr's Pink. It is thus

more resistant than Kerr's Pink, and, presumably like Kerr's Pink, it is tolerant of

the short-day conditions. Kerr's Pink, so

extremely popular in Ireland, appears suited

174
equally well to the extreme long-day conditions of Iceland (see Iceland section) and

to the extreme short-day conditions in Kenya.

The second nnost widely grown variety in

Kenya is now the native Kinongo. A few

European growers plant a little Majestic and

Up-to-Date.

Protective spraying as a means of control

has been considered and full directions have

been issued (Nattrass, 1944) for the conduct

of the operation, both with freshly prepared

bordeaux mixture and with cuprous oxide and

copper oxychloride preparations. Only a very

little spraying has been done, and the real

hope of control is through the introduction of

resistant varieties. Blight has been a serious

disease on tomatoes in Kenya ever since its

introduction, and it is on tomatoes that control

by spraying has real possibilities. A full

account of the disease on tomatoes is given

in Nattrass (1950).

COURSE OF EPIDEMICS AND MAGNITUDE

OF LOSSES

In Kenya the potato plants may be attacked

at almost any stage of growth, and conditions

are too variable over the range of potatogrowing areas for any estimate of loss of

crop from the date at which the haulm is

destroyed. The worst blight years have been

1941 (the first year of attack), 1946, 1951,

and 1955. No surveys have been attempted to

establish the actual timing of the attacks or

the extent of losses in various years; indeed,

such surveys would be extremely difficult to

carry out, owing to the scatter of the cultivations and the many factors other than blight

contributing to low yields. In general, it can

be said that slight attacks tend to coincide

with partial failure of the crop from lack of

rain; while with abundant rain and luxuriant

crops the blight attack becomes severe.

The introduction of potato blight has certainly not prevented the continuation and expansion of potato growing in Kenya; it has

merely introduced yet another factor contributing to the variability of production in various

years and districts. The situation in 1951 to

1955, 10 years or more after the first epidemics, may be seen from the notes on potato

production in the Annual Reports ofthe Department of Agriculture. In 1951 excessive rains

reduced yields in the Nyanza Province and

blight was prevalent, both there and in the

Central Province. The Dutch Robijn showed

greater resistance thaji other varieties to the

adverse conditions and to blight.

In 1952, owing to the failure of the short

rains, plantings were small in the Nyanza

Province. In the Central Province there was

a shortage of planting material created by

the heavy blight attacks of 1951, and throughout Kikuyuland the short-rains crop suffered

a good deal from blight. In Meru good crops

were harvested. In 1953, yields were heavy in

the Meru district and moderate elsewhere. In

the Nyanza Province a fair crop was harvested.

Owing to dry conditions, blight was less

severe than usual, and planting of Robijn was

increasing. In 1954, the crop did well in the

Central Province, with Kiambu coming into

production again. Production in Meru slumped

for security reasons. Blight and paucity of

resistant seed were reported as the limiting

factors to increased production. Yields in

Kericho were above average. Stocks of Robijn

were being built up gradually in all districts.

In 1955, a steady increase of acreage was

reported in the Central Province, except

Meru. Long-rains crops yielded well in all

districts but short-rains crops wBre severely

hit by blight. Production was increasing in

Nyanza, where the output at Kericho had

quadrupled since 1950. Sales from African

areas increased from 687 tons valued at

£14,410 in 1954 to 800 tons valued at £17,300

in 1955.

SUMMARY: KENYA

Blight was first found east of the Rift Valley

in 1941, and by 1942 it had appeared in both

European and African cultivations in the

equatorial highlands on both sides of the Rift.

Mean temperatures between 60 and 65 F. all

the year round, with heavy and variable rainfall, double cropping and perennial groundkeepers, all favor the incidence ofthe disease;

but attacks are very variable from district

to district and season to season. Much work

has been done on the determination of blight

races, and Kenya has beenfoundto be a country

in which much specialization of the fungus

can occur. Several races have been found on

solanaceous forest trees and shrubs. Rresistant varieties have been extensively

tested, many have gone down under local

conditions, but some are still of value and

others of promise. Robijn, a variety with no

R-genes but with some useful measure of

field resistance has largely replaced Kerrys

Pink as the most popular commercial variety.

UGANDA

The authors are indebted to Margaret A.

Keay, of Makere College, Kampala, for a

review of the available information for Uganda.

Potatoes are grown entirely by African peasant

cultivators, and the acreage is very small

compared with that planted to other crops. At

one time there were some potatoes in Buganda,

but the climate in this central province is the

175
TABLE 101. —Potato acreage in Uganda, 1946-55

Year Western

Province Buganda Eastern

Province

1946

1,000 acres

1.7

2.1

4.3

6.1

7.9

8.0

8.9

9.0

9.2

11.0

1,000 acres

1.5

1.1

2.4

.7

.7

.8

1,000 acres

1947

1948 0.4

1949 .7

1950 4.0

1951 4.4

1952 2.5

1953 2.6

1954 3.1

1955 4.1

least suited for them, and cotton and coffee

provide all the cash crops required. At the

present time the largest acreage is in the

Kigezi district in the Western Province, at

altitudes of 5,000 to 7,500 ft., but there is

also a smaller acreage at the same altitude

range in the Eastern Province near Mount

Elgon. Table 101 shows the acreages in the

three provinces over the 10 years 1946-55,

as given by the Annual Reports of the Department of Agriculture.

According to the Annual Reports, blight

spread rapidly into Uganda after it was first

found in Kenya in 1941, and in 1942 and

succeeding years it destroyed much of the

crop in the Kigezi district. In 1948, when the

rainfall at Kabale in this area was 30.2

inches, as against the long-term average of

39.2 inches, there was less damage. In 1945

and 1946 the disease was recorded at Kawanda,

near Kampala. Although blight appears to be a

serious factor in reducing yields in Uganda,

and particularly in Kigezi, it has not prevented a steady increase of potato growing iii

that area, and this is clearly brought out by

the acreage figures. The increase is attributable partly to greater consumption of potatoes

by the African producers themselves, partly

to their need for a cash crop, and partly to

a greater demand for potatoes from an increasing urban population. Under good conditions yields are 2 to 3 tons per acre, but

most yields are probably lower, owing to

the poor standard of peasant cultivation.

The long-term average rainfall and mean

temperature conditions for Kabale in the

Kigezi area are shown in figure 84. As at

Nairobi in Kenya, the rainfall distribution

is typically bimodal. There are two cropping

periods--planting in April for lifting in August

before the September rains; and planting in

October for lifting in February before the

March rains. In some places the main rains

occur about March-April and in other places

that are quite near they may be in OctoberUCANDA

^S.W.Uqanda

KABALE

6,138 ft.

RAINFALL

Inches

A M J J

I CROPPING PERIOD - - CROPPING PERK»-»

FIGUES 84.—Liong-term average monthly rainfalland mean temperatures in relation to potato cropping at Kabale, in southwestern

Uganda.

November. Mean temperatures are somewhat

lower than at Nairobi, being between 61° and

63 F. in every month of the year. The district

is, moreover, notorious for its mists. It is

hilly, with very steep slopes to the valleys,

the bottoms of which are filled with swamps.

The valleys are narrow and there is considerable shadow effect. A very high incidence

of cloud occurs; the mean total amount of

cloud, on a 0 to 10 scale, at 14.30 hr. of daylight being from 7.5 to 8.5 in every month of

the year. Apart from the high diurnal' range

of temperatures (from 49° to 74°, averaged

over the year), the climate would appear to

be almost ideally favorable for blight.

No information is available to indicate the

timing of the epidemics, or their varying

severity in differing years. Potatoes were

introduced into Kigezi about 1913, andthey are

considered to have done better before blight

arrived. Sonne part of the increased acreage

may be due to the need to plant more acres

to compensate for blight losses, but the main

reason for the increase is because more

potatoes can be sold.

176
Control by spraying is out of the question

for the very poor subsistence cultivators. The

small plots are scattered over a wide area, and

it would be burdensome in the extreme to

carry spraying or dusting equipment up the

precipitious slopes. The variety Toro Blue

(probably the old variety Skerry Blue) has

shown a useful degree of field resistance.

Blight on tomatoes has now been reported

in Uganda. Baldwin( 1956) records it at Kawanda

near Kampala, where he found it late in 1954,

but not in 1953. Dr. Keay found it on tomatoes

on Makerere Hill in the second half of 1954

and again in 1955.

TANGANYIKA

According to F.A.O. (1955) the acreage of

potatoes in Tanganyika is very small indeed,

the figure given for 1948-50 being only about

2,500 acres, with a production of about 2,000

tons. E. A. Riley, at Lyamunga, reports that

the real extent of potato cropping is probably

quite unknown. The cultivations are mainly by

Africans, and are located in areas about

the Usambara and Ulugurus Mountains,

Kilimanjaro, Meru, the Porotos Mountains

near Mbeya, Bukoba, and North Mara by Lake

Victoria. (See key map, fig. 80.)

Blight first occurred in the territory in

the Kilimanjaro area in July 1942, following

the outbreak in Kenya in 1941, and it spread

rapidly to other areas in Tanganyika,

practically wiping out potato growing in

the Usambara.

Control has been attempted by spraying,

but this is almost impracticable under the

conditions of peasant cultivation, and nothing

is known of the circumstances that lead to

outbreaks. A number of fairly resistant potato

varieties were introduced into Tanganyika

from Kenya about 1945, including one labeled

**Blight Résister No. 914.•' This later went

down with blight, and Dr. Black in Scotland

found blight race 1,3 on it. Before this only

races 0, 1, and 2 had been recorded. After

1945 eight of Dr. Black's R-resistant varieties

were introduced from Scotland. One of these,

1521 (c), has withstood blight so far and has

resulted in a revival of potato growing in the

Usambara. Seed is being multiplied there for

distribution elsewhere. Dr. Black has told

the authors that this seedling when tested

with race 1,2,3,4 was graded 1 for field

resistance, and this bears out its performance

in Tanganyika, where it has now been grown

for 7 years with 2 crops each year and no

blight has appeared on it.

In the Annual Report of the Department of

Agriculture for 1955 it is stated that the main

potato markets in Tanganyika are Dar es

Salaam and Tanga, and that these are supplied from the Ulugurus and Usambara

Mountains, respectively. Potatoes are a staple

food of Africans in the Porotos and Ukinga

Mountains of the southern Highlands, where

some 7,000 tons were said to have been produced in 1955. There was no export market,

but some were sold in tins to drivers of longdistance transport. The quality of the nativegrown potatoes has never been good, partly

because of blight and partly because the crop

is usually allowed to grow from tubers left

in the ground without any further cultivation.

Nevertheless, the native growers expect a

very high price, and in some seasons they

lose their market, because it is cheaper to

import potatoes from South Africa.

Moreau (1944b) records that in trials in the

Kilimanjaro area potatoes of native type yielded

3 tons per acre, while in the Ulugurus up to 5

tons were obtained with Abundance, and in the

Usambara Mountains even 8 tons with Arran

Chief.

NORTHERN RHODESIA

Riley (1955) has reported that potato blight

was first found in Northern Rhodesia at Nkana,

on the Copperbelt, by Mr. Hadfield of the

Nkana Mine garden on April 19, 1955. On that

date it was seen on the foliage of tomatoes but

not on potato plants nearby, which were then

quite young (about 6 weeks). The disease

progressed rapidly on the tomatoes, and later

the potatoes (Up-to-Date) were affected. Complete destruction of the 4 acres of tomatoes

and 2 acres of potato haulm ensued. The

temperatures during the period 12 to 19 days

before the outbreak were about 81° F. daily

maximum, and 55° daily minimum (mean 68°),

and there had been much cloudy weather.

A. Angus, at Mount Makulu Research Station, Chilnga, has informed the authors that

the disease appeared again, on an epidemic

scale in 1956. It was found on the Copperbelt

at Nkana on potatoes and tomatoes on April 1,

and at Luanshya on potatoes on March 3. It

was also found on potatoes at 2 places and on

tomatoes and potatoes at 1 place in the Broken

Hill area between April 4 and May 18, 1956;

and on tomatoes at 3 places and on tomatoes

and potatoes at 1 place in the Lusaka district,

between April 18 and September 26, 1956.

Moreau (1944b) notes that potatoes were

grown along the railway line from Chôma to

Broken Hill (latitudes 17© to 14o S.)at altitudes

between 3,200 and 4,500 ft. The disease would

now appear to be established in this area, and

also further along the railway to Nkana. In

Northern Rhodesia there is also some potato

growing at latitude 9° S. near Abercom. F.A.O,

(1955) puts the total potato acreage of Northern

Rhodesia at about 2,500 acres in 1953, with a

production of about 2,000 tons.

177
WEST AFRICA

Few potatoes are grown in West Africa, but

the first occurrence of blight innative cultivations of the crop in the highlands of Baxnenda,

in the Southern Cameroons, 6^ N«, so graphically described by T. A. Russell (1954), is

one of the incidents of special interest in the

recent history of the disease about the world.

J, M. Waterston at Ibadan and J. D. Tallantire

and his colleagues at Buea provided information supplementing that given by Russell and

carrying the story up to 1956.

As recounted by Russell, potatoes are to

be found mostly above 6,000 ft. in small tilled

patches carved out of the prevailing grassland.

The cultivation is very primitive and is done

entirely by the women of the tribes with no

other tool than a hand hoe and a stout knife.

In the area to be planted the grass is burned

at the beginning of the dry season, about the

end of November, and the surface soil is

scraped into raised beds, commonly about 6

feet by 4 feet. In this loose soil the potatoes

which have been saved for seed are planted

with the first rains in March, and seed of

other crops such as maize and kidney bean,

is often inter-planted. The first crop is taken

in July, and the second crop, planted in late

August, is harvested in December. Yields of

2 tons per acre are regarded as good.

Figure 85 shows the location of the area

and figure 86 the rainfall and temperature

conditions in relation to the croppings. The

climagram is for 1952, but the general pattern

of rainfall and temperature has been similar

in succeeding years, except that the months

with the heaviest rainfall (reaching 15 inches

CAMEROONS

/

8'N

NIGERIA

4**N

// 3000ft contour

bOOOft. contour

FRENCH

C A MEROONS

GULF OF GUINEA

Fernando

ISO miles

FIGURE 85.--Cameroons, West Africa, showing the Bamenda locality.

178
FIGURE 86.—Monthly rainfall and mean temperatures in relation to

potato cropping in Bamenda Province, South Cameroons, 1952.

or more) have been sometimes in July, sometimes in June, or sometimes in September.

There is a dry period from December to

February. The niean temperature is near 70°

F. for most months of the year, falling slightly

in May or June to September with a low of

about 65° in August. The mean diurnal variation in May and June is about 15° (from 60° to

75°). Comparisons with the climagrams for

Kenya and Uganda (figs. 83 and 84) show that

the conditions are similar to those in other

equatorial mountain regions where blight

now occurs, but mean temperatures in the

Cameroons are distinctly higher, except in

the June-September period. Rainfall and temperature figures for May and June in each of the

individual years 1952-56 are given in table 102.

Potatoes were probably brought to Bamenda

and the adjoining land about Dschang in the

French Cameroons during the German occupancy of the territory before 1915. Since then,

they have been adopted by the native tribes as

an important part of their diet. Until 1952 the

crops were free from the disease, but in May

of that year there was a report of destruction

of the first crop by a disease starting near

the French boundary and sweeping over

Bamenda Province. The symptoms suggested

Phytophthora infestans, and during a visit in

November 1952, the presence of the fungus

was confirmed in every plot of potatoes

throughout the area. Russell was able to

ascertain that the disease appeared in the

French part of the Cameroons in 1951, a year

earlier than on the British side. Some ware

potatoes had been imported from France, and

occasionally some of these had been planted.

There was therefore a possibility that the

disease was brought in from Europe. Mr.

Tallantire supplied the data in table 102,

which show the incidence of. the disease on

the British side since 1952.

In the early crops, lifted in July, weather

conditions in May and June probably determine the severity, or rapidity, of the blight

attack. In 1956 the dry June may have been

partly responsible for the light attack, but in

the other years no obvious differences in

rainfall or mean temperature can account for

the differences, which were probably due to

variation in humidity conditions. With the

very high rainfall, and no doubt much mountain

fog, mean temperatures up to 70° F. in May

and June do not appear to have restricted the

development of the disease.

POSSIBILITIES OF CONTROL

Russell records that spraying with 1 percent bordeaux mixture at 10-day intervals

TABLE 102. --Incidence of blight and climatic conditions in South Cameroons, West Africa,

1952-56

Year Blight incidence

Rainfall Mean

temperature

May June May June

1952 Disease aüDeared and croD wiDed out

IÍ1.

4.0

5.9

10.3

9.0

8.1

In.

7.6

9.2

1Ö.6

1.Ö

OF.

70

es

67

66

OF.

69

1953

1954

1955

1956

Widespread attack and heavy losses

Widespread attacks destroying early crops^ September

plantings not so badly affected.

Attack started in late May, 2 or 3 weeks later than

usual. For the first time.(since 1951) fair-sized

tubers were on sale in the markets.

The lightest attack so far since the disease was

recorded.

67

67

65

67

179
over the growing period failed to prevent the

destruction of the crop. With such heavy rainfall it may be impossible to protect the foliage

without an altogether uneconomic number of

applications. Control by spraying or dusting

is hardly a suitable measure for adoption by

impoverished and backward tribes.

The introduction of resistant va^rieties

appeared to offer more hope of success. In

March and April 1953, a number of R-gene

resistant seedlings or varieties from Scotland,

Canada, and the U.S.A. were planted at the

Government Farm at Bambui within a field

of the local susceptible variety. The B.M.S.

key was used for the assessment of blight on

the trial plots, and on June 16, 1953, where

the local variety (planted March 26 to April 8)

showed 50 to 95 percent blight, most of the

resistant seedlings showed only 0.1 to 5 percent. The score in Kennebec, Pungo, Placid,

Canso, and Keswick from the U.S.A. and

Canada was then 0 percent, and in Cherokee

0.1 percent. Later news from Mr. Tallantire

in 1956 indicates that of the 18 varieties tried

only 2 of Dr. Black's seedlings from Scotland, 1521(c) and 1521(d), have shown real

promise of standing up to the disease and

have given yields of up to 2 tons per acre.

SUMMARY: CAMERCX)NS

Blight, probably introduced from Europe,

attacked primitive potato cultivations in the

highlands of the Southern Cameroons for the

first timie in 1951-52, causing almost complete loss of the crops. In subsequent years

the severity of the attacks has decreased,

possibly owing in part to some natural selection of the more field-resistant types in the

native varieties. Two R-gene resistant seedlings have shown promise in trials. The crop

is important, but even a good yield is only

about 2 tons per acre.

180
ASIA

INDIA

Although the potato acreage in India (fig. 87)

has been increasing during recent years, potatoes are still a relatively unimportant food

crop. In 1950, for example, the total area of

cereals (rice, millets, wheat, etc.) in India

was 193 million acres, as against just over

half a million acres of potatoes (Russell, 1954).

INDIA

ASSAM

Figure 87.--Key map of India.

Potatoes are grown for the ware market. They

remain something of a luxury crop, owing to

their low yield, which averages approximately

2-1/2 tons per acre. The average consumption

of starchy roots (principally potatoes) is 241b.

per head per year (F.A.O., 1955), but actual

consumption varies considerably over the

country, as the growing of potatoes is concentrated in relatively few areas and transport

facilities from these areas are not good.

DISTRIBUTION OF POTATO GROV/ING

AND VARIETIES

R. S. Vasudeva, who has provided much of

the information for this section, has included

detaüs of the acreages of potatoes grown in the

different Indian states during the season 1954-

55. These were as follows:

State: 1,000 acres

Uttar Pradesh ^"^^

Bihar - - - 10^

West Bengal 1^5

Assam 11

Other States JLAl

Total ^^5

None of the other States, after Assam,

grew more than 21,000 acres of potatoes.

Thus, production is seen to be concentrated

in northern and northeastern India. Of the

potato crop 95 percent is grown on the plains

of northern India. The remaining 5 percent

is grown in the hills, up to a height of 9,000

ft., partly to provide a source of seed for

some of the plains' crop. Important hillgrowing districts are around Simla and Darjeeling in the north, and the Nilgiri hills in

the south. An account of potato-growing methods for the plains and hill crop in the Uttar

Pradesh has been given by Mitra (1954).

According to Pal and Pushkarnath (1951), 16

local and 38 European potato varieties have

been identified under cultivation in India. The

main varieties, covering the greatest acreage,

are Phulwa, Darjeeling Red Round, Up-to-Date,

Magnum Bonum, Gola, Satha, Great Scot, and

Italian White Round. The poor quality of much

of the seed of these varieties is one of the

reasons for the generally low level of potato

yields.

CLIMATIC CONDITIONS AND

CROPPING PERIODS

The most striking feature of the Indian

climate is the rainy summer monsoon. The

monsoon rains occur throughout the country,

but with a wide range in duration between different localities. In general, winter is a dry

or very dry season. The potato crop on the

plains is grown during the winter months, as

summer temperatures are too high for successful cultivation (see climagram for Patna,

fig. 88). Incidentally, even the winter temperatures are too high for potatoes on the

low ground in most of peninsular India, and

this partly explains the concentration of potato

growing in the north of the country. Two crops

of potatoes are grown on the plains. The first,

planted shortly after the monsoons in September or October and lifted from December to

March, is the more important. This is known

as the '*winter crop" and is planted with seed

from a previous plains' crop, which has been

stored during the summer. Formerly, storage

was in sand in thatched mud houses, when

losses through storage rots were very great,

but this method is now being rapidly replaced

by mechanically operated cold storages. The

second plains' crop--the "summer crop"--

is grown from December to April, and the

source of seed is usually the hills' crop. Both

crops on the plains are grown under short-day

conditions and have to be irrigated. These factors tend to müitate against high yields.

181
INDIA

.Plains

J J A s o N D

.Hills

Figure 88,--Long-term average monthly rainfall and mean temperatures in relation to potato cropping on the plains and in the hills of

northern India.

In the lower hills (3,000 to 4,000 ft.), where

irrigation is possible and snowfall is either

absent or over very early, potatoes are planted

in February and lifted in June for the ware

market. At higher elevations (5,000 to 9,000 ft.),

where the seed crop is produced, the growing

season is during the summer monsoon period

(see climagram for^ Simla, fig. 88). Planting

takes place in late March or April and lifting

is in August and September. This seed cannot

normally be used for the ••winter crop*' on the

plains, owing to dormancy difficulties.

DISTRIBUTION AND FREQUENCY OF

BLIGHT EPIDEMICS

Blight was introduced into India sometime

between 1870 and 1880 in seed potatoes from

Europe. In the north, it was first observed

near Darjeeling in 1883, and the disease spread

rapidly to other potato-growing areas in the

hills and soon became prevalent in the Khasi,

Sikkim, and Nepal hüls (Butler, 1918). The

presence of the disease in the Simla hills was

detected as early as 1902 (Butler, 1903). Since

then blight has been widely spread throughout

the entire sub-Himalayan region, wherever

potatoes are grown, from Simla in the northwest to Darjeeling and Assam in the northeast.

The disease causes heavy losses year after

year, being particularly severe in the northeastern hilly tracts, where the monsoons breaik

early and are of longer duration.

In the plains of northern India, the disease

was first recorded in the Hooghly district of

West Bengal in 1900. It was observed for the

first time in Bihar at Bhagalpur in 1913; the

first record of blight in the plains of Uttar

Pradesh was not made until 1943, when it

occurred at Meerut and Dehra Dun (Lai, 1948).

Until recently, the only source of blight inoculum in the plains crop was infected seed

from the hills; the high summer temperatures

during the storage period of the plains seed

crop did not allow the carryover of the fungus

from one plains crop to the next(Dastur, 1948;

Lai, 1948). Blight epidemics occurred only

occasionally. The introduction of cold storage

for the seed has enabled the disease to bridge

the summer gap on the plains. In 1949, a

severe attack of blight was recorded in a

•'winter crop" of Phulwa at Hapur in Uttar

Pradesh. This variety is confined to the plains,

and the source of inoculum from which the

crop became infected was undoubtedly carried

over in the seed kept in cold storage during

the summer. Since then, sporadic infection as

well as severe epidemics of blight have been

reported from many localities in the plains,

including Meerut and Hapur, Allahabad,

Benares, and Patna. In the Patna area, in

particular, which is an important potato-growing center, blight appears almost every year,

and sometimes causes severe damage. There

would appear to be a strong connection between

the more frequent outbreéiks during recent

years and the introduction of cold storage for

the seed crop (Vasudeva and Azad, 1952a).

BLIGHT ON HAULM AND

BLIGHT LOSSES

HILLS: There is no systematic survey data

on the timing of blight epidemics in the various

182
hül districts. Vasudeva and Azad (1952b),

however, have given details for one center of

blight progress on sprayed and unsprayed

potatoes during a severe blight year (1949)

and a year (1948) when the disease was slight.

Their trials in these 2 years were held at the

Central Potato Research Institute Sub-Station

at Kufri near Simla, situated at a height of

8,500 ft. Figure 89 shows the blight progress

curves on the control plots and on the plots

sprayed with burgundy mixture--the most

successful of a number of different spray

HAULM

per cent SO

Planted

mid-April

1948

Tons¡Q

Sprayed 2-5

Unsprayed 1-7

Planted 1 ^^^^

mid-April JT crop

' /

mature

1949

sprayed 3-0

Unsprayed 0-7

Auqust Seprember Ocrober

Figure 89.--Progress of blight on sprayed and unsprayed Darjeeling

Red Round potato, with corresponding total yields in tons per acre,

Kufri near Simla, India, 1948-49.

treatments. In both years, the yields of the

sprayed potatoes given in figure 89 represent

approximately the full potential yields in the

absence of blight. In neither year was the 75

percent blight stage reached on the sprayed

plots before the haulm died down naturally,

and spray treatments were applied by hand,

so causing the minimum of damage. The loss

of growth due to blight (i.e., the period between

the 75 percent blight stage and the time that

the tops matured) was about 2 weeks in 1948

on the control plots. In 1949 it was about 6

weeks. This premature defoliation led to crop

losses of 32 and 77 percent, respectively,

for the 2 years. The yield losses seem very

high when considered in relation to the number of weeks' growth lost, but it may well

be that in crops such as these, where the

potential yields are very low (2-1/2 to 3 tons

per acre), bulking at the end of the growing

season is relatively very rapid. It would be

interesting to see the results of bulking rate

trials carried out under these conditions.

PLAINS: Information is very scanty. Dr.

Vasudeva has told the authors that at Patna,

where the Central Potato Research Institute

is situated, blight generally makes its appearance in the second half of December.

By February, when the daily temperatures

begin to rise, its further progress is stopped.

The time that the disease is likely to cause

damage is confined to about 1 month, from

mid-December to mid-January. Majid (1952)

has stated that during the season 1950-51,

blight caused losses in parts of the plains of

Assam ranging from 10 to 65 percent of the

potato crop; protective spraying increased crop

yields by 0.7 to 1.6 tons per acre.

Little attention appears to have been paid in

any part of India to the problem of losses due

to blight in the tubers, and information on the

extent of these losses is lacking.

BLIGHT AND WEATHER DATA

HILLS: The long-term average rainfall and

temperature figures for Simla are shown in

figure 88. Total annual rainfall at this center

is 63 inches, of which over 80 percent falls

during the potato-growing season. This is

typical of the hill districts, where potatoes

are grown. Rainfall, particularly during July

and August, is very high, although there are

considerable fluctuations in amounts of rain

from year to year. The very high rainfall at

this time accounts for the almost annual occurrence of severe blight conditions in the

hills. In table 103 humidity and temperature

data supplied by Dr. Vasudeva are given for

the 1948 and 1949 cropping seasons at Kufri.

The figures were obtained by averaging daily

readings made at 9-00 hours and 17.00 hours

(Indian summertime) of wet-bulb and dry-bulb

thermometers fixed in the potato-trial plots.

Details of blight progress there in these years

have been shown in figure 89.

TABLE 103.--Humidity and temperature data at Kufri, India, May-October, 1948-49

Month

Average relative hiomldity Mean monthly temperature

194Ö 1949 1948 1949

i\JIav

Percent

25

42

82

96

Ö8

89

Percent

48

59

89

86

87

74

OF.

71

69

66

62

61

57

OF.

66

^''^**0'

June •• • ••••• •

67

julv 63

ÂiiPii5?'h ........ •.•••••••••••••••• ••••

62

55p"n+PTnl^py ........• •••••••••••••••••••••

60

Dn+oViPT* ........ •••••••••••••••••••••

56

183
The bad blight year occurredin 1949. Higher

relative humidities and lower temperatures

during the first half of the growing season in

1949 as compared to those in 1948 no doubt

partly explain the earlier appearance and

greater effect of the disease in that year. But,

a proper analysis of the 2 years on the basis

of critical humidity periods, such as those

of Beaumont (1947) or Bourke (1955), is required to make a worthwhile comparison

of blight-weather relationships. Table 103

does, however, show the sort of variation in

humidity and temperature conditions that occurs from month to month during the potatogrowing season in the Indian hill districts.

PLAINS: Potatoes are grown under irrigation during the dry winter months (climagram

for Patna, fig. 88). Apart from being very

much less serious under these conditions than

it is in the hill crops, blight is more readily

controlled by chemical methods.

CONTROL MEASURES

No exact information exists on the extent of

control measures practiced against blight in

India, but it must be rememberedthat potatoes

are still largely grown under rather primitive

conditions, and it is unlikely that many of the

growers are able to undertake a spray program against the disease. The problem can

only be seen in its proper perspective against

the background of the present low yields of

potatoes. These are due to numerous causes,

among the most important of which are the

virus-infected seedstocks, poor cultivations,

and variation in water supply for irrigation.

Untu the general level of potato yields has

been raised appreciably, there can be little

point in the widespread adoption of blightcontrol measures in^tke plains area. In the

hill districts, however, measures could be

adopted, but these districts account for only

5 percent of the total potato area.

Trials on the control of blight with spray

and dust materials under hill conditions have

been reported by Vasudeva and Azad (1952b)

for the Simla Hills, and by Choudhuri (1954)

for the Darjeeling district in West Bengal.

The variety chosen in all the trials was Darjeeling Red Round, which Choudhuri describes

as being very susceptible to blight. Satisfactory

control of the disease was obtained in 1948

and 1949 near Simla by 7 fungicidal applications

given at 10-day intervals (see fig. 89). At

Darjeeling a good control was obtained by

applications at 6- to 8-day intervals, during

the 3 seasons 1950-52. Even so, the final yields

on the sprayed plots in these 2 sets of trials

were very low--in no case better than 3.1 tons

per acre--due to causes other than blight. It

becomes obvious that the farmer can meet the

cost of an intensive spray program, at this

low level of output, only if he receives a very

high price for his potatoes, which in effect

mcikes the potato a luxury crop. Potato-growing conditions in the hills of India are very

similar to those in the high valleys of Mexico,

which have been described in this study. In

both areas control is very difficult, owing to

heavy and frequent rains that wash the fungicides from the leaves. However, Dr. Niederhauser and his colleagues of the Rockefeller

Foundation at Toluca, Mexico, have shown that

blight can be controlled by an intensive spray

program. The sprays are applied to field-resistant potato varieties and yields of up to 17

tons of potatoes per acre have been obtained

there. The value of varieties ^that show a high

degree of field resistance does not yet appear

to have been fully appreciated under the Indian

conditions.

For the plains, information is required on

the need for spraying in the different areas, but

the problem of control is nothing like so serious

as in the hill districts. The use of cold storage

to keep the plains' seed during the summer

months has greatly increased the amount of

blight inoculum present in the plains' crop, and

thus the liability of the crop to infection. The

cold storages have so many advantages over

the older methods of storage, however, that

any return to them is out of the question.

SUMMARY: INDIA

Potatoe'ö are a relatively unimportant crop,

and yields^a^.re low. Blight is serious in the hill

districts, and although these districts provide

only a smêill percentage of the total crop, they

are an important source of seed potatoes.

184
AUSTRALASIA

NEW ZEALAND

New Zealand is normally self-supporting in

both ware and seed potatoes. The total acreage

of potatoes is smadl, about 20,000 acres per

annum; but the population of the two islands

is only 2 million, and with almost all the potatoes going to the ware market, this provides

for an annual consumption of potatoes of about

100 lb. per head. This level of consumption is

comparable to that in the U.S.A. The average

potato yield for the country is 5.9 tons per

acre (F.A.O. statistics).

POTATO DISTRIBUTION AND VARIETIES

The most important maincrop potato-producing areas in New Zealand are in Canterbury (chiefly around Christchurch), in the

South Island and Wellington (Palmer ston North

area) in the North Island (fig. 90). Apart

from Wellington, the North Island grows principally early and midseason potato varieties,

and there is an important early-growing district at Pukekohe, near Auckland. In the

South Island, there is another early growing

district near Nelson. A full account of potato

growing in the various parts of New Zealand

has been given by Claridge (1945).

Figures provided by P. W. SmcLLlfield of the

New Zealand Department of Agriculture at

Wellington are shown in table 104, which give s

the average potato acreages for New Zealand

on a land-district basis.

The potatoes grown in New Zeédand are

largely varieties that originated in Great

Britain. More recently, a number of varieties

have been introduced from the U.S.A., and

some of these are now widely grown. According to Mr. Smallfield, 4 varieties have made

up 71 percent of the total potato acreage during

recent years. These have been: Aucklander

Short Top (31 percent); Ar ran Chief (18 percent); Dakota (14 percent); and Ar ran Banner

(8 percent).

Aucklander Short Top is a selection from

Sutton's Supreme. It is an early maincrop

variety that shows some resistance to blight.

Le itch (1947) has given a description of the

principal potato varieties grown in the country

and their behavior under New Zealand conditions. R-gene resistant varieties have not been

grown commercially anywhere in New Zealand.

SOIL, CLIMATE, AND CULTIVATIONS

The crop is grown over a wide range of soil

types, with the largest yields on the heavy,

well-drained silt loams of the Canterbury area.

New Zealand has a mild equable climate of

the temperate oceanic or northwest Europe

type. The sea has a marked effect on weather

TABLE 104. --Potato acreage according to

district, New Zealand

Land district Acreage-"-

North Island:

Nnrth Anf^lcl and , , , 1,49/;

South Auckland 731

Gisbome 374.

Hawkes Bay

Tnranalrî **•••«

1,075

134

Wellington 3,064

Total 6,872

South Island:

Marlborough"\

Nelson 569

Westland J

Canterbury

Otago

Southland

8,715

1,479

646

Total 11,409

G-rariri total ,,*.*...•«.. 18,281

^ 5-year average, up to and including 195¿i-55.

conditions, and there is a considerable variation in rainfall from district to district and in

different years. The prevailing winds are

from the west, and the wettest parts of the

country lie principally in the west, very noticeably so in the South Island (see rainfall

map, fig. 90, based on Stamp, 1944). Generally

speaking, the potato crop is grown in the

drier parts of the country, as in the Christchurch area of Canterbury, where the average

annual rainfall is 26 inches. Climagrams for

two potato-growing areas (Christchurch and

Pukekohe) are given in figure 91.

Maincrop potatoes have a very long growing

season in New Zealand (approximately

6 months). As the country is in the Southern

Hemisphere, seasons are the reverse of those

in the potato-growing regions of northernlatitudes. Thus, in Canterbury, planting takes

place in October (late spring), and lifting is

in May (late Autumn). A full description of

cultivation methods has been given by Leitch

(1947). Potatoes are grown in ridges, 28 to 31

inches apart; in New Zealand, this has the additional advantage of protecting the tubers

from attacks by the potato moth. Up to the

present time, control of blight by spraying is

185
NEW ZEALAND

borne

'30-50in

ANNUAL

RAINFALL

Figure 90,--Key map of New Zlealand and annual rainfall.

186
NEW ZEALAND

RAINFALL

Inches

f, p Sourh Island

CHRISTCMURCH

S O N Dec

• BUCHT SEASON ' |044-5

Jan F M A M J

_' NO BLICHT SEASON * 1945-6

RAINFALL

Inches

N Dec Jo" F M

r North Island

FUKíKOHt

RAINFALL

Inches

Figure 91..—Monthly rainfall and mean temperatures in relation to

potato cropping for Christchurch and Pukekohe, New Zealand, and

"blight** and "no-blight*• seasons at Christchurch.

uncommon except in the Pukekohe district.

The small amount of spraying is a reflection

of the generally sporadic attacks of blight and

the scattered fields of the potato crop, which

has usually made it uneconomical for the

farmers or contractors to invest in spraying

machinery.

REGIONAL, INCIDENCE OF BLIGHT

C. M. Driver, of the Crop Research Division

at Lincoln, has provided an account of the incidence of blight in the various potato-growing

areas. The disease occurs throughout New

Zealand, but except in the northwest of the

North Island, the attacks are sporadic and

usually only toward the end of the growing

season. Short notes on blight in the several

districts are now given, and these are followed

by more detailed accounts for important

Christchurch district, where blight is not

usually a problem, and for Pukekohe, where

blight conditions are severe.

AUCKLAND: Blight is normally present

throughout the year. Crops are regularly

sprayed from soon after emergence until maturity.

GISBORNE; Blight seldom occurs, because

of the usually dry spring and summer.

HAWKES BAY: The growing season is

usually dry, but occasionally blight may cause

some damage. Spraying is practically unknown.

WELLINGTON: Blight occurs in some areas

in most years, but rarely is damage severe

over a wide area. Sometimes it may occur in

epidemic proportions during December, usually

associated with very heavy rains and waterlogged soils, and thus considerable losses

from causes other than blight. In 3 years out

of 4, January and February are dry months

with ample sunshine, so that blight does not

develop. If blight does appear late, tuber infection may be high, as spores are washed

into soil cracked by earlier dry weather. Until

a few years ago, no spraying was done in this

area, but new contract growers, who lease

large areas of land solely for potato growing,

have invested in. spraying equipment.

NELSON and MARLBOROUGH; BHght attacks are infrequent, mild, and late in the

season.

WESTLAND: Rainfall and humidity are high.

Blight is always severe. Potato requirements

for the district are smaJ.1 and it is cheaper to

ship them from Canterbury than to attempt to

control blight.

CANTERBURY: In general, summers are

dry and blight is unimportant. Conditions are a

little wetter in south Canterbury, so that

blight occurs more frequently; even so, the

main variety, Arran Chief, has sufficient resistance to carry it through the attack in most

seasons.

OTAGO and SOUTHLAND: Rainfall is higher

in these districts than in Canterbury. There

are more days with rain and there is less

sunshine. Blight can be severe on King Edward

once in every Z years or more. However, the

most common variety, Arran Chief, is seldom

killed early enough to affect the yield noticeably.

Christchurch District, Canterbury

Ah account of potato growing in Canterbury

has been given by Dingwall (1953). He describes the Christchurch district as the land

area lying between the rivers Wainnakariri and

Rakaia. Over half the potatoes produced in

Canterbury are grown in Christchurch. The

district concentrates on growing maincrop

ware varieties, with some early varieties in

sheltered situations near the coast. Farther

inland, seed crops are grown and these form

the source of certified seed for the ware

areas.

187
Potato crops are widely scattered through

the district. A high proportion are grown as

a sideline and occupy only a small part of the

farm acreage. The principal varieties are

Aucklander Short Top and Dakota, which fluctuate widely in yield between dry and wet

seasons. Driver quotes 4 to 6 tons per acre as

a typical yield for Aucklander Short Top in a

dry year. In a wet year, 14 tons per acre is

typical for the same variety.

Potato soils are medium and medium heavy

silt and sandy loams, which are usually well

drained. Planting extends from late September

to mid-November, with the majority of crops

planted in October. Varying with variety and

season, most crops mature between midMarch and mid-Apr il. Harvesting starts in

late April, and is carried out mainly in May.

Blight Epidemics and Losses

Blight is relatively unimportant in the

Christchurch district. During the last 20

years, the disease has been severe enough

to cause extensive damage in only 2 seasons.

These have been 1935-36 and 1944-45. Every

third or fourth year, blight appears during

February or March. Progress of the disease

is usually slow (except in odd pockets), so

that a crop noticeably affected (10 to 25 percent leaf damage) may take a month to 6

weeks or longer to reach 75 percent damage

or more. The extra rainfall associated with

blight attacks results in greatly increased

yields, so that, in general, yields are much

higher in a blight year, and this, despite the

fact that very few growers make any attempt

to control the disease.

Tuber blight is not a serious problem. In

general, there is very little in the earlier

maturing varieties and usually not more than

10 percent on the late varieties (often nil).

There may be a few crops on heavy, poorly

drained soil where losses are high, but probably most of the loss is attributable to waterlogging rather than blight.

Results of a survey of tuber infection in the

soil, made at the end of the bad blight season,

1944-45, have been reported by I. D. Blair

of the Canterbury Agricultural College, Lincoln, in a article entitled, "Late Blight of

Potatoes in North Canterbury, 1945." Table

105 presents some of the data from this

survey.

It will be seen that heavy tuber blight

losses occurred in 1944-45. Such years, however, have a frequency of only about 1 in 10

in the district. Two of Blair's conclusions

from this admittedly small survey are worth

quoting: (1) Dakota was less severely affected

by tuber decay than the other varieties listed,

whereas Arran Banner showed the heaviest

infection; and (2) the amount of tuber loss that

occurred was related to different soil types.

The heavier and damper loams were associated

with more disease than the sandy loams.

Infection of the tubers at lifting time by contact with blighted haulm is extremely rare.

Most crops have matured some time before

lifting, and the small importance attached to

this problem is a reflection of the long growing

season and also the long period over which

harvesting is possible. In fact, another problem

often arises. This is a strong weed growth,

which makes lifting difficult. Dingwall (1953)

says that some growers adopt the practice of

quickly grazing off this growth with sheep

before lifting.

Blight and Weather Data

The long-term average temperature and

rainfall figures for Christchurch, together

with the planting and lifting times of potatoes

are shown in figure 91. A marked feature of

the disease in the district is its generally

slow progress on the potato haulm after its

first appearance. Hot drying Fohn winds blow

across the district from the northwest during

the summer, so that even after heavy rains,

the haulms may be rapidly dried. In addition,

temperature conditions may not be too favorable to blight. The diurnal range of temperature during the summer months is from

15° to 20° F., and cold nights with temiperature s dropping below 40° are common, with

occasional nights when the temperature goes

below freezing. Conversely, highdaytime temperatures, up to 90° are frequent; periods of

TABLE 105. --Distribution of tuber blight infection among certain potato varieties,

North Canterbury, 1944-45

Variety Crops examined

Range of infection between

crops

Average tuber

infection

Dakota

Aucklander Short Top.

King Edward

Arran Banner

Inverness Favourite.,

Aucklander Tall Top..

Number

15

16

13

5

3

Percent

0-19

1-42

7-34

11-77

5-20

1-30

Percent

5

15

15

34

11

17

188
low humidity are also common. Hence, blight

IS continually receiving severe checks.

In figure 91, temperature and rainfall conditions from November to March in a blight

season (1944-45) and a no-blight season (1945-

46) are compared. In 1944-45, blight was first

reported at the end of December, and most

potato crops were killed by the disease around

the end of February to early March. The very

heavy rainfall in December and February was

one of the main factors associated with the

epidemic.

Control Measures

Protective spraying is uncommon. Driver

says, »'Spraying has not been done because it

has not paid. No damage is caused (by blight)

three years out of four. In the fourth or fifth

year spraying may have the effect of increasing the surplus which cannot be sold.** Potatoes are being grown increasingly under spray

irrigation in the district. The change in the

microclimate of the crop resulting from this

is likely to give rise to conditions more favorable to blight, and protective spraying may

become common practice. In this case, farmers are already well equipped with lowvolume spraying equipment, which they use

for weed control. Destruction of the potato

tops before lifting is generally unnecessary,

but occasionally a 10 to 12 percent solution

of sodium chlorate is used for this purpose.

Pukekohe District, Auckland

This district, centered on Pukekohe Hill,

concentrates on market garden crops. Outdoors tomatoes are grown during the summer

months. The area is almost frostfree throughout the year, and potatoes are grown in the

winter to supply the early ware market. Dawe

(1951) has described potato ' growing at

Pukekohe. He says that during recent years,

approximately 600 to 700 acres have been

grown annually for the ware market. In addition, some 200 to 250 acres for seed are

grown each year during the summer. The

main reason for this home seed production is

the difficulty in getting seed from elsewhere

in New Zealand in time to plant the early

winter crops. Some difficulty may be experienced in breaking the dormancy period of

seed tubers and one method that has been

used successfully is dipping them in a carbide

solution.

Arran Chief is the most popular variety for

the earliest crops. The use of a matlncrop

variety for this purpose is explained by

short-day conditions under which the crop

is grown. Northern Star (Gamekeeper) is the

principal variety for the later crops. Yields

vary according to the maturity of the crop

and the month of lifting. Dawe gives 4 tons

per acre as an average crop lifted in September.

The soils are basalts and, although heavy,

are generally well drained. Figure 91 shows

the long-term average weather conditions at

Pukekohe, together with the planting and lifting times. Planting starts at the beginning of

the winter (May), and continues into the spring;

lifting takes place from August to January.

The seed crop is usually planted in the early

summer (November-December), following an

early ware crop, and harvested the next April

or May.

Blight Epidemics and Losses

Blight is present in the district at all

seasons. Severe epidémica of the disease

would almost certainly develop every year

in the absence of protective spraying, which,

however, is carried out by all ware growers

from the time the potato plants emerge. Blight

is less severe on the seed crop, which is

grown in the summer or drier part of the

year, and very often there is a time during

the growth oí this crop, when spraying can be

curtailed.

There are two main reasons for the severity

of blight in this district. These are the

weather conditions, and coupled with

them, the presence of ample sources of

blight inoculum throughout the year. The

sources of inoculum are (1) the potato crops,

thennselves; (2) groundkeepers; (3) tomatoes;

(4) honne gardens; and (5) weeds. Potatoes

are being grown throughout the year, very

often on the same field, so that there is a

constant carryover of the disease. Unharvested

potatoes sprout very rapidly and groundkeepers are extremely common. Driver writes

that **Large acreages of tomatoes are grown

from November to May. Although in general

tomato blight races may be different from

potato blight races, yet the latter may carry

over quite well on the tomatoes, which often

follow potatoes.'* Large numbers of home

gardens throughout the district often grow

potatoes for the greater part of the year. In

June 1955 Joan M. Dingley found blight infecting a common New Zealand shrubby weed,

Solanum aviculare, in native bush near Auckland. Many other infected plants of this species

have since been found and by May 1957 it had

been shown to be susceptible to blight races 0;

1; 4; 1,3; and 2,4; collected in New Zealand

(Brienand Dingley, 1955; Driver, 1957).

There are no published records of blight

progress on unsprayed potato haulm for the

Pukekohe district. However, Taylor (1945)

has given some data for unsprayed plots of

King Edwards derived from a series of

spraying trials carried out in the Auckland

district (site unspecified) between 1940 and

1945. Some of his results are set out in table

106.

In each season, blight appeared very early.

The speed with which the tops were killed,

189
TABLE 106. --Blight attack on unsprayed King Edward, Auckland district, 1940-43

Year Date of

planting

Date of blight

appearance

Later observations

1940-41 Oct. 15

Sept. 15

Sept. 16

Nov. 14

After Nov. 14

End of Oct.

Dec. 27, plants severely infected.

Dec. Ö, plants severely infected.

Dec. 1, most plants killed.

1941-42

1942-43

however, was not particularly fast in view of

the obvious smallness of the plants, except

perhaps in 1941-42. Conditions in these trials

and those for the important early ware crop

at Pukekohe are not strictly comparable,

since the trials were planted in the spring,

whereas the early crop is planted in the

autumn and winter. They do give a good idea

of the sort of blight conditions facing the

Pukekohe seed crop, which is usually less

severely attacked by the disease than the

ware crop.

Tuber infection before lifting is not a

problem in the district. Crops are usually

dug quite clean. This is, no doubt, in part a

reflection of the good controlof blight achieved

on the tops, but Driver has further suggested

that the high copper content of the soils in the

area, brought about by frequent copper spray

applications, is protecting the potato tubers

from blight infection. He says *'••• potatoes

may be grown 4 to 6 times in six years on one

area. Tomatoes may have been grown for

several of those years, so the total number of

sprays received over that period could quite

well be as many as 80 to 100, and is unlikely

to be below 40." If this supposition is correct, it falls into line with work reported by

Skaptason, Peterson, and Blodgett (1940), from

Long Island, New York, and referred to in the

U.S.A. section of this study.

Infection of tubers at lifting is also of little

innportance. Driver writes, **One would expect

some tubers to become inoculated from the

haulm at harvest but transport has been coordinated so that potatoes harvested one nnorning can be in the shops in Wellington, 400

miles away, by the next morning. Consequently a high proportion of the potatoes

would be eaten within 4 to 5 days of harvest,

not allowing tuber blight a chance to develop."

The serious blight problem at Pukekohe is

therefore the foliage attack. This, however,

would be so devastatingly severe each year in

the absence of spraying as to make potato

growing impossible.

Blight and Weather Data

Mean temperature and rainfall figures for

Pukekohe are given in figure 91 in relation to

the growing seasons of potatoes. There is a

consistently high rainfall each month during

the winter and spring, when the bulk of the

early crop is being grown. For the 6 months.

May to October, total rainfall averages 28

inches. Though rainfall is heavy, it tends to

come in short, heavy showers, with dry, often

sunny periods in between. This has an important bearing on blight control, as it is

unusual even in the wettest season to have

periods longer than 3 days when spray equipment cannot work. Mean temperatures during

the growing season of the early crop are low.

On the other hand, humidities are high--in

part, a reflection of the long-night and shortday growing conditions.

Control Measures

The early crop is sprayed from emergence

onward at approximately 4- to 8-day intervals

until shortly before lifting. Up to 10 sprays

may be given over the growing season. The

potatoes are lifted while they are still very

immature, at a time when the tops are meeting in the drill but have not yet made spraying

difficult. Driver states, **U for any reason,

harvesting is delayed longer than this, and the

weather is wet, blight becomes very difficult,

if not impossible to control. However, this

seldom happens, growers tending to plant a

succession of crops if they want to harvest

over a period, rather than planting one area

and harvesting a portion at a time."

The most comnnonly used spray material

is copper oxychloride applied at the rate of

5 lb. in 100 gallons of water per acre. It is

possible to produce satisfactory early crops

of potatoes by these control measures under

the very severe blight conditions at Pukekohe.

But the cost of control is very high, and only

economic because of the correspondingly high

value of the early crop. It seems unlikely that

maincrop ware potatoes could be produced

economically in such an area, particularly in

view of the difficulty of controlling blight once

the potatoes have met in the drill,

SUMMARY: NEW ZEALAND

Blight is not generally a serious disease in

the country. The bulk of the maincroppotatoes

are grown in the drier areas. The Pukekohe

district in the northwest of the North Island

is an example of an area where blight Is very

bad; but the production of a valuable out-ofseason potato crop justifies an intensive and

expensive control program against the disease.

190
AUSTRALIA

Potatoes are grown in Australia at all

seasons of the year and under a wide range

of soil and climatic conditions. The acreage

is relatively small. During 1939-45, the area

approached a quarter of a million acres, but

in recent years it has been about 125,000

acres. The only major outlet for the potato

crop is the ware market; but annual consumption of potatoes is low, around 110 lb. per

person, which compares very closely with the

position in the U.S.A. The average yield is

also low, no more than 3.5 to 4 tons per acre

(F.A.O. statistics), and there is little sign

that present-day yields have improved appreciably over those obtained during the late

1930's. Bald (1941) has stated that one of the

major problems of potato growing in Australia

is lack of water supply. This is due to an

unequal distribution of the rainfall, rather

than insufficiency in the total amount, in the

areas where potatoes are grown. In addition,

there is a generally high rate of evaporation

from the soil.

DISTRIBUTION OF POTATO CROP;

CLIMATIC CONDITIONS

The most important potato-growing areas

are in the more temperate parts of the

country (fig. 92). Victoria, with about 40 percent of the total Australian acreage, is the

largest potato-producing State, followed by

Tasmania with 25 percent. Figure 93 includes

climagrams for centers in these two States.

The main potato-growing season is during

the summer months (October to April), and

potatoes are grown both under coastal conditions and at higher elevations; for example,

in the Central Highlands of Victoria, around

Ballarat.

New South Wales, with 15 percent of the

potato crop, and Queensland, with 9 percent,

come next in order of importance. In northern

New South Wales and Queensland, climatic

conditions are subtropical, and the main

potato-growing season is in the winter or

summer, depending on elevation. On the coast

near Grafton. New South Wales (climagram,

fig. 93), potatoes are grown during the winter.

Summer temperatures, whichaverageoveT 75®

F. for each of the 3 months, December to

February, are too high for potato growing. On

the tableland near Glen Innes (climagram,

fig« 93), which is on the same latitude as

Grafton, the main potato-growing season is

during the summer.

Potatoes are not an important crop in

Western or South Australia. In Western

Australia, early potatoes form a high proportion of the crop, and these are grown

during the winter and spring months. A summer

crop is grown in South Australia,mainly under

irrigation.

POTATO VARIETIES, SOIL CONDITIONS,

AND CULTIVATIONS

According to White (1952), the most important variety now grown in Australia is

Sebago, which was imported from the U.S.A.

shortly before the Second World War. Sebago

has displaced Carman (Green Mountain) and

Snowflake, which used to be the leading

varieties in Victoria and South Australia; in

New South Wales and Queensland, Sebago has

displaced Factor (Up-to-Date). Sebago shows a

high degree of field resistance to blight, and

in this respect it is superior to the older

varieties it has replaced. Other important

varieties in Australia are Sequoia, Monak,

and Adina. In Tasmania, Bismark (early)

and Brownell are still the chief varieties.

Most potatoes in Australia are grown on

soils derived from basalts and adjacent transitional soils. These are generally fertile, often

very deep, and permeable despite a high clay

content. Bald (1941) has described in some

detail the relation of soil type to potato growing in the country. Methods of cultivation vary

between districts; in general a surprisingly

large amount of cultivation, in particular

planting and lifting, is CJtrried out by hand

labor. Cutting of the seed is general, and the

crop is grown in wide rows (30 to 36 inches).

Potato growing under temperate conditions

in Victoria has been described by Mattingley

(1954); details of the cultivation of the crop

under the subtropical conditions of Queensland are given by CartmiU and Bechtel (1951).

An important pest of potatoes in Australia is

the potato moth (Gno rimoschema operculella).

Caterpillars of this moth feed both on potato

leaves and the tubers. The crop is sprayed or

dusted with DDT to prevent caterpillar damage,

and, in addition, good earthing up of the rows

prevents the moths from laying their eggs on

the tubers. This latter preventive cultivation

serves also to protect the tubers from blight

infection.

FREQUENCY OF BLIGHT EPIDEMICS

Not much attention has been paid to potato

blight, and the disease is usually considered

of little economic importance. According to

Bald (1941), itis muchless serious in Australia

than in most other countries where potatoes

are grown. G. C. Wade, Tasmania; S. Fish,

Victoria; C. J. Magee, New South Wales; and

J. H. Simmonds, Queensland, provided the

following notes on the incidence of the disease

in their respective States.

TASMANIA: Potato blight was first officially

recorded in Tasmania in 1907, and a history

of the disease in the State is givenin an article

by Oldaker (1947). Since 1947, it has caused

fairly general losses in the 1947-48 and 1952-

191
AUSTRALIA

500 miles

'^^ i • ,...

) : VICTOR lÁ

OtwoY Ranqes'

Melbourne

O

Stanlcyi»^nYaj;C'

Midlands_^__N /J

Area \~3w*"^°'*°'"*

C-s/« TASMANIA

FIGURE 92.--Key map of Australia, showing distribution of potato growing and localities referred to in text.

53 seasons. In 1955-56 it was probably nnorc

generally serious than ever before, causing

heavy losses in all districts, including many

districts where it had previously never caused

sxifficient damage to be noted. Except in a few

localities, blight causes economic damage only

in about 1 year in 5 in the important potatogrowing districts of the northwest coast, while

outbreaks rarely occur in the midlands and

south. Attacks are most frequent in the Table

Cape district near Wynyard, and the Scottsdale

district in the northeast.

VICTORIA: Isolated occurrences in individual crops are reported nearly every season.

but the disease very seldom becomes of any

economic importance. It is reported most

frequently from the Otway Ranges and South

Gippsland.

NEW SOUTH WALES: Losses from potato

blight in New South Wales are as variable

as the season and the locality in which the

crop is grown. Whether loss will occur in any

locality seems to depend on weather conditions, the presence or absence of inoculum

not being an obvious conditioning factor. In

seasons favorable to blight it appears to become well dispersed in the air spora. Annual

Disease Survey Records show that outbreaks

192
AUSTRALIA

6 p Victoria

BALLARAT

.New South Wales (COAST)

CRAFTON

RAINFALL 5

Inches

6 j. New South Wales (TABLELAND)

GLEN INNES

FIGURE 93,--Long-term average monthly rainîall and mean temperatures in relation to potato growing periods for four centers in

Australia.

occurred in coastal districts of New South

Wales in 10 of the past 25 years (1930-31 to

1954-55). In the same period in upland or

tableland crops, it was important in only 5

seasons, and in one of these it was recorded

only on the northern tablelands.

QUEENSLAND: The main wet period occurs

during the summer when potatoes are not

normally grown, or, if they are, temperatures are too high for blight. The winter and

spring is normally dry. The appearance of

blight is therefore sporadic and restricted

to the somewhat rare occurrence of suitable

weather conditions.

OTHER STATES: Blight is of little or no

importance in the greater part of the potatogrowing areas in Western and South Australia.

Cass Smith (1952), however, has reported

that serious outbreaks of the disease have

sometimes occurred in the winter-planted

crop near Perth during the period August to

mid-November.

LOSSES FROM BLIGHT

The small importance attached to blight by

Australian growers and research workers is

reflected in the paucity of precise data about

the effects of the disease on the haulm and

tubers. G. C. Wade and S. Fish have provided details of blight progress on unsprayed

potato crops, grown in ''blight" years at

particular centers in Tasmania and Victoria.

Details of these two crops are shown in table

107. These records are, so far as is known,

the only ones of their kind available for the

whole of Australia.

At Toolangi the period between planting

and the 75 to 90 percent blight stage was 128

days. This is about the normal length of the

growing period of Sebago, when it is grown in

the U.S.A., although it may well have a somewhat longer growing season in Australia. The

13-ton yield of this crop, however, shows

that the effect of blight on yield was not veryserious.

The observations on the crop at Wynyard

were made during a series of spraying trials,

which have been reported by Williams, Wade,

and Cartledge (1956). Bismark is an early

variety, and the length of time between planting and 75 to 90 percent haulm destruction in

the crop referred to in table 107 was 101 days.

The unsprayed crop yield of 2.7 tons per acre

compares with a mean yield of 6.2 tons per

acre for the different spray treatments employed in this particular trial. The haulm on

the sprayed plots was destroyed mechanically

112 days after planting, so that there was only

approximately 11 days* difference in effective

growth between the 2 sets of plots, but this

resulted in a fall in crop yield of over 50 percent on the unsprayed plots. To some extent

this loss of yield was caused by tuber infection. Digging of the trial was delayed until all

blighted tubers had rotted in the soil, so that

it is not possible to say how important this

was. G. C. Wade has stated that blight was

probably more serious in the 1955-56 season

than in any season since its introduction into

Tasmania. In similar trials carried out at

Wynyard in 1953-54 and 1954-55, blight caused

no losses.

It is impossible to make any accurate

estimates of the losses caused by blight in

193
TABLE 107. —Data on blight in Tasmania and Victoria, Australia, 1954-56

Area and variety Year Date of

planting

First

appearance

of blight

Date of 75 to

90 percent

defoliation by

blight

YieldToolangi, VICTORIA:

Sebago variety

Wynyard, TASMANIA:

Bismark variety

1954-55

1955-56

Dec. 20

Sept. 26-30

i^ril 1

Oct. 30

i^ril 27

Jan. 5

Tons/acre

13.0

2.7

Australia, on the basis of the available evidence. All that can be isaid is that overall

losses caused by the disease are very small«

Nor is it possible to apportion the losses that

do occur between yield losses resulting from

premature defoliation and tuber blight losses.

The good earthing up that the crop is given in

Australia against potato tuber moth and the

generally long growing and long harvesting

periods are factors operating against serious

tuber blight infection. Bald (1941) has stated

that, in aggregate, the losses caused in

the Australian potato crop by early blight

(Alternaría solani) are probably greater than

those caused by late blight. The Australian

potato growers show their lack of respect for

both diseases by taking little or no protective

action against them.

BLIGHT AND WEATHER DATA

The climagrams (fig. 93) for important

potato-growing areas in Victoria and Tasmania

indicate that low rainfall during the growing

season is an important factor associated with

the general absence of blight. Victoria and

Tasmania together account for 65 percent of

the total Australianpotato acreage. At Ballarat,

the average monthly rainfall is at or below 2

inches throughout practically the entire growing season; at Stanley, the average rainfall is

below 2 inches for each of the 3 months,

January to March, when blight is most likely

to develop. The clinmagrams for New South

Wales show that the coastal crop at Grafton

is also grown during a dry period; the tableland crop at Glen Innes is grown during a

relatively wet season. However, the frequency

of blight years at Glen Innes is only about 1

year in 5, and it seems likely that high temperatures are limiting the disease there, despite

the wetter conditions.

The absolute amount of rainfall is only a

very rough guide to the likelihood of blight

attacks, and in most of the potato-growing

areas of Australia, totctl rainfall figures give

an exaggerated idea of the amount of water

available to the potato crop. This is because

rainfall is poorly distributed. Heavy downpours are followed by considerable runoff

from the soil surface. In addition, there is a

high rate of water evaporation fronn the soil,

associated with low humidity conditions. Low

humidities are chiefly responsible for the

relative unimportance of blight in Australia.

Mean monthly humidity figures during the

potato growing season for Melbourne and

Ballarat in Victoria are shown in table 108.

The dry conditions under which potatoes

are grown suggest the possibilities of irrigation as a principal method to innprove crop

yields. Both overhead and channel irrigation

of the potato crop are practiced in parts of

Australia. It is possible that a marked increase in overhead irrigation would affect

the incidence of blight attacks.

TABLE 108. —Mean monthly relative humidity

at Melbourne and Ballarat, Victoria

Month

October..

November.

December.

January..

February.

March....

i^rll

Melbourne Ballarat

Percent

57 70

56 66

55 62

53 60

56 59

58 64

64 74

Percent

CONTROL MEASURES

The replacement ofthe old blight-susceptible

varieties. Factor and Carman, by the more

field-resistant Sebago has served as a partial

blight control measure. It may well be that in

large parts of Australia where blight is

normally either absent or slight in its effect,

such action in itself is sufficient control,

especially as Sebago is resistant in the tubers

as well as in the foliage. At any rate, very

few Australian growers spray or dust their

potato crops against blight. In Victoria, New

194
South Wales, and Queensland, growers generally consider spraying to be uneconomical.

In Tasmania, matters are different. Blight

caused fairly general losses in the 1952-53

season, and as a result, the Potato Marketing

Board asked the Department of Agriculture to

investigate the control of the disease. Spraying trials were carried out in 1953-54, 1954-

55, and 1955-56, and the results of these trials

have been given by Williams, Wade, and

Cartledge (1956). Some of the figures from

these trials have already been referred to

above. Blight was not serious in 1953-54 and

1954-55. Yield figures from the trial in this

latter season indicated that a considerable

reduction in yield can be caused by spraying

with certain materials in a **no-blight" year.

The yield on plots sprayed with bordeaux

mixture was 30 percent lower than the yield

on the unsprayed plots. In 1955-56 (a severe

blight year), on the other hand, the unsprayed

plots produced only 43 percent of the yield

on the sprayed plots. In connection with the

spraying trials in two of the seasons, the

temperature-humidity rules of Bourke (1955)

and Beaumont (1947) were tested for blight

forecasting under Tasmanian conditions. The

occurrence of blight-weather periods by these

rules in relation to blight developnnent at

Wynyard is shown in figure 94. It was concluded that Bourke's Irish rules apply reasonably well, but that a major difficulty in making

general forecasts of blight in Tasmania lies

in the considerable variations in weather that

occur over short distances. Blight forecasts

were considered to be reliable only in the

vicinity of the weather recording stations.

lOO

BLIGHT

ON

HAULM

per cent SO

BOURKE PERIODS

BEAUMONT PERIODS

2

RAINFALL

Inchei

FIGURE 94.--Development oí blight on unsprayed Bismark haulm in

relation to Bourke periods (Irish rules), Beaumont periods, and

rainfall at Wynyard. Tasmania, 1955-56. (After Williams, Wade,

and Cartledge. 1956.)

In Australia, where severe blight outbreaks

occur at infrequent intervals, the provision of

reliable blight forecasting services for the

main potato-growing areas would help considerably in preventing losses by the disease.

Even in Tasmania, which is relatively compact, there are considerable difficulties in

the way of doing this, owing to variations in

climatic conditions; and, it must be admitted

that in the larger States the present difficulties facing a forecasting service are almost

insuperable. Until recently, a routine control

program against blight, using copper sprays,

would almost certainly have led to a net loss

in potato yields, owing to the depression of

yields in **no-blight*'years. The position would

be different if organic fungicides were used

every year, although provision would have to

be made to keep wheel-damage losses to a

minimum. Potatoes are already grown in wide

rows. However, Australian growers at the

present time are making little use of any

protective sprays. One reason for this is lack

of suitable equipment; although low-volume

sprayers, purchased prinaarily for weed control, are on the increase. It is possible that

in some areas of the country, such as parts

of Tasmania, a routine spray program, based

on the low-volume application of organic

fungicides, may be found to be economical.

Little work has been done on haulm destruction, as the need for this does not often arise.

A recent account by the Tasmania Agronomy

Division (1956) has stated that pentachlorophenol (P.C.P.) has proved an economical and

effective chemical for haulm killing, in experiments that they have conducted. Testing the

blight resistance of named and unnamed potato

varieties, including material from the U.S.A.

and Scotland that could be incorporated in a

breeding program, has been carried out in

Victoria. Phis work was reported by Chambers

(1954) and (1955). The emphasis has been

placed on the incorporation of major gene

resistance in commercial varieties, but up

till the present time no Australian blightresistant varieties have come on to the

market.

SUMMARY: AUSTRALIA

Potato blight is of small economic importance in Australia. This reflects the generally low rainfall and humidity conditions

throughout most of the potato-growing areas.

These conditions also lead to low yields.

Chemical methods of controlling blight are

rarely practiced.

195
CONCLUSIONS

The great interest of this study has been to

see blight epidemics, not in the abstract or

entirely from the viewpoint of plant pathology,

but in their widest relations to the potato

husbandry of the world, the provision of food

supplies, the distribution of the crops, and the

diverse climatic conditions under which

potatoes are grown. It is not the intention to

recapitulate the sectional sumnnaries already

given for each country, or to present any

resume of all the information that has been

gathered together. The general conclusions or

convictions of the authors that are reached

are stated broadly, leaving the reader to fill

in the innumerable necessary qualifications.

Names of countries given in brackets indicate

the preceding sections in which amplification

of the various points, examples, may befo\ind.

THE DEFINITION OF CLIMATIC CONDITIONS

The kind of clinmagram evolved in the course

of this study may prove to be a useful device

in other work on potato technology. Such

diagrams enable quick visual comparisons to

be nnade of some of the main conditions affecting both potato growing and the incidence

of blight in different regions. With the climagram for an area, it is possible to s3e with

fair certainty the period or periods of the

year during which there can be potato cropping, and the favorability, or otherwise, of

the area for both potato growing and for blight,

at least as to mean temperature and rainfall.

Diurnal range of temperature, distribution of

rainfall, and humidity conditions are broadly

dependent upon the type of climate, and dayiength dz;pends on the latitude.

EFFECT OF MSAIT TEMPERATURE ON POTATO CROPPING

The entry of ascertained cropping periods

on the climagrams in this study confirms by

world survey the finding of Bushnell (1925)

[Introduction] that maximum tuber production

occurs over the mean temperature range of

59^ to 640 F. A slightly wider range (58^ to

65^) takes in most of the regions of the world

v/here the highest potato yields are obtained.

The 2 most important months for the bulking

of maincrop potatoes, during which the mean

temperatures lie within this range, are, in

general the Z months preceding the month of

maturity. Those are also the 2 months during

which blight most seriously affects cropyield.

EFFECT OF RAINFALL ON POTA^^O CROPPING

Although lack of adequate rain limits potato

cultivation in many parts of the world and the

rainfall during the growing period is the

principal single factor responsible for variation in yield from year to year in most of the

great potato-growing areas, there is no such

clear-cut relation between cropping and mean

rainfall as there is for cropping and mean

temperature over the world as a whole. The

distribution of rainfall affects its utility, as

also does the porosity of the soil and the rate

of evaporation [Introduction] . The climagrams

show that regions with the higher potato yields

generally Imve, on a long-term average, 2 to4

inches of rain during eachmonthof the growing

period [Belgium, Netherlands, Ireland, Prince

Edward Island], but that potatoes are grown

where the rainfall in some months reaches 15

inches or more [ India, East and West Africa]

and under irrigation in the virtual absence of

rainfall, given suitable soil and temperature

conditions [Western U.S.A., Peru, northern

Chile ] .

DEFINITION OF BLIGHT EPIDEMICS

The losses caused by potato blight, both in

reduction of total yield and by infection of the

tubers, are consequences of the attack on the

foliage. To define the incidence of blight inany

area, the necessity is first to assess the course

of the unrestricted epidemic on unsprayed

foliage. If this is done over a sequence of years

it becomes possible to determine the frequency

with which epidemics of defined course and

severity occur in the region. The modifying

effect of control measures can then be ascertained by similar assessments on the treated

crops.

The use of a standardkey, suchas the B.M.S.

key [England and Wales], which defines the

degree of attack by the approximate percentage

of the total foliage killed by the diseas'^e,

enables assessments of blight to be made

quickly and easily in potato fields. Such

assessments can be made in any part of the

196
world, and workers in other countries can

understand what is meant by them. If these

assessments are nnade at intervals over the

growing period, the whole course of the

epidemic may be plotted on a time scale. This

progress curve will show the starting date of

epidemic spread, the date at which further

growth of the tubers is arrested, the period

during which spores are washed down on to the

soil and possibly will infect the tubers, andthe

amount of blight that is left on the haulm at

lifting»

In the absence of such complete information,

the eventual severity of the epidemic on the

haulm may best be defined by the date at which

the disease h?5 alm.ost completely defoliated

the plants and therefore stopped further growth

of the tubers. This stage ic easily recognizable

in the field, even from the air, and the date

introduces the num.erical element of precision.

DETERMINATION OF THE EFFECTS OF BLIGHT EPIDEMICS ON YIELD

Very frequently about the world, attempts

have been made to base estimates of national

losses from blight upon the results of sporadic

spraying trials. Such trials, usually made for

the purpose of comparing different spray treatments, tend to be carried out in the worst

districts for blight and on the most susceptible

varieties. By the tacit and usually wholly

incorrect assumption that similar results

would be obtained in all years and in all

districts, grossly exaggerated and m.isleading

figures are obtained.

In sonne countries [Northern Ireland, Denmark] spraying trials to a uniform plan have

been carried out at many centers year aiter

year over a long period, and the yields of

sound and blighted tubers have been recorded.

Such sequences of trials constitute excellent

surveys of the consequences of blight epidemics. If carried out at enough centers they

provide information on the average gain from

spraying that could scarcely be surpassed.

They are very laborious, however; they do not

reveal the defoliation losses unrecovered by

spraying, and unless they include systematic

assessment of the disease on the foliage they

provide no information on the courses of the

epidenaics in the several years--except perhaps by inference.

Following the work of Murphy [ Ireland ] in

relating effects of spraying to the development

of the potato plant, a method has been worked

out [England and Wales ] whereby the results

of a few carefully conducted spraying trials

with complete observations of the disease on

the foliage, coupled with periodic liftings to

obtain mean bulking curves for the prinèipal

varieties, can be used to interpret foliage

observations made on a large r imber of crops

over a very wide area. On test throughout

England and Wales over the past 10 years,

1946-56, this method proved to give mean

estimates of defoliation losses at least as

accurate as those that can be obtained by

lifting and weighing the produce from, sprayed

and unsprayed plots. Application of the principle of the method for the interpretation of

available data from other countries [Netherlands, Republic of Ireland, Arcostook County,

U.SeA] indicates its general applicability.

Similar methods are alrepxly in use in Poland

and under trial in Norway «

The basic principle is to ascertain the time

at which blight on the haulm has progressed so

far as to stop further growth of the tubers (75-

90 percent blight, according to the B.M.S. key)

and then to read off the proportion of the potential yield nornaally present at this date from a

mean bulking curve for the variety.

The mean bulking curve must be obtained

for the variety in the region for whichloss assessments are to be made, and it must be used

in such a way that account istakienof the relative length or shortness of the growing season.

FREQUENCY OF ^'BLIGHT YEARS" IN DIFFERENT REGIONS

The incidence of blight on the potato haulm

in any region is a product of two factors: The

severity of the epidemics and their frequency.

A "blight year" may be defined as one in which

there is at least 3 to 4 weeks' loss of growth

on maincrop varieties in along growing season

or 2 weeks in a short growing season, with a

consequent loss in yield of 10 percenter more.

The frequency with which such **blight years*'

occur is ranked in table 109 for some of the

potato-growing centers of the world for which

climagrams have been given.

This ranking shows clearly that not only are

most of the world's potato crops grown where

the mean temperature range in the 2 months

preceding maturity lies within 58*^ to 65° Fo,

but that the incidence of blight is also greatest

within this range. There is little correlation

betv/een the frequency of blight years and longterra average rainfall, except at the extremes

of the frequency scale. The centers fall, however, into quite distinct groups according to

the type of climate. Blight years are most

frequent in maritime areas and at high altitudes

in tropical and subtropical countries where

there is much cloud and mountain fog. Where

maritime influences modify continental conditions, as in Denmark and northern Germany,

blight years become less frequent; and where

true continental climatic conditions prevail,

as in the Wa^rsaw and Moscow regions, the

frequency is lowest«

197
TABLE 109. --Frequency of "blight years*' in some potato growing centers of world, ranked in

relation to mean temperatures and rainfall^ in the 2 months preceding maturity of the

maincrops

Frequency of

"blight years"

(years in 10)

10.

9 to 7.

6 to 3.

4 to 2.

1 or less.

Locations v/here mean temperatures in last 2 months before maturity were5QP to 37^ F.

(

Aberdeen, Scotland (3).

Reykjavik, Iceland

(2-1/2).

'Puno, Peru (4-1/2)

38^ to 63° F.

Mexico City, Mexico (6-1/2)

Kabale, Uganda (3)

Songer, Kenya (8)

George, S. Africa (3-1/2)

Birr, Ireland (3-l/2)

Penzance, England (3)

Brest, France (2)

Flushing, Holland (2-1/2)

MaMo, Sweden (2-1/2)

Chariottetown, P.E.I. (3-1/2)

Fens, England (2)

Jutland, Denmark (3)

Aroostook County, U.S.A. (3-1/2)

Cambridge, England (2)

Berwick, Scotland (2-1/2)

Ghent, Belgium (3 )

Gemert, Holland (2-1/2)

Hanover, Germany (3)

Leningrad, U.S.S.R. (2-1/2)

Moscow, U.S.S.R. (3)

Warsav;, Poland (3)

Stanley, Tasmania (2)

Christchurch, New Zealand (2)

Ballarat, Australia (1-1/2)

66° to 70° F.

Glen Innes,

Australia (3)

Long Island, U.S.A.

(3-1/2)

Red River Valley,

U.S.A. (2-1/2)

Saratov, U.S.S.R.

(1-1/2)

Numbers in parentheses after centers refer to inches of rainfall per month obtained from a

long-term average for the 2 months.

INCIDENCE OF BLIGHT AND WEATHER CONDITIONS

Although weather conditions inthe early part

of the growing period affect the date of outbreak of blight, it is, in general, the conditions

during the 2 months preceding maturity of the

crop that decide the progress of the attack on

the haulm and the amount of defoliation loss.

This is shown by the blight progress curves

available for different parts of the world, and

it is substantiated under the previous head in

these conclusions. Long-term average rainfall

during these 2 months does not determine the

relative frequency of blight years in different

areas, but in any given area annual variations

in rainfall profoundly affect both blight incidence and the level of crop yield. Rain provides water on the leaves in which sporangia of

the blight fungus can germinate, and it assists

spread of the disease by washing the sporangia

about; but its greater effect, directly and

indirectly, is to raise the relative humidity of

the atmosphere about the plants.Highhunnidity

rather than rainfall is the main connponent of

**blight weather,*' and the determination of

the frequency and extent of occurrence of such

weather demands detailed humidity observations [Netherlands, England and Wales, Ireland, Chile] . Average humidity records are of

little value for indicating the prevalence of

**blight weather," and if broad indications are

required they are best derived from consideration of the locality, the altitude, and the general

type of climate.

198
THE DUALITY OF THE DISEASE

The duality of potato blight, as a disease of

the haulm and also of the tubers, complicates

the estimation of blight losses and makes it

difficult for growers to grasp the rationale of

control measures. Sonnetimes, as among the

Indians of Peru, the foliage and tuber infections

are thought to be caused by two separate and

distinct diseases. Often, and even among the

most technically advanced of European and

North American growers, there is a tendency

to overestimate the importance of losses

from blight in the tubers--which are immediately obvious and sometimes spectacular--and to forget the more insidious

and unseen loss of yield due to premature

defoliation. Very frequently, loss estimates for defoliation and tuber infection are

lumped together and it becomes difficult

or impossible to disentangle them for separate consideration.

THE DISEASE IN THE TUBERS

Estimates of tuber infection cannot be derived from climagrams or progress curves for

blight on the haulm. These often show conditions under which tuber infection is likely to

occur, but whether it occurs or not and the

extent of its occurrence depend upon soil conditions, the thoroughness of the earthing-up,

the tuber suceptibility of the variety, the time

of lifting in relation to the blight attack on the

haulm, and the weather conditions at the actual

time of lifting. The incidence of tuber blight in

relation to all these and other factor s has been

little explored. Two routes of tuber infection

must be distinguished: Infection in the soil

before lifting and infection at lifting in the

presence of blighted haulm. Infection by the

first route mostly beconies apparent by lifting,

and as it is relatively easy to count or weigh

the conspicuously blighted tubers at this stage,

it is on this type of infection that most information is available. Infection in the soil before

lifting may occur where there is any blight on

the haulm to provide inoculum. It is therefore

a very general consequence of blight epidemics

on the haulm. Infection at lifting occurs only

where blighted haulm is still present or

sporangia are surviving in the surface layers

of the soil. Infection at lifting is therefore an

occasional consequence of blight epidemics on

the haulm.

Where there is a long growing season, with

time for blighted haulm to die down completely

well before lifting, tuber infection before

lifting is the main cause of loss. Where the

growing seasons and harvest periods are short

[ Iceland, northern Sweden, Prince Edward

Island, Aroostook County, U.S.A.] infection at

lifting may predominate. It may also be a very

serious source of loss inprimitive cultivations.

Tuber infection before lifting tends to be

greater in soils of high water-holding capacity.

such as the sea clays of the Netherlands, than

in drier soils, such as those of the great sandy

plain of northern Europe. The distribution

pattern and magnitude of losses from blight

in tubers before lifting, with a long growing

season, is exemplified by the results of detailed surveys in England and Wales. Here,

even in a bad blight year, the mean loss, over

the whole country, is only 3 to 4 cwt. per acre

(less than 2 percent of the crop). This is made

up by a very few crops with high losses,

sometimes of several tons per acre, numerous

crops in which the loss is moderate or slight,

and well over half the crops withno loss at all.

This pattern of distribution may be fairly

general. Certainly throughout the world, occasional heavy and spectacular losses from

blight in tubers tend to color the picture and

give rise to an unbalanced impression of the

relative importance of the losses caused by

tuber infection and those caused by premature

defoliation. Even in Ireland the mean loss from

blight in tubers (in unsprayed crops) is less

than a third of that due to premature defoliation, and in many regions the proportion is

much less. In the Costa region of Peru, where

coastal fogs favor infection of the haulm

and there is little or no rain to carry spores

down to the tubers, tuber infection is unknown.

Although the loss from blight in tubers may

often be much less than that from premature

defoliation, it tends to occur more constantly

from year to year. Thus, quite a small general

reduction in the level of tuber infection,

as by the adoption of more tuber-resistant varieties, may be of more value to

the national economy than much larger

but sporadic reductions of defoliation

losses in severe blight years [England and

Wales].

COMPENSATORY EFFECTS ON YIELD OF BLIGHT EPIDEMICS AND RAINFALL

Throughout the great potato-growing regions

of the world, in middle to high northerly latitudes, potato yields in *'blight years" are not

markedly lower than in years when there is

little blight, and they may indeed be higher.

This is perhaps the most striking single finding from the study: The compensatory effect

of yield increase from the more abundant rainfall in the blight years. To some extent the

good yields obtainedinblight year s are attributable to protective spraying. In some circumstances (see p. 204) protective spraying reduces

199
tuber infection, and this is part of the gain.

But the overall effect of spraying may be much

less than is generally supposed. In England and

Wales, where the situation has been very fully

analyzed, it is found that only about a third of

maincrops are sprayed, and that even if all

this spraying were fully efficient, it could not

be responsible for more than 0.35 ton per

acre increase of the mean national yield in a

bad blight year; whereas the mean yield inbad

blight years is about 8 tons per acre as against

7 tons in the drier years when there is little

or no blight. On a long-term average over all

types of seasons, protective spraying contributes only about 1 percent of the total potato

production. The part of spraying in maintaining high yields in blight years is greater in

Ireland, in the sea clay regions of Holland, and

some other maritime regions where the

frequency of blight years is higher and more

spraying is done. In Aroostook County, U.S.A.,

where all the crops are sprayed, it is likely

that yields in blight years might be lower than

in no-blight years if no spraying were done.

But in Denmarkand Germany, where the amount

of spraying done is much the same as in England and Wales, the contribution of spraying to

the production in blight years is probably about

the same. Certainly the general evidence shows

that throughout the main potato-producing

countries of the world, blight years are years

of greater potential yield than the drier years

when there is less blight. In other words,

drought or shortage of rain is of far greater

consequence than blight in reducing potato

yields, and this is true not only in northern

Europe and North America, but throughout the

world as a whole.

ECONOMIC EFFECTS OF BLIGHT LOSSES: TROPICAL AND SUBTROPICAL REGIONS

Blight epidemics are occasionally catastrophic, and it has been commoninthe past to

lay great stress on this fact. Even today, when

blight first invades an area where an impoverished and backward peasantry has become too largjely dependent on the potato for

food supplies [Chile, Cameroons] , there may

be acute temporary privation. In some regions

[ Mexico, Africa] high incidence of blight may

prevent potato cultivation in a number of

districts where it would otherwise be possible,

and thus deprive the population of a valuable

supplement to other and more important food

resources. Here and there the disease may

cause the loss of part of an otherwise favorable

growing season, with consequent loss of a

valuable market [Transvaal]. In most of the

recently invaded potato-growing areas, however, the disease has by no means led to any

cessation of potato growing, and production

has increased in spite of it [India, Kenya,

Uganda]. The tropical and subtropical regions

in which blight now causes the most spectacular occasional destruction are in general ones

in which the potato is only a minor, and sometimes only a luxury, crop and in which potato

yields are very low from causes other than

blight. Not infrequently, in such circumstances,

spectacular blight epidemics can do positive

good, for by focusing attention upon the potato

cultivations, they can lead to substantial improvements in the methods of husbandry,

manuring, choice of variety, and suppression

of virus diseases, which are the real needs of

the area, and can have much greater effect

upon the level of yield than all the losses from

blight.

ECONOMIC EFFECTS OF BLIGHT: MAIN POTATO-GROWING COUNTRIES

The economic consequences of blight epidemics in the great potato-growing countries

are conditioned by the salient fact that yields

tend to be highest in the blight years and

lowest in the years with least blight. They also

depend upon the national utilization of the

potato crop. In the majority of those countries

with a high human consumption of potatoes the

ware market is effectively buffered against

shortages and surpluses by a heavy usage of

potatoes for live stockfeeding and manufacture.

Peru is an exception to this. Where outlets for

the use of surpluses are limited [U.S.A.,

Canada, England, and Wales] , reduction of

abnormally high yields by blight in some years

may be positively advantageous to the national

economy, though not to individual growers.

Early defoliation may, however, lead to a high

percentage of small, unsalable tubers [Aroostook County, U.S.A.]. Where there is a large

and flexible usage of potatoes for fodder or

industry, all reductions of yield by blight,

even in the years when ware surpluses are

greatest, may constitute real national loss.

In the great potato-producing countries there

is very little likelihood of any recurrence of

famine due to potato blight, and shprtages of

potatoes, where they occur, are more likely to

follow the dry years with little blight, than the

wetter blight years. There nnay, however, be

many blighted potatoes coming on the market

after intermediate types of season, where the

total yield is maintained but the tuber infection

is heavy.

In all years and in all countries there is a

national loss chargeable to blight equivalent

to the total cost in labor and materials of all

the spraying done; and in years with little or

no blight there may be quite a serious loss due

to reduction of an already light crop by

mechanical and phytotoxic damage from unnecessary spraying. Where blight causes a

200
yield reduction regularly every year, or almost every year, there Is a national loss

equivalent to the acreage that has to be planted

to compensate for the proportionofthe regular

loss not recovered by spraying--and spraying,

where It Is done. Is rarely more than 50 percent efficient. The economic consequences of

blight are thus highly complex. They benefit

some and cause hardship to others; Indeed,

they Involve the whole structure of society,'

and they certainly cannot be expressed In

ternns of tons per acre.

COURSE OF ESTABLISHMENT OF BLIGHT IN NEW AREAS

Since 1935 potato blight has Invaded a nunnber of countries where It was previously unknown [Chile, Peru, Kenya, Rhodesia, Cameroons]. The Introduction has almost certainly

been brought about In each case through the

Importation of Infected tubers In seed or ware.

The courses of establishment of the disease In

the new areas throw light on the first epidemics In Ireland and other parts of Europe In

1845-46, and they have followed a fairly constant pattern. First, there Is aperlodofa year

or two when the disease Is present here and

there among the crops of the new.country, but

It passes unnoticed or unrecognized. Then

comes a year In which weather conditions are

unusually favorable for the Infection of

the tubers--It may be a late attack

causing no very great loss from premature

defoliation. Then by 111-fortune the following

year Is also one In which the weather Is

favorable for blight, and favorable this time

for an early attack. Many blighted tubers have

been planted In seed saved from the previous

season; there are many foci of Infection; and

the epidemic spreads over a wide area,

causing spectacular destruction of the haulm

and heavy loss of crop. At this stage there Is

alarm, the Impoverished peasants who have

been subsisting In greater or less degree on

the potato suffer acutely, and commissions of

Inquiry are set up--usually too late to prevent

lifting of the crops while blight Is sporlng profusely on the diseased haulm, with consequent

further widespread Infection of the tubers.

In the following years the disease then acts

as an Instrument of natural selection among

the potato varieties that have become Indigenous In the country. The more bllghtsusceptlble sorts gradually pass out of cultivation, and this process Is aided by the

Importation of more resistant sorts from other

parts of the world. In the meantime there may

be quite a run of years during which weather

conditions are unfavorable for the recurrence

of a severe epidemic [Chile]. Alarm then

subsides, and during these years the disease

becomes endemic over the whole terrain;

always with sufficient foci to start up the

disease again on an epidemic scale when the

weather Is favorable. Sooner or later, according to the climate, the next severe epidemic

does occur, but with le s s susceptible varieties;

with some spread of education about the nature

of the disease, the losses are less than In the

first year of epidemic attack. The cultivation

of potatoes then goes on, losses are considerable from time to time, but It Is usually

a case of better blight than lack of rain, and

the extent of the cultivation comes to depend

rather upon nnarket demands and the general

suitability of the area for potato growing than

upon the Incidence of blight. Some old and

popular varieties continue to be grown no

matter what their blight susceptibility.

CARRYOVER OF BLIGHT FROM SEASON TO SEASON

The cllmagrams for various parts of the

world show clearly the gaps between cropping

periods In which the fungus naust survive. In

some areas [George, S. Africa; Pukekohe,

New Zealand] potato growing goes on throughout the year, and new potato foliage Is always

subject to Infection by airborne sporangia from

older crops In the vicinity. In other areas

[Kenya, East and West Africa] where there are

two cropping periods In the year, the short

gaps between them may be bridged by potatoes

left In the ground and growing on like weeds.

Blight on solanaceous trees and shrubs [Kenya;

New Zealand] may also have a bridging effect.

In some hot regions [Plains, India] where potatoes can be grown only Inthe winter, the fungus

must survive a summer gap, withtemperature s

that may be lethal to the mycelium In the

tubers, but the Introduction of cold stores for

the seed In the summer has made survival

possible. In other regions where potatoes are

grown in the winter [ Southern States, U.S.A.],

tomatoes, for which the optimum temperature

range Is higher than for potatoes, may bridge

the summer gap. In still other areas [Transvaal; Australia] the winter gap Includes a

period, sometimes of several months, during

which the tubers are left In the ground. In

warm or hot soil, which causes more or less

complete rotting of any with blight Infection,

thus effecting their eradication. Go spore s of

the blight fungus have now been found In potato

foliage In one country of the world [Mexico],

and Is conceivable that these spores, If they

survive In leaf debris, may play some part In

the carryover of the fungus.

In the great northernpotato-growing regions

of the world (northern Europe, U.S.A., Canada)

there Is a long winter gap during which there

Is no foliage on which the fungus can survive.

201
and so far as is known this gap Is bridged

solely by survival of mycelium of the fungus

in some of the infected tubers. The proportion

of the infected tubers, in which the fungus does

survive is very small [Netherlands, England

and Wales]; most, but never all, of the

infected tubers are riddled out of seed stocks;

and the infected shoots arising from blighted

tubers planted are generally few and far apart.

Greater concentrations of blightedtubers occur

in cull piles [ U.S.A.] or in waste heaps left by

clamp sites [ England and Wales] , and thus the

probability of primary foci occurring ingrovsrth

from such heaps is high. The proportion of

infected plants among groundkeepers also tends

to be high, as some ofthese arise from blighted

tubers rejected at lifting. In many districts

where both early varieties and maincrop

potatoes are grown, the infection appears first

on the foliage of the early potatoes and these

contribute to the first infection of the maincrops. Undoubtedly, the long period required

for the buildup of infection after the winter

break retards the onset of epidemic spread

of the disease; there is some evidence that

when the level of blight in the tubers is low in

a dry year, the outbreak in the following year

will be later than usual. But in the main there

appears always to be sufficient inoculum to

start a blight epidemic when weather conditions

are favorable for its development. Although

important progress has recently been made in

the study of the early stages of blighf epidemics

[Netherlands, England and Wales], there is

much about this phase of the disease that is

still obscure.

EFFECTS OF VARIETAL RESISTANCE IN MODIFYING BLIGHT EPIDEMICS

That sonne potato varieties are more resistant to blight than others was observed during

the first epidemics of blight in Europe in 1845-

46. Ever since that time the choice, or the

breeding and introduction, of varieties having

some measure of blight resistance as one of

their desirable qualities has been the principal

factor in reducing the incidence of blight about

the world. The introduction of Championgreatly

lessened the severity of the blight epidemics

in Ireland during the nineteenth century, and

the same process is still going on. In recent

years the substitution of Sebagofor Up-to-Date

in Australia, of Robijn for Kerr's Pink in

Kenya, of Sebago for Green Mountainin Prince

Edward Island, and of Alpha and Voran for

indigenous varieties in Mexico are all cases in

point. For whatever reasons these substitutions

were made, and by no means all of them were

deliberately made for the purpose of reducing

blight losses, they have all had this effect.

During the past quarter of the century the

long mooted idea of breeding potato varieties

totally immune from blight by crossing the

domestic potato with Solanunn demissum and

other wild species has been very strenuously

explored [u.S.S.R., Germany, Scotland, Netherlands, U.S.A., Mexico, England and Wales] .

The work has revealed that the blight fungus

is not homogeneous, as was once supposed, but

that it has a number of races or strains.

differing from each other inthe potato varieties

that they can attack. New varieties, with ^.

demissum in their parentage, have withstood

attack by all known races of the fungus for a

time, only to go down eventually with other

races not previously distinguished or perhaps

not previously existent. The situation has been

analyzed in genetical terms (see fig. 16,

Scotland). The ntiajority view of potato breeder s

now appears to be that R-gene resistance is

not enough, and it may be of less value than the

long familiar kind of partial resistance possessed to a varying degree by such varieties

as Champion, Voran, Kerr*s Pink, Alpha, and

Sebago. This kind of resistance is now distinguished by the name of '*field resistance."

Although only a partial resistance, this is

effective against all blight race s.Its attractive -

ness to potato breeders has recently been

enhanced by the discovery [Mexico] of an operative sexual phase in the blight fungus, which

may be capable of producing an almost infinite

range of new blight races.

A very important outconie of all the work

with S. demissum is the discovery that, apart

from R-gene resistance, this species also

possesses field resistance in a very high degree, and thatitmay beusedas a source of this

quality in potato breeding for field resistance.

The nature of field resistance, now it has been

given this name, is under world investigation.

POTATO BLIGHT FORECASTING

A general relationship between the development of potato blight epidemics and the occurrence of wet or humid weather has been

recognized since the famine years of the last

century, but it was not until the formulation of

the Van Everdingen rules [Netherlands] in the

mid-1920's that real progress was made inthe

attempt to define specific meteorological conditions that would provide warnings of blight

outbreaks for the purpose of spray timing. The

work was advanced by the researches of

Crosier in the U.S.A. onthe humidity and moisture requirements of the blight fungus, published in 1934 [introduction], and by the derivation, over the period 1929-39 of the simple

Beaumont rule [England and Wales] .

202
In the past decade much practical success in

regional forecasting has been achieved by the

application of the Beaumont and other temperature-humidity rules--notably those of Post and

Richel, Bourke, and Wallin--in ways already

summarized in this study under the countries

concerned [Netherlands, England and Wales,

U.S.A., Irish Republic, Germany]. The essentially different cumulative-rainfall method of

Cook has also been successfuly employed in

eastern Virginia, and with certain modifications Hyre has found it successful under test

in some Northeastern States [U.S.A.] .

In general critical weather periods for the

development of blight have now been empirically defined, and the recording of them

has been reduced to fairly simple and practical

procedures. Critical periods, according to one

system or another, can now be recordedin most

parts of the world, but the way in which the

observations are to be interpreted and used to

form the basis of a valid forecasting system

can be worked out only by investigation and

experience in the country concerned. Account

must be taken of the number and distribution of

the weather stations available, the nature of

the terrain, and the phenology of the crops.

Effective and extensive collaboration between meteorologists and plant pathologists is

a prime requirement in the development of

a forecasting service, as the meteorological

findings (whatever criteria of "blight weather**

are employed) must be checked and correlated

with the results of adequate field surveys of

blight outbreaks in the region to be served.

The use of synoptic weather maps and airmass analysis [irish Republic] may find an

application in many parts of the world, where

forecasting has to be attempted for a wide

area with no very close network of weather

stations.

PROTECTIVE SPRAYING: MATERIALS

The fungicides employed for potato spraying

have changed during the past quarter of a

century. Up till the early 1930's the use of

bordeaux or burgundy mixtures was universal.

Preparations of copper oxychloride and of

cuprous oxide then began to come on the

market, having the advantage that they greatly

simplified the preparation of the spray fluid

in the field, but the disadvantage that they did

not give such lasting protection of the foliage.

Bordeaux and burgundy mixtures, in point of

retentivity under rain, are still unsurpassed.

The copper oxychloride and cuprous oxide

sprays made great headway, nonetheless. The

introduction of low-volume potato spraying in

the late 1940* s--following the development of

light low-volume machines for the application

of MCPA and 2,4-D weedkillers to cereals--

was greatly to the advantage of the new sprays,

as they were suitable for application at low

volunne while bordeaux and burgundy mixtures

were not. By the 1950's practically all the

potato spraying in England and in Holland was

being done with cuprous oxide or copper oxychloride. In the meantime, the use of organic

sulfur fungicides, chiefly zinc or manganese

dithiocarbamates, had largely superseded copper oxychloride and cuprous oxide in the

U.S.A. It was found that under the prevailing

conditions in the U.S.A.--with frequent applications, high temperatures, and much insect

damage conducive to copper injury--the dithiocarbamates controlled the epidemics satisfactorily, and their use led to increased

yields. By 1957 the dithiocarbamates had taken

a place among the available potato spraying

materials of the world; they were replacing

copper oxychloride to a considerable extent in

the Netherlands, at least for the first sprayings,

and they were just being launched on the market for potato spraying in England and

Wales.

PROTECTIVE SPRAYING: EXTENT OF USE

The amount of protective spraying done in

various parts of the world depends upon the

nature of the terrain, the frequency and severity

of the losses caused by blight, the value of the

crop, and the amount of technical development

in the country concerned. The method is impracticable in primitive cultivations, up steep

mountains, and in areas where torrential rains

may wash off the spray deposits as soon as

they are applied [East Africa] . It is employed

to best advantage in maritime areas where

there is a high level of yield, and where without it there would be heavy defoliation losses

almost every year [Netherlands, Ireland,

Prince Edward Island] . It is used, sometimes

to excess, where cultivations are very fully

mechanized, and numerous applications

already required for insect pest control make it

easy to incorporate fungicides [parts of U.S.A.].

The need for spraying against Colorado beetle

[France] may also promote its adoption.

The value of spraying as a routine measure

is uncertain and its use is local and discretionary in countries where there is an

irregular alternation of blight years and years

with little or no blight [Germany, England and

Wales]. It is in such countries that blight

survey and forecasting work is most needed,

to distinguish the localities in which spraying

is profitable and to save wasted expense and

crop damage in years when it is not. In such

countries there is a strong tendency to spray

203
only the more blight susceptible and valuable

varieties. In general there is a tendency for

less and less spraying to be done as the

climate becomes more and more continental,

and the need decreases [U.S.A., northern

Europe], But, in the absence of guidance from

objective and impartial blight surveys, there

is also a decided tendency for the amount of

spraying done to be influenced as much by the

commercial enterprise of spray manufacturers

and spraying contractors as by the incidence

of blight.

PROTECTIVE SPRAYING: EFFECT ON CONTROL OF BLIGHT IN TUBERS

Although it is common to regard protective

spraying as a measure for the reduction of

defoliation losses rather than for the control

of blight in the tubers, one point that emerges

from this study is that protective spraying

sometimes effects a very considerable reduction in tuber infection. In some countries

[Ireland, sea clay regions of Holland] the

available figures show that protective spraying

generally about halves the mean percentage

of blighted tubers in the crops. Where spraying

unduly prolongs the growth of partially blighted

haulm at the end of the season, it often has the

opposite effect; but in some countries,

especially those with a short growing season

[Iceland] and in some seed-growing areas

[Scotland, Brittany], protective spraying appears to be used for the main purpose of preventing tuber infection. There is a very noticeable preference for the use of copper rather

than organic sulfur fungicides for the later

applications where the reduction of tuber

blight is an objective; in Ireland strong applications of burgundy mixture are recommended

for this purpose.

It is understandable that if protective spraying is continued late in the season and is so

effective that it keeps blight off the haulm

entirely, there will be no inoculum to wash

down to the tubers and their infection will be

prevented. This ideal state of affairs is rarely

attainable in practice, and despite all practicable spraying there will almost inevitably

be some blight on the haulm toward the end of

the season in a blight year. Then the effect of

the spraying would appear to be one of degree,

the longer blight is kept off the haulm by protective spraying, the shorter the period during

which sporangia can wash down and infect the

tubers. The greater persistence of the copper

sprays, and particularly of bordeaux and

burgundy mixtures, inpart explains the preference for copper rather thanfor the dithiocarbamates, as copper sprays provide the longest

possible protection of the haulm from the last

spraying and therefore the greatest possible

avoidance of the chance of tuber infection.

There is evidence [Prince Edward Island; Long

Island, U.S.A.; Pukekohe district. New Zealand] that the direct application of copper

fungicides to the soil or accumulation of

copper in the soil after years of protective

spraying with copper may kill sporangia washing down to the soil, and therefore effect a

direct protection of the tubers. If this is, in

fact, an important part of the action of late

copper applications in reducing tuber infection,

then the inferiority of the dithiocarbamates

for this purpose becomes understandable, for

they may break down and lose their

potency as fungicides when in contact with

soi].

Where the growing period is short, and

haulm destruction some 10 days or a fortnight

before lifting would entail a considerable

sacrifice of crop, reduction of tuber infection

by late protective spraying with copper may be

a better technique. But it has the disadvantage

that it nnay leave the haulm green and with

blight sporing on it at lifting, and thereby lead

to infection of the tubers when they are exposed. Perhaps with copper in the soil, the

usually stipulated 10 to 14 days' delay between haulm destruction and lifting is not

necessary, and haulm destruction much

nearer the date of lifting would suit

the case. This is a matter for investigation.

HAULM DESTRUCTION

Removal of blighted haulm before lifting to

prevent infection of the exposed tubers is one

of the oldest of blight control measures, but

destruction of the haulm by chemical or

mechanical means is a comparatively new

technique. It is now employed extensively in

North America, the Netherlands, Great Britian,

and to some extent in other European countries,

and particularly on seed crops. The principal

materials now employed are sodium and potassium arsenites, sulfuric acid, s odium chlorate,

DNC, and other di-nitro compounds. Mechanical haulm-cutter s and pulverizers are als o in

limited use.

Haulm destruction is employed on seed

crops to prevent translocation of viruses fronn

the haulm to the tubers and also for the

limitation of seed size. It is often also useful

on ware crops as a farm practice for clearing

the ground in preparation for lifting.

As a measure for the reduction of blight

Infection of the tubers it is of most value where

partially blighted haulm would otherwise be

surviving at lifting. This most commonly

204
happens where the growing season Is short

(under continental conditions and at high latitudes) and the method is certainly used on a

very large scale in North America at the

present time for the purpose of preventing

tuber infection at lifting. It has the disadvantage that a period, generally taken as 2

weeks, has to be allowed between haulm

destruction and harvest. Thus, some sacrifice

of an already short growing season has to be

made if lifting is to be completed successfully

within the limited harvesttime.

As a measure for the reduction of tuber

infection in the ground before lifting--the chief

need where there is a long growing seasonand

most crops die down naturally well before

harvest--haulm destruction is often disappointing [England and Wales, Netherlands].

Unless the risk of tuber infection in the soil

is considerable, as when a variety very

susceptible to blight in the tubers is being

grown, it is generally uneconomic to employ

haulm destruction for the purpose of preventing tuber infection before lifting. Good

earthing up and the avoidance of tuber-susceptible varieties are better expedients.

CONTROL BY ^^DISEASE-ESCAPE" METHODS

One of the main findings of this study has

been that the optinrium conditions for potato

cropping are in general also the optimum

conditions for the incidence of blight. Oppor-

' tunities for evading blight losses by moving to

other production areas are therefore few.

Sometimes it may be preferable to grow

potatoes in areas where yields are low, rather

than in those where yields would be nnuch

higher but for the heavy or catastrophic incidence of blight [Sierra versus the high

valleys, Mexico]. Surface irrigation may provide the water required for good potato yields

without giving rise to the humidity necessary

for the development of blight on the foliage;

thus, potato cultivation under surface irrigation in arid regions may be attended with

disease-escape [Western States, U.S.A.,].

Planting of maincrops as early as possible

^ and sprouting before planting [England and

Wales], or **vernalization'* [U.S.S.R.], may

also effect an important measure of diseaseescape, by obtaining as much of the crop as

possible before weather conditions become

favorable for epidemic spread of blight. Detailed study of the climagram for an area may

help to reveal other possibilities. The choice

of early-maturing, or early-bulking, varieties

[ Ar ran Banner, in Ireland; Bintje in France

and the Netherlands] may contribute to the

same end. But early-maturing varieties usually

also sprout early and are not good keepers.

The combination of early bulking and late

sprouting, even if it is not attainable by

breeding, could perhaps be secured by judicious

use of chemical sprout-depressants. The opposite method of disease-escape by choice of

variety is to grow certain late varieties, which

are not generally attacked by blight until late

in the season [Ackersegen, in Germanvl.

INDICATIONS OF SUBJECTS FOR FURTHER INQUIRY

In the course of this study the authors have

inevitably found many matters concerning the

epidemiology and control of potato blight on

which there is a paucity of information in the

world. Some of the directions in which further

investigation appears to be most needed are

as follows.

1. There is a need in most countries for

systematically conducted field surveys

of the incidence of blight, over a number

of years, to establish the true losses

caused by the disease and to guide control policy.

2. Such surveys can be made most economically by organizing countrywide observations of the disease on the haulm, and

interpreting the findings in the light of

periodic lifting and spraying trials at key

centers. Aerial reconnaissanee - -with

suitable checks from the ground--could

be employed in this work.

3. More joint agronomic and plant pathological inquiry couldbe devotedto realizing the full potentialities of diseaseescape methods of control.

4. In potato breeding there is scope for

•^improving" varieties popular for their

quality, such as King Edward in England

and Bintje in the Netherlands, by breeding in a greater measure of field resistance.

5. Breeding for tuber resistance deserves

more attention.

6. The possibilities of breeding for diseaseescape, e.g., for early bulking coupled

with good keeping quality (without early

sprouting), though perhaps intrinsically

difficult, should be further explored.

7. Work in progress on the investigation

of the mechanism and true nature offield

resistance is of fundamental importance

and should be extended.

8. There is need for further investigation

of heat treatment of seed-potato tubers,

to ascertain whether or not there is any

practical possibility of eliminating in205
fection from seedstocks by this means

and to reveal the effects of such treatnnent, at various times between lifting

and planting, on dormancy breaking and

sprouting. Such inquiries would be relevant to observed effects of natural heating on seedstocks in several parts of the

world.

9. In regions of the world where blight is

still absent or confined to small areas,

a sharp watch should be kept for new

outbreaks to prevent the dispersal of

infected local seed, which generally precedes first and catastrophic epidemics.

10. Modern spray materials wouldbe greatly

improved if their capacity to resist

washing off by rain could be brought up

to equality with that of bordeaux and

burgundy mixtures.

11. The possible efficacy of copper application to the soil as a means of reducing

blight infection of tubers deserves critical investigation.

12. The whole question of the need for 10 to

14 days' delay between haulm destruction and lifting requires re examination.

There is also a dearth of real knowledge

concerning the relations between soil

conditions and tuber infection.

13. There is need for further study of the

early stages of blight epidemics. Conditions governing spore dispersal, distances traveled by sporangia without

losing their viability, and mechanisms

of spread in the field have not been adequately explored.

14. The establishment of blight forecasting

services is particularly desirable in

countries where there is great variation

in the incidence of blight from year to

year. The purpose of such services

should be as much to save unnecessary

spraying and sprayer damage in years

when the incidence is slight as to promote correct spray timing in those when

it is likely to be severe.

15. The working out of a successful forecasting scheme in any country demands

investigation of the significance of critical weather periods by direct blight

survey in the field and is best linked with

surveys proposed in paragraph 1.

16. The economics of chemical control need

close examination in relation to the

frequency of blight years and to the loss

from crop damage in protective spraying, and from crop sacrifice in haulm

destruction.

17. There is need for much more spread of

education about the nature of the disease

over the world as a whole. The need for

more enlightenment concerning the interactions between haulm and tuber infection

is not confined to backward countries.

18. In potato marketing and the organization

of potato supplies, there is need for

economic study of the opposed effects

of rainfall and blight losses on potato

yields.

206
LITERATURE CITED

Aebi, H.

1956, Principes de lutte contre le mildiou de la

pomme de terre. Rev. Romande 12: 4, 29-31.

Akeley, R. v.. Lombard. P, M., and Stevenson, F. J.

1952. HLight-resistant potato varieties can save

copper and cut cost of production. Amer.

Potato Jour. 29: 49-52.

. Stevenson, F. J., and Cunningham, C. E.

1955. Potato variety yields, total solids, and cooking

cpiality as affected by date of vine killing.

Amer. Potato Jour. 32: 304-313.

Stevenson, F. J., and Schultz, E. S.

1948. Kennebec: a new potato variety resistantto late

blight, mild mosaic, and net recrosis. Amer.

Potato Jour. 25: 351-361.

Allan, W.

1956. Cyprus. Annual report of the department of

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219
INDEX

SUBJECT INDEX

Aerial reconnaissance, 205

Africa, 5, 166-180, 196, 198, 200, 201, 203

Aircraft, use for spraying in

England, 25

Netherlands, 83

Red River Valley, 130

Alabanna, 129, 133

Alternarla solani> SEE Early blight

Aphids, control of, 133, 146

Argentina, 5, 161

Aroostook County, Maine, 121-126, 136, 197,

198, 200

Arsenites, use for haulm destruction in

Canada, 146

England, 32

Netherlands, 84

U. S. A., 133

World, general, 204

Austria, 107-109, fig. 48

Australia, 191-195, 201, 202, fig. 92

Beaumont periods. SEE Forecasting

Belgian Congo, 173

Belgium, 3, 67-71, 196, 198, fig. 27

Blight assessment on haulm, methods

B. M. S. key, England and Wales, 11

Dutch modification of, 78, fig. 33

Use in Norway, 93, fig. 41

Use in other countries. SEE Blight

progress curves

Horsfall and Barratt probit scale

U. S. A., 134

Use in Peru, 1 56

International needs, 196

Scales for incidence and severity

Austria, 107, 108

Denmark, 86

Western Germany, 104

Blight epidemics

Definition of

By frequency and severity of blight years,

3, 134, 136, 197

By progress curves, 11, 196, 197

Determination of effects on yield, 19, 39,

54, 79-81, 87, 88, 97, 98, 103, 104,

122-124, 197

Conapensatory effect of rainfall, 16, 86,

199, 200

Early stages of, 7, 17, 77, 147

Economic consequences of, 200

In Ireland 1845 and Chile 1950 compared,

161

In particular countries« SEE under country

In relation to weather. SEE blight weather

In world, general review, 197, 198

Modification of, by resistant varieties, 202

Blight in tubers

Conditions favoring. SEE Tuber infection

Control measures, 31, 204, 205. SEE ALSO

Haulm destruction; Heat treatment

Blight in tubers--Continued

Effect of storage conditions, 8, 84

General review, 199

Incidence of. SEE under countries

Varietal resistance, 29, 35, 73, 106, 143,

188, 194, 199, 205

Blight progress curves

Chile, 162, 164

Denmark, early use in, 88, fig. 38

England and Wales, 11, 32, figs. 8,9, 12

France, 63, figs. 24, 25

India, 182, fig. 89

Long Island, 132, fig. 65

Netherlands, 78, 79, figs. 31, 32

Prince Edward Island, 143, fig. 70

South Africa, 168, fig. 81

Switzerland, use in, 108

Tasmania, 195, fig. 94

World, general, uses, 197, 198

Blight races: biotypes of Phytophthora infestans

Discovery of, 3, 34

In Kenya, 174, 175

In Mexico, 154

In Netherlands, 84

In Northern Ireland, 47

In Peru, 160

In South Africa, 169

In Tanganyika, 177

In U. S. A., 125, 140

In Western Germany, 106

International nomenclature, 42, 140, 150,

fig. 16

Significance of oospores, Mexico, 154, 202

Survey of, Canada, 150

World, general review, 202

Blight-resistant potato varieties. SEE Breeding, Potato varieties, and Field resistance

Blight weather

Characteristics of, 9, 198

Critical periods, 17, 56, 74, 138, 145, 158,

194

SEE Forecasting

Blight zones

England and Wales, 11, 12, figs. 4,5

France, 62-65

Poland, 109, figs. 51, 52

Principles of zoning, 11

Republic of Ireland, by rainfall, 49, fig. 18

'*Schadgebiete'' in Western Germany, 102,

105. fig. 46

U.S.S.R., 113, fig. 56

B. M. S. key, for blight on haulm, 11

Bordeaux mixture

Copper-lime ratios, 146

Need for equal retentivity in other sprays,

206

Properties of, 203, 206

220
Bordeaux mixture--Continued

Use for potato spraying in

Belgium, 70

Canaeroons, 179

Canada, 143, 146, 147, 150

Chile, 165

Cyprus, 117

England, 21-25

France, 66

Kenya, 175

Long Island, 133

Mexico, 153

Netherlands, 82, 83

Norway, 95

U.S.S.R., 115

World, general review, 203

Bouisol, colloidal copper spray, 54

Brazil, 5

Breeding potatoes for blight resistance

Effect in modifying epidemics, 202

History of, general, 3, 202

In Australia, 195

In Belgium, 70

In Canada, 141, 150

In Chile, 165

In England and Wales, 34-36

In France, 66

In Kenya, 174

In Mexico, 153, 154

In Netherlands, 84

In Northern Ireland, 48

In Poland, 110

In Scotland, 42

In South Africa, 169

In U. S. A., 125, 140

In U.S.S.R., 112, 115

In Western Germany, 106

Need of breeding for tuber resistance, 205

Burgundy mixture, use for potato spraying in

England and Wales, 25

Iceland, 91

India, 183

Irish Republic, 51, 52, 54

Netherlands, 82, 83

Northern Ireland, 45, 46

World, general review, 203

Calcium carbide, use for domancy breaking, 189

Calcium cyanamide, use for haulm destruction

Canada, 146

U. S. A., 138

Cameroons, 178-180, 200, 201, fig. 85

Canada, 140-151, 200, 201, fig. 68

Carryover of blight, from season to season

By continuous cropping, 159, 166, 189, 202

By imported seed, 116, 127, 134, 169, 182

By outdoor tomatoes, 134, 189, 201

Early stages of epidemics, 17, 75, 206

Effect of cold storage on. 182, 184, 201

Effect of soil temperature on, 169

In U. S. A., 131, 134

In world, general review, 201, 202

Indications from climagranns, 201

Investigations needed, 206

Life history of fungus, 7, 8

Carryover of blight--Continued

Primary foci from blighted tubers. 17 75

77, 128, 202

SEE ALSO Cull piles, Groundkeepers, Solanaceous wild plants

Chile, 5, 57, 160-165, 196, 198, 200, 201,

fig. 77 '

China, 5

Christchurch district. New Zealand, 187-189

** Chuño," 155

Climagrams

Australia, 194, fig. 93

Austria, 107, fig. 49

Belgium, 68, 70, fig. 28

Cameroons, 178, 179, fig. 86

Canada, 142, 148, figs. 69, 71

Chile, 161, fig. 78

Cyprus, 117, fig. 58

Denmark, 88, fig. 36

England, 15, fig. 6

Explanatory notes, 3, 4, fig. 2

France, 65, fig. 26

Iceland, 90, fig. 40

India, 181, 182, fig. 88

Kenya, 173, fig. 83

Mexico, 151, 153, fig. 73

Netherlands, 7 3, fig. 30

New Zealand, 185, fig. 91

Northern Ireland, 43, fig. 19

Norway, 92, fig. 42

Peru, 157, fig. 75

Poland, 109, fig. 50

Republic of Ireland, 49, fig. 19

Scotland, 40, fig. 15

South Africa, 166, fig. 81

Sweden, 95, 98, fig. 44

Uganda, 176, fig. 84

U. S. A., 121, 128, 130, 132, figs. 61, 63, 64

U.S.S.R., 111, 114, fig. 55

Uses in potato technology, 3, 196

Western Germany, 105, fig. 47

World, general review, 196, 197, 198

Climate, in relationto potato growing and blight

General considerations, 1, 3, 9, 196-199

SEE ALSO under countries

Colorado beetle and blight control

Austria, 109

Belgium, 70

Canada, 146

France, 66, 203

Netherlands, 83

U. S. A., 120, 133

Western Germany, 106

World, general review, 203

Common scab of potatoes, surveys in

England and Wales, 19

Consumption of potatoes

In world, general, 3, 4, 200, 201

SEE ALSO under countries

Copper carbonate, use in potato spraying,

U.S.S.R., 115

Copper, in soil, effect on blight in tubers

Long Island, 131

Prince Edward Island, 147

Pukekohe district. New Zealand, 190

221
Copper, in soil--Continued

Need for further investigation, 206

Copper oxychloride sprays, use in

Belgium, 70

Cyprus, 117

Denmark, 88

England and Wales, 22-25

France, 66

Kenya, 175

Netherlands, 82, 83

New Zealand, 190

Northern Ireland, 46

Scotland, 41

U.S.S.R., 115

Western Germany, 106

World, general review, 203, 204

Copper sprays, phytotoxic effect

SEE Spray damage

Cost of potato spraying

SEE Economics of spraying

Cull piles, waste heaps, and clamp sites, as

sources of blight* infection, 7,75,125,134,

145, 202

Cuprous oxide sprays, use in

Belgium, 70

England and Wales, 25

France, 66

Kenya, 175

Netherlands, 83

Northern Ireland, 46

Western Germany, 106

World, general review, 203

Cyprus, 115-117, figs. 57, 58

Daylength, effects of

Equatorial Africa, 171, 174, 175

General considerations, 6

Hastings section, U. S. A., 129

Iceland, 90, 175

Norway, 92, 94

Sweden, 95, 97, 98, 99

New Zealand, 190

DDT spraying, 66, 70, 83, 125, 130, 133, 135,

169, 191

Denmark, 85-89, 197, 200, fig. 35

Disease-escape nnethods of blight control

England and Wales, 36

France, 66

General considerations, 3, 205

Mexico, 153, 205

Peru, 160

South Africa, 169

Southern Rhodesia, 171

U.S.S.R., 115, 205

Distribution of potato growing, 1, 3, 196, 200,

fig. 1

SEE ALSO under countries

Dithane. SEE Dithiocarbamates

Dithiocarbamates, use for spraying in

Australia, 195

Belgium, 70

Canada, 146, 147

Cyprus, 117

England, 26

France, 66

Dithiocarbamates- - Continued

Mexico, 153

Netherlands, 82, 83

Peru, 159

South Africa, 169

Southern Rhodesia, 171

U. S. A., 125, 129, 133, 136

Western Germany, 106

World, general review, 203

DNC, DNOC, DNBP, di-nitro compounds, use

for haulm destruction in

Netherlands, 84

U. S. A., 133, 138

Western Germany, 106

World, general review, 204

Dormancy breaking in seed tubers, 116, 189

Dusting, for blight control in

Canada, 150

England and Wales, 25

France, 66

Northern Ireland, 45

Peru, 159

Red River Valley, Minn, and N. Dak., 130

U.S.S.R., 115

Early blight (Alternarla solani) in

Australia, 194

Canada, 146

Prince Edward Island, 146

Red River Valley, 130

South Africa, 169

Economic effects of blight losses

Compensatory effects of rainfall, 199, 200

In main potato-growing countries, 201

In tropical and subtropical regions, 200

Need for further investigation, 206

Economics of potato spraying

Belgium, 70

England and Wales, 26-28, 33, 34, 36

Netherlands, 83

U. S. A., 137

Egypt, 115

England and Wales, 6, 10-36, 196, 197, 198,

199, 200, 202, 203, 204, 205, figs. 3.4

Exploration, for wild Solanum species, 35, 115

Field resistance of potato varieties to

Phytophthora infestans

As in Norway, 95

Ackersegen and Voran in Chile, 165

Alpha and Voran in Mexico, 153, 202

Black's Seedling 1521(c) in Tanganyika, 177

Comparison with R-gene resistance, 154,

169, 202

Definition of, 35, 154, 202

Further investigations needed, 205

Pimpernel and others in Southern Rhodesia,

171

Robijn in Kenya, 174, 202

Sebago in Australia, 191, 195

Sebago in Canada, 145, 2Q2

Skerry Blue (Toro Blue) in Uganda, 177

Solanum demissum as source of, 34, 154, 202

World, general review, 202

Flame throwers, use in haulm destruction, 138

222
Flea beetle, 120, 133, 146

Florida, Hastings section, 126-129, 136

Fohn winds, effect of. New Zealand, 188

Forecasting, potato blight

Beaumont rule, 17, 202, 203

Concept of zero date, 41

Cook's cumulative rainfall method, 138, 139,

203, fig. 67

Cooperation between pathologists and meteorologists, 203

In Aroostook County, Maine, 125

In LK>ng Island, New York, 133

In Netherlands, 74, 75, 202, figs. 31, 32

In Red River Valley, 130

In Scotland, 41

In Switzerland, 108

Investigations in Western Germany, 105, 106

Irish rules, 56, 203

Moving graph method, U.S.A., 139, 203

Nocturnal humidity rule, Peru, 158

Nomogram for incubation period, 115

Operations charts, 17, 203, fig. 7

Requirements for success, 203

Thran accumulated temperature rule, 106

'•Tizon" (blight) warning service, Chile, 165

Trials of Beaumont and Irish rules

In Chile. 164, fig. 79

In Tasmania, 195, fig. 94

Use of synoptic maps, 57, 203, fig. 21

Van Everdingen rules, 74, 115, 202

Warning services in France, 66, 67

Warning service in Prince Edward Island,

145, 147

World, general review, 202, 203

France, 60-67, 198, 203, 204, 205, figs. 22, 23

Frosts

Effect on blight infection of tubers, 99, 121,

150

Effect on overwintering of blight, 124

Frost-resistant varieties, 90

Limiting potato-growing period, 6, 90, 99,

121, 166, fig. 81

SEE ALSO Climagrams

Gains and losses from potato spraying. SEE

Spraying trials and Economics

Germany. SEE Western Germany

Gnorimoschema operculella. SEE Potato moth

Greece, 115

Groundkeepers as sources of blight infection,

131, 134, 174, 189, 201

Growth curves for potato tubers

Bintje and Voran, Netherlands, 79, fig. 34

Late varieties in Maine, 122, 123, fig. 62

Majestic and King Edward, England and

Wales, 19, fig. 8

Up-to-Date, Republic of Ireland, 54

Use in loss estimations, 19, 20, 54, 196,

197, fig. 8

Haulm cutters and pulverizers, 33, 84, 89,

125, 131, 133, 138, 204

Haulm destruction

For control of virus diseases, 48, 66, 71

In world, general review, 204, 205

Haulm destruction--Continued

Investigations needed, 206

Loss of yield caused by, 33, 34

Methods and objects of, 32, 204

On cull piles, 125

On seed crops, 39, 40, 41, 48, 66, 70

Practice in

Australia, 195

Canada, 150

Europe, 66, 70, 84, 89, 106

Peru, 159

U. S. A., 125, 131, 135, 138

U.S.S.R., 115

SEE ALSO Sulfuric acid, Arsenites, Tar oil

fractions. Sodium chlorate. Calcium

cyanamide, D.N.C, and Haulm cutters

and pulverizers

Trials in England, 31, 32

Vascular discoloration caused by, 125

Heat treatment of seed tubers

By high soil temperatures, 169, 201, 206

Lethal temperature for mycelium, 8

Need for reinvestigation, 205, 206

Proposed use in South Africa, 169

Holland, SEE Netherlands

Humboldt current effects

Chile, 164

Peru, 157

Humidity conditions governing blight epidemics. SEE Blight weather and Forecasting

Iceland, 89-92, 198, 199, 204, fig. 39

India, 181-184, 196, 200, 201, fig. 87

Introduction of blight into new areas

Belgian Congo, 173

Cameroons, 178, 201

Canada, 147, 148

Chile, 160, 161, 201

Cyprus, 116

India, 182

Ireland, 160, l6l

Kenya, 172, 173, 201

Need for lookout services, 206

Northern Rhodesia, 177, 201

Parallel between first Chilean and Irishepidemics, 160, 161

Peru, 156, 201

Southern Rhodesia, 170, 201

Tanganyika, 173

Tasmania, 191

Uganda, 173, 176

U. S. A., 133

World, general review, 201

Ireland, 196, 197

Blight and weather in, compared with Chile,

160, 161, 164

Republic of, 48-59, 198, 199, 203, figs. 17,18

Irrigation, practice and effect on blight

Australia, 194

Chile, 160, 164, 165

Cyprus, 115, 117

General considerations, 6, 196

India, 182, 184

Mexico, 153

New Zealand, 189

223
Irrigation- -Continued

Overhead irrigation

Australia, 194

New Zealand, 189

Peru, 155

Southern Rhodesia, 170, 171

U. S. A.. 133, 135

Israel, 115

Italy, 115

Japan, 5

Kenya. 172-175, 198, 201, 202, figs. 80, 82

Leafhopper, 120, 130

Leaf roll, 145, 170

Long Island, 131-133, 136, 198, 204

Losses

Due to premature defoliation by blight

Econonxic effects of, 200

In particular countries. SEE under country

Methods of computation, 19-25, 38,39,55,

79, 86, 97, 98, 124, 142, 143, 156.

157, 183. fig. 8

Due to tuber infection

Frequency diagram, England and Wales,

fig. 11

General review, 199

In particular countries. SEE under country

The duality of the disease, 199

Due to spray damage. SEE Spray damage

Low-volume spraying, technique, 22-25, 66,

83, 104, 106, 125. 138, 189, 195, 203

MCPA and 2.4-D weedkillers, 203

Mexico. 1, 151-154, 184, 198, 200, 201, 202.

205, fig. 72

Morocco, 115

Mozambique, 170

Netherlands, 3, 5, 71-85, 196, 197, 198, 199.

202, 203, 204, 205. fig. 29

Net necrosis, 145

New Zealand. 5, 185-190. 198, 201. 204, fig. 90

Northern Ireland, 43-48, 197, figs. 17, 18

Northern Rhodesia. 177. fig. 80

Norway, 92-95, 197, fig. 41

Pentachlorophenol. use for haulm destruction.

195

Peru. 5, 153, 155-160, 196. 198. 199. 200,201.

fig. 74

Phytophthora infestans

Biotypes. SEE Blight races

First records. SEE Introduction into new

areas

Germination conditions, 7, 8

Incubation. 7, 8

Infection through upper and lower leaf surface, 99

On tomatoes. SEE Tomatoes

Oospores. in Mexico, 1. 154, 201. 202

Primary foci, 7, 84, 212

Spore dispersal, 7, 8, 9, 77

Sporulation. 7. 8

Phytophthora infestans - - Continued

Survival

At higl temperatures, 8, 130

In stems, 8, 147, 157

In tubers. 8. 73. 77, 182

Tuber infection, routes of, 7

Wild host plants. 134, 154, 174, 189

World distribution, 1

Poland, 109-110, 197, 198, figs. 50, 51

Population

In relation to potatoes, 5

SEE ALSO under countries

Potatoes

As world crop, 3, fig. 2

Breeding. 3, 202

SEE ALSO Breeding for blight resistance

Cultivations. SEE under countries

Factors affecting yields, 3, 5, 196. 197, 1?9

Human consumption

Use for fodder and manufacture. 5. 200

SEE ALSO under countries

Length of growing season, 3. 6

Rate of bulking. SEE Growth curves

Seed crops. SEE Seed-potato growing

Potato moth, 191

Potato varieties

As 92 95

Ackcrsegen, 60, 70, 90, 101, 102, 106, 109

110, 161, 165. 205, fig. 52

Adina, 191

Agnes, 102

Akranes, 90

Alma, 109

Alpha, 49. 68. 86. 90. 91. 151. 153. 202

Amarilla de Pueblo. 151

Aquila. 70, 101, 102, 106

Arran Banner, 10. 29, 35, 43, 48. 49. 52. 5 6

60. 116. 117, 185, 188, 205

Arran Chief, 185, 187, 189

Arran Consul, 43

Arran Peak, 10, 35, 43

Arran Pilot, 10, 37. 91, 92

Arran Victory, 43, 45. 52

Arran Viking, 35

Aucklander Short Top, 185. 188

Aucklander Tall Top. 188

Augusta. 102

Ballydoon, 43

Berlichingen, 112

B.F. 15, 60

Bintje, 60, 63-66, 67-70, 73. 76, 78-84, 85

87, 108, 151, 205

Birgitta, 95

Bismark. 191

Black's Resistant Seedling 1521(c). 177. 180

Black's Resistant Seedling 1521(d). 180

•'Blight Résister'' 914, 177

Bliss Triumph, 119, 129

Blue Icelandic. 90

Bona, 101, 102, 105. 108

British Queen. 35. 43, 46, 47, 49, 52

Brownell. 191

Canso. 141, 142, 150, 180

Capella. 102

Carman. SEE Green Mountain

224
Potato varieties--Continued

Champion, 49, 84, 202

Cherokee, 180

Chippewa, 119

Concordia, 102

Corahila (nnixture), 161, 165

Craig's Alliance, 35

Craig's Bounty, 35

Craig's Royal, 35

Craig's Snow White, 35

Dakota, 185, 188

Darjeeling Red Round, 181, 184

Dianella, 86

Dr. Mclntosh, 10, 35

Duke of York, 10, 68, 73, 75, 77, 101, 108

Dunbar Standard, 43

Early Puritan, 95

Early Rose, 90, 91, 95, 109, 112

Eclipse, 10

Eersteling. SEE Duke of York

Eigenheimer, 68, 73, 77. 78, 81, 82, 90

Empire, 140

Epicure, 10, 37, 40, 49, 112

Epron, 112

Erste von Nassengrunde, 66

Erstling. SEE Duke of York

Etoile du Le^on, 60

Factor. SEE Up-to-Date

Flava, 102, 105, 109

Furore, 90, 151, 171

Gamekeeper. SEE Northern Star

Gineke, 151

Gladstone, 29, 35, 43

Gloria, 95

Gola, 181

Golden Wonder, 37, 49, 52

Grahm. SEE Up-to-Date

Great Scot, 90, 181

Green Mountain, 119, 121, 123, 124, 131,

138, 140, 141, 142. 143. 145. 147, 191,

194, 202, 212, fig. 71

Gullauga, 90, 91

Heida, 101, 102, 105

Home Guard, 10, 35

Hybrid 42 (U.S.S.R.), 112

Ijsselster. 68, 73

Industrie, 70, 161

Institut de Beauvais, 60

Inverness Favourite, 188

Irish Cobbler, 92, 119, 131, 141, 142, 147,

157

Italian White Round, 181

XeTssing, 92

Kameraz. 112

Katahdin, 119, 121, 123, 124. 131. 138. 141,

142

Kennebec, 6, 119. 121, 125, 139. 140, 142,

166, 169, 171, 180

Keswick, 141, 142, 150, 180

Ker Pondy, 60

Kerr's Pink, 37, 38, 43. 49, 52. 55. 90. 92.

94, 173, 174, 175, 202

Kigezi Blue (Skerry Blue), 174

King Edward, 10, 19, 22, 23, 24, 28, 29, 32,

34, 35,37,38,43,49,187,188,189, fig. 9

Potato varieties--Continued

Kinongo Kerai (Northern Star). 174. 175

Krasava, 60

Lenino, 109

Leona, 151

Lochar, 45

Lorkh, 112

Magna, 102

Magnum Bonum, 95, 97, 181

Majestic, 10, 19-24, 29, 34, 35, 37, 39, 43,

46, 48, 49, 54, 81, 117, 175, fig. 12

Maritta, 101, 102, 106

May Queen, 10

Mittelfrühe, 102, 109

Monak, 191

Ninetyfold, 10

Noordeling, 73

Northern Star, 174, 175, 189

Oberambacher Frühe, 101. 108

Olympia, 102

Onikwa 52, 169

Onikwa 53, 169

Orion, 35

Papa Amarilla, 157

Papa Blanca. 157

Pentland Ace, 35, 42, 171

Pentland Beauty, 35

Phulwa, 181, 182

Phytophthora-Resistant, 112, 115

Pimpernel, 171

Placid, 180

Pontiac, 119

Preferment, 1 74

President, 95, 161

Prestkvern, 92

Prummel, 151

Pungo, 180

Record, 68

Red Icelandic, 90, 91

Red McClure, 119

Red Pontiac, 119, 129, 138

Redskin, 10, 37

Red Warba, 119

Robijn^ 169. 171, 174, 175, 202

Robusta, 102

Roode Star, 174

Russet Burbank, 119

Russet Rural, 119

Saskia. 60, 108

Satha, 181

Sebago, 119, 123, 126, 140, 141. 142, 143.

145. 169, 191, 193, 194, 202

Sequoia, 90, 191

Sharpe*s Express, 10

Sieglinde, 101, 108

Silesia, 109

Sirtema, 66

Skerry Blue, 174, 177

Snowflake, 191

Sutton*s Supreme, 185

Switez Dolkowski, 109

Toro Blue (Skerry Blue), 177

Triumph. SEE Bliss Triumph

Tylstrup Odin, 86

Ulster Beacon, 48

225
Potato varieties - -Continued

Ulster Chieftain, 10, 48

Ulster Dale, 35

Ulster Emblem, 48

Ulster Ensign, 48

Ulster Knight, 48

Ulster Premier, 35, 48

Ulster Prince, 10, 48, 171

Ulster Supreme, 35, 48

Ulster Tarn, 48

Ulster Torch, 35, 48

Up-to-Date, 35, 49, 52, 53,54,55,86,90. 92,

94, 95, 97. 116, 117, 151, 166, 169, 170,

171, 175, 177, 181, 191, 194, 202

Urgenta, 60

Varieties with field resistance to blight.

SEE Ackersegen, Arran Consul, Arran

Viking, Cherokee, Furore, Kigezi Blue,

Kinongo Kerai, Pimpernel, Placid,

Pungo, Robijn, Skerry Blue, Voran

Varieties with R-genes for blight resistance.

SEE Aquila, Black's Resistant Seedling

1521(c), Canso, Craig's Snow White, Kennebec, Keswick, Maritta, Onikiva 52,

Onikiva 53, Orion, Pentland Ace, Pentland Beauty, Phytophthora-Resistant,

Ulster Torch

Vera, lOl

Virginia, 102

Voran, 49, 68,73, 77, 78,79,81,82, 84, 102,

153. 161, 165, 169, 171, 174, 202

White Rose, 119

Wohltmann, 94, 109, 112

Powdery scab, 124

Primary foci of blight infection, 7, 8, 75, 77,

84, 202

Prince Edward Island, 131, 142-147, 196, 198,

199, 202, 203, 204

Pukekohe district, New Zealand, 189-190,201,

204

Rainfall

Compensatory effect on yield, 16, 86, 199, 200

Need for economic study, 206

Data, in relation to potato yields and

severity of blight

Cameroons, 179, 180

Canada, 148

Chile, 163

Cyprus, 117

England and Wales, 15

Netherlands, 74

Northern Ireland, 43

Republic of Ireland, 51, 53

U. S. A., 128, 129, 135, 136

Western Germany, 105

Effect on potato cropping, 196

In relation to frequency of blight years, 197

SEE ALSO Climagrams

Red River Valley, 129-131, 136

R-gene resistance of potato varieties to

Phytophthora infestans

Comparison with field resistance, 154, 169,

202

SEE Breeding potatoes for blight resistance

Russia. SEE U.S.S.R.

Scotland. 5. 6. 36-43, 198, 202, 204, figs. 13,

14

Seed potatoes

Chitting (green-sprouting), 10, 114, 115

Cutting of, 120, 191

SEE ALSO Vernalization

Seed-potato growing in

Austria, 109

Belgium» 67, 68

Canada, 140, 141, 142, 146

Chile, 161

Denmark, 83, 88

England and Wales, 1 0, 11

France, 60

India, 181

Netherlands. 71

New Zealand. 185, 187, 189

Northern Ireland, 43

Republic of Ireland, 49

Scotland. 36

South Africa, 169

U. S. A.. 118, 121, 129

U.S.S.R., 112

Western Germany, 99, 100, 105

Self-sets. SEE Groundkeepers

Sodium arsenite. SEE Arsenites

Sodium chlorate, use for haulm destruction

Denmark. 89

France, 66

Netherlands, 84

New Zealand, 189

Northern Ireland, 48

Republic of Ireland, 52

World, general review, 204

Soil conditions affecting

Potato yields, 6, 196

Tuber infection, 7, 199, 204, 206

Solanaceous wild plants, as sources of blight,

134, 201

Solanunn aculeastrunn, Kenya, 174

Solanum aviculare. New Zealand, 189

Solanum incanum, Kenya, I 74

Solanum indicum, Kenya, 1 74

Solanum panduraeforme, Kenya, 174

Solanum andigenum, use in potato breeding,

171, 174

Solanum demissum hybrids.

SEE Breeding for blight resistance

SEE Field resistance

South Africa, Union of, 36, 49, 166-169, 198,

200, 201, fig. 80

Southern Rhodesia, 170-171, fig. 80

Spain, 36, 115

Spongospora subterránea. SEE Powdery scab.

Spray damage

By copper injury, 26, 52, 55, 125, 129, 137,

146, 195, 203

By sprayer wheels, 25, 26, 41, 81, 120, 133,

138, 195

Crop sacrifice in haulnn destruction, 33, 34,

91, 205

Spraying practice

SEE Low volume spraying

226
Spraying practice - -Continued

SEE under countries

World, general review, 203, 204

Spraying trials, results of

Australia, 193, 195, fig. 94

Catneroons, 179, 180

Canada, 143, 144, fig. 70

Denmark, 87, 88, figs. 37, 38

England and Wales, 20-25, 34, fig. 9

France, 64, figs. 24, 25

Iceland, 91

India, 184, fig. 89

Mexico, 153

Netherlands, 78-81, figs. 32, 33

New Zealand, 189, 190

Northern Ireland, 45-47

Norway, 94

Peru, 156, 157

Republic of Ireland, 52-54

Sweden, 97

U. S. A., 123, 127, 128, 129, 131, 137, fig. 65

Use in national surveys, 197

Western Germany, 103

Storage losses, caused by blight, 30, 31, 84,

124

Sulfuric acid, use for haulm destruction

History of, in England, 31, 32

In France, 66

In Scotland, 41

In world, general review, 204

Surveys for potato blight

Austria, 107

Canada, 147

Denmark, 86

England and Wales, 12-25, fig. 5

Methods, 11, 74, 84, 107, 108, 109, 206, 216

Netherlands, 78

New Zealand, 188

Northern Ireland 46

Norway, 92, 93, 94

Poland, 109

U. S. A., 121-124, 133-135

Western Germany, 102

Sweden, 6, 95-99, 198, 199, fig. 43

Switzerland, 107

Synchytrium endobioticum. SEE Wart disease

Tanganyika, 171, 177, fig. 80

Tar oil fractions, use for haulm destruction,

33

Tasmania, 191, 195, 198

Temperature

Range for severe blight incidence, 197, 198

Ränge for tuber production, 5, 196, 197, 198

Temperature - - Continued

Requirements of Phytophthora infestons. 7,

o

Requirements of potato crops, 5

Significance of diurnal range, 196

SEE ALSO Climag rams

Tomatoes, blight on

Carryover to potatoes, 134, 189, 201

Chile, 161

Kenya, 174, 175

New Zealand, 189

Northern Rhodesia, 177

South Africa, 169

Uganda, 177

U. S. A., 201

World survey of, 1, 134

Toxaphene, 130

Tuber infection, by blight

Absence in lack of rain, 156, 157

At or before lifting, 28, 135

Conditions favoring, 30, 124, 125, 130, fig.

Control measures, 31, 204, 205

Duality of the disease, 156, 199

Effect of.copper in soil, 131, 147, 190, 204

Effect of drying tubers after harvest, 115

Effect of protective spraying on, 52, 54, 91,

204

In relation to maturity, 28, 40

In relation to soil conditions, 7, 29, 65, 69,

81, 82, 83, 84, 104,188,199,204

In seed crops, 39, 84, 134

Incidence of, general review, 199

Routes of, 7

SEE Haulm destruction

Varietol differences, 29, 188

Tuber moth, 169

Uganda, 171, 173, 175-177, 198, 200, 201,

fig. 80

U. S. A., 6, 118-140, 196, 197, 198, 200, 201,

202, 203, 204, 205, figs. 59, 60

U.S.S.R., 6, 111-115, 197, 198, 202, 205, fias.

53, 54

Vernalization of seed pototoes, 114, 115

Vine killing. SEE Haulm destruction

Wart disease, in Newfoundland, 141

Weather conditions favoring blight. SEE Blight

weather

Western Germany, 99-107, 198, 200, 202, 203,

205, fig. 45

Zineb. SEE Dithiocarbamates

227
NAME INDEX

Aebi, H., 108

Akeley, R. V., 122. 123, 125, 137. 140

Allan, W., 116

Andren, F., 97

Andries, E., 68

Angus, A., m, 177

Amautov, V., 115

Azad, R. N., 182, 183, 184

Baerecke, M. L., 106

Bald, J. G., 5. 191. 194

Baldwin, B. J. T., 177

Bant. J. H., 21

Barbotin. F., 63

Barratt, R. W., 134. 156

Barrus, M. F., 135

Bary, A. de, 8

Bates. G. R.. lU. 31. 170

Bazin de Segura. C. III. 153. 156. 158. 159,

160

Beaumont. A., 17, 21, 74. 184. 195. 202. 203

Bechtel, W. H., 191

Beck, J., 70

Beer, W. J., 25, 26

Beran, F., in, 107

Biggar, W. A., 38, 41

Bjórknnan, T.. 96

Björling, K., 99

Black, W., III. 35. 42. 84, 140. 174, 177. 180

Blair, I. D., 188

Blodgett, F. M., 131, 190

Bonde, R., UI. 121, 122. 123. 124, 125, 134,

139

Bordukova. M. V.. 115

Bourke. P. M. Austin, UI. 56. 57. 58, 160, l6l,

162, 163, 164, 165, 184, 195. 203

Boyd, A. E. W.. 28, 39, 40. 41

Boyd, D. A., 11

Boyd, O. C., 135

Brentzel, W. E., 130, 133

Priejer, C. J., Ill

Brien, R. M., 189

Brooke, D. L., 126, 129

Brown, B. E., 119

Bruin, T., 84

Bruyn, H. L. G. de, 84

Buchholtz, W. F., 129, 132

Bukasov, S. M., 115

Burton, W. G., 5

Bushnell, J., 5, 196

Butler, E. J., 182

Cairns, H., 45

Callbeck, L. C., IH, 142, 143, 145, 146, 147,

150

Campagna, E., 154

Cartledge, E. W., 193, 195

Cartmill, W. J., 191

Cassell, R. C, 122, 124

Cass Smith, W. P., 193

Caudri, L,., m

Cervantes, J., 152, 153, 154

Chambers, P. C, 117

Chambers, S. C, 195

228

Chester, K. S., 134

Chouard, P., 65

Choudhuri, H. C, 184

Claridge, J. H., 185

Clark, C, F., 119

Clarke, J., 48, 1 74

Clayson, A. M., 8

Clayton, S. N., 127, 128, 129, 137

Clouston. D., 38, 41

Cook, H. T., Ill, 130, 134, 136, 137, 138, 139

Cox, A. E., 26, 136

Crosier, W., 8, 56, 202

Crucq, J., 84

CuUen, J. C, III, 35

Cunningham, C. E., 122

Darling, H. M., Ill, 139

Darpoux, H., Ill, 63, 65

Dastur, J. F., 182

Davidson, W. D., 49

Davidsson, I., 90

Dawe, T. C. R., 189

Deasy, D., 49

Delaney, D., Ill, 49

Demesmay, H., 60

Dickey, R. S., Ill, 13

Dingley, J. M., 189

Dingwall, A. R., 187

Doling, D. A., 48

Doncaster, J. P., 19

Drew J, P. 49

Driver, C. M., m, 6, 187, 189, 190

Dubniak. H., Ill, 109, 110

Ducomet, V., 63

Ehimas, P., Ill, 63, 66

Dyke, G. v., 20

Dykstra, T. P., 119, 129

Eddins, A. H., Ill, 126, 127, 128, 129, 137

Edmundson, W. C., 120

Eide, C. J., Ill, 130, 139

Emilsson, B., 99

Ext, W., 104

Fish, S., Ill, 191, 193

Foéx, E., 63

Foister, C. E., UI, 37, 38

F^rsund, E., lU, 92, 94

Forsyth, J. C, 141

Fridriksson, S., Ill, 90

Galindo, J., 154

Gallegly, M. E., 140, 154

Georghiou, G. P., Ill, 116, 117

Graham, K. M., 150

Grainger, J., Ill, 38, 41

Grainger, P. N., m, 150

Gram, E., IH, 86, 87, 88, 89

Gray, E. G., HI, 38, 39, 41

Gregory, P. H., 19

Guntz, M., m, 63, 66

Gwinn, A. B., 138

Hânni, H., 9, 108

Hârle, A., 102, 103
Hagberth, N. O., 95

Hammarlund, L., 88

Hascoet, M., 66

Hawkes, J. G., 6, 35, 115, 155

Henderson, J., 28, 40

Hirst, J. M., Ill, 8, 9, 17

Hix, H., 101

Hoctor, D., Ill

Hodgson, W. A., 150

Hôljer, E., 96

Hogen Esch, J, A., 73

Holz, W., 106

Honey, J. K., 19

Hopkins, J. C. F., m, 170

Horsfall, J. G., 134, 139. 156

Howard, H. W., 35

Howatt, J. L., Ill, 148, 150

Hoyman, W. G.. Ill, 129, 130, 131, 137, 139

Hudson, S. C, 35, 141

Hyre, R. A., 121, 124, 125, 132, 139

Jaivenois, A., 70

Jensen, J. L., 8, 169

Johannes, H., Ill, 9, 102, 103, 104, 105, 106

Jonasson, J., 96

Jouin, C, 66

Journet, P., Ill, 66

Juzepczuk, 115

Kameraz, A., 113

Keay, M. A., Ill, 175, 177

Kendrew, W. G., 3

Kerr, W. E., Ill, 170

Klatzman, J., 61

Krostrowicka, M., 35

Lai, T. B., 182

Lamas Carrera, J. M., 156

Landis, B. J., 120

Lansade, M,, 64, 66

Large, E. C., 11, 17, 19. 20, 22, ¿5, 26, 32,

33, 35

Leeuwenburgh, J., 83

Leitch, C. G., 185

Lessens, W. J., 11

Lihnell, D., Ill, 97, 99

Lint, M. M. de. III, 78, 82, 84

Littlejohn, L. J. S., 116, 117

Llaveria, M., 157, 159

Löhnis, M. P., 74

Lombard, P. M., 119, 125

Lorkh, A. G., 112, 114

Lunden, A. P., 95

Lutz, J. M., 130

Maan, W. J., 83

McCulbin, E. N., 126

Mclntosh, T. P., Ill

McKay, R., Ill, 49, 50, 52, 54, 55, 56

Mackelvie, D., 48

Magee, C. J., IH, 191

Majid, S., 183

Malmonté, M. J., 62

Malmus, N.. 104

Martin, L. D., 31

Mastenbroek, C., 42, 84, 140

Mathew, Father, 50

Mattingley, G. H.. 191

Meijers, C. P., 78, 82, 84

Melhus, I. E., 138

Miller, P. R., IH, 1, 130, 132. 134, 139

Mills, W. R., 42, 84, 140. 153, 154

Mitra, A. K., 181

Monot, G., 66

Montaldo, A., I6l, 165

Moore, W. C., IH, 11, 29. 30

Moore, W. D., 138

Moreau, R. E., 6, 171, 177

Müller, K. O., 35, 102, 160, 161, 165

Murphy, P. A., 28, 49, 52, 54, 55, 142, 197

Muskett, A. E., IH, 43, 45

Nattrass, R. M., ni, 116, 172, 173, 174, 175

Naunnova, N, A., 115

Niederhauser, J. S., in, 151, 152. 153, 154,

184

Nijdam, F. E., 73

Novikov, F., 115

O'Brien, M., 1, 130, 132, 134

Oehrens, E., 161

Oldaker, C. E. W., 191

Opitz, K., 101

Ormel, H. A., 82

Pal, B. P., 181

Parks, N. M., 140, 142

Patterson, J. B. E., 25

Peikert, F. W., 125

Petersen, H. I., 88

Peterson. C. E., 138

Peterson. L. C., HI, 42, 84, 131, 132, 134, 140,

190

Pethybridge, G. H., 35, 49. 52

Pettersson. S., 97

Polykov, I., m, 113

Post. J. J,, 74. 203

Pouchet. J.. 66

.Pushkarnath. 181

Racicot, H. N., IH. 147

Ramsey. G. B.. 128

Raucourt. M.. 64. 66

Reddick. D.. 140, 174

Reid, W. J.. 119, 129

Revilla, v., 157, 159

Rhoads, A. S.. 128

Richel. C.. 74. 203

Richter, H., HI

Rigot, N., 69

Riker, A, J., IH

Riley, E. A., UI, 177

Robertson, N. F., 8

Robinson, J. A., 124

Roer, L.. 94

Roland, G., 70

Rose, D. H., 138

Ross, H., 106

Rosser, W. R., 19. 22, 34

Rozhestvenski, N.., 113, 114

229
Rudorf. W., 102, 106

Ruehle, G. D., 129

Ruprecht. R. W., 126

Russell, E. J., 60, 111, 181

Russell, T. A., 178. 179

Ryan, M., 174

Salaman, R. N., 34, 35

Samson, R. W., 139

Sanchez, L,., 157, 159

Schaal, L.. A., 120

Schaper, R., 106

Scheibe, K., UI, 103, 104, 106

Schick, 102

Schreier, O.. 107

Schrumpf, W. E., 121

Schultz, E. S., 122, 124, 125, 140

Sellgren, K. A., 99

Servin, L., 154

Shands, W. A., 121

Siebeneick, H*. 73

Simmonds, J. H.. III. 191

Simpson. G. W., 121

Skaptason, J. B., 131, 190

Smallfield, P. W.. III. 185

Smith, la. P., 17, 57

Smith, O., 118

Snell, K., 101

Sommer, K., 106

Spurlock, A. H., 126, 129

Stamp, L. D., 185

Stapel, G.. 87, 88. 104

Stevens. N. E., 133

Stevenson, F. J., 122, 123, 125, 126, 137, 139

140

Storey, I. F., 21, 22

Stutt, R. A., 141

Tallantire, J. D., III, 178. 179, 180

Taylor. G. G., 25, 189

Taylor, R. E., 22

Ten Houten, J. G., III

Thran, P., 8, 106

Thurston, H. D., 130, 139

Torka, M., 106

Townsend, G. R., 129

Toxopeus, H. J., m, 84

Uhlig, S., 8. 9, 102, 105, 106

Ullrich. J., m, 106, 107

Vandenplas, A., 70

Van der Plank, J. E., III, 5, 166, 169

Vanderwalle, R., 69, 70

Van der Zaag, D. E., 8, 75, 84, 169

Van Everdingen, E., 74, U5, 202

Van Hamelen, M. G., 99

Van Vleit, W., 141

Vasudeva, R. S.. lU, 181, 182, 183, 184

Vavilov, N. I., 115

Ventura, E., m, 64, 66

Voelkel, H., 102, 103, 105

Wade, G. C., m. 191, 193, 195

Wade, G. K., 139

Waggoner, P. E., 139

Walker, J. C., 133

Wallace, R. W., III, 43

Wallin, J. R.. m, 8, 129, 130, 137, 139,203

Waterston. J. M., III. 178

Watts Padwick, G., 171

Webb, R. E., 123, 125, 137

Welvaert, W., m, 69, 70

White, L., 191

Wilkins, V. E., m

Williams, P. G., 193. 195

Wiltshire, S. P., 74

Wood, J. I., 132, 135

Yule, A. H.. 22

Zaitsev, N. D., 112

230

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