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AGRICULTURE HANDBOOK NO. 267

AGRICULTURAL RESEARCH SERVICE

U.S. DEPARTMENT OF AGRICULTURE
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PRODumon

AGRICULTURE HANDBOOK NO. 267

AGRICULTURAL RESEARCH SERVICE

U.S. DEPARTMENT OF AGRICULTURE

BY AUGUST E. KEHR, ROBERT V. AKELEY, AND GEOFFREY V.C. HOUGHLAND

Washington, D.C. Issued July 1964

For sale by the Superintendent of Documents, U.S. Government Printing Office,

Washington, D.C., 20402 - Price 60 cents
CONTENTS

Page

INTRODUCTION 1

History 1

Botany of the potato 2

Morphology and anatomy 3

Plant introduction and maintenance 3

Per capita consumption of potatoes 8

Value as food 8

PRODUCTION 9

Production areas in the United States 9

Production factors in relation to profits 11

VARIETIES 13

Anatomical features of potato varieties used for

identification 13

Stem and stem wing 13

Leaves and terminal leaflets. _ 13

Stipules 13

Flowers 14

Tubers 14

Sprout characters 14

^" Disease resistance —15

Varieties from which to choose 15

Cheroliee 16

Chippewa 16

r)azo(' 16

Early Gem 16

Early Ohio 17

Fundy 17

Green Mountain 17

Irish Cobbler 17

Katahdin 17

Kennebec 17

Keswieli 18

Norland 18

Onaway 18

Ontario 18

Plymouth 18

Pungo 18

Red LaSoda 18

Red McClure 1!)

Red P(mtiac 19

Red Warba 19

Rushmore 19

Russet Burbank 19

Russet Rural 19

Russet Sebago 20

Seliago 20

Teton 20

Triumph 20

Warba 20

Waseca 20

White Rose 20

Varieties introduced since 1960 20

Arenac 20

Blanca 21

Catoosa 21

Emmet 21

LaChipper 21

LaRouge 21

Navajo 21

Ona 21

Penobscot 21

Redsl<in 21

Reliance 21

Shoshoni 21

Snowflake 21

Viking 22

Varieties for disease resistance and special purposes - 22

Varieties resistant to specific diseases 22

Varieties with special food values and other

qualities 22

Varieties for special uses 22

Page

GROWTH REQUIREMENTS OF THE

POTATO 22

Soil requirements „ 22

Climatic requirements 23

Temperature 23

Moisture 23

Day length 24

Length of growing season 24

Rotations 24

Soil preparation and improvement 24

Building up soil organic-matter content 25

Liming 26

FERTILIZERS 26

Importance 26

Kind and amount 27

Fertilizer placement 27

Avoidance of nutrient deficiencies 28

SELECTING AND PREPARING SEED 28

Selecting seed stock 28

Certified seed 28

Effect of geographical source of seed 29

Effect of storage 29

Effect of greensprouting 29

Breaking rest period 29

Thiocyanate treatn)ent 30

Ethylene chlorohydrin treatment 30

Seed treatment 30

Whole versus cut seed 31

Cutting seed potatoes 31

Suberization of cut seed 31

Care of cut seed 32

PLANTING THE CROP 32

"—'Time of planting —-32

Seed spacing 32

Depth of planting 33

Planters 33

Amount of seed to plant 35

GROWING THE CROP 35

Cultivation 35

Irrigation—humid areas 36

Furrow irrigation—dryland areas 36

Overilead irrigation 38

Hail injury 38

Spraying and dusting 39

Weed (control 43

HARVESTING THE CROP 44

Vine killing 44

_ Season of harvest •—45

Harvesting machinery 45

Avoiding mechanical bruises at harvest 46

Washing 47

Dry cooling 47

STORING THE CROP 47

GRADING AND MARKETING 50

BREEDING IMPROVED POTATO VARIETIES. 50

POTATO QUALITY 52

Production factors affecting solids and mealiness

of potatoes 52

Determin i ng solids in potatoes 54

Air-and-water method 54

Potato hydrometer method 54

Blackening after cooking 54

Internal blackening of raw tissues 55

Tulier greening 55

GROWING AND DEVELOPING POTATOES

FOR SPECIAL USES 57

Potatoes for chips 57

Potatoes for dehydrated products 59

Potatoes for frozen products 59

Potatoes for certified seed 59

m
ACKNOWLEDGMENTS

The authors wish to acknowledge the helpful cooperation of .Joe W. Jarvis,

Supervisor of Livestock and Agriculture, tlnion Pacific Railroad Co., and

Ben F. Vance, Iowa State University, for providing several of the illustrations.

Some of the information in this handbook is similar to that formerly contained in Farmers' Bulletin 1958, "Potato Production in the Northeast and

North Central States"; Farmers' Bulletin 2034, "Potato Growing in the Western States" ; and Farmere' Bulletin 2098, "Potato Growing in the South."

IV
commERcmi

POTHIO

PRODUCIIOn

By:

AUGUST E. KEHR,

Horticulturist

ROBERT V. AKELEY,

Horticulturist

GEOFFREY V. C. HOUGHLAND

Plant Physiologist

Crops Research Division

Agricultural Research Service

INTRODUCTION

Commercial potato production is a liijrlily specialized business requiring large capital investments. If the enterprise is to be ])rotitable, farm

operators must have both technical training and

business experience. liach year since 1950, a progressively greater proportion of the annual crop

has been processed in comparison to the proportion sold as fresh or stored raw potatoes. This

trend has created new problems in potato production and marketing.

]*robiems facing potato growers today are far

more complex than any innnediately i)receding

the Second World War. Despite existing problems, yields per acre rose from a national average

of 80 hundredweight per acre in 1940 to almost

200 luuidredweight per acre in 1962 (table 1). It

T.Mii.E 1.—Arerage fofal fotato prodvrtion and

yleJdii, xelerfed ye<ir>í. Íí)30-C>2

Yoar

Total acres

harvested

Yield per

har\'ested

acre

Total production

1930

1935

1940 ._

1,000 acres

3, 138. 9

3, 468. 8

2, 832. 1

2, 664. 3

1, 697. 9

1,40.5. 0

1,371. 0

1, 359. 4

1,428. 4

1,336.3

1, 396. 9

1,496. 0

1, 385. 0

Cwt.

65. 7

65. 5

79. 9

94.4

152. 6

162. 1

179. 3

178.4

186. 9

183. 9

184. 3

196. 3

193. 7

1,000 cwt.

206, 290

227, 337

226 152

1945 251 639

19.50...

1955

259, 112

227, 696

245, 792

242 522

19.56

1957 _

19.58 . . 266, 897

245, 799

257, 435

293, 594

268, 280

1959

1960

1961

1962

has been predicted that the national average acre

yield should rise to as much as 250 hundredweight

by 1975. Although iiational total production did

not change markedly from 1940 to 1961, the acreage requii'ed to grow the normal annual crop has

decreased by about 50 percent. Shifts in the geogra])hical areas of production, changes in varieties

grown, shifts to fewer and larger farms, maiked

changes in mechanization and crop utilizat ion, and

imjn-oved cultural methods all have contributed to

intensification in production and increased yields

per acre.

History

Most botanists agree that the potato originated

in the New World, though the exact locality of

origin is uncertain. Historians record that the

Spaniards found potatoes in Peru at the time of

their conquest of that country beginning in 1524.

The native home of the ]iotato is often claimed to

be in the Andes of Peru and Holivia at altitudes of

4,000 to 6,000 feet, where its vei-y close botanical

relatives Hourisli even today.

Historians disagree about the first introduction

of the potato into Kurop. There is good evidence,

however, that the Spaniards introduced the potato

from South America into Spain by 1580, or even

as early as 1565. Hieronymous Cardan, a monk, is

su])posed to have been the first to introduce it from

Peru into Spain. From Spain the plant was taken

into Italy about 1585, into Belgium and Crermany

by 1587, into Austria by 1588, and into France

soon after 1600. Philip (le Sivry, Prefect of Mons,

Belgium, sent two potato tubers to Carolus Clusius

in Vienna, Austria, where they were received on

January 26, 1588. Philip de Sivry had received a
AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICXILTURE

plant from an attaclic of the Papal Legation; he,

in turn, had obtained plants from Italy. De Sivry's colored drawing, now in the Plantin-Moretus

Museum at Antwerp, Belgium, was probably made

soon after 1588 and is considered by some writers

to be the first illustration made of the potato.

However, the first published illustration and description was by Gerard, an English botanist. It

appeared in 1597. Gerard, unfortunately, gave it

the misleading name of Batata virgm/ana, and

thereby created confusion in the origin, history,

and proper identification of the potato. The accepted botanical name Sohmum tuheromim was

first used in 1596 by the Swiss botanist, Kaspar

Bauhin, and this was the name adopted in 1753

by Linnaeus in his Species Plantarum.

Introduction of the potato into England probably was independent of its spread in Europe.

The exact time of importation into England is

clouded by the confusion that existed between the

potato, the sweetpotato, and other tuber- and rootforming plants. In 1586, Sir Francis Drake introduced into England a plant that he incorrectly

called potato. On this trip, Drake had stopped in

Virginia and picked up survivors of Raleigh's

Colony on Roanoke Island. Heriot, one of the

colonists Drake brought back to England, later

became farm manager of an estate in Ireland

owned by Sir Walter Raleigh. However, the

tubers Heriot took back to England could not

have been potatoes because none of his written

descriptions of six root- or tuber-forming plants

even approximates the potato.

It could very well be that the Spaniards first

brought the crop to Ireland, because trade was

brisk between Spain and Ireland in the I7th century. Without question, the newly introduced

plant first became an important agricultural crop

in Ireland. Soon after its introduction there, the

potato was readily accepted as a staple food, demonstrating for the first time the potential commercial value of a plant that previously was no

more than a botanical curiosity. Between 1650

and 1840 potatoes had become a vital part of the

basic food supply in Ireland. When late blight

disease wiped out the crop in the 1840's, famine

forced many Irish people to emigrate to America.

Because of its early food use and importance in

Ireland, the potato plant is sometimes erroneously

called the Irish potato.

When or where the potato was introduced into

the continental United States is not known. It is

believed that potatoes did not exist in Virginia

when Drake landed there in 1586 ; if he had potatoes aboard his flagship, it is thouglit that they

came from Cartagena (Colombia). One of the

early colonial records shows that potatoes were

ordered to be taken from England to South Carolina by colonists settling on the Edisto River in

1674. There is no record that potatoes were actually introduced at that time, however. Potatoes

were introdiiced from Ireland into Londonderry,

N.H., in 1719 by a groiip of Presbyterian Irisli.

This is the first introduction into New England

and possibly the United States. Acceptance of

potatoes as food was very slow in North America.

However, total production had reached 1,603,730

hundredweight in 1840, wlien potatoes were first

mentioned in the U.S. Census.

Botany of the Potato

The name "potato" is believed to have originated from the Indian name, "Batatas."

The potato is one of about 2,000 species in the

family Solanaceae. This family includes sucli

plants as tobacco, tomato, eggplant, pepper, horse

nettle, bittersweet, ground cherry, and petunia.

Botanically, the potato cultivated in North America, Europe, and other lands is Solanum tuberosum L. There are nearly 160 wild species and

20 cultivated species of the tuber-bearing Solanums. All these relatives of the potato are of

New World origin. Probably the closest wild

relative is S. andigenum Juz. & Buk. which produces acceptable yields under the short-day conditions of the Andes Mountains. Some botanists

consider S. andigenum to be a subspecies of

S. tuherosum.

The potato may be classified as a dicotyledonous

annual, although it can persist in the field vegetatively (as tubers) from one season to the next

even in northern Maine. In fact, volunteer plants

growing from unharvested potato tubers, unintentionally left in the field, create many problems

in pest and disease control that affect the production of certified seed.

Being a dicotyledonous plant, the potato has

the characteristics of all dicotyledons including

stems with vascular bundles placed in a circular

arrangement and containing definite layers of

xylem and phloem. Because it is a dicotyledon,

the potato may be grafted within the species

Solanum tuherosum as well as upon related

species. Frequently one reads of tomatoes being

grafted upon potato root stocks to obtain plants

bearing tomatoes on the tops and potatoes underground. Such plants have no commercial value.

However, research workers, particularly in Russia, have performed many grafting experiments

with potatoes and tomatoes and claim to have obtained "graft hybrid" effects. These experiments

have been repeated many times elsewhere with

negative results. In the United States and in

liurope some potato breeders have grafted varieties of potatoes upon tomato stocks (or vice versa)

to induce better flowering and seed setting and

for disease studies.
COMMERCIAL POTATO PRODUCTION 3

Morphology and Anatomy

The potato tuber is an enlarged portion of an

underground branch of a stem called a stolon or

rhizome (fig. 1). Technically, these undergroiuid

stems of the potato most nearly approximate

rhizomes, but the term stolon is more common.

The stolons have leaf scales located ajternately

on their surface in the same manner as the aboveground stems. The tubers originate from the tips

of stolons, and occasionally tubers form along tlie

stolon itself. The potato tubers contain all the

characteristics of normal stems, including dormant true buds (eyes) formed at the base of a leaf

(rudimentary in this case) with detectable leaf

scars (the eyebrows). Lenticels or stem pores

through which air penetrates to the stem interior

are plainly found on most tubers. Lenticels often

become enlarged to objectionable size when soils

are overly wet and access of air is restricted.

The buds (eyes), from which further growth

initiates, are found in a spiral pattern on all parts

of the tuber. The eyes tend to be concentrated at

the distal end (also called the seed or eye end)

of the tuber. They are fewer in number and

farther apart toward the proximal (or stem) end

where the tuber is attached to the stolon.

The buds (eyes) of the seed end possess apical

dominance and will normally sprout first, a condition characteristic of buds at or near the apex

of all conventional stems. When the apical buds

are removed or die, other buds are stimulated to

sprout in the same manner as lateral buds on a

woody stem are stimulated to sprout when the

"leader" is removed. When whole tubers are

planted, the effect of apical dominance is especially important because with whole seed, only

one to three large productive stems per hill

will usually emerge. The effect of apical dominance is reduced if a large tuber is cut into smaller

seed pieces. Stems from cut seed pieces tend toward greater uniformity in rate of emergence, but

a noticeable difference will still exist between stemend and seed-end sprouts.

The outer layer of single cells of the tuber, the

epidermis, is usually colorless. Anthocyanin, the

pigment that colors red and blue potatoes, is

fo>md in the periderm (several layers of corky

cells immediately below the epidermis). The epidermis and periderm together comprise the "skin"

of the mature tuber (fig. 2). The varieties Dakota Red, Red McClure, Triumph, Pontiac, and

Bed Warba are commercial varieties with red pigment (anthocyanin) in their periderm. In a few

varieties the colored pigment is in the outer layers

of the cortex, the region immediately inside the

periderm that extends inwardly to the vascular

ring. Varieties known to have red pigment in the

outer cortex are Spaulding Kose, Early Rose, and

Early Ohio.

The remainder of the tuber from the vascular

ring inward, designated as the medullary area, is

divided into outer and inner medulla and constitutes the fleshy part of tlie tuber. The outer medulla generally contains the denser portion of this

area; the inner medulla includes the watery and

more translucent part. Persons unfamiliar with

the internal structure of the potato sometimes mistake the inner medulla for an abnormality or defect. The inner medulla extends toward each

eye, fonning a continuous tissue that connects all

the eyes of the tuber (fig. 2).

The initiation of young tubers at the tips of

the stolons usually occurs when the plants are 6 to

8 inches high, or from 5 to 7 weeks after planting

(fig. 3). Tuberization is affected by many environmental factors and depends largely on translocation and storage of food in excess of that

needed by other parts of the plant in their growth

and metabolism. Growth of the young tuber is

the result of lx)th cell division and elongation and

storage of translocated food constituents within

the cells.

Contrary to a commonly accepted notion, tuberization is not dependent upon flowering. Potato

plants will form tubers without any flowers ever

appearing on the tops. Some experimenters have

even conversely suggested tliat tuberization is actually enhanced by removing the flowers or flower

buds. The notion that tuberization depends on

flowering arose because unfavorable climate such

as hot and dry growing conditions, which are

normally unfavorable for flowering, also retard or

even inhibit tuber formation. Because climatic

conditions that favor flowering also favor tuberization, it seems natural to make the mistake of

associating flowering with high yields and poor

flowering with low yields.

Descriptive morphology of leaves, stems, and

flowers is included in the section Varieties.

Plant Introduction and Maintenance

For a long period after the potato was introduced into the United States, little effort was made

to improve it or to introduce new kinds. Thus

the period from 1719 to 1850 was characterized by

no marked or lasting improvement in the cro]).

Improvement of potatoes near the end of this

period became imperative because the available

varieties "ran out" to the extent that yields decreased to low levels and production was uneconomical. "Runningout" was caused largely by

high levels (>F~tùrt5éMransmitted virus diseases in

existing stocks and lack of proper seed maintenance methods.

In 1851, C. E. Goodrich, a clergyman of Utica,

N.Y., introduced a small amount of potatoes received from the American consulate in Panama.

A selection called Garnet Chili resulted from this
AGR. HANDBOOK 2 6 7, U.S. DEFT. AGRICULTUKE

Flower—

Leaf stipules

Underground stem

-Terminal leaflet

Developing tuber

Eyebrow

(rudimentary leaf scar

Eye

Old 'seed' piece^ ^-^

True roots

Í—Stem

Adventitious roots

Scale leaf on root stolon

Lenticels

N Distal end

tj

Stem or proximal end

Young tuber

FIGURE 1.—Diagrammatic drawing of the

parts of a potato plant. Note that the

stolons and tubers arise from stem tissue,

while the true roots arise at the base of the

stem.
COMMERCIAL POTATO PRODUCTION

introduction. Nearly 200 named potato varieties,

includiiifi sevei'al important commercial ones still

grown, iiave this single variety represented in

their ancestry.

Cortex

Vascular ring

Corky epidermis

Periderm

Eyes or buds

Inner medulla or pith

(branching to each eye

or bud)

Outer medulla

FIGURE 2.—Cross section of a potato tuber. Red pigment

occurs in the i)erlderm or outer cortex of some red

varieties. Tlie water-soalved appearance of tlie inner

medulla is normal, though it may be more pronounced

in some tubers than others.

Since 1925, several plant explorers have made

plant exploration trips into Mexico, Central

America, South America, and other areas where

the potato is believed to have originated, in search

of new forms of the potato for use in impi'oving

commercial types. In addition, many named varieties of potato have been introduced into tlie

United States, from most of the potato-producing

areas of the world. The continual flow of new

material coming into this country is effectively

coordinated by the New Crops Research Branch

of the Agricultural Research Service in the U.S.

Department of Agriculture.

The most challenging problem following the

introduction of foreign potato varieties into the

country is the maintenance of the collection in a

vigorous, disease- and insect-free condition. Most

of the introductions must be propagated and

maintained vegetatively because they do not reproduce or breed true to their original type when

grown from true seed, and many produce few, if

any, true seeds under our conditions of culture.

Vegetative propagation is the only sure means of

tfíaÍíTFáining' iïïichangèd original lines that possess

important characteristics that otherwise might be

lost. This vegetative method of propagation

necessitates planting and growing the many introductions each year—a time-consuming and expensive process, which also exposes the plants to the

continual danger of infection with tuber-borne

diseases.

To help solve these problems and maintain the

valuable characteristics of foreign plant introductions, a special potato^inti-oduction and maintenance sfatTön was established by the U.S.

DeparTment of^griculture and cooperating State

agricultural experiment stations. At this station,

located at Sturgeon Bay, Wjs^^^ all introductions

of the tvrber-bearing /S'ö/«nMm species, valuable

parental stocks, and varieties are maintained as

true seed or tubers. A list of vegetatively propagated varieties is given in tables 2 and 3. Requests for two- or three-tuber samples of Solanum

material from the Sturgeon Ray collection may be

addressed to the Potato Introduction Station,

Sturgeon Bay, Wis.

FIGURE 3.—Photomicrograph of the longitudinal section through a developing tuber at the tip of a stolon : A, epidermal

cells; B, cortex; G, vascular ring; D, medulla.

717-780-—64 2
6 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

TABLE 2.—Vegetatively propagated foreign varieties of Solanum tuberosum maintained at Sturgeon

Bay, Wis.

Variety Origin or source Remarks

Ackersegen Germany Resistant to common scab and late blight; immune to

Akebia do

virus A.

Resistant to common scab.

Albion Netherlands Resistant to leaf roll virus.

do Resistant to common scab and late blight.

Ambra Czechoslovakia

Germany Resistant to leaf roll virus.

do Resistant to late blight and leaf roll virus.

Arran Banner British Isles Resistant to \irus Y and dry rot.

do Field immune to viruses A, B, and C.

Arran Consul do

do

Field immune to virus C; resistant to late blight.

AsDotet Norway

Germany Resistant to leaf roll virus.

Tasmania Resistant to Verticillium wilt.

Avanzi Italy - - - --

Netherlands Early maturity.

Belle de Fontenev France Do.

Netherlands _ Resistant to late blight.

do Popular in Europe for chipping and cooking quality; imAustralia _ .

muñe to virus A.

Resistant to leaf roll virus.

Germ any -

Netherlands

British Isles Field immune to viruses A, B, and C; resistant to dry rot.

Oanso Canada Resistant to late blight.

Germ any Resistant to common scab and late blight.

Craigs Snow'white British Isles Field immune to viruses X, .\, B, and C; foliage resistance to

Doon Earlv do

late blight.

Field immune to viruses A and B; resistant to virus Y.

Dore Netherlands Early maturity.

1 )iiivelander do Resistant to late blight.

Eersteline British Isles _ _ Immune to virus A; early maturity.

Netherlands Popular in Netherlands for cooking quality.

Ella Russia - - -

Elsa

Erdeold Germany - Resistant to common scab.

do Resistant to late bliglit, wet rots, and viruses.

... do Early maturity.

Resistant to late blight.

Factor

Falke

Scotland Resistant to potato wart.

Resistant to common scab, late blight, and leaf roll virus.

Flava

Forelle _ ... do Resistant to late blight, loaf roll virus, and virus Y.

do ... _ Do.

Fruhmolle do . - __ Resistant to late blight and leaf roll virus; early maturity.

Fnihnudel do

. do

Resistant to late blight.

Resistant to common scab; early maturity.

Resistant to ring rot.

do Early maturity.

Gladstone Britisli Isles

do

Field immune to viruses A and B; resistant to dry rot.

Field immune to virus A ; resistant to leaf roll virus and dry

Grummel N(^therlands

rot.

Heida - _ . Germany. Resistant to leaf roll virus.

Hindenburg do Resistant to common scab.

do Resistant to common scab and late blight; early maturity.

Norway . _

Jubel Germany . _ _ Resistant to common scab and late blight.

Czechoslovakia

do - . . . . Resistant to drought.

British Isles _ _ Field immune to viruses Y, A, and B; resistant to dry rot.

Kittine Czechoslovakia Early maturity.

Kotnov do

do

Libertas Netherlands Resistant to late blight; high dry-matter content.

Tjimosa do Early maturity.

Majestic British Isles Field immune to virus C.

Maritta Germany Resistant to common scab, late blight, and virus Y.
COMMERCIAL POTATO PRODUCTION

TABLE 2.—Vegetatively propagated foreign varieties of Solanum tuberosum maintained at Sturgeon

Bay, Wis.—Continued

Variety

Marins

Markt-RedwitzerFruhe.

Marta

Matador

Meerlander

Mirka

Mittelfruhe

Monak

Moona

Noordeling

Oberarnbacher Frühe- . . Olympia

Ostbote

Panther

I'arnassia

I'entland Ace

Pimpernel

Popnlair

President

Primula

Record

Rheingold

Rode Eersteling. _

Rosanel

Saskia

Sharps Express

Sientje

Sikersegen

Sirtema

Skerry Champion _

Southcsk

Stewart Dawn

Tebon

Triumf

Ulster Ensign

Urgenta

Urtica

Vera

Victor

Voran

Ysselster

Zaldi

Zeeburger

Origin or source

Norway

Germany

Czechoslovakia.

Netherlands _.. _

do

Czechoslovakia.

Germany

Australia

do

Netherlands

Germany

do.'.

do

do

do

British Isles

Netherlands

do

do

Germany

Netherlands.

German3'

Netherlands _

Costa Rica_.

Netherlands -

Scotland

NetherlandsGermany

Netherlands _

British Isles _

do

do

Norway

Netherlands _

British Isles _

Netherlands

Germany

do

Spain

Germany

Netherlands _

Spain

Netherlands.

Remarks

Early maturity.

Resistant to common scab

Immune to virus A.

Resistant to late blight and nematode.

Resistant to common scab and late blight; early maturity.

Resistant to common scab and late blight.

Resistant to late blight.

Field immune to viruses X, A, and B; resistant to late blight.

Resistant to late bhght.

Resistant to VerticiÜium wilt.

Field immune to viruses B and C; resistant to ring rot

and late blight.

Immune to virus A ; early maturity.

Immune to virus A; resistant to late blight.

Resistant to common scab.

Early maturity; immune to virus A.

Immune to virus A; early maturity.

Immune to virus A.

Resistant to common scab; early maturity.

Resistant to leaf roll virus.

Resistant to common scab; early maturity.

Resistant to leaf roll virus.

Resistant to dry rot.

Resistant to scab, late blight, and leaf roll virus.

Early maturity.

Resistant to late blight.

Resistant to common scab.

Resistant to late blight and nematode; resistant to drought.

TABLE ^.~Vegetatively propagated American varieties of Solanum tuberosum maintained at

Sturgeon Bay, Wis.

Variety Remarks Variety Remarks

Antigo Resistant to common scab.

Resistant to late blight and common scab; tolerant to Fusarium

solani var. eumarlii.

Resistant to late blight, common

scab, virus A, and net necrosis.

Resistant to virus A.

Early maturity.

Resistant to lat(> blight.

Resistant to virus A.

Earlaine__

Earlv Blue

Resistant to virus A; early ma- Canus turity.

Cayuga..

Early Gem. . ._ Resistant to common scab; early

maturity.

Cherokee Earlv Rose..

Chippewa.

Erie

Resistant to late blight.

Cliisago . - Garnet Chili

Glenmeer

Golden ... . .

(¡oluinbia Russet

Cortland

Resistant to late blight; drought

and frost tolerant.

DeSoto
8 AGR. HANDBOOK 267, U.S. DEPT. AGRICtTLTURE

TABLE 3.—Vegetatively propagated American varieties of Solanum tuberosum maintained at

Stvrgeon Hay. Wls.—Continued

Variety Remarks Variety Remarks

Harford Resistant to late blight; drought

tolerant.

Resistant to virus A, net necrosis,

and drought.

Resistant to virus A ; early maturity.

Tolerant to F. solani var. eumartii.

Resistant to virusesYand A ; drought

tolerant.

Resistant to late blight and net

necrosis.

Resistant to common scab.

Early maturity; yellow flesh.

Resistant to late blight and common scab.

Resistant to late blight, ring rot,

early blight, and net necrosis;

field immune to virus A.

Resistant to common scab.

Resistant to late blight and common scab; tolerant to F. solani

var. eumartii.

Resistant to common scab.

Early maturity.

Resistant to late blight and common scab.

Resistant to late blight; early

maturity.

Resistant to Verticillium wilt; tolerant to purple top virus.

Redglo - ..

Redkote Resistant to common scab and

Houma

Red LaSoda

Red Pontiac

Red Warba

Rushmore _

internal discolorations.

Irish Cobbler

Kasota Early maturity.

Katahdin __

Kennebec

Russet Burbank

Russet Rural

Russet Sebago

Saco

Resistant to common scab.

Do.

Knik

Resistant to late blight, common

Manota Immune to viruses X and A ;resistant

Marveold

Saranac

to late blight and net necrosis.

Resistant to late blight and ring rot.

Resistant to late blight, common

scab, virus A, and vellow dwarf

Menominee

Sebago

Merrimack

Seneca.. . . Resistant to late blight, scab, and

F. solani var. eumartii.

Resistant to late blight on the

foliage, flea beetles, and leaf

hoppers.

Resistant to late blight.

Norland Sequoia

Snowdrift

Osage. Spaulding Rose

Tawa

Osseo - to potato wart.

Pawnee Resistant to late blight, scab, and

Plymouth

Teton -.

mosaic.

Resistant to ring rot.

Waseca

Red Beauty Whitecloud.. .

Yampa . . Resistant to early blight, leaf roll,

Redburt - and mosaic.

Per Capita Consumption of Potatoes

The rate at whicli potatoes are used for food is

determined from the average consumption of potatoes per person. Per capita consumption figures

(table 4) show tliat the average American in 1963

ate about half as many potatoes per year as his

forebears did in 1910.

TABLE 4.—Per capita corwiuinptlon of potatoes in

the United States, selected years, 1910-63

Year Potatoes Y(>ar Potatoes

1910

Pounds

198

185

140

157

132

142

123

122

106

1955

1956 .

Pounds

106

1915 .. . 100

1920 1957 106

1925 1958 99

1930 1959 101

1935 1960

1961

1962

102

1940 --_ 103

1945 103

1950 1963... 100

Despite the gradual decrease over the years,

there is some evidence that per capita consumption may now be increasing slightly. This change

is probably largely the result of newly developed

methods of processing, which make it much easier

to use potatoes in preparing meals in the home and

in commercial and institutional kitchens. A growing realization of the high nutritive value of potatoes may also have contributed to the recent uptrend in per capita consumption.

Value as Food

It is an erroneous idea that potatoes are fattening. Nutritionists recognize that all foods eaten in

excess are fattening. Actually potatoes are less

fattening pound for pound than most items in the

daily American diet. For example, about 11

pounds of potatoes are required to produce 1

pound of body fat, or about the same amount as

milk, canned corn, canned peaches, or frozen peas

(table 5). On the same basis, the "fattenmg"

tendency of potatoes is 1/10 that of margarine,

1/5 that of dry cereal, 1^ that of bread, and 1/2 t'^«'**

of beef or hamburger. Potatoes alone actually are

comparatively low in calories per pound. Wlien

fried or served with a generous amount of butter or other fat preparation, however, the total
COMMERCIAL POTATO PRODUCTION 9

caloric intake may be high. A unit weight of

french-fried potatoes has about five times the

caloric v-alue as the same weight of mashed

potatoes.

TABLE 5.—Potatoes compared to other foods in

cost -per food unit and relative ''fattening''''

potential

Food

Sugar

POTATOES

Margarine

Oatmeal (dry)

Macaroni (dry)

Bread

POTATOES

Cereal (dry)

Butter

Peanut butter

POTATOES

Corn (canned)

Milk (whole)

Peaches (canned).

Hamburger

Beef steak (round)

Cabbage

Peas (frozen)

Turnips

Beans (green

canned)

Market

cost per

pound

$0. 15

.03

.29

. 20

.23

. 21

.06

.32

.74

. 60

. 09

. 14

. 13

. 18

.59

1. 09

. 12

. 53

.25

. 22

Calories

per

pound

Number

1,800

325

3, 300

1, 770

1, 670

1,250

325

1,650

3, 300

2,615

325

350

310

270

830

830

80

340

125

85

Cost

per 1,000

calories

$0.08

. 09

. 09

. 11

. 14

. 17

. 18

. 19

. 22

. 23

. 28

. 40

. 42

. 67

. 71

1.31

1. 50

1. 56

2. 00

2.58

Amount

eaten in

excess for

1 pound

of body

fat 1

Pounds

1. 9

10. 8

1. 1

2, 0

2. 1

2. 8

10. 8

2. 1

I. 1

1. 3

10. 8

10. 0

11. 2

13. 0

4. 2

4. 2

43. 7

10. 0

28.0

41. 1

' Based on 3,500 calories = I pound body fat.

Potatoes are an economical food; they provide

a source of low-cost energy to the human diet,

a fact well recognized in many parts of the world.

When the price of potatoes is around 3 cents per

pound ($1.80 per bushel), they are one of the

cheapest sources of food energy (table 5). Even

at 6 cents per pound, only sugar, margarine, oatmeal, and bread are less expensive per unit of food

value.

The fact that potatoes also provide significant

amounts of vitamins and minerals at low cost has

been stressed too little by some food specialists and

others. The nutrients supplied by potatoes are

shown in table 6. Potatoes are outstanding as

a source of vitamin C. If properly processed and

prepared, potatoes can provide large amounts of

the minimum daily requirement of this vitamin

in the American diet. Potatoes are also an economical source of vitamin Bi (thiamin). Tlie

average American spends approximately 2 cents

of each food dollar on potatoes and for this small

expenditure receives an intake of 2 percent of

his energy, 4 percent of his iron, 6 percent of his

thiamin (Bi), 6 percent of his niacin, and 10 percent of his vitamin C.

TABLE 6.—Nutrient value of potatoes prepared

in various ways ^

rt £S OT^ ^

« g

o 3

g

0) e

<u Si Xî o

T3 s 'S

O) 3

Nutrient

""3

o O

oiled; peel

boiling: 12K inch

g

o

.—1

ä

'Srozen fren

10 pieces

inch

^'1

s 03

ffl « o &H <^

Food energy cal._ 90 90 110 95 145

Protein g.. 3 3 1 2 4

Fat g.. Trace Trace 7 4 1

Carbohydrates g_. 21 21 10 15 30

Calcium mg_ _ 9 9 6 4 47

Iron mg__ 0.7 0.7 0. 4 0.8 1. 0

Vitamins:

A I.U- Trace Trace Trace Trace 50

Thiamin mg__ 0. 10 0. 11 0. 04 0.08 0. 17

Riboflavin mg_ _ 0.04 0. 04 0. 02 0. 01 0. 11

Niacin mg__ 1. 7 1. 4 0. 6 1.2 0. 2

C mg_. 20. 0 20. 0 2.0 10. 0 17.0

' Source: 1959 Yearbook of Agriculture.

If potatoes were the only source of the total

daily food energy requirement, they would provide the full requirement of riboflavin, fi^ times

the iron requirement, 3 to 4 times the thiamin and

niacin requirement, and more than 10 times the

vitamin C requirement. Some calcium, protein,

and vitamin A would be supplied, but the full

requirements would not be met. A diet of whole

milk and potatoes would supply almost all the

necessary food elements for the maintenance of

the human body.

PRODUCTION

Production Areas in the United States

Potatoes are grown commercially in every State

in the country (fig. 4). Throughout the United

States there is not a single month in the year when

potatoes are not being planted or harvested somewhere. In the Northeast, production of the fall

crop is highly developed m Aroostook County,

Maine (fig. 5). In the Atlantic coastal areas from

Long Island to northeast North Carolina, there

are numerous smaller but important districts of

production. Inland centers include western New
FIGURE 4.—Principal potato-producing areas.
COMMERCIAL POTATO PRODUCTION 11

—«i^^sB

^^n

rá***

^ -»-«alcOíBtó

FiGUKE 5.—Potato-producing area in Aroostoolî County, Maine. About 85 percent of tiie total potato production in

Maine is grown in Aroostoolî County. Planting begins as soon as the soil can be prepared, usually from May -j

to June 1. In tlie baclvground are fields of clover and oats used in the rotation with potatoes.

York, Michigan, north-central Wisconsin, and the

Red River Valley of North Dakota and Minnesota.

The potato is a major farm crop in nearly all

the Southern States. It is grown chiefly as an

early crop during the winter montlis because it

yields better when grown at temperatures averaging not more than 70° F. The yield per acre of

early-crop potatoes in the South is usually less

than tliat of late-crop potatoes grown in otlier

areas, but the market price of the southern crop

is generally high enough to help make up for the

différence m yield.

Potatoes are grown extensively in the Western

States, chiefly on irrigated land. In recent years

dam construction and well drilling have brouglit

into cultivation additional land available for

growing tills crop under irrigation. Potatoes are

also grown without irrigation on some acreage in

the humid parts of the West. Idalio now leads

all other States in potato production, and Bingliam. Cassia, and Minidoka Counties, Idaho, are

famous for their potato farms (fig. 6).

Production Factors in Relation to Profits

Profitable potato production depends on several

factors, including size of the enterprise and market price received by growers. Growers who obtain high yields generally have lower production

costs per hundredweight. However, among growers in any given production area with tlie same

yields per acre, there is considerable range in operation costs per unit or hundredweight.

It is often assumed that high yields alone indicate efficient production and economical land use,

but a valid estimate of real or net income profit

from a farm operation cannot be obtained until

the cost of seed, fertilizer, labor, and otiier ¡)roduction costs are charged against the selling price

of the crop. This point is well illustrated in figure

7, in which fertilizer costs are used as an example

of what happens when gross returns and net returns per acre beyond the cost of potato fertilizer

are considered.

Increasing the amount of fertilizer per acre

does not necessarily increase tlie yield at a fixed

rate since the productivity of the land is limited.

Tlie gross return in busliels and dollars per acre

from the application of 1,200 to 1,800 pounds of

fertilizer per acre (fig. 7) increased, but the net

return remained about the same even with a 50-

percent increase in fertilizer applied per acre.

Fui-ther increase over 1,800 pounds per acre did

not increase yields and lowered the net return per

bushel.
12 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

FiGUKE 6.—A potato seed field in Teton County, Idaho. Overhead irrigation is very common in this area. Idaho is

nov*' the leading potato-producing State. Production in Bingham, Cassia, and Minidoka Counties represents a

large portion of total national i)otato production.

The upper curve in fijrure 7, representing gross

return, is based on "the law of diminishing need."

Simply stated, this law operates because there is

a physiological limit to the amount of fertilizer

that can be utilized by the crop in producing an increase in yield. The lower curve representing net

return illustrates the "law of diminishing returns." It follows that for each set of conditions

maximum net returns can be obtained at a certain

level of fertilization, but further increase in the

fertilizer level will result in decreased net profits.

A small number of farms produce most of the

potato crop. In the 1960 census, farms with 50

acres or more comprised less than 1 percent of the

684,853 potato farms but produced almost 65 percent of the total U.S. crop (table 7).

Farm management studies usually prove that

the larger the acreage of crops per farm, the

higher the profits per farm. The financial advanTABLE 7.—Acres of potatoes harvested in 1959 on

farms of different sizes ^

Acres of

potatoes per farm Farms

Acres

harvested

Percentage of

total

harvestefl

Under 5 _ -

Number

670, 499

4,366

7,688

5,828

6,472

Number

11, 380

28, 843

120,746

205, 765

767, 697

Percent

6. 4

5-9.9 - 2.4

10-24.9 10. 0

25-49.9

50 or more

17. 1

64. 1

Total 684, 853 1,200,431 100

' Agricultural Census, 1960.
COMMERCIAL POTATO PRODUCTION 13

^EÑTfUZEft LBS/A.

19.00 'la.OO U7.00 iJ&OO 145.00 '54.00 esTiMAreo COST

tage of large-scale operations is usually related to

greater efficiency in use of labor, machinery, and

land. Large enterprises are also more conducive

to savings in buying and selling practices.

FIGURE 7.—Gross and net returns per acre beyond the

cost of potato fertilizer applied at different rates per

acre (based on 7 years' data on Long Island). The

curve for gross returns flattens because of the diminishing needs of the crop for plant food. The curve

for net returns illustrates the law of diminishing

returns. Note that applications of 1,200 and of 2,100

I)ouuds per acre both gave the same net return per

acre. The theoretical linear relationship between return per acre from yields and fertilizer application

(A) is valid only with small applications of fertilizer.

VARIETIES

Anatomical Features of Potato Varieties Used

for Identification

Sometimes it becomes necessary to identify varieties of uncertain identity, or those that may have

become mixed or mislabeled. Certain varieties

fortunately can be identified by their plant growth

habits in the field. However, different environments markedly influence the expression of many

characteristics of growth. Yov varietal identification the plant characteristics least affected by

environment are: (1) stem and stem wings, (2)

terminal leaflets, (3) stipules, (4) flower characters, (5) tuber characters, and ((>) sprout cliaracters. Although not anatomical, known disease

reaction characters often help to identify some

varieties.

Stem and Stem Wing

The angles of the potato stem are extended to

form structures called wings. The wings may be

prominent or inconspicuous, straight or waved,

far apart or so close they appear as double wings.

(See fig. 8.)

The stem pigments are more variable. Stems

may be entirely green, reddish, or purple. The

anthocyanin pigments present may be mottled or

uniformly distributed along the entire stem or

they may be localized at nodes or certain

internodes.

Leaves and Terminal Leaflets

The leaf of the potato is compound and consists of a petiole, a terminal leaflet, and several

pairs of primary leaflets, interspersed with smaller

secondary leaflets and sometimes tertiary ones arranged along the midrib. When the pairs of primary leaflets are widely separated and there are

few secondary leaflets, the leaf is designated as

open. Wlieii primary leaflets are close together

717-760'—64 3

FIGURE 8.—Potato stems showing types of wings : A,

straight, double (I'ontiac); It, waved, double (Garnet

Chili) ; C, slightly waved, double (Irish Cobbler).

Wing cliaracters are not greatly changed by differences

in environment.

and the secondary ones are numerous, the leaf is

described as closed (fig. 9).

The terminal leaflets are readily classified as

lobed, partially truncate, or decurrent (fig. 10, A,

In the Chippewa variety, terminal leaflets are

lobed only on one side, usually on the lower leaves.

Since this distinctive feature does not occur consistently, it is best to make observations only on

older leaves that are completely formed.

stipules

Stipules, the leaflike growths at the base of the

peduncles and petioles of the lowest inflorescence, provide one of the most useful plant charac-
14 AGR. HANDBOOK 2 67, U.S. DEPT. AGRICULTURE

' \—Terminal leaflet

Primary leaflets

Tertiary leaflets

Secondary leaflets

Petiole

FIGURE 9.—Tyix-s of potato leaves and enumeration of

leaf parts: A, cl()sed-tyi)e leaf; IS, oi)en-type leaf.

teristics for varietal identification (fig. 10, D,

E, F). Stipules may range from large to small

in size. They may be spreading or may entirely

clasp the peduncles or petioles.

Flowers

Flower color is probably the most distinctive

plant characteristic used for variety identification. Flower color ranges from white for Burbank, White Rose, and Green Mountain to reddish

for Pontiac, Wai-ba, Katahdin, and Sebago to

purplish for the Rural varieties. Some varieties

produce an abundance of blossoms while others

bloom sparsely. Certain varieties may bloom profusely in one environment and not at all in a different one. Anther colors also vary ; they may be

orange, lemon yellow, or greenish yellow.

Tubers

The skin color of mature tubers is one of the

best single means of variety identification. Most

FIGURE 10.—Leaf characteristics of importance in the

classiflcation of potato varieties: ,4, lolied terminal

leaflet (Katahdin) : U, partly truncate terminal leaflet

(Chippewa) ; C, decurrent terminal leaflet (I'resident) ;

D, very large leaflike stipule—at base of peduncle of

lowest inflorescence (Earlaine) ; E, stipule medium in

size, spreading (Katahdin) ; F, stipule medium in size,

clasping (Green Mcmntain).

varieties may be classified as either russets, whites,

pinks, red.s, or purples. The intensity of skin

color and russeting varies with location, and cultural practices, but the layer in which the tuber

color occurs in red varieties is distinctive. Color

pigmentation is located in the periderm layer of

Triumph, Red Pontiac, Red Warba, and Red

McClure and in the underlying cortical layer of

Early Rose, Spaulding Rose, and liarly Ohio varieties. The depth of eyes and shape of tubers, although distinct varietal characteristics, are not

always reliable means of identification l)ecauí5e of

their wide variability caused by differences in environment. Comparative chipping tests and total

solids determinations also can be helpful for identification if the potato varieties compared were

grown under similar conditions.

Sprout Characters

When potatoes are allowed to sprout under certain conditions of light, temperature, and humidity, they develop distinct characteristics at definite

stages of growth that are valuable aids in identification. If potato tubers are sprouted in darkness or in diffused daylight, sprout characters
COMMERCIAL POTATO PRODUCTION 15

TOP

MIDDLE

.^^TERMINAL LEAF SCALES

Íí'v';?^ BASE OF LATERAL LEAF SCALE

BASE

Q- .' ,

'• •' 'j'V^i'r--PAPILLAE

ROOT PROTUBERANCES

FIGURE 11.—A sprout developed in the dark showing distinctive features useful in variety identiflcation.

develop that are typical for certain varieties

(fig. 11). These features are the most useful for

identification.

Disease Resistance

Reaction tj certain planjjiathpgens is the basis

for another method of yarietaT idëîîtrfication.

Tests for resistance to specialized" races of thelate

blight organism and to scab and viruses X, A,

Y, and S can readily be used to determine variety

identityJJicougk-diiïgrences in susceptibility.

Varieties From Wliicli To Clioose

A relatively large number of varieties of potatoes are grown in the early-, intermediate-, and

late-crop sections of the United States. They differ in time of maturity, yield, appearance, cooking and marketing qualities, and resistance to various diseases and insects. All other characteristics

being equal, a variety resistant to even one disease

or insect is better than a susceptible one.

A variety tliat is good in one geographical area,

however, may be of little value in another. The

grower is warned, therefore, against buying large

quantities of high-priced seed stock of a recently

introduced variety or one reported superior in

other localities until he learns whether it is

adapted to his particular environmental conditions. He had best obtain the needed varietal

information from his county agent or State agricultural college or to test the variety himself on a

limited scale.

Descriptive information for 30 leading varieties

is given in table 8 and as follows :

TABLE 8.—Varieties of most importance in potato production in the United States and Canada, listed

according to acres of seed certified in 1960

Variety

Katahdin

Russet Burbank.

Rod Pontiac

Sobago

Kennebec

Irish Cobbler

Red LaSoda

Norland

White Rose

Chippewa

Early Gem

Green Mountain.

Red McClure

Cherokee

Keswick

Pungo

Early OhioFundy

Russet Rural.

Russet Sebago.

Plymouth

Triumph.

Onaway_

Waseca._

Acres

certified,

1960

Number

48, 393

36, 145

25, 175

20, 820

20, 107

18, 770

5, 996

4, 657

4, 156

3,401

2,708

2,073

1, 529

1,242

1, 109

852

769

675

669

525

503

492

482

475

Origin: date and place

1932, USDA

Unknown

1949, USDA, Michigan....

1938, USDA

1948, USDA, Maine

Unknown

1954, Louisiana

1959, North Dakota

1893(?), Private

1933, USDA

1955, USDA, Idaho, North

Dakota.

1878, Private

Unknown

1953, USDA, Iowa, Indiana.

1951, Canada, E.F.S

1954, Virginia, USDA

1871, Private

1960, Canada Department

of Agriculture.

Private release by E. S.

Carmen.

1947, Wisconsin

1956, USDA, North Carolina.

1878, Private

1954, USDA, Michigan

1949, Minnesota

Parents

USDA Seedling 40568 X USDA Seedling 24642.

Mutation from Burbank, a seedling of Early Rose.

Mutation from Pontiac (Triumph X Katahdin).

Chippewa X Katahdin.

USDA B 127 X USDA 96-56.

Unknown.

Triumph X Katahdin. l

ND 626 X Redkote.

Seedling of Jackson.

USDA 40568 X USDA 24642.

Russet Burbank X USDA 96-56.

Dunmore X Excelsior.

Mutation from Peachblow.

USDA 96-56 X USDA 528-170.

S. demissum BCa X Green Mountain to Earlaine

USDA 96-44 X USDA 528-170.

Seedling of Early Rose.

Keswick X USDA 96-56.

Mutation from Rural New Yorker.

indefinite origin.

Mutation from Sebago.

Mohawk X USDA 96-66.

Peerless X Early Rose.

Katahdin X USDA 96-56.

Bliss Triumph X Minn. 15-2-10.

Parentage of
16 AGR. HANDBOOK 2 6 7, XJ.S. DEPT. AGRICULTURE

TABLE 8.—Var'tetien of most importance in potato production in the United States and Canada, listed

according to acres of seed certified in 1960—Continued

Variety

Acres

certified,

1960

Origin: date and place Parents

Number

448

414

361

306

263

239

1953, Nebraska--- . Minn. 5-10-3-23-2 X Minn. 29-32-

Ilichter's Jubel X USDA 44537.

Triumph X Minn. 4-16.

Katalidin X Green Mountain.

Mutation from Warba.

USDA 45146 X Earlaine.

-1-34.

Ontario 1946, USDA, New York

1927, Minnesota (selected

in Iowa).

1954, Louisiana

TîPH Wnrha 1939, Minnesota

Teton -- 1946, Maine, Wyoming,

USDA.

Cherokee ""

Medium early. Adaptation not widely determined; performance in Maine and on midwestern

muck soils has been better than that of Irish Cobbler. Tubers medium size, roundish with blunt

ends; skin smooth, creamy white; eyes medium

deep; flesh white. Flowers white. Resistant to

common strains of late blight fungus, to common

scab, and to mild mosaic. Resistant to net necrosis caused by leaf roll infection. Starch content

of Cherokee compares with that of Irish Cobbler. Cooking quality good. Valued most for

its resistance to scab, especially on some of the

muck soils of Indiana, Iowa, and Minnesota.

Chippewa

Midseason, 10 to 15 days later than Irish Cobbler. Widely distributed, reaching its greatest

importance in Indiana, Maine, Michigan, Minnesota, New Jersey, New York, North Dakota, and

Wisconsin. Does well on peat or muck, especially adapted to such soils in Indiana and New

York, where large, good-quality crops are \)V0-

duced; but in most other sections table quality

poor. Tubers (fig. 12) large, elliptical to oblong,

medium thick; skin smootli, dark creamy butf;

eyes shallow, same color as skin; flesh white.

FIGURE 12.—Tubers of Chippewii variet.v. Widely distributed. Greatest importance in Indiana, ilaine,

Michigan, ¡Minnesota, Xew Jersey, New York, North

Dakota, and Wisconsin.

Flowers lilac with white tips. Susceptible to

wart, spindle tuber, and leaf roll, but to date no

net necrosis found in tubers as a result of leaf

roll infection ; resistant to mild mosaic. Cooking

quality good to poor.

Dazoc

Very early. Grown mostly in Minnesota,

Nebraska, and Wyoming. Small, medium compact plants with medium thick, erect stems; medium to small, open, dark-green leaves. Tubers

medium size, oblong ovate and thick ; eyes medium

to deep; skin deep, bright red, smootli; flesh

white. Medium in solids content and among the

best early red varieties for tuber type. No known

disease resistance; foliage and tubers highly susceptible to late blight.

Early Gem

Early. Usually grown in Idaho but also in

Minnesota, North Dakota, and Wisconsin as an

early-maturing russet type that can be marketed

before Russet Burbank matures. Tubers tend to

develop growth cracks under wet growing conditions. Plants medium to small. Tubers medium

long to long, medium thick; eyes shallow; skin

russeted; flesh white (fig. 13). Flowers white.

^

FiuuRE 13.—Tubers of Early Gem. Greatest imiM)rtance

in Idaho, Minnesota, North Dakota, and Wisconsin.
COMMERCIAL POTATO PRODUCTION 17

Resistant to common scab; apparently susceptible

to all other common potato diseases. Cooking

quality medium to good; Iveeps well in storage.

Early Ohio

liarly; matures about 10 days later than Triumph. Grown to a limited extent in Iowa, Minnesota, North Dakota, Oregon, South Dakota, and

Wisconsin. Adapted to the Ked River Valley

gumbo soils and to the sandy soil districts of

Minnesota. Tubers medium size, round-oblong to

somewhat cylindrical (largest tubers), seed and

stem ends rounded; skin smooth, pink, lighter at

base than at apex; lenticels prominent; eyes

numerous, moderately shallow, sometimes ])rotuberant, pink; flesh white, sometimes with color in

cortex. Flowers white. Susceptible to virus

diseases and wart. Cooking quality good.

Fundy

Early to midseason. Plants moderately vigorous with a slightly spreading habit of growth.

Iveaves slightly crinkled; tend to rim roll as maturity advances. Tubers elliptical, medium thick ;

skin smooth, slightly netted; eyes shallow; type

excellent; appearance attractive. Flowers white

and sparse. Resistant to common races of late

blight. Medium dry-matter content; excellent

cooking qualities for boiling and baking. Fundy

compares favorably in yielding ability to Irish

Cobbler and Katahdin when grown as a main

crop in New Brunswick, Canada.

Green Mountain

Late. Grown in the Northeastern States, where

it produces high yields of good-quality tubers.

Being replaced in some States by newer varieties,

chiefly Katahdin, Chippewa, Seljago, and Kennebec. Production limited to cool and reasonably

moist climate and to lighter soils. Tubers large,

short-oblong to oblong, broad, flattened ends usually blunt; skin smooth or often netted, white;

eyes medium deep, white; flesh white. Flowers

white. Immune to wart but susceptible to most

of the other common potato diseases; net necrosis develops extensively in storage from current-year leaf roll infection. Cooking quality

usually very good but potatoes tend to blacken

after cooking.

Irish Cobbler

Early. Wide adaptation; grown in nearly

every State. Adapted to muck and the lighter

soils; best adapted to cool weather and ample

moisture. Tubers large to medium, roundish

with blunt ends, the stem end often notched

rather deeply, giving a shouldered appearance to

the tuber; skin smooth, creamy white; eyes shallow to rather deep, particularly in bud-eye cluster; flesh white (fig. 14). Flowers lilac wifh

white tips, bleaching nearly white under prolonged, intense heat. Suscej)tible to most virus

diseases and very susceptible to common scab;

resistant to mild mosaic and innnune to wart.

Cooking quality usually good.

FIGURE 14.—Tubers of Irish Cobbler. Grown in nearly

every State. Grown principally for seed in JIaine anil

Minnesota and for table stock in Southeastern

States. One of the most widely adapted potato varieties grown in this country. High in quality but very

susceptible to scab.

Katahdin

Late; matures a little later than Green Mountain and at about the same time as Smooth Rural.

Adaptation wider than that of Chippewa. Tubers large, elliptical to roundish, me(lium thick;

skin smooth, dark creamy bufl'; eyes shallow, same

color as skin; flesh white (fig. 15). Flowers lihic

with white tips. Resistant to mild mosaic, net

necrosis, and brown rot and immune to wart ; not

infected with leaf roll as readily as most other

varieties. Cooking quality medium to good.

FIGURE 1.").—Tubers of Katahdin. AVidely grown. Greatest iniiMirtance in Maine and other Atlantic Coast

States, heads all other varieties in number of acres

grown for eertiñed seed.

Kennebec

Late; matures a little earlier than Katahdin.

Adaptation wider than that of Chippewa but

less than that of Katahdin. Tubers large, ellij)-

tical to oblong, medium thick; eyes shallow; skin

smooth, creamy bufl'; flesh white (fig. IG). Flow-
IB AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

ers white. Resistant to common strains of the

late blight fungus, mild mosaic, and net necrosis

in the tubers caused by seasonal infection with

the leaf roll virus. Cooking quality: A generalpurpose potato that bakes, boils, and fries satisfactorily. Very desirable for french frying or for

chipping.

FIGURE 16.—Tubers of Kennebec. Widely adapted and

grown in most States for processing and table stock.

Graduallj' replacing Katahdin.

Keswick

Medium to late. Plants large, erect; stems medium to tliick and slightly angled, green. Leaves

moderately long and broad. Tubers ellii)tical to

oblong; eyes medium deep; skin dark cream; flesh

white. Flowers white. Susceptible to leaf roll

and bunch top viruses; resistant to conunon

races of late blight. As a main crop in New

Brunswisk, Canada, it has equaled Green Mountain in yield and cooking quality.

Norland

Early. Smooth, shallow-eyed, red-skinned with

early yielding ability (fig. 17). Moderate resistance to common scab; susceptible to infection by

the common potato viruses and to the fungus

causing late blight. Norland possesses good cooking and chipping qualities and produces a high

percentage of U.S. No. 1 tubers.

FiouKE 17.—Tubers of Norland. Grown mostly in the

North Central States.

Onaway

Early. Grown mostly in Michigan as an early

crop. Medium to large upright plants with thick

stems and large leaves. Tubers elongated; skin

smooth, creamy white; eyes medium deep; flesh

white. Tubers rough when large and not good

for chipping. Resistant to common scab and considerable field resistance to late blight. Tubers

susceptible to injury from early blight and from

Fusarium wilt.

Ontario

Late. Very high-yielding variety; highly resistant to scab. Tubers oblong; skin smooth, dark

creamy bulï; eyes shallow except at bud end,

which is sometimes sunken, same color as skin but

turning pink or purple on exposure to light; flesh

wliite. Flowers pale lilac in center of each petal,

^yhite at edges. Has an undesirable tendency at

times to form sprouts and often secondary tubers

at the bud end of the tubers. Cooking quality

ranges from poor to good, depending on conditions under which it is grown. Highly recommended for scabby soils.

Plymouth

Midseason. Grown mostly in Maine, Minnesota, New York, Ohio, and Wisconsin. Production IS increasing in otlier areas. A high-yielding variety. Tubers oblong-flattened; shallow

eyed; skin cream buff. Flowers white. Moderately resistant to common scab; immune to the

common race of late blight. Cooking quality

good and chipping quality good to excellent.

Pungo

Midseason. Adaptation not fully determined;

yielding ability in Maine and Virginia very good.

Acreage is continually increasing among the Atlantic Coastal States. Tubers large, elliptical to

round-elliptical, medium thick, blunt ends; skin

flaked, self-colored, creamy buif; flesh white.

Flowers white. Resistant to the common strain

of late blight fungus and to mild mosaic; good

resistance to net necrosis caused by leaf roll

virus—similar to Sebago. Dry-matter content

liigh, similar to Green Mountain.

Red LaSoda

Medium early. Similar to LaSoda except

darker red skin. Grown mostly for seed in Minnesota, North Dakota, South Dakota, and Wisconsin and for table stock in otlier areas. Tubers

round to slightly oblong; skin bright red, very

smooth; eyes medium to shallow; flesh white (fig.

18 ). Flowers purple. Some resistance to mosaic.

High yielder.
COMMERCIAL POTATO PRODUCTION 19

É|^ Ijj^l^ ^m^

FIGURE IS.—Tubers of Red LaSoda. Grown mostly in the

North Central States of Minnesota, North Dakota, and

South Dakota ; and in the South Central States of Alabama, Louisiana, and Mississippi.

Red McClure

Apparently a mutation of Perfect Peacliblow,

difïerin<r only in tuber color. Red McClure has a

netted red skin, whereas Peacliblow has a pinkish

skin. Grown mostly in Colorado and Wyoming.

Plants large with purple pigmented stems.

Tubers round, flattened; eyes few, shallow; budeye cluster frequently depressed ; flesh white. Immune to potato wart but susceptible to virus

diseases.

Red Pontiac

Late. High-yielding variety of value in sections where red varieties are in demand ; especially

adapted to muck soils in Michigan, in the Everglades area of Florida, and in Minnesota, North

Dakota, and Wisconsin. Similar to Pontiac except darker red skin. Tubers large, oblong to

round, blunt at ends; skin smooth or sometimes

netted, uniformly red; eyes medium deep, red;

flesh white (fig. 19). Flowers light reddish purple with white tips. Not markedly resistant to

common scab or virus diseases; in some trials in

Michigan and Oliio more drought resistant and

freer than other varieties from hollow heart and

misshapen tubers. Cooking quality good.

RedWarba

Very early. Tubers medium size, short, round ;

skin smooth, red, uniformly colored except for an

occasional tuber with small white areas; eyes

mostly deep, red; flesh white. î^lowers pink.

Suspectible to virus diseases. Good cooking

quality.

Rushmore

Early to medium early. Medium yielding ability; adapted to the northern potato-producing

area. Tubers oblong, russet color, brighter and

lighter than Russet Burbank. No claim is made

for disease resistance in either foliage or tubers.

Cooking and baking qualities very good to excellent, similar to those of Green Mountain.

Russet Burbank

Late. Usually grown in the Northwestern

States, but production is increasing in Maine,

Michigan, North Dakota, and Wisconsin. Much

of this increased production of Russet Burbank is

due to its fav-orable marketing and processing

qualities. Tubers large, long, cylindrical or

slightly flattened; skin russeted, heavily netted;

eyes numerous and shallow; flesh white (fig. 20).

Flowers white and few in number. Some scab

resistance but susceptible to most other potato

diseases, especially leaf roll, net necrosis, and Verticillium wilt. Cooking and processing qualities

very good ; keeps well in storage.

FIGURE 19.—Tubers of Red I'ontiac. Grown mostly in

Florida, Michigan, Minnesota, North Dakota, and WisFiGXJRE 20.—Tubers of Russet Burbank. Widely grown

in the Northern States, especially the Northwestern

States, and Maine, Michigan, North Dakota, and Wisconsin. Sometimes called the Netted Gem. Knobby

tubers may develop under growing conditiona of uneven moisture and temperature.

Russet Rural

Late. Grown mostly in Colorado, Maine, Michigan, New York, Pennsylvania, and Wisconsin.

Tubers large, broadly oblong, flattened; skin

somewhat netted, russet, occasional tubers only

partly russet; eyes few, shallow; bud-eye cluster

frequently depressed; flesh white. Flowers violet with lilac tips. Very susceptible to wart but

resistant to common scab. Cooking quality good ;

keeps well in storage.
20 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

Russet Sebago

Late. Characteristics identical witli those of

Sebago except for two important differences: It

has russet skin and is more higlily resistant to scab

than Sebago. Because of its scab resistance, its

production is increasing in Wisconsin, where it

originated.

Sebago

Late. Firmly established commercially and increasing in production as an early variety in Alabama, Florida, and South Carolina, and as a late

variety in the North Central and Northeastern

States. Higli yielding. Tubers large, elliptical

to round-elliptical, medium thick; skin smooth,

ivory yellow; eyes shallow, same color as skin;

flesh white (fig. 21). Flowers lilac with slightly

lighter tips. Resistant to mild mosaic and net

necrosis; vines and tubers moderately resistant to

late blight; resistant to brown rot in Florida;

resistant to yellow dwarf, indicated by tests in

New York and Wisconsin; susceptible to wart.

Cooking quality variable but usually good if

tubers are allowed to mature fully before being

dug.

FiciUHE 21.—Tubers of Sebago. Grown mostly in Alabama, Florida. South Carolina, and the North Central

and Northeastern States.

Teton

Medium late. Adaptation good in Maine,

Pennsylvania, and Wyoming. Tubers round to

oblong, medium size; skin flaked, self-colored,

light buff; eyes few, medium deep to shallow,

same color as skin; flesh white. Flowers white.

Kesistant to ring rot and mild mosaic. Cooking

quality and starch content are medium to good

and compare favorably with those of Katahdin.

Triumph

Early. W^idely adapted; grown commercially

in 25 States; of greatest inq^ortance in Southern

and Midwestern States. Tubers large to medium,

round, thick; skin smooth, uniformly red; eyes

medium deep; flesh white. Flowers ¡jink. Sus«

ceptible to most diseases, including wart. Fairly

mealy when cooked.

Warba

Early. Plants medium size, spreading, with

thick, prominently angled, green stems and long,

medium-wide dark-green leaves. Tubers medium

size, short, round; skin creamy white; eyes rather

deep and red; flesh white. Flowers pink. Susceptible to mosaic and wart diseases.

Waseca

Very early. Plants small to medium size, compact to spreading with medium thick, slightly pigmented stems. Long, narrow, slightly rugose,

dark-green leaves. Tubers medium to large, oblong to round ; eyes medium shallow ; skin reddish ;

flesh white. Flowers white, tinged with purple.

Very few small-sized tubers produced.

White Rose

Late. Grown primarily in California and

Northwestern States. Large, spreading plants;

stems prominently angled, green or slightly reddish purple pigment on the upper part of medium

thick stems; leaves medium in length, breadth,

and type. Tubers large, long, elliptically flattened; skin smooth, white; eyes numerous, medium

deep (fig. 22) ; flesh white. Flowers white. Susceptible to most common potato diseases, including

wart.

1

FIGURE 22.—Tubers of White Rose. Grown mostly in

California and the Northwestern States.

Varieties Introduced Since 1960

Arenac

Late-maturing variety; immune from latent

mosaic and resistant to scab and late blight.

Tubers white, smooth. Arenac is rated relatively

:h in yielding ability, total solids content, chipping qualities after harvest and after reconditioning out of storage, and general cooking qualities..
COMMERCIAL POTATO PRODUCTION 21

Blanca v

Hijfh-yielding, mediiim-matiirinji, scab-resistant

variety adaptable to Colorado. Tubers smootli,

white, oval to round, slijihtly flattened. Piquai to

Katahdin in yielding ability and usually superior

in solids content, (iood bakei-; tubers ¡)roduce

high-quality chips after harvest and when reconditioned after storage.

Catoosa

High-yielding, medium-early red variety; resistant to scab, immune to common races of late

blight fungus. Tubers smooth, oblong to round.

First introduced red variety tiiat is resistant to

both late blight and scab and should do well in

areas where Pontiac is grown successfully. Catoosa has good cooking quality and, as grown in

Florida, makes good french fries directly after

liarvest.

Emmet

î^mmet, a sib of Aronac, possesses most of the

characteristics listed for Arenac. Usually Emmet yields less than Arenac and is lower in solids

content. It is not acceptable for chipping after

reconditioning out of storage.

LaChipper

A variety of the Irish Cobbler type; mediummaturing; flesh very white. It has some resistance to late blight; has proved superior to other

standard varieties in chipping qualities when

grown in the South.

La Rouge

Medium- to late-maturing; skin smooth, attractive, red; flesh white to cream. Laliouge has

some scab resistance, matures 10 days earlier than

LaSoda when grown in the North, and is well

adapted to production in the north-central and

southern potato-growing areas.

Navajo

Similar to Blanca in most plant characteristics. Less desirable than Blanca for resistance

to scab and for chipping after storage and

conditioning.

Ona

High-yielding; resistant to scab, late blight,

mild mosaic, and Verticillium wilt; desirable

variety, especially in Florida, Maine, and Minnesota, or wherever common scab, late blight, or

Verticillium wilt are production problems. Skin

smooth, white; eyes shallow. Ona has produced

satisfactory chips after harvest and after 10

months of storage with reconditioning. When

grown in Maine, its solids content compares favorably with standard varieties.

717-700^—64 4

Penobscot

Evaluation tests have shown Penobscot to be

satisfactory in yield, consistently high in solids

content, and more resistant to leaf roll than standard varieties now being grown. It possesses field

innnunity from mild mosaic and produces oblong

to ovate tubers with white flesh and cream to

bun' skin and medium to shallow depth of eye.

It has shown good chipping and french frying

qualities at harvesttime and at intervals after

storage and reconditioning.

Redskin

Smooth skin, medium-red color, medium-early

maturity, resistance to wind damage and scab,

high yielding ability, high solids content, and recovery from frost damage are characteristics of

Redskin that make it a very dasirable potato for

production in the South, aspecially in areas where

Eed Pontiac is grown. Its yielding ability, its

resistance to scab and to net necrosis in the tuber,

and its high resistance to Verticillium wilt when

grown in the North make it especially adaptable

to grow for seed production.

Reliance

Medium maturing; has shown moderate resistance to late blight, common scab, russet scab, silver

scurf, aster yellows, and leaf roll. Immune to

latent mosaic, and net necrosis has not been observed in leaf roll infected tubers. Tubers white,

smooth, oval; eyes shallow. Although acceptable

chips have been made directly after harvest. Reliance does not recondition as well as some other

varieties and is being released primarily as a

fresh-market variety.

Shoshoni

Late-maturing variety; tubers round to oblong;

skin, medium russet; eyes, medium-deep, sliglitly

piidv. Tubei-s grown in areas of high summer

temperatures have a tendency to become rough

and form hollow heart. Evaluation tests have

shown that Shoshoni is a high-yielding variety

with good processing characteristics for dehydrated products and that it has field resistance to

scab and Verticillium wilt.

Snowflake

Matures about the same time as Irish Cobbler.

Tubers smooth and shallow-eyed, oblong to round,

and seldom develop knobs or growth cracks. Eye

distribution is comparable to that of Kennebec.

Highly field resistant to virus Y, but*susceptible

to otiier potato viruses. Cooking qualities excellent for baking, boiling, or processing into potato

chips or flakes.
22 AGE. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

Viking

High yield and good culinary quality are two

important characteristics of Viking. Medium- to

late-maturing; tubers range from medium red

to bright red; slightly resistant to scab and

drought. Flesh extremely white. Excellent for

boiling and baking.

Varieties for Disease Resistance and

Special Purposes

A few varieties are grown for special uses. The

information given below is intended as a broad

classification of these varieties to be used as a

guide, keeping in mind that different environmental conditions can appreciably alter the characteristics listed,

Yar|fiîies.fifi$islajiUoJpecific^ise^

Scab: Blanca, Cherokee, Early Gem, Menominee,

Navajo, Ontario, Plymouth, Pungo, and Redskin.

iate blight: Catoosa, Cherokee, Fundy, Kennebec, Menominee, Merrimack, Ontario, Plymouth,

Pungo, Saco, ,and Sebago.

'Hiarly blight: Merrimack.

Verticillium wilt : Houma, Menominee, Ontario,

Red Beauty, Saranac, and Sequoia.

Mild mosaic: Cherokee, Chippewa, Katahdin,

Kennebec, Merrimack, Redskin, Saco, Sebago,

and Teton.

Latent mosaic : Saco and Tawa.

Rugose mosaic: Chippewa, Katahdin, Kennebec, Norland, and Snowflake.

Yellow dwarf: Sebago.

Leaf roll : Houma and Katahdin.

Net necrosis : Cherokee, Chippewa, Houma, Katahdin, Kennebec, Merrimack, Redskin, Saco,

and Sebago.

Wart : Katahdin, Mesaba, Mohawk, and Sequoia.

Ring rot: Merrimack and Teton.

Corky ring spot: Delus, Merrimack, Plymouth,

Pungo, and Saco.

Southern bacterial wilt: Early Gem, Katahdin,

Saco, Sebago, and Sequoia.

Varieties With Special Food Value and Other Qualities

High solids : Delus, Green Mountain, Irish Cobbler, Keswick, Merrimack, Russet Burbank, and

Saco.

High vitamin C content: Houma, Katahdin,

Mohawk, Sebago, Sequoia, and White Rose.

Nondarkening after cooking: Chippewa, Russet Burbank, and Triumph.

Varieties for Special Uses

Home gardens: Irish Cobbler, Katahdin, Kennebec, Plymouth, Red Pontiac, and Redskin.

Chip manufacture: Chippewa, Irish Cobbler,

Katahdin, Kennebec, Russet Burbank, Rural

New Yorker, Russet Rural, Sebago, and Teton.

Starch recovery: Delus, Irish Cobbler, Green

Mountain, Merrimack, Russet Burbank, and

Saco.

Frozen french fries: Katahdin, Kennebec, and

Russet Burbank.

Early market: Chippewa, Early Gem, Plymouth, Pungo, Red LaSoda, Red Pontiac,

Sebago, and Triumph.

Baking : Delus, Green Mountain, Kennebec, Merrimack, Mohawk, Saco, and Snowflake.

Boiling: Chippewa, Katahdin, Kennebec, Norland, Plymouth, Pungo, Red LaSoda, Red Pontiac, and White Rose.

GROWTH REQUIREMENTS OF THE POTATO

Soil Requirements

The yield of tubers and their shape and general

attractiveness depend to a great extent on the

texture and physical nature of tlie soil. Among

the best soils for potatoes are well-drained, sandy,

gravelly, and shale loams well supplied with organic matter and available plant food. Peat or

muck soils are particularly adapted for potato

production if they are well drained. Potatoes

should not be planted in soils that tend to stick

to the tul)ers. Above all, the soil must be well

drained because potatoes cannot tolerate conditions that restrict the air needed by the roots. In

poorly drained soils, water fills tlie pore spaces

and replaces soil air. When air is excluded, the

chemical reactions necessary for maintaining a

proper environment for potato roots cannot take

place, plant material cannot decompose normally,

and the potato roots cannot make vigorous

growth.

Even a sandy loam topsoil cannot produce satisfactory crops of potatoes if the subsoil is slow

to drain. Unless a soil has good natural drainage, it should be drained artificially before it is

used for growing potatoes. Heavy clay soils,
COMMERCIAL POTATO PRODUCTION 23

which are hard to prepare and tend to puddle

when wet and produce poorly shaped tubers, can

be made satisfactory if they are adequately

drained and a high organic-matter content is

maintained.

Soils tliat lack desirable physical and chemical

qualities for potatoes or are infested with diseaseproducing organisms should be avoided. Potato

soils that have produced many scabbed tubers in

the past are likely to produce similarly diseased

tubers unless resistant varieties are grown. Premature dying of vines and subsequent reduced

yield are likely to occur on deep sandy soils that

tend to blow, are subject to leaching, and possess

low water-holding capacity. The potato needs a

steady, adequate supply of soil moisture, particularly when the tubers are forming. Any interruption in growth occurring anytime between

tuberization and harvest can result in knobby

tubers caused by second growth.

)

\ Climatic Requirements

The amount of food produced per acre depends

on both the yield of potatoes and the amount of

dry matter they contain. The_totiil_yield of dry

aatter per acre depends on a complex of variable

,ctors including moisture, soil properties, variety

anted, fertilization, temperatures, and light.J^

The amount of storage of elaborated food in the

tubers is a function of the difference between the

food produced by photosynthesis and that needed

by the plant.

The yield per acre of total dry matter is becoming increasingly important with the rapidly expanding potato-processing industry.

The rate of photosynthesis may be governed by

the following: Light intensity, light quality, and

light duration; temperature of the air and soil;

carbon dioxide supply; moisture supply; chlorophyll content of the plant tissues, especially the

leaves; and the accumulation of the products of

photosynthetic reaction. Kate of photosynthesis

is limited when any one of these factors or a

combination of factors is insufficient. Thus if

temperature is a limiting factor, increasing air

temperature may increase photosynthesis to the

point where carbon dioxide becomes limiting or

chlorophyll is destroyed. Any single factor may

increase only to a certain optimal point, beyond

which it may be harmful. When air temperature

increases above the optimal point, photosynthesis

may be reduced while the plant may use more food

in respiration. Under these conditions little or

no stored food is accumulated in the tubers. Tliis

/interaction of climatic factors has been largely

/ responsïETe for the concentration of potato pro-

^ duction in speciñc geographical areas.

7

Temperature

The potato has made its greatest development

as a source of food in areas where the average daylight temperature seldom exceeds 70° F., and

night temperatures are cooler. Cool night temperatures are more essential than cool day temperatures. Experimental evidence indicates that the

optimum air temperature for tuber formation is

about 60° to 65° F. (15° to 18° C). At temperatures of 68° to 85° F. (20° to 29° C.) tuber development is markedly reduced, while attemperatures

of 85° F. or above few tubers are formed and those

that do form are poorly developed.

Tlie optimum temperature for photosynthesis

in potatoes is about 68° F. At higher temperatures photosynthesis increases slowly within certain limits, but the rate of respiration increases

in accordance with the increase in temperature.

Hence the higherthe meaa temperature above the

photosynflîëtic optimum, the greater the proportion of elaborated food used in respiration, and

the smaller the proportion that can be accumulated and stored in the tubers. These effects of

temperature explain why potatoes can l)e grown

profitably only in areas where the average air

temperatures are seldom higher than 70° F. The

critical period in the growth of the potato plant

insofar as yield per acre of dry matter is concerned occurs during tuberization.

Heat tolerance differs with varieties. The

varíetyüákota Ked (Jersey Redskin) is known

to be especially heat tolerant, y Attempts are being

made to develop other variefiies resistant to heat

and drought.

Soil temperatures are related to air temperatures, so lx)tli are of great importance in the storing of dry matter in the tubers. Unfavorable

heat and drought also modify shape and appearance of the tubers.

Moisture

Moisture, especially if it is a limiting factor, I

greatly affects yield and quality of the crop. At 1

the other extreme, excess moisture can cause the

soil to be waterlogged and poorly aerated, and /

result in poor root growth and reduced yield. <"

Ix)w soil moisture will reduce yield and also total

dry matter. From 300 to 636 pounds of water

are required to produce 1 pound of potato dry

matter in humid areas, whereas in irrigated areas

this ratio may increase five times.

The average water supply and its distribution

during the growing season are critical factors

determining yields. In the best potato-growing

areas in humid regions, total rainfall ranges between 12 and 18 inches for the period between

planting and harvesting.

The optimum water needs for potatoes may be

affected by fertilizer treatment as well as by soil,
24 AGR. HANDBOOK 2G7, U.S./DEPT. AGRICULTURE

air temperature, winds, and cultural practices.

It is important to recognize the interaction of

these factors in potato production. For example,

if moisture is a factor limiting yields, large applications of inorganic fertilizers may actually

depress rather than increase yields.

Day Length

/ Although growth and production of the potato

are affected By light—its intensity, quality, and

duration—the crop is nevertheless capable of utilizing relatively low light intensities for long

durations.

Potatoes are greatly influenced by the length of

the light period. During long days vegetative

top growth, particularly stem elongation, is increased, while during short days stems are relatively short. T^ng days tend to encourage formation of long-branched tuber stolons in certain

varieties, while sliort days tend to encourage the

production of shorter stolons and increase tlie

rate of tuberization. Under_short days potato

leaves tend to be soft and more susceptible to late

blight, while the same varieties grown imdér long

days are more resistant to blight. For example,

the Sebago variety grown in the winter in Florida

is notably susceptible to late blight, although when

grown under longer summer days in Maine it is

acceptably resistant. Blossom buds form under

short days but readily drop off, while under long

days there is commonly full development of flowers

if the mean temperatures arg-below 70° F.

Species of the potato collecteiaruTTiëscnbéd in

Mexico and Central and South America have entirely different growth habits when grown in the

northern United States.

Temperature effects frequently overshadow day

length effects. Thus, inj^oöl-elimates even early

potato varieties tubenzejvelljmäeclshorL^ys.

Botli late and early potatoes tuberize well under

extremely long days in Alaska, largely because

of the cool temperature and ability to utilize light

^' of low intensity.

The interaction between length of days, tem-

/ peratures, genetic factors for maturity, and length

/ y of growing season is complex. For this reason

( / maturity of a variety can be compared among

varieties at only one location.

Length of Growing Season

With any potato variety grown under optimal

/ conditions, the yield of total solids per acre is

/ usually a direct function of the length of grow-

/ ing season. Every effort should be made to plant

/ as early as feasible and delay harvesting the

\ , crop as long as possible in order to lengthen the

1 growing season. A long growing season usually

\ results in higher solids and better quality potatoes

\ than does a short season.

One often hears reports about the high solids

/

/produced by certain varieties of potatoes grown in

/Europe. "When European varieties are grown in

/this country, however, the production of solids

I per acre is relatively low because yields are sigI nificantly below those of our higher producing

American varieties. The probable explanation is

ythat the growinj5_S£sa^onjn_most potato areas of

I EuTQgeJs long and the temperature is ideal for

j tuberization (low day and night temperatures

^ with adequate moisture), whereas temperatures

\ in this country are likely to be unfavorably high

,, or the growing seasoii may be too short.

Rotations

Potatoes should be grown in a planned crop

rotation to keep the soil fertile, maintain a loose

friable texture, check weeds, build up organic

matter, and reduce crop loss from insect damage

and plant diseases, thus making possible the largest yields per acre.

No detailed information can be given about

crops to be grown in rotation with potatoes, as

combinations vary from place to place. Potato

growers must develop the rotation best suited to

local conditions, getting assistance if needed from

a county agricultural agent.

In general, long rotations (potato crops planted

3 or more years apart on the same land) are

particularly good for reducing potato losses caused

by soil-borne organisms causing such diseases as

scab, Verticillium wilt, Fusarium wilt, and black

leg. Wheat wireworms {Agriotes mancus) increase in abundance in fields planted for several

years to hay crops and may be a serious threat

to a potato crop immediately following the plowing under of a heavy sod. On the other hand,

the Pacific Coast wireworm (Llmonhn canus) increases in fields under intense cultivation, but the

population decreases in soils planted to liay crops.

Dryland farmers normally plant potatoes on

summer-fallowed land or after a cultivated crop

such as corn or beans. A small grain crop is

considered unsuitable preceding potatoes on dry

land because it depletes the soil moisture to depths

of 4 to 61^ feet or more. The best soil moisture

storage conditions for dryland potatoes are maintained by summer-fallowing tlie land before potatoes are grown. In some places where summerfallowed land is liable to blow badly, growers may

find it desirable to sow the land to grain in the

fall and then plant potatoes in the spring.

Soil Preparation and Improvement

In general, except where a winter cover crop

is planted, plow the land for the late-potato crop

in late summer or early fall. This recommendation holds not only for green-manure crops, such

as combinations of oats, peas, and vetch, but also

for clover and alfalfa sod. Fall plowing aids
COMMERCIAL POTATO PRODUCTION 25

decay of the sod, and leaves the soil exposed to

the natiiral weathering processes during the winter months. How deep to plow depends largely

on the depth of the surface soil and the character of the subsoil. Plow a deep and fertile surface soil to a depth of 8 to 10 inches, but plow a

shallow soil not deeper than an inch below the

plow sole. A 14-inch plow or a larger one is

required to do a good job of plowing for potatoes.

If much trash is to be turned under, plows with

10- and 18-incli bottoms and with large clearance are needed. Strip-plant fields with a decided

slope. It is preferable to plow these sloping lands

in very late fall or in the spring and leave them

rough, to help prevent erosion.

Disk the fall-plowed land the following spring

as soon as the land is dry enough to work. On

most potato soils the disk harow should be followed by the spring-tooth harrow. It penetrates

the soil deep enough to tear up clods, and the flexible teeth are damaged very little from stones or

other obstructions. On some fields it may be necessary to finish the land with the spiked-tooth

harrow to break clods and level the ground. The

spiked-tooth harrow is also effective in eradicating

small weeds.

In general, heavy soil benefits most from fall

plowing because the action of frost, snow, and

winter rain on exposed soil makes it more mellow.

Fields from which soil is likely to be washed away

by winter rains and rapidly melting snow, however, should be plowed in the spring.

Prevention of soil erosion cannot be overemphasized. Any v.aste of topsoil ultimately means a

serious loss of capital to the potato grower. Erosion problems should be discussed with your county

agent or local soil conservation specialist.

In some parts of the West, alfalfa is turned

under just before potato-planting time, and neither

the cutoff crowns nor the surviving alfalfa plants

give trouble. Thorough plowing of alfalfa soil

IS essential. The plow should be equipped with

alfalfa shares, so that no plants will be left uncut.

Many growers find it beneficial to loosen heavy

soil to a depth of 16 to 20 inches with a land chisel.

Soil should not be plowed when too wet. If

plowed when too wet, the land is likely to remain

m poor condition throughout the growing season.

In the South where heavy rainfall may occur, potato land is ridged to facilitate better rimofï of

excess water and to provide better drainage.

Basin listing is sometimes a good way to prepare an area for dryland potato growing. The

basin lister makes a furrow with small earthen

dams at regular intervals. The dams keep rainwater from running off the surface and thus increase tlie quantity of moisture that is taken up

by the soil. If fields are to be irrigated, proper

leveling of the land is essential before final preparation of the seedbed (fig. 23).

* vl'

FioUKE 23.—Careful leveling of land is essential when

potatoes are to be grown under irrigation.

The importance of thorough seedbed preparation under all conditions cannot be overemphasized. Poor land preparation cannot be remedied

by later cultivations.

Building Up Soil Organic-Matter Content

One of the most important soil problems in

practically all potato regions, except those with

muck and peat, is that of supplying and maintaining soil organic matter. In warm regions with

long seasons, the decomposition of organic matter in the soil is rapid and almost continuous. In

colder regions decomposition is less rapid in Stimmer and does not occur appreciably during the

winter. An ample supply of decaying organic

matter helps to keep the soil loose and mellow and

thus prevents packing. Organic matter also facilitates plowing and cultivating; it enables roots of

potato plants to penetrate the soil more readily,

and it retains raiitwater; it provides food energy

for tlie growth of desirable soil micro-organisms ;

and it supplies plant nutrients. Potato tubers

develop and maintain their normal shape better

where there is adequate soil organic matter. Tuber shape is an important consideration in marketing the crop.

Each potato farm must ordinarily produce its

own organic-matter supply. There are four general methods of getting organic matter into the

soil: (1) Follow a suitable crop rotation in which

a leguminous crop such as clover or alfalfa is

grown; (2) grow and plow under green-manure

crops (including catch and cover crops), such as

crimson clover, vetch or a combination of peas

and vetch, soybeans, cowpeas, rye, oats, barley,

wheat, millet, Stidan grass, field corn, or other suitable crop plants (fig. 24) ; (3) apply barnyard

manure; and (4) plow under any other organic

residues that will increase the humus content of

the soil.
26 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICXILTURE

FIGURE 24.—I'lowins uiidi'i- crhiison clover in Aroostook

County, Maine. Organic matter thus added Improves

the physical condition of the soil. Plowing under

legumes adds nitrogen as well as organic matter.

Barnyard manures improve the physical condition of the soil, add plant nutrients, and increase

the bacterial activity. Handle manure proi)erly

so as to prevent as much as possible the loss of

valuable nutrients. Never pile manure loosely in

the open yard or under the eaves of the barn, because both storage methods permit leaching of

valuable plant nutrients.

If left in the open, build the manure pile with

a slight depression at the top and with nearly

straight sides. Keep the manure moist throughout to avoid excessive heating, for, if the pile is

allowed to dry out too much, firing results with

loss of nitrogen. To be used most efficiently for

potatoes, haiü the manure directly to the field and

spread it uniformly on sod land before fall plowing. Rainwater will wash the soluble plant

nutrients of the manure into the soil. On sloping

land, losses from runoff can be reduced by spreading the manure just before plowing.

Light applications of manure are better than

excessively heavy ones. From 5 to 8 tons of manure applied per acre gives better returns than 12

to 15 tons. Generally it is a good practice to add

40 to 50 pounds of superphosphate to each ton of

manure.

Fresh manure, especially horse manure or

chicken manure, should not be applied just before

the potato crop is planted. These manures may

produce a favorable environment for common scab

development, particularly if the soil is near neutral

or pH 7.0 reaction. There is less danger from

scab development on soils having a pH of 5.0 tlian

on those having a pll near 6.0. The best rule to

follow in using fresh manure is to get it plowed

under as soon as convenient, so that it will decay

before the crop is planted, and to avoid using

liorse manure or chicken manure on potato land

by applying these manures to other crops in the

rotation.

Liming

Liming is important in successful potato production in most humid regions. When the soil is

too acid, clover or other soil-improvement crops in

the rotation begin to thin out and bare spots are

liable to appear in the fields. The question that

concerns potato growers, however, is how much

lime to apply to benefit the clover without favoring the development of common scab on the

potatoes. Growth of the common scab organism

in the soil is promoted by alkaline or mildly acid

soil reactions.

Increasing the soil acidity will usually check

the development of the organism to a point where

mostly clean tubers are obtained. Too much

acidity (below pH 4.8) is undesirable for the

potato crop, and a reaction above pH 5.2 will favor

scab.

If lime is needed, it can be used effectively in

potato rotations when applied before the land is

seeded; then clover does well, and common scab

is held in check. In Aroostook County, Maine,

potato growers have greatly increased their use of

liming materials. In dryland and irrigated areas

of the AVest, the soil reaction is usually above pH

7.0. It is seldom economical to attempt to increase acidity by using soil amendments such as

sulfur. Where scab is a problem, rotation practices, especially those involving the turning under

of considerable amounts of organic matter, are

more practical.

It is a good policy to have potato soils tested

periodically to determine soil amendment requirements. Soil tests are made by State agricultural

experiment stations for a nominal fee and recommendations are made on the basis of the soil test.

FERTILIZERS

Importance

Fertilizing potatoes is a highly important

practice in all sections, especially where they are

groM'ii for commercial trade. Since satisfactory

answers to many fertilizer i^roblems require firsthand knowledge about the soil and its previous

treatment, only general information can be given.

Best advice on local problems of kind, amount,

ratio, and placement of fertilizer is available

through your county agent and State agricultural

college. In many States soil-testing services
COMMERCIAL POTATO PRODUCTION 27

furnish fertilizer recommendations based on tlie

analysis of a soil sample from tlie field to be

planted.

Numerous comparisons of fertilized and unfertilized plots have shown the importance of

supplying nutrients to the potato crop. Potato

fjrowers can easily reco<rnize a row or part of a

row that has been planted without fertilizer, because the effects are generally indicated by retarded, spindly growth, liglit-green color, and

reduced yield. The efficient use of fertilizer is

liighly important economically to growers.

In some sections of Western States, sulfate of

ammonia or ammonium nitrate is tlie only fertilizer material used on potatoes.

Potato plants require an ample supply of plant

nutrients to insure rapid, steady growth and

proper tuber development. Moreover, these requirements need to be met at the beginning of

growth; any delay in applying the fertilizer much

beyond planting time may reduce the yield. Although some available nutrients are naturally

present in the soil, fertilizer is usually needed to

supply readily available plant nutrients for maximum production. Decaying organic matter and

barnyard manure also supply nutrients, but the

rate of availability usually is inadequate for the

needs of the potato crop during the early growth.

Kind and Amount

The kind and amount of fertilizer required for

successful potato production depend to a considerable extent on the kind of soil and its fertility,

on the available manure and its condition, on the

rotation practiced, on tlie potato variety, and on

seed-spacmg distances. It is also generally agreed

that the effectiveness of fertilizer is greater when

the supply of moisture in the soil is adequate.

The most satisfactory fertilizer for the potato

crop in most sections contains nitrogen, phosphoric acid, and potash in proper proportions. In

sections where their need is indicated, magnesium

and calcium compounds are usually included.

Complete fertilizer is generally used in commercial

potato-producing sections, altliough sometimes

nitrogen may not be considered necessary on muck

or peat. In some sections where well-kept manure is used, the only additional material needed

may be superphosphate. Good yields occasionally

may be obtained with manure reinforced with

superphosphate, but a complete fertilizer usually

gives better yields.

The nitrogen, phosphoric acid, and potash ratio

of potato fertilizers is important. No one ratio

will suit all potato soils or all potato varieties

grown, but 1-2-1, 1-2-2, 2-3-3, and 1-1-1 are the

ratios most widely recommended and used.

The rate of fertilizer application varies from

one potato-producing section to another, depending to a large extent on rainfall, soil, potato variety, spacing, and other growth factors. In most

sections of New England, particularly in Aroostook County, Maine, growers customarily use a

ton or more of fertilizer to the acre, the rate depending largely on the closeness of seed spacing

and on the variet}^ planted.

Many growers in the highly commercial potatoproducing districts use from 2,000 to 2,500 pounds

of fertilizer to the acre. However, on lighter

sandy soils in sections where lack of moisture is

a problem, 500 pounds per acre is often considered ^

the maximum rate.

The use of liigh-analysis and double-strength

fertilizers for potatoes has increased rapidly in

recent years, particularly in sections where heavy

applications of fertilizers are customary. Growers

using the higher analysis fertilizers appreciate the

reduction in weight attained, the smaller storage

space required, the savings on transportation and

bags realized, and the greater efficiency in all labor

involved.

In addition to cutting in half the costs of bags,

freight, storage, and handling, use of doublestrength fertilizer saves time and labor at planting

time because the hopper on the planter does not

have to be filled so often.

Fertilizer Placement

The most effective method of applying fertilizer

for potatoes is to place it in a band on each side

of the seed pieces, about 2 inches away from them,

at their level or slightly below. On sloping land,

in order to prevent fertilizer from getting too close

to the seed pieces through angling of the planter,

apply the fertilizer bands about an inch below the

seed-piece level.

"When applied in the row and mixed with the

soil, some of the fertilizer is liable to come in contact with the seed pieces and result in retarded

germination, weak plants, and greatly reduced

yields. In Maine, for example, a 4-year average

yield when fertilizer was placed at the side was

43 bushels per acre higher than when the fertilizer

was mixed with soil m tlie row; in Michigan, 29

bushels higher; in New Jersey, 18 bushels higher;

and in Virginia, 21 bushels higher.

Comparisons in other States gave similar results. The additional yields were obtained not

from any change in the kind or quantity of fertilizer used, but simply from a difference in the

placement of the fertilizer.

Do not apply fertilizer so it will come in direct

contact with seed pieces or deposit it directly over

them. Either position may mean a poor stand.

To avoid possibility of fertilizer injury when applying more than 2,500 pounds per acre, some

growers apply one-third of the total application

to the plow sole with a distributor that attaches
28 AGR. HANDBOOK 2 67, U.S. DEPT. AGRICULTURE

to the plow or they broadcast part of the fertilizer

before planting. The remainder of the fertilizer

is then applied in bands at planting time.

Avoidance of Nutrient Deficiencies

Magnesium, an essential plant food element, is

included in the group of plant nutrients with nitrogen, phosphorus, and potash. It is an essential

component of chlorophyll, the green coloring matter of plants. If available magnesium is lacking

in the soil, the formation of chlorophyll in the

plant is checked and normal growth activities

are seriously disturbed. The first symptoms of

magnesium deficiency appear on the lower leaves,

but in advanced stages the entire plant may be affected. Yellowing first appears at the margins

of the leaflets and spreads between the veins toward the midrib, where a characteristic bulging

develops. The leaflets become thicker and brittle, unlike leaves that are yellowing natui-ally

through age. Ii/regular, brown, dead patches develop, and thei^ the lower leaves often drop olï

the plant.

Potato fertilizers can now be obtained with 30,

40, or 50 pounds of soluble magnesium oxide per

ton, sufficient to help prevent magnesium deficiency in many soils. Potato vines showing early

symptoms of magnesium deficiency can be sprayed

with a solution of epsom salts. Dissolve 20

pounds of epsom salts in 100 gallons of water and

apply this amount of spray per acre. Although

spraying is a quick-acting remedy, soil applications of magnesium compounds produce more lasting effects.

In areas where soils lack sufficient manganese,

such as the Homestead area of Florida, applications of 100 pounds of 65-percent manganese sulfate per ton of fertilizer have improved yields.

The boron requirement of the potato is relatively

low; manganese and zinc, like iron, are available

in acid soils in sufficient amounts. Certain mucks

and peats are sometimes deficient in copper, but

the amount naturally present plus that added in

some fungicidal sprays appears sufficient to prevent a deficiency of this element in most potato

soils.

SELECTING AND PREPARING SEED

Selecting Seed Stock

Although much has been written on the importance of good potato seed stock, some growers

continue to use inferior seed. Seed is satisfactory

only if it is an adaptable variety, is true to name,

and is sound, not oversize, and free from frost

injury and disease.

Commercial potato growers should always use

certified seed. It costs more than uncertified seed,

but the extra cost is small in view of the greater

yield that can be expected. Using quality seed

practically free of infection is very important in

potato production because a number of potato

diseases affecting both yield and quality are catised

by infections carried inside the tuber and are not

affected by external treatments. Many diseases

show no visible sign of infection on the outer surface of the tuber. This is true of wilt, some of the

bacterial diseases, and nearly all the viruses.

When an infected seed piece is planted, not only

does the disease reappear in the plant grown from

that seed piece but the entire field is exposed to

spread of infection.

Certified Seed

A grower cannot tell by looking at potato tubers

whether they will be good for seed. However, it

is true that a few symptoms of diseases which will

endanger the next potato crop can be seen on the

tubers. Among these are scab, Rliizoctonia, and

late blight lesions, or actual breakdown caused by

ring rot, bacteria, and Fusarium. But the most experienced grower cannot detect by tuber inspection, infection by mosaics, which cause losses as

high as 20 percent of the crop, or leaf-roll disease,

often reported to cause losses of 10 percent. Other

seedborne diseases can be detected only through

plant symptoms in the field. Examples of diseases tliat can be introduced on poor seed and

later may permanently infest the soil include

Fusarium wilt, wart, corky ringspot, and powdery

scab. The use of certified seed is the best guarantee against the introduction of seedborne

diseases.

Certified potato seed is grown chiefly in northern areas where lower growing temjjeratures not

only favor the expression of virus disease symptoms but also aid in the control of insect vectors

that spread these diseases. Certified seed potato

gro\yers use all the methods available for eliminating infections from their seed stock so that their

fields will meet the rigid requirements of a State

certification service. Infected plants in seed potato fields can be detected and removed before

infection spreads. In 1915 seed-potato certification was begun in the United States to provide

growers of tablestock with disease-free seed true

to varietal name. Since that time the enforcement of more and more rigid certification standards has helped considerably to improve the quality of certified seed.
COMMERCIAL POTATO PRODUCTION 29

Certified seed is produced on both irrigated and

nonirrigated land in all potato-seed-producing

States. For many years potato growers were

prejudiced against seed stock grown under irrigation, but experiments have shown that irrigation

has little or no effect on either the vitality of

the seed or the vigor of plants grown from it.

Potatoes of average size are more desirable for

seed than larger ones, partly because seed i)ieces

cut from them have smaller cut surfaces. Seedpiece rot, the most common cause of poor stands of

potatoes, is generally more severe when the seed

pieces have large areas of cut surface exposed

to infection. Many growers today prefer to buy

small whole tubers for seed at i)remium prices to

avoid the risk that exists when cut seed pieces

are used for planting.

Effect of Geographical Source of Seed

/ Experiments conducted, particularly in South-

/ ern States, have indicated the desirability of buy-

/ ing seedstocks from certain geographical areas.

L Yield differences found between seed of the same

11 variety^^riSinating in dJiferenLJßcationsjire considered to be-JîausedJ:iy~^"jïïacéeffèct?^~This "ef-

/

fecT" is sometimes attributed to differences in seed

dormancy which can cause retarded and uneven

1 emergence, buti other possible influences, such as

\ tiiber-tTarisüaitted,diseases ditfeiTngJiL4-lieir_vi^t-u-

\ lèn^IJ^n^j)la£etoj)lace, val^IabTesoil and moisIture coïïditiorïsTUTffering fertilizer practices, and

possible natural selection of somatic mutations,

/ cannoir be^ruÎHd^xraTas contriburing canses^ FurIther research is needed to study the nature of

I "place effect" and to determine its probable causes.

I Seed potatoes grown under adverse climatic en-

/ vironments seldom produce a good crop. It is

/ seldom profitable to use seed potatoes grown in

/ certain areas of the South even though they are

/ disease free. Crops grown from such seed can-

•""^ not be expected to give good yields. The nature

of this adverse pliysjologicaL,"place ejTectrMs not

known aTjiresCTLtL "Place effect" is not easily

remedied in planting stock; several seasons of repropagation and growth under ideal climatic conditions are necessary to overcome this effect on

seed potatoes.

Effect of Storage

Storage temperatures and humidities are known

to affect seed quality. High temperatures in

storage encourage sprouting too early in the season. These sprouts should be removed. Potatoes

that have been desprouted once usually make satisfactory seed even though they may be somewhat

shriveled. Repeated desprouting, however, impairs the propagating value of the tubers. Varieties differ in the number of desproutings they can

stand before their tubers are unfit for seed. For

example, the variety Ontario is unfit for seed

after two desproutings, but Kennebec can be

desprouted four times before its vigor is seriously

affected. A storage temperature of 40° F. or

slightly less is considered ideal for keeping seed

potatoes.

Seed potatoes stored at low temperatures (below

40° F.) should be warmed up at 60° or higher for

about 10 to 14 days before planting. Warming up

the potatoes will start sprouts growing and emergence after planting will be quicker. This treatment is especially recommended if the soil temperature is also low (45° to 50° ).

Effect of Greensprouting

Greensprouting is seldom used commercially in

this country, but it is a common practice in northern P^urope. (ireensprouting is accomplished by

exposing the seed tubers to indirect daylight at

temperatures of 60° to 65° ¥. for 14 to 21 days before planting. Tubers treated this way form short

stubby green sprouts. Under unfavorable growing conditions, the sprouts from these small tubers

(seed pieces) emerge rapidly with little incidence

of decay. When growing conditions are favorable,

there is no marked advantage from greensprouting. Much of the beneficial effect of greensprouting is simply the result of warming the tubers.

(See "Effect of Storage.")

Breaking Rest Period

The true rest period occurs after harvest when

the tuber buds will not develop sprouts even

though environmental conditions are favorable.

Dormancy is that stage when the sprouts will initiate growth only when environmental conditions

are favorable. Potato tubers when freshly harvested are normally in a pronounced state of rest.

In cold storage, potatoes normally do not sprout

until late winter or early spring, although they

may have already emerged from their rest period.

Potato varieties differ markedly in the length of

time they will remain in the rest period or in a

nonsprouting conditicm. For example, Kennebec

and Katahdiu are noted for a long rest period,

whereas Calrose and Golden are varieties with a

relatively short rest period (table 9).

iVequently potatoes must be planted soon after

harvest and it is necessary to break the rest period. The two chemicals used most successfully

for this purpose are potassium or sodium thiocyanate and ethylene chlorohydrin. The thiocyanate treatment is more convenient when the

tubers are to be cut and planted soon afterward.

The ethylene chlorohydrin treatment is more satisfactory where potatoes must be treated whole

and shipped after treating.
30 AGR. HANDBOOK 267, U.S. DEPT. AGRICULTURE

TABLE 9.—Time required to sprout potatoes stored

at various temperatures (^average of 3 storage

seasoTisY

Variety

Calrose

Cayuga

Cherokee

Chippewa

Earlaine

Earlaine 2

Erie

Golden

Green Mountain

Houma

Irish Cobbler

Kasota

Katahdin

Kennebec

Menominee

Mesaba

Mohawk

Norkota

Pawnee

Pontiac

Potomac

Red Warba

Russet Burbank

Russet Rural

Rural New Yorker

Sebago

Sequoia

Teton

Triumph

Warba

White Rose

Average, all varieties

Storage temperature

70° F.

Weeks

5

7

7

8

8

6

6

5

8

8

8

8

8

9

6

7

9

7

7

8

9

7

8

9

9

6

8

8

8

7

8

60° F.

Weeks

7

7

9

8

8

7

7

5

10

8

9

8

8

9

6

8

10

8

8

9

10

8

10

9

9

6

8

8

9

7

12

50° F. 40° F

Weeks

9

10

2.38

12

11

10

13

7

15

12

13

11

13

19

12

11

14

10

12

18

2 41

12

13

12

12

11

15

20

11

12

15

14

Weeks

24

25

2 38

2 36

26

25

2 39

10

33

27

20

25

MO

2 38

30

29

27

28

26

2 35

2 41

29

27

25

25

2 41

237

2 41

35

29

27

231

1 Adapted from Wright, R. C, and Whiteman, T. M.,

1949. "The Comparative Length of Dormant Periods

of 35 Varieties of Potatoes at Different Storage Temperatures." Amer. Potato Jour. 26: 330-335.

2 No sprouting was visible at termination of this storage

test.

Thiocyanate Treatment

Either potassium or sodium tliiocyanate can be

used. Soak the cut tubers in li/^ percent of solution for 1% hours. Cut the seed tubers tlirougli

the seed or bud ends, or at least through one of

the eyes. Plant the seed in moist soil soon after

it is treated.

CAUTION

Potassium and sodium thiocyanate solutions

are highly poisonous. Great care must be

taken that they do not get into the mouth.

Do not leave treated potatoes where animals

have access to them, or where they may be accidentally mixed with untreated tubers. All

vessels used should be thoroughly cleaned before storing. Dispose of old solutions so that

humans or animals cannot get them. Do not

dispose these poisons in streams or ponds.

Ethylene Chlorohydrin Treatment

Because ethylene chlorohydrin treatment is a

gas treatment, airtight gas containers are required. ¥ov small lots, glass desiccators or large

garbage cans with tight-fitting lids sealed with

potter's clay can be used; for large lots, a large

airtight room is necessary. The sacks sliould be

arranged in the gas chamber so that the gas can

circulate freely.

The potatoes should be heated before the gassing

is started and must be kept at 75° to 80° F. during

the 5-day gas treatment; higher temperatures are

likely to develop blackheart in the tubers. During

the early part of the 5-day treatment, the anhydrous chemical is diluted by adding six parts of

water to fovir of the chemical by weight. An attempt is made to have the ethylene chlorohydrin

evaporate throughout the entire 5-day treatment.

One quart of 4()-percent ethylene chlorohydrin

should be evaporated for every 600 to 720 pounds

of potatoes in the gas chamber.

If the temperature rises above 82° F., or if the

humidity rises to the saturation point, it is advisable to provide outside ventilation. This may be

done by opening the door of the gas chamber a

few inches for 5 or 6 hours, depending on conditions. The gas chambers are ventilated for 12

hours after the 5-day treatment. The gassed potatoes are kept at a temperature of 75° to 78° until

loaded for shipment.

Not all varieties react to the gas treatment in

the same way. The rest period of most of them

can be broken by the 5-day treatment, but varieties like Irish Cobbler, Mohawk, and Russet Burbank require a second 5-day treatment after an

interval of 12 to 15 hours.

CAUTION

Ethylene chlorohydrin gas is poisonous and

also explosive in certain mixtures with air;

keep lights and fire away from the room or bin

where it is being used. Thoroughly ventilate

the room or bin after treatment and before

permitting anyone to enter. In some areas local or State permits may be required.

Seed Treatment

Surface disinfection of potato seed with chemicals such as mercuric chloride, formaldehyde, and

commercial preparations was once considered a

necessary practice. Alore efficient production

methods and ample supplies of disease-free certified seed have made this practice unnecessary, ex-
COMMERCIAL POTATO PRODUCTION m

cept in special or small localized production areas.

Seed disinfection can affect only the disease

organisms borne on the surface of the tuber. It

cannot control virus diseases transmitted by mechanical means, by insects, by wind and water, or

by organisms such as the one causing scab which

also infests the soil. Seed disinfection tends to reduce seedborne scab and Rhizoctonia and hence

improves the chances for a better stand of plants

under adverse growing conditions. It is difficult to

justify seed treatment as an efficient practice for

conmiercial potato production when relatively disease-free seed is available. In addition, if seed

treatment is not done properly with warm acid or

caustic solutions, it may damage the seed, especially some varieties such as Sebago or Chippewa.

Whole Versus Cut Seed

Many growers prefer to plant uncut B-size seed

potatoes because uncut seed has the advantage of

saving the expense and inconvenience of cutting

the larger No. 1 size tubers for planting. Use of

whole seed also helps to insure a better stand and

to prevent the spread of certain diseases by contaminated seed-cutting knives. Potato growers

in some irrigated areas prefer whole seed tubers,

because whole tubers ai'e less likely to rot or become dry in the soil before the sprouts are rooted.

Using certified B-size whole seed is generally a

good practice because, on an equal seed-piece-size

basis, whole seed produces more tubers per plant

than does cut seed. Some growers are reluctant to

use B-size whole seed because of a belief that tiiis

seed often contains a higher incidence of virus

disease than do cut seed pieces from large tubers.

Virus-infected plants tend to produce smaller

tubers but certified seed lowers disease incidence,

making the use of B-size seed a safe practice.

Standard production practices are designed to

produce seed potatoes at nominal cost per lumdredweight and result in a high percentage of the potatoes going into grades larger than B-size. More

B-size whole seed would be used if it could be produced more economically. Seed buyers usually

expect to pay at least 50 cents per hundredweight

more for whole seed.

Cutting Seed Potatoes

Formerly most seed potatoes were cut by hand,

but recently more and more cutting is being done

with mechanical seed cutters. Tlie continued improvement of these seed-cutting devices is increasmg their use. Best results are obtained with mechanical seed cutters when the seed lias been

graded to a uniform size. Such varieties as

Katahdin and Chippewa, having few eyes on the

basal half of the tuber, produce considerable waste

when the large tubers of these varieties are cut for

seed. Before cutting is begun, always allow the

seed potatoes to warm up, especially if they have

been held in cold storage.

Blocky seed pieces are desirable (fig. 25). The

planter can handle the blocky pieces to better advantage, and they are less liable to decay in the

ground if weather is unfavorable. The seed piece,

in general, should weigh II/2 to 2 ounces and should

have from one to three eyes. The Katahdin and

Kennebec varieties usually set a smaller number

of tubers per plant than most varieties; therefore,

each seed piece of these varieties should have two

or more eyes, or the spacing in the row should be

reduced in order to help lower the percentage of

oversized tubers that might otlierwise develop.

FicuHE 25.—Even stands are more likely to be produced

by large, blocky seed pieces than by smaller ones.

Suberization of Cut Seed

The general practice among potato growers is

to cut seed about the time it is needed for planting;

usually not more than a 2-day supply is cut in advance because rain may interfere with planting

and unplanted cut seed is likely to deteriorate unless given special care. However, it is safe to cut

seed some time in advance of planting if storage

conditions are provided to heal the cut surfaces, a

process known as suberization.

Some growers who have large acreages to plant

and are limited in time and labor have found it

more economical and also convenient to cut the

seed potatoes well in advance of planting. Wlien

seed is to be cut and stored for 10 days or more,

proper care should be taken to heal, or suberize,

the cut surfaces of tlie seed pieces. If the fresh-
32 AGR. HANDBOOK 2 67, U.S. DEPT. AGRICULTURE

cut seed is not suberized, it should be planted as

soon as possible after cutting.

Properly healed, or suberized, seed has several

advantages over freshly cut seed (ftg. 26). Cutting can be done before the rush of spring planting

begins. Suberized seed is less lilcely to shrivel if

the soil is dry at planting time or to rot if conditions are too wet. With suberized seed the danger

of heating, which often happens in the case of

freshly cut seed improperly stored in barrels or

bins, is greatly reduced. Many a bad stand has

been attributed to the poor quality of seed, when in

reality it probably was the direct result of improper handling.

FIGURE 2G.—Comparison of A, freshly cut, and /}, suberized, or forked-over, seed pieies.

Suberization is best accomplished by holding the

cut seed for a week or 10 days at a temperature

from 60° to 70° F. and a relative humidity of approximately 85 percent. Recently it has been

found feasible to cut potato seed at the point of

origin and ship it by railroad in specially heated

refrigeration cars. Suberization takes place during transit and the seed arrives at its destination in

excellent condition for planting.

Slatted crates are the best type of container in

which to suberize cut seed; and if the crates are

kept covered with damp sacks, suberizing is usually good at the temperature range recommended.

To prevent the seed pieces from sticking together,

rack the containei-s from side to side within 24

hours after cutting the seed. If the seed is suberized in burlap sacks, fill these only half full and

pile the sacks in such a way as to avoid undue

lieating. The piles may be covered with moist

sacks that are kept damp by occasionally sprinkling with water, but care should be taken not to

get the seed too wet ; otherwise, rot may start on

some of the seed pieces.

Care of Cut Seed

If cut potato seed is exposed to the sun or wind

on a hot day foi- even a short time, decay may start

before or soon after planting. On such a day it

is good practice to sliade all seed taken to the field

for planting, including seed left in the planter

during the noon lioni-. A canvas shade, if used,

should be placed above the potatoes so that hot air

will not be trapped under it.

Freshly cut seed if sacked and piled may develop

heat, one of the i)rincipal causes of seed-piece

decay.

Cut seed should never be put into unwashed used

fertilizer bags.

Some growers dust freshly cut seed pieces with

lime or gypsum to keep them from sticking

toirether.

PLANTING THE CROP

Time of Planting

Planting time depends chiefly on local climatic

conditions. In general, potatoes should be

planted as soon as the soil has begun to warm

up and can be tilled without danger of compaction. The most favorable time to plant in this

country differs from North to South. Growers in

the Northern States usually plant the bulk of their

crop in May, the Central States in xVpril, and

the Southern States from November to P^ebruary. Planting the crop too early in the season

often results in frost damage and poor stands because of wet or cold soils. i^ITsing whole seed or

cut suberized seed lessens the chances of poor plant

stands caused by seed-piece decay. "Within reasonable limits the earlier the crop is planted the

greater will be the total yield and the solids content of the tubers.

In sections where the growing season is longer

than required to mature the potato croji, it is

customary to select a planting date that will provide the most favorable environment during the

critical period of tuberization.

Seed Spacing

Tlie natural fertility of the soil, its moistureholding capacity, the climate, the variety grown,

and the amount of fertilizer applied usually govern the spacing witliin tlie row and the distance

between rows. In the best potato sections of the
COMMERCIAL POTATO PRODUCTION 33

Northeastern States tlie rows are occasionally

spaced as close as M inches. To make better nse

of mechanized e(i[nii)ment, some growers prefer a

wider row spacinji;, up to iîé inches.

Within the rows, hill si)acinf>: averajjes about 12

inches, and the tendency is to siiorten the distance

to 10, 8, or even (> inches when such varieties as

Kennebec and Katahdin are planted. These varieties usually set a small number of tubers, and

closer spacing helps prevent the development of

a high percentage of oversize tubers. In Aroostook (^ounty, Maine, the combination of good

seed, good cultural practices, and heavy fertilization tends to i)roduce a large percentage of oversize tubers unless the spacing within the row is

reduced to less than 12 inches. Close spacing in

the row also tends to reduce loses from hollow

heart, and on good land it generally increases

the number of marketable tubers.

In dryland potato production in the Western

States, the distance between rows is usually 42

inches and the seed pieces are spaced 14 to 30

inches apart in the row. Under irrigation, the

rows and seed pieces can be spaced much closer

together; usually, for convenience in cultivating,

ditching, irrigating, and digging, the rows are

spaced ?A to 36 inches apart and the seed pieces

are planted 9 to 14 inches apart in the row, according to variety and the fertility of the soil. For

early potatoes, the distance between rows is sometimes reduced to 32 or even 30 inches.

Some varieties when grown under irrigation require close spacing to keep the tubers from growing too large, to minimize growth cracks and hollow heart, and to produce maximum yields.

Where rainfall in the West is sufficient to produce

a good crop without irrigation, as in the western

parts of Oi'egon and Washington, the seed pieces

are spaced the same as on irrigated land.

Wide ridges are desirable for growing potatoes

on most irrigated land, because they allow the

tubers ample room for development and also protect them from frost in the fall.

In the South, spacing generally depends on the

type of cultivation equipment vised. In the sugarcane regions of Louisiana, spacing between rows

is often 72 inches. Most commonly, however, row

spacings of 34 to 36 inches are used with 9- to 14-

inch spacing in the row.

Larger seed pieces and closer spacing are used

in sections with soil and weather that are optimum

for potato culture than are used in less favorable

sections.

Depth of Planting

No specific deptJi of planting will give equally

good results under all conditions. Seasonal, cultural, and climatic conditions that prevail during

the growing season are the determining factors.

If seed pieces are planted shallow, it is necessary

to ridge the row after early cultivations so that

stolons will be formed well below the ridge surface. Where ridging is practiced, the custom is

to plant the seed pieces 2 or 3 inches deep, measured

from the tot) of tlie seed piece to ground level.

The row ridge is built up gradually, starting as

soon as the plants break the ground and contniuing at 7- to 10-day intervals until the proper ridge

is formed.

Tubers formed at a depth of 2i/^ to 5 inches

usually develop under satisfactory temperature

conditions favorable for high quality. If the seed

pieces are planted too shallow, the tubers formed

may be subject to "sunburn" and quality is

lowered.

In fields where Rhizoctonia cankers occur, the

seed pieces should be planted shallow or planted

deep with a shallow covering. This practice insures quick emergence of the sprouts above ground

and avoids the early attack of this fungus disease

which is less damaging to a healthy, fast-growing

plant. For level culture where Khizoctonia is

present, plant the seed pieces 3 to 4 inches below

the field level but w4th a covering of not more

than 2 inches of soil over the seed piece.

In the West seed pieces are usually planted from

3 to 5 inches deep to make sure that they will be

covered at all times with moist soil. Deep planting is more necessary in dryland farming than

under irrigation. "Irrigating up" is a common

practice in furrow-irrigated areas. (See p. 37.)

Planters

Four general types of potato planters are now

used in this country: the automatic picker, the

assisted feed, the cup-type, and the automatic

tuber-unit planter. The automatic picker type is

the one most widely used in the major producing

areas (fig. 27). This type of planter has been

gradually improved for operating at higher speeds

by increasing the number of pickers from 12 to 16

FiGUKE 27.—A two-row pieker-type planter.
34 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

per wheel and by redesigning other parts to

achieve more uniform delivery of seed pieces and

to obtain more regular seed spacing at high operating speeds. Greater economy of operation is possible when two-row automatic planters can be

operated up to 5 or 6 miles per hour. Good results

may be obtained with planters of this type if the

seed potatoes are cut in blocky pieces of about the

same size. Punctures made by the pickers jabbing

into the seed pieces, however, may spread diseaseproducing organisms and viruses from one piece

to another.

Planting costs are reduced by using four-row

planters for large acreages (100 acres or more).

Two two-row planters can be assembled to make a

four-row planter by means of conversion kits now

TABLE 10.—Seed pieces required to plant an acre of potatoes at different spacings

Distance between rows

Pieces required for an acre at stated spacing distances within rows

6 inches 8 inches 10 inches 12 inches 14 inches 16 inches

30 inches

Number

34, 848

32, 670

30, 748

29, 040

Number

26, 136

24, 502

23, 061

21, 780

Number

20, 909

19, 602

18, 449

17, 424

Number

17, 424

16, 335

15,374

14, 520

Number

14, 935

14, 001

13, 178

12, 446

Number

13, 068

32 inches 12,251

34 inches _- 11,531

10, 890

available commercially. The assembled four-row

machines are adequately flexible for operating on

uneven ground and are capable of planting 50

acres a day while operating at speeds of 5 miles

per hour. A much larger acreage can be planted

in a day with a picker-type planter than with an

assisted-feed type, but the assisted-feed type

usually insures a better stand of potatoes.

A tractor driver and helper constitute the field

crew reqviired to operate either a two- or a fourrow automatic planter. The helper rides the

planter, keeps a constant check on the seed feeding

to the picker wheels, and watches other details

of the machine operation.

The task of refilling the seed hoppers is being

mechanized to an increasing degree. Much seed

formerly handled manually in bags is now being

transported to the field in bulk by truck and loaded

into the seed hoppers of the planter in a few minutes by suitable high-capacity elevating conveyors.

Hopper extensions are also added to increase the

hopper capacity so that enough seed can be carried

to plant four rows 4 miles long.

Equipment and methods for hauling seed to the

field and handling it from truck to planter are

illustrated in ARS 42-21, "Mechanical Seed Cutting and Handling of Potiitoes." Copies may be

obtained from : Red River Valley Potato Research

Center, East Grand Forks, Minn.

The two-row assisted-feed planter requires a man

in the rear for each unit, besides the tractor driver,

to see that there is one seed piece in each pocket

of the revolving, horizontal disk before it discliarges the individual seed pieces as it passes

over the dropping tube. Although assisted-feed

planters are simple in construction and their purchase price is low, their low capacity tends to limit

TABLE 11.—Seed potatoes required to plant an

acre at different spacings with seed pieces of

various weights

Seed required per acre when seed

Spacing of rows and pieces weigh an average of—

of seed pieces within

rows (average)

1 iH IH 1% 2

ounce ounces ounces ounces ounces

Rows 30 inches

apart: Cwl. Cwt. Cwt. Cwt. Cwt.

6-inch spacing 21.7 27.2 32. 6 38. 0 43. 6

8-inch spacing 16.3 20. 4 24.5 28.6 32. 6

10-inch spacing 1,3. 1 16.3 19. 6 22. 8 26. 1

12-inch spacing 10. 9 13. 6 16. 3 19. 0 21. 8

14-inch spacing 9. 4 11.7 14. 0 16. 4 18.7

16-inch spacing 8.2 10.2 12. 2 14.3 16.3

Rows 32 inches

apart:

6-inch spacing 20.5 25. 6 30.7 35.9 40.9

8-inch spacing 15.3 18.7 22. 9 26.8 30.6

10-inch spacing 12. 2 15.3 18.4 21. 4 24. 5

12-inch spacing 10. 3 12.8 15.4 17.9 20. 5

14-inch spacing 8.7 10. 9 13. 1 15. 2 17.5

16-inch spacing 7. 7 9.6 11.5 13.4 15. 4

Rows 34 inches

apart:

6-inch spacing 19.2 28.0 28. 8 33. 6 38.4

8-inch spacing 14. 4 18. 0 21. 6 25.2 28.8

10-inch spacing 11.5 14.4 17.3 20. 2 23.0

12-inch spacing— 9. 6 12. 0 14. 4 16.8 19. 2

14 inch spacing 8. 2 10. 3 12. 4 14. 4 16.5

16-inch spacing 7.2 9.0 10.8 12.6 14. 4

Rows 36 inches

apart:

6-inch spacing 18. 1 22. 7 27. 1 31. 7 36. 1

8-inch spacing 13.6 17.0 20.4 23. 8 27.2

10-inch spacing 10. 9 13.6 16.3 19. 0 21. 8

12-inch spacing 9. 1 11. 3 1.3.5 15.8 18. 1

14-inch spacing 7. 7 9. 7 11. 6 13.6 15. 5

16-inch spacing 6.9 8. 5 10.2 11.9 13. 6
COMMERCIAL POTATO PRODUCTION 35

their use to smaller acreages. They are sometimes

used by foundation seed growers for tuber-unit

planting.

Cup-type planters are automatic in operation

and require about the same attention in operation

as picker-type planters. They are well adapted

to use whole seed and for this reason are used

extensively, although not exclusively, in areas

where whole seed is preferred for planting.

Automatic tuber-unit planters are available

commercially. The operation of a two-row tuberunit planter requires a tractor driver and two feeders who place the individual tubers on the fourway knives of the cutter wheels. The four seed

pieces from each tuber are dropped and spaced in

the usual manner, except that a wider space is left

between each group of four seed pieces. On some

tuber-unit planters liquid disinfectant is applied

to the cutting blades in each cycle between tuber

cuts.

Every potato planter should be carefully inspected and put in good, dependable Vorking condition before the planting season begins. Dependable operation not only increases planting efficiency but also avoids delays from breakdowns.

Breakdowns can cause serious trouble, especially

in Northern States where early planting is essential to fully mature the crop. Planting adjustments for proper seed-piece spacing and accurate

fertilizer application should be made in accord

with the variety grown, the climatic conditions,

the type of soil, and the cultural practices to be

followed.

Amount of Seed to Plant

The amount of seed needed to plant 1 acre of

potatoes varies considerably and depends on the

spacing and the size of the seed pieces. The number of seed pieces required to plant an acre of

potatoes at different distances (table 10) sliows

that the closest spacing, 30 by 6 inches, requires ,

34,848 seed pieces to the acre, as compared with

14,520 for a spacing of 36 by 12 inches.

The general practice in the North is to use large

seed pieces. On land well supplied with organic

matter^ moisture, and available plant food, large

seed pieces or whole tubers from 1 to 2 ounces in

weight usually are profitable. The larger the seed

pieces the greater tlie total amount of seed required

per acre. Table 11 shows the weight of seed potatoes required when an acre is planted with any one

of five different weights of seed pieces at given

spacings. Table 11 also emphasizes the extremely

wid& seed-piece requirements—from 6.9 to 43.6

hundredweight depending on spacing and size of

seed piece.

GROWING THE CROP

Cultivation

The objectives in cultivation are to destroy

weeds, aerate the soil, and ridge the rows to provide soil covering for the developing tubers. Cultivation really starts, however, when the soil is

being prepared for planting. Thorough preparation of the land before planting makes later cultivation easier and more effective. One good harrowing before the potatoes come up is worth

several cultivations afterward. A good practice

is to roll the ridges immediately after planting.

This tends to compact the soil about the seed

pieces, hastens germination, and makes the work

of the smoothing harrow and weeder more effective. The use of a weeder lengthwise on the rows

once or twice is very effective in controlling weeds,

especially when the potato plants are small.

Both level and ridge cultivations are practiced

in the areas producing late potatoes. In the level

system of cultivation, use the weeder or the

smoothing harrow once or twice just before the

plants emerge. The teeth of the harrow are angled

back to avoid breaking off the young potato

sprouts. As soon as the plants can be seen in the

rows, cultivate deeply in the middles between the

rows but shallow close to the plants. For two

successive cultivations, use cultivator wings or hilling attachments in order to forra a small ridge

2 or 3 inches high close to the plants. Make later

cultivations, if necessary, to control weeds, but

take great care to avoid breaking many potato

roots while cultivating, as this may reduce the

yield.

In the row-ridge system, use the cultivator to

loosen the soil and to destroy weeds between the

rows. After each cultivation use disks or hilling

attachments to form a ridge 6 to 8 inches high.

With modern equipment, cultivation and hilling

are done in one operation. It is important to

commence ridging early with varieties like Katahdin, which form tubers shallower than some other

varieties and produce abundant low-growing vines.

In the early cultivation the potato sprouts or

young plants are commonly buried once or twice

to smother small weeds near the plants and to

protect the young potato plants from late killing

frosts. However, do not cover the plants too

deeply, as this may reduce yields.

Discontinue cultivation as soon as the plants

come into full bloom or when the vines meet between the rows. If weeds persist after the plants

have reached this stage and tlie rows are ridged,

use a "spade"' with adjustable wings or other adjustable hilling attachments. The spade removes

the weeds from the middle between the rows and
m AGR. HANDBOOK 267, U.S. DEPT. AGRICULTURE

the sides of the ridges, and it disturbs very little

soil close to tlie plants.

Irrigation—Humid Areas

Í Unquestionably, the amount and distribution

of rainfall are the most important factors in potato production over which the potato grower has

no direct control. In some seasons, insufficient

rainfall or uneven distribution fjreatly reduces

late-i)otato yields, even tliougli seed, fertilizer, and

cultural operations have been tlie best. ^ For tliis

reason increasing numbers of table-stock and seedpotato growers are depending on supplemental

irrigation systems to insure against crop losses

from drought.

i Irrigation will increase yields and quality when-

/-^ever the soil moisture falls below 50 percent of

tlie soil water-holding capacity. Great care

must be taken to avoid supplying more irrigation water tlian needed to bring tlie soil only to its

optimum moisture capacity. Overirrigation not

(only maj^ cause unnecessary leachingTrofitlsoTnay

increase incidence of potato diseases, which could

result in losses more sévet-erttetir-the losses by

drought. There is always danger in humid areas

tiiat heavy rains may follow a normal ii-rigation

to cause the soils to become supersaturated.

In the Northeast and North Central potato

areas, overhead sprinkler-type irrigation is most

common. Lightweight seamless aluminum pipe

with quick couplers is used for movable main

lines and laterals.

The questions relating to installation of an irrigation system are too numerous to be dealt with

liere, but if the grower considering such a move

has an adequate supply of water available, mucli

of his problem is solved. Kind of soil, slope of

land, value of crop, acreage, engineering details,

and other points will need to be considered. Tlie

grower should consult the engineering start' of liis

agricultural college, and also find out what his

iieigiibors who have installed irrigation systems

have learned firsthand about irrigation.

Furrow Irrigation—Dryland Areas

Furrows for irrigating potato fields are usually

run between rows or pairs or rows (fig. 28). The

kind of furrow to use depends on the type of soil,

the row length, and the slope of the land. Ordinarily, the furrows are made with two-row cultivators on which the shovels have been replaced

with special ditchers. On flat land with heavy

soil, the irrigation furrows must be deep and narrow, so that the water will be applied to the soil

below the tubers and will not wet the ridge tops

before it has reached the far ends of the rows. If

deep furrows are used, the soil in the potato rows

is less likely to pack. On a steep slope or in soil

that washes badly, small furrows sliould be used.

FiGuiiE 28.—Furrow irrigation of potatoes. This method

has been used tor many years in the western potatoproducing areas.

To prevent washing, only small streams of water

should be applied on steep slopes. Length of run

should be governed by the rate at which the soil

absorbs water. As a general rule, it is best to avoid

runs of more than 600 or 700 feet. Longer runs

would waste too much water and cause excessive

wetness.

Tlie usual method of applying water for row

irrigation is to run a small head in each furrow by

making a cut into the bank of the liead ditch. One

cut may serve either for a single row or for several

rows. Instead of cutting into tlie bank, many

growers take water out of the head ditch through

horizontal tubes or siphon it over tlie bank. An

adjustable canvas dam may be used in a head ditch

to check the flow and thus raise the water level.

Two or three such dams are sometimes used in a

relatively short lengtli of ditch to obtain even distribution of water in the furrows.

The yield and quality of potatoes grown under

irrigation depend largely on how the water is used.

Iirigation does not lower the quality of the crop

if the water is used correctly. In some parts of

the "West, Í5 or 4 applications of water may be sufficient to grow a crop; in others, 5, 6, or even 10
COMMERCIAL POTATO PRODUCTION 37

applications may be necessary. The nunibet- of

irrigations needed varies from year to year as rainfall and other natural conditions vai-y. In a 14-

year study at Greeley, Colo., consistently larger

yields were obtained where the soil moisture was

favorable for vigorous plant growth until tlie

tubers reached full size. Frequent light applications (2 to 4 acre-inches) of water gave better

results than infrequent heavy ones (4 to 5 acreinches) .

I5ecause of differences in water-holding capacity of soils and variations in temperature, rainfall,

and other natural conditions, it is impossible to seti

definite rules as to the time to begin irrigating, the

number of irrigations, or the time to stop irrigating. Tliis general rule should be followed : Apply

the first water whene\-er tlie plants seem to need it

in order to maintain vigorous growth, and thereafter keep the soil moist until the tubers have

reached full size. Ideally, all the soil except the

tops of the ridges should be kept moist all the time.

On the peat lands of the delta area at the confluence of tlie San Joaquin and Sacramento Rivers,

in California, irrigation ditclies 60 to 75 feet apart

and 24 to 30 inches deep are cut between rows of

potatoes. The head ditch connecting these ditches

receives its water supply through a head gate or

siphon direct from the river or canal. The soil is

open and porous, so that a higher water level in the

ditches also raises the water table throughout the

irrigated area. When a tract of land has received

the right amount of water, the supply is cut off

and tlie water level is lowered by pumping the excess water back into the river. Some potato lands

in the San Luis Valley of Colorado, and in the

Egin Bench district of Fremont and Madison

Counties in eastern Idaho, are ii'rig-ated by raising

and lowering the water table in much the same

way.

Sometimes when snowfall or spring rain has

been insufficient, or when there Jiave been drying

winds or continued hot weather, or both, it is necessary to irrigate before planting potatoes or to

"irrigate up"' the crop (fig. 29). "Irrigating up"

means applying water immediately after planting.

If spring plowing has been delayed, the land may

be irrigated before it is plowed. When alfalfa sod

has been crowned in the fall (the tops of the plants

have been cut off by very shallow plowing) and the

land has been plowed early in the spring, it is not

FiGUBE 29.- "Irrigatiug up" a field of potatoes in Colorado. The field is irrigated immediately after the seed has

been planted.
88 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

practical to irrigate by flooding so the land may

be furrowed, or ditched, and irrigated before being

planted. Irrigating in the fall or in the early

spring before plantnig, when water is available, is

a good practice, preferable to irrigating immediately after planting.

"Irrigating up" must be done very carefully,

especially in hot weather, or it is likely to cause

seed-piece decay. Good germination and little decay of seed pieces can result from "irrigating up"

if the water is applied by the furrow method and

moisture is allowed to reach only the seed pieces

or sprouts, without wetting the soil above the seed

pieces. This practice should be used only if the

soil lacks enough moisture for seed germination.

Overhead Irrigation

The use of portable, overhead sprinkler irrigation on potatoes has steadily increased in the intermediate- and late-crop sections of the Eastern and

Midwestern States. More than 80 percent of all

irrigation in 28 Eastern States is now done by

portable sprinklers. There is also a growing preference for portable, overhead sprinkler irrigation

in Southern and Western States, wherever this

method can be used. Overliead irrigation of several kinds may be used effectively on potatoes, but

all are the portable type, except on some farms

where permanent lieader pipes are installed beside

each field. Water is generally pumped from wells,

streams, and ponds and supplied directly to the

system at pressures required to operate the different types of sprinklers used.

With potatoes, the type of overhead irrigation

installed is not nearly so important as the manner

in which it is used. To obtain maximum benefits,

exacting decisions must be made as to the amount

of water to apply and the time to apply it. These

decisions and the results obtained are influenced

to a great extent by the amount and distribution

of ramfall each season. With very little to guide

him but experience, the potato grower must regulate the amount and timing of supplementary

water applied to meet the exacting requirements

of potatoes for uniform but not excessive moisture

during the growing season. If potatoes are irrigated after the soil moisture has fallen below 50

or 60 percent of the soil water-holding capacity,

there is generally a risk of causing second growtli

•in the tubers. Conversely, soil moisture in excess

of optimum is liable to reduce both yield and

quality of the crop. Although scientific facts

underlie much of the available information on

overhead irrigation of potatoes, successful operation still remains as much an art as a science.

The cost of overhead irrigation equipment and

its seasonal operation varies between farms, but

these financial considerations and other incidental

expenses determine the increase in yield required,

at current potato prices, before tlie investment can

be justified. Average increases of from 45 to 85

busliels per acre have been reported from overhead

irrigation over an 8-year period, and tliis would

seem an adequate return from a nominal investment. When used properly to supplement natural

rainfall, overliead irrigation can be a profitable

investment for any potato grower with a suitable

source of water on his farm.

Overhead irrigation results in greater incidence

of foliar diseases than does furrow irrigation.

Spores of disease organisms causing sucli diseases

as early and late blight are rapidly disseminated

by splashing droplets of water. In addition, the

wet leaf surfaces provide ideal conditions for primary and secondary infection, conditions not

usually found in furrow irrigation. The widespread use of overhead irrigation in the Northwest, therefore, creates conditions for potential

losses by leaf diseases which in the past have been

almost nonexistent in the irrigated areas of the

West.

Hail Injury

Hailstorms during the growing season may

seriously damage potato plants (fig. 30). A fall

of hail, generally accompanied by wind and heavy

rain, often defoliates the plants and breaks their

stems. Potato plants badly injured by hail may

still produce a good yield if sufficient time remains

during the growing season and conditions are

favorable for recovery of vine growth and tuber

development. Stem wounds caused by hail heal

quickly, and stems that have been beaten to the

ground gradually become erect and develop new

branches and leaves. Healthy, vigorous plants

with well-developed root systems have a much

better chance to recover than weak or diseased

ones. Plants that are destroyed down to ground

level have little chance to recover; generally their

underground stems rot and further growth is impossible. Hail-damaged potatoes are readily susceptible to infection by a few disease organisms

that normally are not serious on potato vines.

When plants have been injured by hail but not

completely destroyed, it is advisable to put the

soil in good physical condition immediately, if this

can be done without further injuring the plants.

Other cultivation should be delayed until the

plants become more erect. After that, regular

cultural practices should be followed.

Plants that are badly injured by hail after the

tubers have formed must develop new tops before

the tubers can make any further growth. If the

tubers are only partly developed at the time of

injury, they may mature without growing any

larger, or may develop knobs known as second

growth (discussed later), or they may send out

sprouts or stolons. Generally, the best potatoes

produced by hail-injured plants come from small
COMMERCIAL POTATO PRODUCTION 39

FIGURE 30.—Hail injury to a field of potatoes.

tubers that normaly would not develop. An

early crop has a better chance of good recovery

after hail injuiy tlian a late one; the late one may

not have sufficient time to develop fully before

frost. If hail injury occurs near the normal

harvest period, the crop usually should be

harvested early.

Spraying and Dusting

To control insect pests and leaf diseases such

as late bliglit and early blight, spray or dust the

plants with insecticides and fungicides, or a combination of both. Good control of insects can be

obtained with either a dust or a spray, if properly

applied; however, a spray is much more effective

for controlling late blight.

Sprays tend to drift less than dusts and may be

applied more satisfactorily on windy days. With

a well-adjusted spray boom, adequate spray coverage can be obtained in winds up to 7 or 8 miles

per hour.

Dusts are sometimes preferred where the water

supply for sprays is not convenient. Dusting

equipment is also light in weight and causes little

compaction of the soil. When the wind velocity

exceeds 3 miles per hour, the drift of dusts can be

reduced by attaching to the duster boom a trailing

apron 8 to 15 feet long made of 9-ounce cotton

ducking. With the duster moving 3 to 4 miles per

liour, the dust is conñned long enough to allow

good foliage coverage, even in winds up to 7 or 8

miles per hour. The apron will remain spread

out over the plants if its outer rim is weighted

with a rope. Dust coverage is also affected by the

velocity of the dust leaving the nozzle oritíce, the

wind velocity of the air around the plants, and

whether there is a brief period when the expelled

dust is confined around the plants. With most

row-crop dusters, at least two nozzles per row are

needed, preferably three : one on each side of the

row and one above.

With both sprays and dusts, adequate coverage

of the foliage is necessary for good insect or disease control. For most insects this means coverhig both the upper and lower surfaces of the leaves.

Thoroughness of spray coverage depends largely

on the number, kind, and placement of the nozzles,

the speed the sprayer travels, and the pressure.

The sprayer should be equipped with three or four

nozzles per row, preferably the hollow- or solidcone type rather than the flat, fan-type nozzle.

Two nozzles should be placed on drop pipes, or on

trailing arms that regulate their position with one

nozzle on each side of the row between the plants.

Both of these nozzles should be directed sliglitly

forward so as to cover foliage of the lower half of

the plants. Direct one or two of the upper nozzles

downward and slightly forward to spray the upper

half of the plants. If there are two upper nozzles,

place one on each side of the row. To get best

spray coverage, raise the spray boom to increase
40 AGR. HANDBOOK 26 7, U.S. DEPT. AGRICULTURE

the height of the nozzles as the phiiits grow.

AVhen the boom is raised, it may be necessary to

substitute longer drop pipes.

The amount of spray or dust for adequate coverage increases as the plants grow. Some pesticide

can be saved if the application rate is low at first

and then increased as the plants become larger.

However, any saving may be offset by the time required to make the necessary adjustments. Instead

of adjusting the rate of application as the plants

grow, most growers prefer to use the same rate

required for large plants all season. With ground

equipment, use 25 to 35 pounds of dust per acre;

or if spraying, use 75 to 150 gallons at 200 to 400

pounds' pressure per square inch, or 20 to 40 gallons at 40 to 60 pounds' pressure. With aircraft,

by using nozzles with a small orifice and 50 to 100

pounds' pressure, 20 to 40 gallons per acre may be

applied. With aircraft, use Í55 pounds of dust

or 5 to 8 gallons of spray per acre early in the season and 9 to 12 gallons during the latter part.

Thorough coverage with insecticides or fungicides

by aircraft is difficult.

Carefully adjust the machinery to deliver the

proper amount of spray or dust, and check adjustment each ttme before use to insure proper application rates throughout the season. Also check,

clean, and adjust tlie nozzles frequently. Make

necessary corrections or replacements as soon as

you detect any malfunctioning of the equipment.

Use prepared spray concentrates or dust mixtures of fungicides and insecticides in preference

to home mixes because of safety factors. Reliable

commercial formulators usually furnisli a finished

product of uniform quality that costs less than a

home mix. Be Sure You Fully Understand

the Manufacturer's Directions for Use. They

Are Usually Printed on the Label of the

Container.

The proportion of active ingredient in stock

materials is shown on the container label as the

percentage of total weight, but the active ingredient in emulsifiable concentrates is generally

stated as pounds per gallon. Use this information

to determine the proper amount of active ingredient needed per acre. To prepare a spray, fill

the spray tank with a measured amount of water

and add to it the amount of emulsion concentrate

or wettable powder required to supply the desired

concentration of active ingredient. Tlioroughly

mix. Apply the finished spray at a rate sufficient

to supply the amount of active ingredient recommended per acre. The active ingredient content of dusts and granules is also used in

computing application rates per acre.

Properly timed applications of sprays and dusts

not only increase their efficiency against pests but

also save time, effort, and money. For best control of insects and diseases, start applications each

year before damage occurs. Start applying fungicide to control late blight when the plants are 4

to 6 inches high. For greatest protection against

aphids and flea beetles, start insecticide applications when about one-fourth of the stand of potato

plants has emerged. Until danger of late blight

lias passed, fungicide should be applied weekly to

protect the new growth and to renew the fungicide

on the old growth. Most of the newer organic

fungicides for late blight should be applied about

every 5 days during an epidemic, or when tlie

weather is favorable for an outbreak of tlie disease. For specific advice about disease and insect

control, consult your agricultural extension specialist, county agent, or other local agricultural

authority.

How often to apply insecticides for best insect

control depends on the degree of control needed

and also on the number of pests remaining on the

plants-after each application. Since relatively little additional cost is involved, some growers, to be

safe, include an insecticide with each weekly application of fungicide. Outbreaks of many insect

pests are difficult to forecast, so it will pay to examine carefully the leaves and tops of the plants

in your potato fields for insect buildup. Be prepared to apply insecticide at the first sign of damaging numbers of the insects on the plants.

A convenient reference list on potato diseases,

their occurrence and control, is given in table. 12.

Insect control information is available in USDA

Agriculture Handbook 264, "Potato Insecfs."

Market diseases of potatoes are described in

USDA Miscellaneous Publication 98, "Market

Diseases of Fruits and Vegetables: Potatoes."

FIGURE 31.— I'otato tubor cut in half showing internal

sprouting, a physiological abnormality. In this unusual

case the sprout has tubcrized.
COMMERCIAL POTATO PRODUCTION "i =

TABLE 12.—Reference table of some potato diseases

«

Name Causal organism Part of plant

affected

Mode of

transmission

Seasonal

carryover

Control

Bacterial brown Bacterium (Pseudomonas solan acearum).

Bacterium {Erwinia

rarotovora).

Bacteria and fungi

Physiologic

Bacterium (Erwinia

phytophthora).

Tubers and

soil.

Tubers

do

do

Tubers and

foliage.

Soil Soil

rot.

Bacterial soft rot__.

Bacterial tuV)er rot.

Blackheart

Blackleg

Soil and

contact.

Soil and wash

water.

None

Soil, seed, and

seed-corn

maggot.

do

do

None

Soil, seed, and

seed-corn

maggot.

resistant varieties

(Sebago, Katahdin).

Allow tubers to suberize

after harvest; do not

wash tubers, or dry 4

minutes at 150° F.

after washing.

Same as for l)acterial

soft rot.

Store at temperatures

below 70° F. with

proper ventilation.

Use disease-free seed;

prevent wovmding in

tillage; control seedcorn maggot; rotate

crops; suberize seed

Calico « _ - Virus (alfalfa

mosaic).

Virus (tobacco

rattle).

Virus (beet curly

top).

Vines and

tubers.

Tubers

Vines

Aphids and

seeds.

Nematodes

Insects (beet

leafhopper).

Seed and

other crops.

Soil

Eliminate "volunteer"

alfalfa plants; use

disease-free seed;

control aphids.

Corky ring spot Avoid infested fields* use

Curly top Weed hosts

resistant varieties;

control nematodes.

Avoid beet leafhopper

areas; use disease-free

seed; control leafDry rot. . . Fungus (Fusarium

spp).

Fungus (Alternaria

solani).

Physiologic

Tubers

Vines and

tubers.

Tubers

Vines and

tubers.

Tubers

Vines

Soil Soil

hoppers.

Prevent wounding at

harvest; allow tubers

to suberize and then

store at 40° to 50° F.

Spray with organic

dithiocarbamate fungicides or bordeaux

mixture; practice crop

rotation and sanitation.

Promote soil aeration

and drainage.

Avoid infested fields; use

disease-free seed ;

practice crop rotation.

Early blight Air Soil; plant

debris.

None

Soil and seedNone

Weed hosts

and seed.

Enlarged lenticel Not transmitted.

Soil and seedNot transmitted.

Insects and

seed.

Fusarium wilt

Growth cracks. _

Fimgus (Fusarium

solani var.

einiiartii and F.

ox\jsporum).

Plivsiologic .

Haywire- See purple-top wilt..

Physiologic -

keep soil moisture optimum and uniform.

Heat necrosis..

Heat sprout

Hollow heart

Internal black spot

Genetic and

physiologic.

do

Physiologic

Tubers

do

do

Not transmitted.

do

Not known

None

do

Not known

provide adequate

moisture; harvest early.

Use resistant varieties.

Space plants closer in rows;

use resistant varieties.

Reduce harvest and grading bruises; prevent potassium deficiency; keep

soil moist prior to harvest.

Use nonsusceptible varieties; provide adequate

soil moisture.

Internal brown

spot.

do do Not transmitted.

None
AGR. HANDBOOK 267, U.S. DEFT. AGRICULTURE

TABLE 12.—Reference table of some -potato diseases—Continued

Name Causal organism Part of plant Mode of Seasonal Control

affected transmission carryover

Internal sprouting. _ Physiologic Tubers Not trans- None Do not store at high temmitted. peratures. Probably

caused by interaction of

various storage conditions (fig. 31).

Jelly-end rot Fungus {Fusarium Tubers (espe- Soil Soil Provide uniform and adespp.). cially long

ones).

quate soil moisture continuously through season

from 30-40 days after

planting.

Late blight Fungus Use resistant varieties; (Phytoph- Vines and Air and seed__ Seed; cull

Ihora infeslam). tubers. piles. spray with dithiocarbamato fungicides or

:>.•/*

.^.:..

bordeaux mixture; spray

cull piles with herbicides or eliminate entirely; kill potato foliage

: !. 10 days to 2 weeks before digging.

Latent mosaic Virus (potato virus

X).

Vines Mechanical Seed Use resistant varieties or

disease-free seed.

Leaf roll Virus (potato leaf

roll).

Viníís and Aphids and

seed.

Seed (minor

weed hosts).

Use disease-free seed; contubers. trol aphids; use resistant varieties; rogue diseased plants; isolate from

other potatoes; market

potatoes early.

Leak Fungus (Pythium Tubers Soil Soil Prevent harvest injury ;

debaryanum). cool tubers to 40°-45°

F. after harvest.

Mahogany brown- Physiologic do Not trans- None Use resistant varieties;

store at 40° F. or above. ing. mitted.

Mild mosaic Virus (potato virus Vines and Aphid and Seed I SO flisoaso-froe CIPPH nr

A+X). tubers. seed. immune varieties; control aphids; rogue dis-

(iased plants early.

Nematode (.eel- Nematodes: Roots Soil and Soil, seed, and Fumigate soil with nemaworm) diseases. Golden (Heterodera roslochiensis).

tubers. equipment. tocides; rotate crops or

fallow fields; prevent

movement of material

Potato tubor Tubers Drainage from infested fields; use

. - , ;;-,■■ rot (Diiylenchiis water. nematode-free seed; use

destructor).

resistant varieties. For

Root knot Tubers and golden nematodes, ob-

{Meloidogyne roots. serve quarantine regulaspp.). tions.

Root lesion, or Tubers

meadow

{Pratylenchïis

spp.).

Net necrosis See leaf roll do Not known Seed and

culture.

Psyllid yellows Insect {Paratrioza Vines and Insect Seed and Control psyllids.

cockerelli). tubers. (Psyllids). weed hosts.

Purple-top wilt \'irus (aster yellows) _ do Insects (leaf- Crop and Control leafhoppers; ro-

(haywire). hoppers) . weed hosts. tate susceptible perennial crops.

Rhizoctonia Fungus {Rhizoctonia Stems and Soil Soil and seed. . Disinfect seed tubers;

solani). tubers. avoid planting in cold

soils; rotate crops; use

disease-free seed.

Ring rot Bacterium (Corynebacterium sepeVines and Mechanical Sped find Plant disease-free seed;

tubers. use whole seed or disin-

.i-T-Ll^ V^l.C4f AAÂ VC4li( ^ i» ^

storage.

donicum). fect cut seed pieces;

disinfect storages and

equipment; use resistant

varieties.
;.; COMMERCIAL POTATO PRODUCTION .

TABLE 12.—Reference table of sotne potato diseases—Continued

u

Name Causal organism Part of plant Mode of Seasonal Control

affected transmission carryover

Rugose mosaic \'irus (potato virus Vines Aphidsandseed- Seed (minor Plant disease-free seed; Y + X). weed liosts). control aphids; use resistant varieties; rogue

diseased plants early.

Scab, common Fungus {Strepto- Tubers Soil

myces scabies). nisistant varieties; rotate crops; disinfect

s(K>d ; acidify soil or

treat with soil fungicide

(pentachloronitrobenzene).

Avoid infested fields; use Scab, powdery Fungus {Spongospora do do do

subterránea). disease-free seed; rotate

crops; plant where average soi temperature exceeds 57° F.

Second growth Genetic and physi- do Not transmit- None Provide adequate and uniologic. ted. form soil moisture; use

resistant varieties.

Silver scurf. Fungus {Spondyloc- do Soil Use disease-free seed; pracladium atrovirens). tice crop rotation; store

at 40° F.

Southern blight Fungus {Sclerotium

rolfsii).

Vines and

tubers.

do Soil Avoid infested fields* treat

soil with pentachloronitrobenzene ; practice deep

plowing where feasible;

use disease-free seed.

Spindle tuber Virus (potato spindle tuber).

do Seed and mechanical.

Seed-.

duce machinery use in

fields after plants

Spindling sprout See purplo-top and Tubers Seed Probably

leaf roll. aphids.

Stom-ond brown- Unknown do Not trans- None Use resistant varieties;

Ulg. mitted. store at normal cool

Verticillium wilt Fungus {Verticillium Vines and Soil and drain- Soil and

storage temperatures.

Fumigate infested soil; use

albo-atrurn). tubers. age water. seed. resistant varieties; practice long rotations; plant

disease-free seed. Wart Fungus {Synchylrium

endobioticum).

Tubers Soil Soil Do not plant in infested

fields; areas known to

have been infested in

Maryland, Pennsvlvania, and West Virginia

. ,

are under strict quarantine. Resistant varieties are available. Yellow dwarf Virus (potato yellow Vines and Seed, leaf- Seed and Use disease-free seed; condwarf). tubers. hoppers, and

mechanical.

clovers. trol leafhoppers; rotate

clover fields.

Weed Control'

If good cultural practices are followed in growing potatoes, weeds are not often a serious problem. Use of a weeder when the crop is ñrst

emerging controls most early-season weeds; the

usual cultivations along with the shading provided by the vines normally control weeds later

' Prepared in cooperation with Dr. L. L. Danielson,

Crops Protection Research Branch, Crops Research Division, Agricultural Research Service.

in the season. Weeds that emerge following the

last cultivation are a serious problem in some areas

of production because they reduce the efficiency

of mechanical harvesting.

Perennial grasses and sedges including bermudagrass, nutgrass, and quackgrass cannot be completely controlled by cultivations (fig. 32). They

compete with the potato crop for nutrients, moisture, and light. They also cause severe quality

losses by rhizome and root penetration of the potato tubers. Some perennial grasses and sedge
Ww AGR. HANDBOOK 26 7, U.S. DEPT. AGRICULTURE

FIGURE 32.—^Potato tubers damaged by nutgrass. Nutgrass is the iiKist serious weed in some potato areas, especiall.v along the Eastern .seal)oard. Nutgrass is very

difficult fo control and persists in the soil for long

I)eriods. The arrow points to a "nut" imbedded in

the tuber at the right.

can be controlled by applying herbicides to the

soil before planting or during the early stages

of growth.

Directions for safe use of herbicides are

given on the manufacturer's label and should

be carefully read and followed. These directions give the correct time, rate, and method of

application, crops on which use is approved,

and precautions for the user. When in doubt

about correct use, consult your State experiment station weed control specialist.

A number of early-season annual weeds can be

controlled by herbicides applied immediately after

planting and before tlie potatoes come up.

Where late-season annual weeds are a problem,

herbicides may be applied immediately after tlie

last cultivation to prevent further germination

of weed seeds.

Herbicides for use on potatoes are given in

table 13. The choice of herbicide depends on the

weeds to be controlled and the specific- recommendations of the local State experiment station.

TABLE 13.—Herbicides for potatoes

Common name '

DATC, CP15336.

CDAA

I^alapon.

Diuron..

DNBP.

EPTC.

PCPPetroleum solvent.

Scsone

2,4-1:)

2,4-DEP

Chemical name

2,3-dichloroallyl diisopropylthiolcarbamate.

2-chloro-A^, A^-diallylacetamide.

2,2-díchloropropíonic acid

3-(3,4-d ichloropheny1) -

1,1-dimothylurea.

4,6-dinitro-o-sec-butylphenol.

Ethyl A',A'-dî-w-propylthiolcarbamatCi

PentachlorophenoL

Sodium 2,4-dichlorophenoxyethyl sulfate.

2,4-dichlorophcnoxyacetic

acid.

tris-(2,4-dichlorophenoxyethyl) phosphite.

Time and method

of application

Preplanting soil incorporation.

Immediately after

lay-by.

Preplanting, lay-by. .

Preemergence

-do.

Preplanting soil incorporation, layby soil incorporation.

Preemergence

do.

Preemergence, lay-by.

Preemergence

Lay-by

Type of weed

Wild oats.

Annual broadleaved

weeds and weed

grasses.

Annual and perennial

weed grasses.

Annuals broadleaved

weeds and weed

grasses.

do

Annual broadleaved

weeds and annual

and perennial weed

grasses.

Annuals broadleaved

weeds and weed

grasses.

do

-do.

Annual broadleaved

weeds.

do

Growth stage of weed

Germination.

Do.

Active growth only.

Germination and young

seedlings.

Germination.

Seed germination and

rhizome sprouting.

Germination and seedling stage.

Young emerged seedlings.

Germination.

Young emerged seedlings.

Germination.

' Taken from Nomenclature of Committee Report of the Weed Society of America, 1962.

HARVESTING THE CROP

Vine KiHing

Now that insecticides are widely used to control insects and better and less toxic fungicides

are used to control blights of the potato, the leaf

injury formerly caused by insects and fungus

diseases is reduced to a mmimum. Under these

conditions, maturity of the vines is delayed and

unless the green plants are killed by early frosts,

it becomes necessary to destroy the vines by other
COMMERCIAL POTATO PRODUCTION 45

means in order to harvest the crop. This can

be done by mechanical or chemical means or by

a combination of the two methods. Machines

such as beaters with rubber flails or chains are

extensively used and do a good job of destroying

the vines, but they cover only two rows at a time

and are relatively expensive (fig. 33). Certain

chemical sprays such as dinitro compounds and

arsenites can be applied with regular 4- to 12-

row spray equipment, preferably fitted witli fantype nozzles. A good vine kill can be obtained

with chemicals in 4 to 10 days, depending on the

weather after application. If the vines are infected with late blight, spraying with a contact

chemical vine killer is preferable, especially if

the crop is to be stored, because these chemicals

lessen the danger of late bliçrht tuber rot. For

mechanical harvesting operations, vines are often

cut even after they have been killed by frosts or

chemicals (fig. 84.).

1'ua ¡IK :',:',.—Oiu' type of boater used tor deslroyiiij:

potato vines.

FiGL'UE 34.—\iiie cutter in operation after vines have

died. Vine cutting is essential in mechanized harvesting.

Certain dusts, including anhydrous copper sulfate and calcium cyaiiamide, have been used to a

limited extent as vine killers, but they are usually

\

slower acting and less effective than sprays and

mechanical methods. Although all methods of

vine killing at times tend to produce a definite

type of flesh discoloration in the tuber, this trouble has so far had little retarding effect on the

growing use of vine-killing methods on late potatoes. Vine killing is regarded favorably by

many growers, because it gives them a means of

artificially hastening the maturity of their crop

whenever, for any reason, they decide to dig it

Season of Harvest

Thejmiper time for harvesting potatoes is determined chiefly by the maturity of the crop. Actually, the decision as to when to dig is often

influenced by such considerations as market prospects, availability of help, and weather. The/

principal reason for waiting until the crop is fully(

matured before digging is that the skin of tlie\

tubers "sets" after the vines die and then there is \

less likelihood of skinning and bruising. Even

\yhen digging a mature crop, however, precau- I

tions sliould be taken to avoid skinning and bruis- /

ing by padding the rough {¡laces on the digger I

and by handling the harvested potatoes carefully.,^

If late potatoes are dug while tlie vines are green,

besides the chances of skinning and bruising, tliere

is danger of transferring spores of the late blight

fungus from the vines to the tubers" where fhey

can later produce rot in storage. ' In iJae-^n'oduction of certified seed potatoes, however, early

harvesting has been found desirable in order to

avoid current-season infection with the virus disease leaf roll, which is transmitted by certain N

insects. In general, early harvesting reduces the )

yield, but this loss may often be offset by other (

advantages—disease-free seed and desirable tuber )

size. ^

The usual harvesting seasons are as follows:

Winter, January through March; early spring,

April through the first half of May; late spring,

second half of May and June; early summer, June

through first half of August; late summer, second

half of August and September; fall, October

through December. The beginning and end of

these harvesting seasons are not fixed dates but

approximations, and overlapping occurs.^ Generally, the marketing season follows close to the

harvesting season, excej)t for the fall crop where

marketing from storage extends into the first 6

months of the following year.

Harvesting Machinery

The crop is harvested with a wide variety of

equipment and methods in different parts of the

country. The type of equipment and predominant

harvesting methods vary greatly with the size of

individual operation, local topography, soil and
46 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

field characteristics, labor supply, and market disposition of the crop. Harvesting equipment used

by small operators generally consists of one-row

or two-row elevator-type diggers and flat-rack

trucks for hauling the potatoes in bags, barrels,

boxes, or crates filled by handpickers following

the digger.

The digger aprons are usually driven from the

power-takeotf of the tractor, sometimes through

a three-speed transmission used to change the

speed of the apron relative to digger speed whenever the operation of tlie tractor is adjusted for

different field conditions. Potato diggers powered

independently by 1-, 2-, and 4-cylinder engines

equipped with a clutch and sometimes a 3-speed

gear set are common in some areas. Certain advantages are obtained through independent control of tractor speed and digger-apron speed whenever digging conditions vary considerably.

Wlien properly operated, the digger should

convey enough soil to cushion the potatoes against

bruising for three-fourtlis or more of the digger

apron. If the digger apron does not retain sufficient soil because it is sandy or too dry, change

the agitator sprockets to reduce agitation or substitute idler rollers and reduce apron speed.

When the soil is very dry, irrigating the field

several days in advance of digging usually provides a satisfactory soil cushion on the apron and

makes digging easier and faster.

During the last decade great interest has been

shown in the design and construction of various

types of potato harvesters. Potato growers themselves have devised and built many of these machines. Mechanical harvesters are now being used

to harvest 80 to 95 percent of the crop in some of

the important producing areas where conditions

are favorable for their use. The machines represent a great variety of designs, special features,

optional accessories, and specifications, all of

which are rapidly evolving and changing from

year to year.

A few have ingenious devices for removing

potato vines and stones. Some harvesters consist of supplementary equipment that attaches to

conventional diggers; some are more elaborate

self-propelled outfits that permit workers on the

niacliine to grade and bag the potatoes; and other

macliines deliver tlie potatoes in bulk into a truck.

.Potato harvesters are coming into wider use in

sections where there are few stones and long level

fields.

Various types of bag loaders also have been

devised. Some are used for loading in the field

and may also be used in the storage house.

All these laborsaving devices have reduced

tremendously the extremely heavy work and the

time formerly required to harvest potatoes.

Avoiding JMeclianical Bruises at Harvest

Methods of liandling potatoes after they have

been dug vary with the locality. In Maine, the

tubers are picked up in baskets, from which they

are poured into barrels and hauled to the storage

house where they are stored in bulk, the empty

barrels being returned to the field. In many sections of the Northeast the potatoes are picked up

in slatted crates, hauled to the storage house, and

either stored in the crates or stored loose in bins.

On Long Island, N.Y., and in New Jersey the potatoes are hauled to the storage house or grading

shed in burlap bags, except those that are loaded

in bulk from a harvester. Handling potatoes in

crates, however, is perhaps the most widespread

method in the South and Northeast, and to a

lesser extent in the North Central States. Potatoes in crates are usually bruised less than those in

bags or barrels. Careful handling of the tubers

at harvest can pay big dividends when the crop

is sold.

Mechanical harvesting and bulk handling are

preferred practices in most of the western production areas, as well as on the larger potato farms

throughout the rest of the country.

In the mechanical harvesting of potatoes, great

care must be taken to avoid excessive bruising and

injury. Special attention should be given to provide proper adjustments of all conveyors and

drops. Bruises are not always noticeable at harvest, but may show up after storage and result in

heavy losses by lowering the grade and increasing

the nvimber of culls. Considerable knowledge and

practical experience plus constant alertness aiul

skill are required to operate any machine properly

so as to obtain the best possible performance with

minimum damage to the crop. Mechanical harvest injuries result in millions of dollars in losses

annually to potato growers. For a full discussion, see ÜSDA Agriculture Handbook 171, "Increasing Potato-Harvester Efficiency."

A few simple rides will go far in reducing mechanical injury. Set rotobeaters so that flails just

clear the ground ; set digger points deep enough to

carry soil three-quarters of the length of the digger chain ; and maintain a digger groundspeed of

100 to 125 feet per minute. The digger belt sliould

operate at about this same speed, with the agitators set so the tubers do not bounce. Chain agitation should be slowed to a minimum consistent

with good separation of tubers from soil and trash.

A tight digger chain causes leas bruising by whipping than a loose one. Use padded deflectors and,

when feasible, rubber-covered chain links. Avoid

dropping the potatoes more than 5 to 6 inches.
COMMERCIAL POTATO PRODUCTION 47

Washing

Washing potatoes before sliipping them is

widely practiced in most important potatoproducing areas. Washing improves the appearance of tlie tubers and makes grading easier.

In tlie past it \vas customary to run tlie potatoes

tlirougli a soaking tank and then tliroiigli a rinsing

spray. Such wasliing generally left a consideraijle percentage of the tubers infected with a soft

rot organism introduced into the tank water by infested soil clinging to tubers. The potatoes were

readily infected through bruises in their skin.

Spraying jets have now largely replaced the soaking tank, and this change has greatly reduced rot

infection. Some rot still occurs, however, since

the washing does not remove all soil particles infected with rot organisms.

From tlie washer, the potatoes are carried on a

conveyor-belt system to a grader and then past

manual sorters, who remove all decayed or otherwise undesirable tubers.

Altliough the potatoes are not dry when packed,

they generally arrive at market in excellent condition, provided tliey were washed and loaded when

tlie relative Inmiidity of the air was low. It is

unsafe to sliip washed potatoes immediately,

without drying them, if the relative humidity of

the air is above 70 percent. If shipped under

tliese conditions, the potatoes are liable to decay

badly before they reach their destination.

Dry Cooling

Dehydrocooling, a treatment which withdraws

moisture from the air and rapidly moves cold, dry

air through the load, has proved fairly effective

in preventing washed potatoes from decaying in

transit. The benefits are probably due more to

the drying than to cooling.

Drying potatoes with air from a large electric

fan definitely reduces soft rot, but the rapidity

and thoroughness of fan drying are limited by

sucli factors as air temperature, relative humidity,

and size of tubers.

Commercial hot-air potato driers are used to

dry large quantities of potatoes after washing.

Heating the air increases its capacity to absorb and

hold moisture. Potatoes are not damaged when

dried for 4 minutes in air heated to 150° F., and

bacterial soft rot is less likely to develop.

Most commercial potato driers are insulated

chambers heated by a forced blast of liot air

through which the potatoes move slowly. Speed

of travel is adjusted to allow the potatoes about

4 minutes in the drier. Air temperature inside the

drier may also be adjusted to suit variations in air

relative humidity. When the drier is properly adjusted, the temperature of the tubers is increased

only slightly because of the cooling effect of

evaporation.

STORING THE CROP

Practically all the fall crop potatoes that are

stored on farms are produced in the northern half

of the United States. Possibly 75 percent of this

crop is stored in farm and commercial warehouses.

Potato storage designs have been changed in the

past few years to take advantage of modern bulk

liandling methods. Potatoes are still handled in

barrels in Maine, but in some areas most of the

potatoes are handled in bulk trucks (fig. 35).

Hulk trucks are unloaded over mesh belts and the

potatoes are conveyed down into storage bins, often

with less than i/^ of 1 percent of injury. As tlie

draper chain conveyor in the truck unloads the potatoes, it sifts out small clods and soil. In addition, vines and trash are removed manually as the

potatoes are carried into tlie bin over mesh or

draper chain conveyor belts, which also sift out

small clods and soil.

The type of farm storage used in Maine (fig.

36) is often filled 12 to 14 feet deep. This can be

- This section was prepared by A. D. Edgar, agricultural

engineer ; Herbert Findlen, horticulturist ; and P. H.

Heinze, plant physiologist, Agricultural Marketing Service, U.S. Department of Agriculture.

done without danger from heat damage because of

the cool weather at harvesttime and the average

annual soil temperature of approximately 40° F.

The Idaho type of storage (fig. 38) may be filled

only to a depth of 8 to 10 feet because the temperatures at harvesttime and the soil temperatures are

higher. More effective ventilation may be required with deep storage to keep the potatoes

FIGURE 35.—A mechanized potato-harvesting operation.
48 AGR. HANDBOOK 26 7, U.S. DEPT. AGRICULTURE

FIGURE 3(i.—iJeep-bin storage in Maine. Potatoes are unloaded from barrels or from bulk trucks (fig. 37)

standing in the ui)i)er drive alley. I'otatoes may be

removed from storage through the lower cross-alley.

FiouKE 37.—Trash and clods being removed from the

mesh conveyor l)elt. Soil sifts through, as potatoes

are conveyed from bulk trucks into the deep bins (tig.

36) or conveyed into truck-level bins (fig. 38).

below sprouting temperatures. The earth-covered storage (fig. 38) is practical only in the semiarid sections of the West; the insulated roof

storage (fig. 36) is adapted to more humid sections.

Windows are not recommended in potato storages,

because sunlight and artificial light cause greening

of the potatoes. All lights should be used

sparingly. (See '"Tuber Greening," p. 55.)

As potato storages liave increased in size, forcedair circulation and ventilation have become a necessity ; and with electricity and air-inoving equipment now available, forced-air systems are now

practical.

In the cooler part of the fall crop area, forced

shell circulation is adequate for talile- and seedstock storage. Shell circulation may be described

as perimeter air distribution with wall airspaces

that lead air up from the wall-floor ducts between

the insulation and adjoining potatoes. From the

top of the wall airspace, the air returns over the

bins of potatoes to the control center. At the control center, dampers route the air either to be recirculated l)y fan or to be exhausted, as more air

is drawn into the distribution system.

Forced-through circulation is required for the

higher temperature storage of processing potatoes,

and for large masses of table or seed stock in the

warmer parts of the fall crop area. Forcedthrough circulation systems distribute air from the

control center to bin-floor ducts. From these

ducts, it is forced up through the bin between the

l)otatoes to tlie airspace above. From above the

bins, air returns to the control center as in the

shell circulation systems.

Ventilation is required to remove potato heat

in the fall, but little or no ventilation is required

for this purpose in the winter. However, there is

usually enough ventilation during the period of

temperature regulation in the fall to remove excess

water vapor and prevent condensation and dripping. Some ventilation is always needed in winter

to remove the water vapor produced by the potatoes and to prevent ceiling condensation and dripping. Heat is sometimes required in extremely

cold weather and in a partly filled storage in average winter weather. The more abovegrade wall

and ceiling insulation used, the higher the storage

humidity possible before it is limited, in cold

weather, by condensation and dripping; and tho

higher the humidity, the lower the potato shrinkage. More ventilation is needed, therefore, in

poorly insulated than in well-insulated storage to

remove water vapor before it condenses and wets

the potatoes. Tlie amount of insulation needed

varies with the ratio of exposed (abovegrade) wall

and ceiling area to the usual amount of potatoes

in storage during cold weather. A well-designed

storage when full will have about 0.35 square foot

of exposed wall and ceiling area per hundredweight of potatoes. If a storage is one-third full

in cold weather, the ratio is equivalent to 3 times

0.35, or about 1 square foot per hundredweight.

Usually a 6- to 8-inch mineral wool or fiber glass

insulation is practical in colder climates, 4 inches

in moderate climates, and less in warmer parts of

the fall crop area. To be fullv efl^ective, the insulation should have a vapor barrier between it

and the warm, relatively moist storage. A 4-mil

polyethylene membrane, carefully lapped, and

stapled between studding, rafters, or ceihng joists

and inside storage sheathing, makes a good vapor

barrier.

Potatoes when stored should be dry and reasonably free from soil. If excess soil accumulates in

the bin, air circulation is impaired with the result

that temperatures in some places in the storage

may be high enough to cause sprouting and excessive decay, while in other places freezing damage

may occur.

Ventilation should be regulated to hold the

potatoes at about 50° to 60° F. and high relative
COMMERCIAL POTATO PRODUCTION 49

FIGURE 38.—Interior of earth-covered storage in Idaho.

humidity during the first 10 to 14 days. Under

these conditions, cuts and bruises heal rapidly,

and losses from shrinkage and decay are reduced.

A low temperature at the beginning of the storage

period is not necessary to prevent sprouting because the potatoes are then in a state of rest and

are unable to sprout.

If field fro.st or late blight is present in the

held, the temperature in the storage should be

lowered to between 88° and 40° F. as soon as possible to retard decay. Lower humidity also is

desirable under these conditions to <lry the surface

of potatoes and reduce the spread of decay.

After the preliminary curing period, temperatures may be lowered. It is desirable to store seed

potatoes at ;'>8° to 40° F., since at this temi)erature

they will not sprout. The relative humidity

should be maintained at 75 to 90 percent.

Potatoes stored at 50° to 60° F. have better

cooking and processing qualities because tliey contain less sugar than those stored at lower temperatures. At 50° to 60°, however, sprouting will

occur after about 2 to 3 months. Although limited

sprouting does not affect potatoes for food purposes, badly sprouted stock shrinks and is difficult

to market. Certain growth-reguhiting chemicals

have been approved by the U.S. Food and Drug

Administration to control or reduce sprouting on

potatoes. One of these (maleic hydrazide) is

applied on the plants in the field 2 to ?> weeks after

full bloom. Others are occasionally applied as

tlie potatoes are put into storage. More often the

inhibitors are applied after a period in storage to

avoid interference with wound healing or are

applied as the potatoes are removed from storage

to control sprouting during the marketing period.

Do not use these chemicals on seed potatoes as

they may retard or completely prevent sprouting.

Before applying chemicals to stored potatoes,

obtain specific ijistructions from your county agricultural agent on the proper way to use them.

Potatoes stored at 40° F. are acceptable for most

l)urposes, but not for processing. Holding such

potatoes at 60° to 70° for from 2 to 4 weeks before

they are used usually improves their cooking and

processing qualities. This change in storage permits some of the sweetness to disappear. High

reducing sugar content of potatoes is the cause of

luidesirable dark-brown color of potato chips and

french fries, and is responsible for the browning

of dehydrated potatoes following processing.

Potatoes stored throughout the winter at temperatures of 38° to 40° F. should not shrink more

tlum 5 jiercent. At higher temperatures, shinkage

will be greater.
se AGR. HANDBOOK 267, U.S. DEPT. AGRICULTURE

Before the potatoes are removed from storage,

tlieir temperature should be raised to about 50° F.,

if this can be done without raising the temperature of potatoes in adjoining bins. The higher

temperature will reduce injury from bruises and

internal black spot during grading and other

liandling.

Early-crop, or summer-harvested, potatoes

usually are not stored or held except for relatively

short periods. For table stock, however, early

potatoes that are nearly free of disease and injury

can be stored 4 to 5 months at 40° F. if they are

cured 4 days or longer at (¡0° to 80° before storage.

They can be stored about 2 months at 50° without

curing. Careful handling to avoid injury and exposure to sun and heat during harvesting will help

minimize losses cavised by decay.

A considerable portion of the early crop is used

for making chips. Holding these potatoes in cold

storage, or even at moderate temperatures of 50°

to 55° F., for only a few days causes an excessive

accumulation of reducing sugars in the tubers,

which results in the production of dark-colored

chips. Subsequent holding of these potatoes at

70° or higher usually fails to lighten the color

appreciably. Transit and storage temperatures

near 70° are required to maintain chipping quality

in early-crop potatoes.

GRADING AND MARKETING

It is distinctly to the advantage of the potato

grower, and likewise anyone offering potatoes for

sale to the consuming public, to see that the potatoes going on the market present a good appearance. Tliis is good advertising for the potato industry. Potatoes having cuts, braises, and insect

or disease injuries neither sell well nor command a

premium price. Such potatoes should be promptly

diverted from market channels so that the consumer will not suffer undue loss in preparing them

for the table. Potato growers everywhere must do

everything possible to safeguard consumer goodwill and work continually to prevent the possible

development of consumer prejudice against potatoes in ireneral. Tubers of undesirable size or

shape and with excess soil on them should also be

culled out when preparing potatoes for market.

Only in this way can the individual grower hope to

build up a reputation for good table stock and,

similarly, can any potato-producing region hope

to establish a good reputation for its product.

The ITnited States Standards for Potatoes,

issued by the U.S. Department of Agriculture,

ftilly describes officially designated potato grades

and the tolerances for defects pertaining thereto,

and presents a definition of terms used in the

prescribe<i standards. Copies of the latest Standards may be obtained from the U.S. Department

of Agriculture, Washington, D.C., 20250.

BREEDING IMPROVED POTATO VARIETIES

Old potato varieties are continually being replaced by new ones developed through breeding

programs. The potato breeding program of the

TT.S. Department of Agriculture and cooperating

State experiment stations has been highly effective

in developing new, improved varieties of potato.

Most of the varieties being grown commercially in

this country today originated through this program. Only a few important varieties such as

Irish Cobbler, Eusset Burbank, White Rose, and

Bliss Triumph were developed prior to the establishment of the potato breeding work in the

Department of Agriculture.

The parts of the potato flower are large and it

is a simple technique to remove the anthers and

apply pollen for making controlled ci'osses. The

steps are illustrated in figures 39 and 40.

Flower production and seed formation occur in

the field or garden only when the plants have ideal

growing conditions, with especially cool temperatures. Such ideal conditions are easily produced

in the greenhouse in winter, especially in areas

where the incident sunlight is high during the

winter months.

Whenever cross-fertilization is successful, the

ovaries develop within 10 days, and approximately

60 days later the potato fruit, resembling a small

tomato, is mature (fig. 41). As the fruit matures, there is only a slight fading of the green

color; the fully ripened fruits Ijecome light green

to cream in color, depending on variety. Most seed

balls contain from 100 to 300 mature seed (fig. 42).

The seed, which resembles miniature tomato seed

without fuzz, germinates erratically soon after
COMMERCIAL POTATO PRODUCTION 51

maturity. After storage for a few months it will

germinate and grow more readily. Each of the

small seedling plants originating from the seed

constitutes a potential new variety. Small tubers

(fig. 42) form on the seedling plants and they can

be harvested within 3 to 4 months, if growing temperatures are cool and light intensities are high.

From tliese seedling tubers new potato varieties

are produced. At the Plant Industry Station,

Beltsville, Md., from 40,000 to 50,000 seedlings are

grown each year. Comparable numbers are grown

in the Federal program at Prosser, Wash., Baton

Rouge, La., Greeley, Colo., and Aberdeen, Idaho,

and in many State programs including Iowa,

Michigan, Minnesota, Nebraska, New York, North

Carolina, North Dakota, West Virginia, and

Wisconsin.

The objectives of the National Potato Breeding

Program are (1) high yields, (2) high quality for

fresh and processing uses, and (3) high resistance

to all important potato diseases and insects.

FIGURE 39.—Collecting pollen in a glass vial, using a

mechanical vibrator to shake the iwUen from the

anthers.

FiüuiíE 40.—Applying pollen to the receptive stigma of

au eumsculated bud.

FniuiiE 41.—Potato seed balls formed after successful

fertilization. The .seed balls resemble small green tomatoes, but do not change color appreciably on ripening.
52 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

FlouBE 42.—True potato seed (left) and seedling tubers (right). (Natural size.)

POTATO QUALITY

Production Factors Affecting Solids and

l\Aealiness of Potatoes

Occasionally one hears that potato quality is

not as good today as it was in years past. However, before comparing the quality of present-day

potato varieties with that of varieties grown years

ago, one should be careful to define the qualities

being compared. Although mealiness is often assumed to indicate good quality, it is not always

the consumer's criterion of desirable quality in

a potato. Quality in potatoes is usually referred

to as a specific characteristic, but it is well established that desirable cooking quality can mean

several things to several people. Because of this

lack of agreement on tlie factors that make a

potato good, it is difficult to decide which qualities, if any, need improvement before attempting

to finds ways of improving them. In all fairness to growers, shippers, and retailers, it should

be stated that the general appearance and, to a

large degree, the cooking quality of potatoes being sold on many retail markets have markedly

improved during the last decade. Mvich credit

for this improvement is due the potato breeder

who has been criticized in some quarters for selecting for shape, shallow eyes, and disease resistance

at the expense of quality and flavor.

The task of improving cooking quality in potatoes is not an easy one. It takes 10 years or

more of constant selecting and testing before a

promising seedling can establish itself as superior

to those already introduced. Furthermore, as

one attempts to dissect the components of good

culinary quality and palatability in potatoes in

order to make selections, the problem appears to

be a paradox. A rational approach to the problem inevitably leads to the conclusion that quality

is the result of an equilibrium between a combination of factors and not one factor alone. If one

factor is abnormal, it can have a controlling influence on the resulting quality of even the best

variety available. Some of the factors known to

influence potato quality are listed below :

(1) Temperaturefi of soil or air.—High temperatures exceeding 70° F. during the period of

tuberization, especially toward the end of the

season, lower potato quality.

(2) Rainfall or moisture.—In general, high

levels of moisture at maturity and low levels

earlier in the season lower quality. The relation

of moisture levels to potato quality is not fully

understood, but it is believed that high levels of

irrigation on heavy soils tend to decrease total

solids.

(3) Soil type.—In general, well-drained loam

soils have produced crops with higher solids than

heavy clay or poorly drained soils. In most cases

total solids are higher in potatoes grown on mineral soils than on muck or peat soils.

(4) Variety.—Given identical environmental

and cultural conditions, some varieties will consistently give higher solids than others. (See section under choice of varieties.)
COMMERCIAL POTATO PRODUCTION 53

(5) Date of planting.—Earlier planting of the

crop at any jfiven location will increase potato

quality over that of later planting. Probably no

other single cultural practice so greatlj' affects

potato quality. AVarming seed at 70° F. for 2

weeks just before planting will result in quicker

emergence and early-season growth, factors contributing to higher solids and higher yields.

(6) Date of harvest.—As long as the vines are

green, harvest should be delayed, if feasible, to

lengthen the growing season. Early planting and

late harvest provide a maximum growing season,

one of the prime requisites for high quality.

(7) Ferttllzation practices.—Usiuilly, high nitrogen and potash levels result in higher yields

but lower total solids and quality than do moderate levels. Soils of low fertility will often yield

small crops of high quality. Today's agricultural

competition does not permit growers with low

yields to remain in business.

(8) Spraying practices.—Control of insects and

diseases by spraying with insecticides and fungicides undoubtedly increases yield. However, it is

believed that gradual dying of the vines and slow

"maturing" of the tubers in the fall from inadequate control of pests in former years resulted

in higher potato quality. If this observation is

correct, spraying practices used today may tend

to lower potato quality by delaying maturity and

making vine killing necessary. These relations

are not well understood and need more research.

In the meantime there is a growing tendency to

discontinue some of the later sprays to allow

earlier dying of the vines by natural means.

(9) Vin^ killing.—Vine killing has become necessary for most varieties because improved insect

and disease control keeps the vines lush and green

and the tubers immature up to harvest!ime. Sudden killing of vines by any means (chemical or

mechanical) obviously allows little time for

normal maturity, and consequently potatoes harvested directly after the vines are killed are not of

the highest quality. Growers, however, usually

have little choice in this regard if they plan to

complete harvest of large acreages on schedule,

unless the vines are killed naturally by frost. For

highest solids, slow-killing chemicals are preferable to fast-killing methods.

(10) Day length.—Maturity is hiistened by

short days and vuie growth is encouraged by long

days. This has been more fully discussed under

climate. Full maturity gives higher quality.

(11) Light intensity and temperature.—Bright

sunny weather late in the season, especially if accompanied by day temperatures below 70° and

slightly cooler night temperatures, will result in

higher solids and higher quality than continued

cloudy weather.

(12) Geographic location.—Geographic location has a mai'ked effect on potato quality. Ortain

areas have a combination of climatic conditions

which especially favor high potato quality; for

example, southeastern Idaho, the Red River Valley

of North Dakota and Minnesota, and Aroostook

County, Maine. Geographical location has more

effect on total solids than does variety. The solids

variation in nine potato varieties grown at four

locations in Maine in 1952 is illustrated in table

14. Differences caused by location are higher for

each variety than the greatest difference between

varieties grown at one place. Ontario, tlie variety

lowest in the test at Presque Isle, Maine, was

higher in percent of solids when grown there than

Green Mountain or any of the other varieties

grown at Pittsfield or Exeter, Maine.

TABLE 14.—Solids in 9 varieties of potatoes groion at 1^ locations in Maine ^

Variety —'"

Solids in potatoes grown at—

Greatest

Pittsfield

(Mav 26-

Sept. 25)

Exeter

(Mav 27-

Sept. 26)

Sherman

(May 28-

Sept. 29)

Presque Isle

(Mav 6-

Sept. 30)

difference in

solids

Irish Cobbler -

Percent

16.2

1.5. 6

14. 5

14.6

15. 0

14. 4

14. 0

15.0

14. 7

2.2

Percent

17.7

17.4

16.7

15.7

17.7

16.5

14.8

16.7

16. 5

2. 9

Percent

19.2

19. 9

17. 7

18. 2

19.4

16.2

16.7

19.9

16.9

3.7

Percent

21.9

21.8

19.9

19.9

21.8

20.4

18.2

20.9

19. 9

3.7

Percent

5.7

Green Mountain _ 6.2

Katahflin 5. 4

Konnebec - 5.3

Cherokee . . _ 6. 8

Triumph 6. 0

Ontario - - - - _ 4.2

Pungo - - . 4. 9

Earlv Gem 5.2

Greatest difference in solids

' Locations represent different environmental conditions of potato-growing areas in Maine.
H AGR. HANDBOOK 2 67, U.S. DEPT. AGRICULTURE

Determining Solids in Potatoes

Growers occasionally need to determine the

solids content of their potatoes for special marketing purposes. But potato breeders usually have a

special need for this kind of information in order

to evaluate family lines segregating for solids content. The determination of solids by the laboratorj^ method based on the loss of moisture after

drying is not fast enough and has proved too

costly. As an alternative, a specific gravity

method may be used to obtain a good estimate of

total solids. Two specific gravity methods are

available, one requirmg the weight of the sample

in air and in water and another known as the hydrometer method. Table 15 shows the evaluation

scale used by U.S. Department of Agriculture

potato breeders to convert specific gravity readings

to solids and starch equivalents.

Tuber samples selected for specific gravity determinations should represent several locations in

the field and should be a random sample with respect to size. The tubers should be strictly clean

and entirely free from hollow heart. Soil on potatoes will make the specific gravity reading too

high and hollow heart will cause the reading to

be too low.

TABLE 15.—Chart for converting specific gravity

readings to total solid and. starch readings ^

TABLE 15.—Chart for converting specific gravity

readings to total solid and starch readings '—

Continued

Specific Total Starch Specific Total Starch

gravity solids gravity solids

1. 040 11. 2 1. 073 18.2 12. 5

1. 041 11. 4 1. 074 18. 4 12.7

1. 042 11. 6 1. 075 18.6 12. 9

1. 043 11.8 ^ 1. 076 18. 8 13. 1

1. 044 12. 0 1.077 19. 0 1.3. 3

1.045 12. 2 1. 078 19. 2 1.3.5

1. 046 12. 5 1. 079 19. 4 13. 7

1. 047 12. 7 1. 080 19. 7 13.9

L048 12. 9 1.081 19.9 14. 1

1. 049 13. 1 1. 082 20. 1 14. 3

1. 050 1.3. 3 1. 083 20. 3 14.5

1. 051 1.3. 5 1.084 20.5 14. 7

1. 052 1.3.7 1. 085 20. 7 14. 9

1. 053 1.3.9 1. 086 20. 9 15. 1

1. 054 14. 1 1.087 21. 2 15.4

1.055 14. 3 1.088 21.4 15. 6

1.056 14.5 1. 089 21. 6 15.8

1.057 14. 8 1. 090 21.8 16. 0

1.058 15. 0 1. 091 22. 0 16.2

1.059 15. 2 1. 092 22. 2 16. 4

1.060 15.4 1. 093 22. 4 16. 6

1. 061 15. 6 1.094 22. 7 16. 9

1. 062 15.8 iô. 2 1. 095 22. 9 17. 1

1. 063 16. 0 10. 4 1.096 23. 1 17. 3

1. 064 16. 2 10. 6 1. 097 23. 3 17.5

1.065 16. 5 10. 8 1.098 23. 5 17.7

1.066 16. 7 11. 0 1.099 23.7 17.9

1.067 16. 9 11. 2 1. 100 24. 0 18.2

1.068 17. 1 11. 4 1. 101 24.2 18. 4

1. 069 17.3 11. 6 1. 102 24. 4 18.6

1. 070 17.5 11.8 1. 103 24. 6 18.8

1. 071 17.7 12. 1 1. 104 24. 8 19. 0

1.072 18.0 12.3 1. 105 25.0 19.2

Specific Total Starch Specific Total Starch

gravity solids grnvity solids

1. 106 25. 2 19. 4 1. 125 29. 3 23. 5

1. 107 25.5 19.7 1. 126 29. 5 23.7

1. 108 25. 7 19.9 1. 127 29. 8 24. 0

1. 109 25. 9 20. 1 1. 128 30. 0 24. 2

1. 110 26. 1 20. 3 1. 129 30. 2 24. 4

1. Ill 26.3 20. 5 1. 130 30. 4 24.6

1. 112 26. 5 20. 7 1. 131 30.6 24. 8

1. 113 26. 7 20.9 1. 132 30.8 25.0

1. 114 26. 9 21. 1 1. 133 31.0 25. 2

1. 115 27.2 21. 4 1. 134 31.3 25.5

1. 116 27. 4 21.6 1. 135 31.5 25. 7

1. 117 27.6 21. 8 1. 136 31.7 25.9

1. 118 27. 8 22. 0 1. 137 31.9 26. 1

L 119 28. Q 22.2 1. 138 32. 1 26.3

1. 120 28.3 22. 5 1. 139 32.3 26. 5

1. 121 28. 5 22.7 1. 140 32.5 26. 7

1. 122 28. 7 22. 9 1. 141 32. 8 27.0

1. 123 28.9 23. 1 1. 142 33.0 27. 2

1. 124 29. 1 23.3

' Adapted from M. Maercker.

107. 1880.

Air-and-Water Method

Landwerths Band 25:

Two advantages of the air-and-water method of

determining specific gravity are : (a) samples may

be very small to large, and (b) very simple, ordinarily available equipment is needed. The

samples are weighed first in air and then in water.

Specific gravity is determined by the equation:

Specific gravity=- welght of sample in air

( weight of sample in air) — ( weight

of sample in water)

Accuracy of determinations by this method may

be increased by increasing the sample size to 25

pounds or more (fig. 43). Scales must be accurate

and readings made in decimals of pounds and

ounces.

Potato Hydrometer Method

A potato hydrometer devised by the American

Potato ("hip Institute ^ gives direct specific gravity

readings. Exactly 8 pounds of potatoes are

weighed in air and placed in a basket suspended

below the hydrometer. The assembly with potatoes is placed in a container of water deep enough

to float it, and the specific gravity is read directly

from the scale on the hydrometer.

Blacl(ening After Cooldng

One of the most prevalent complaints of housewives is the blackening of the flesh of market po-

' Address : National Potato Chip Institute, Cleveland,

Ohio.
COMMERCIAL POTATO PRODUCTION 55

^«» j?'*»

j

FIGURE 43.—Simple apparatus, made from stock materials, for determining six"ciflc gravity of potato samples

of different sizes by weighing in air and water.

tatoes that occurs sometimes after cooking (fig.

44). Experts do not agree on the reasons for potatoes turning dark after cooking, but certain relations to cultural practices have been reported.

The condition is worse in some seasons than in

others. More research is needed on this very important problem. The following relations have

been noted :

1. Variety.—Some potato varieties are more

susceptible than others to darkening. Varieties

with a tendency to darken are Irish Cobbler, Green

Mountain, Rural New Yorker, Russet Rural, and

Russet Burbank. Slight tendency toward darkening after cooking occurs in Katahdin, Red Warba,

Warba, and Red Pontiac. Varieties with a tendency to remain white after cooking are Bliss

Triumph, Chippewa, and Sebago. The varieties

that show the greatest tendency to after-cooking

darkening include most of the older potato varieties developed before 1920. The varieties Sebago

and Chippewa—more recent introductions by the

USDA—are among those least susceptible. Resistance to after-cooking blackening is a characteristic that has been given increased emphasis in

most breeding programs. For many years, potato

bi'eeders have tried to avoid introduction of varieties known to be susceptible to blackening. Exhaustive testing and selection are necessary to

eliminate or minimize this undesirable tendency

in varieties.

2. CUmatic conditions.—Low air temperatures

(50° F. or lower) during long periods in the month

prior to hai'\'est have been associated with aftercooking blackening. Cloudy weather is likewise

claimed to increase the tendency to blacken.

3. Maturity of vines and tubers at harvest.—

There is some indication that harvesting before

the vines and tubers are fully matured tends to

increase the incidence of after-cooking blackening.

4. Fertilizers.—No concrete evidence can be

given directly linking fertilizer practices with

darkening of cooked potatoes, except that heavy

fertilization delays maturity and therefore may

indirectly favor blackening.

FIGURE 44.—After-cooking blackening of two potato

samples from a breeding program : A, no appreciable

blackening ; B, rather marked external and internal

blackening.

Internal Blackening of Raw Tissues

Blackening of the flesh in uncooked potatoes is

referred to by various workers as internal black

spot, internal bluing, or internal blackening. It

has been studied rather extensively but the exact

cause is still obscure. Most frequently it is considered to be a physiological disorder. It occurs

in raw as well as in cooked potatoes, as dark

areas in the flesh beneath the skin. Harvesting

bruises, or pressures, "trigger" the development

of this ailment in subsequent storage. It is probable that cultural practices, climatic conditions,

or even a virus may be predisposing factors of

this disorder. Irrigation practices, moisture relations, and fertilizer nutrient balances have at

various times been correlated with it to some

degree, but much more needs to be known about

blackening before it can be prevented entirely.

Under similar conditions, varietal differences in

degree of blackening are apparent, but under favorable conditions all varieties show some

symptoms.

Tuber Greening

The development of green color in the tubers

of potatoes exposed to light is a familiar defect

commonly referred to as sunburnt, light-struck,

or greening. Most potatoes will not be green

when properly cared for during growth, through

harvest, in storage, and in the market, but it is

often impossible to avoid all traces of greening

before potatoes reach the consumer. The first
56 AGR. HANDBOOK 2 6 7, U.S. DEPT. AGRICULTURE

sig-ns of greening may occur in tlie field because

of poor soil coverage and subsequent exposure to

sunlight. Later, other sources of light may add

to tuber greening while the potatoes are being

stored, graded, and packaged for the retail market

where further light exposure may occur in varying amounts and intensities. Although some of

these exposures may be of short duration, or the

light weak, the effects are cumulative so that the

ultimate intensity of greening may be serious.

It has been known for some time that tubers

from varieties and seedlings differ in their greening tendencies, but only recently has significance

been attached to these differences obtained under

light exposures similar to those existing in retail

stores. These differences are now believed to be

inherited so that it is recognized that a solution

to the problem lies in potato breeding. The economic importance of a nongreening commercial

potato variety, especially for the fresh market

trade, is without question.

In a study of genetic differences in potato tuber

greening, the tubers of certain varieties and seedlings were exposed intermittently to combinations of daylight and fluorescent light similar to

those existing in retail grocery stores. Figure 45

shows the comparative results obtained wlien the

tubers of two commercial varieties (Kennebec and

Katahdin) and four seedlings (B4523-8, B922-6,

B922-3, and B3696-13) were exposed to combined

daylight and fluorescent light intermittently for

different periods. The greening in' ensity in general was only slightly increased after 123 hours

of light exposure, but the tubers from seedlings

B4:523-8, B922-6, and B922-3 were remarkably

less green than those from Kennebec and Katahdin. This experiment and others like it with the

tubers obtained from progenies of crosses between

greening and nongreening parents have indicated

that multiple genes and incomplete dominance

are involved in the inheritance of the nongreening

character. This finding is the first step toward

the production of nongreening commercial potato

varieties, which is the objective of a continuing

project being conducted by the National Potato

Breeding Program.

B

361 hrs.

278 hrs.

123 hrs.

/^'

FIGURE 45.—The effects of combined daylight and fluorescent light on the greening of potato tubers of

varieties and seedlings exposed intermittently for différent periods: A, Katahdin; B, B4523-8; G, liennebec ; D, B922-6 ; E, B922-3 ; and P, B3696-13.
COMMERCIAL POTATO PRODUCTION 57

GROWING AND DEVELOPING POTATOES FOR SPECIAL USES

One of tlie fastest growing developments in the

food industry today is the trend toward processing

of potatoes. Production practices and varietal

characteristics must be altered to meet the demand

for potatoes better adapted to this rapidly expanding processing industry. The processing industries now use about 25 percent of the total U.S.

production (table 16) as compared to only 1.4

percent in 1940.

Potatoes for Chips

Large amounts of potatoes are being manufactured into cliips (table 16). Potatoes intended

for chipmaking must meet definite specifications

of the manufacturer. From a production standÇoint, farmers growing potatoes for chip manuacture should follow only the best practices

recommended for commercial potato production.

Particular emphasis must be placed on (1) use of

cultural practices that produce tubers of maximum solids content, and (2) selection of varieties

best for chipmaking that are adapted to a given

production area.

High solids content is especially important in

growing potatoes for chips because the yield of

chips per 100 pounds of potatoes is in direct proportion to their solids content. Potatoes with

solids of 15 percent or less will usually yield less

than 21 pounds of chips per 100 pounds of fresh

weight. However, the same fresh weight of potatoes with solids of 24 percent will yield in excess

of 32 pounds of chips. Also, chips made from

potatoes of high solids content absorb less frying

oil than those made from potatoes of low solids

content. High solids potatoes not only reduce

tlie cost of the finished product but also improve

chip keeping quality because chips low in fat

content, made from such potatoes, remain in good

condition longer than those high in fat.

Potato varieties vary considerably in their adaptability for chip manufacture. Some varieties

consistently will produce dark-colored chips even

at harvesttime, largely because of their relatively

high sugar content. Many varieties will produce

light-colored chips at harvest when the sugar content is usually low, but will always chip dark after

storage when sugars have accumulated. Chip

TABLE 16.—Potatoes: Ut'dizntion of the 1957-61 crops

Utilization items 1957 crop 1958 crop 1959 crop I960 crop 1961 crop

Sales:

Table stock

1,000 cwt.

148,408

1,000 cwt.

148, 868

1,000 cwt.

149, 013

1,000 cwt.

149, 376

1,000 cwt.

154 834

For processing :

Chips and shoestrings 17, 356

3,776

4,215

612

1,216

1,390

12, 691

17, 063

5,917

7,352

911

1,250

1,614

18, 387

20, 195

7,656

8,745

I, 173

1,185

1,262

7,718

21,310

10, 104

13,373

1,669

1,572

1,237

10, 177

22, 556

Dehydration

Frozen french fries

8,518

15 911

Other frozen products

Canned potatoes

Other canned products (hash, stews,

soups)

Starcli and flour _. _

2,227

1,458

1,317

20, 493

TotaL__ 41, 256 52, 494 47, 934 59, 442 72, 480

Other sales:

Livestock feed 8,950

13,641

18,918

13, 079

6,607

13, 583

5,348

14, 547

18 918

Seed 13, 850

Total. - - - -- - --- 22, 591 31, 997 20, 190 19, 895 32, 768

Total sales .-. .. ... 212, 255 233, 359 217, 137 228, 713 260, 082

Nonsales:

Seed used on farm where grown. . 7,577

8,176

2,718

11,796

7,086

7,279

3,916

15, 257

7,166

5,920

3,085

12, 491

7,504

5,459

2,887

12, 872

7,436

Household use . 5,236

Feed

Shrinkage and loss

4,234

16, 606

Total nonsales . . 30, 267 33, 538 28, 662 28, 722 33, 512

Total production.. . . 242, 522 266, 897 245, 799 257, 435 293, 594
g

4

9 10

FIGURE 46.—Color reference standards for evaluating varieties and breeding lines for chip manufacture. These color standards have been officiall.v apl)roved by the American Potato Association. Grades 1 and 2 are preferred by most chip manufacturers. ( Courtesy of 15. L. Thomas Cinciiiuati

Ohio.) '

>

a

Cd

o

o

»

d

je

Ö

H

>

O

M

O

d

f

d

ft) H
COMMERCIAL POTATO PRODUCTION m

manufacturers prefer varieties that will produce

light chips at harvest, will accumulate sugars

slowly at the usual storage temperatures, and will

lose their high sugar content readily upon "reconditioning" after storage.

Potato breeders, especially in the IJSDA, are

actively engaged in making selections for new

potato varieties wliich will produce chips of liigh

quality both at harvesttime and after "reconditioning" from storage. Chips from promising

selections are ranked according to a scale of colors

shown in figure 46. Because of the special requirements of potatoes for chip manufacture, j)ar( icular

emphasis in the National Potato Breeding Program is being jilaced on developing new varieties

for cliipmaking.

Potatoes for Dehydrated Products

Dehydrated products including flakes, granules,

and dices require potatoes meeting special needs.

In addition to a high solids requirement, there is a

special need for greater uniformity in solids within

the same variety grown in one geographical location. To fulfill this requirement, growers must

have maxinnnn control of moisture, fertilization

])ractices, spacing, and other cultural practices.

On the other hand, breeders are attempting to develo]) varieties with desired qualities and less influenced by environment than are present varieties.

Considerable research is needed before growers

can be supplied the help needed to produce potatoes specially adapted for manufacturing dehydrated products.

Potatoes for Frozen Products

Frozen potato products consist largely of frozen

french fries, but there are other frozen products

being developed such as diced potatoes, mashed.

hash brown, j)()tato pufl^s, au gratin, rissole, cakes,

shreds, and dehydrofrozen products (partial dehydration combined with freezing). Flavor is

especially important in these products. Until the

basic components of flavor are more fully understood, development of potatoes adapted for these

uses will of necessity be delayed. Tests are being

conducted with cooperating processing firms to

enable selection of lines best aclapted for these uses

which will also maintain favorable flavor and texture after being thawed and cooked.

Potatoes for Certified Seed

Production of certified seed potatoes is also a

specialized kind of potato enterprise. The term

"certified" as applied to potatoes means that the

fields and harvested crops have been periodically

inspected by a stañ' trained to detect evidences of

seedborne diseases and varietal mixtures. Potato

seed certification standards and their enforcement

are the responsibility of each State. There are no

Federal standards for certification. The primary

purposes and advantages of certified seed are discussed under "Selectmg and Preparing Seed,"

p. 28.

The certified seed grower must purchase seed

for his own use from specified sources and then

continually protect it from contamination by

diseases and prevent variety mixture. lie must

have had training in identifying disease symptoms

and nnist be capable of roguing infected plants

from his fields. Growers interested in producing

certified seed should write to the official agency in

charge of potato seed certification in their State.

The names and addresses of such officials and other

information on growing certified seed may be

found in the 1956 Potato Handbook of the Potato

Association of America, New Brunswick, N.J.

•irU.S. GOVERNMENT PRINTING OFFICE: 1964 O 717-7 60