^/57^^f^

FIRE BLIGHT

A BACTERIAL DISEASE

OF ROSACEOUS PLANTS ti^

^^ UNITED STATES

ik ^M DEPARTMENT OF

AGRICULTURE

AGRICULTURE

HANDBOOK

NUMBER 510

PREPARED BY

SCIENCE AND

EDUCATION

ADMINISTRATION
FIRE BLIGHT

A BACTERIAL DISEASE

OF ROSACEOUS PLANTS

By

TOM VAN DER ZWET and

HARRY L KEIL

Research Plant Pathologists

Fruit Laboratory

Beltsville Agricultural Research Center

Beltsville, Maryland 20705

UNITED STATES AGRICULTURE PREPARED BY

DEPARTMENT OF HANDBOOK SCIENCE AND

AGRICULTURE NUMBER 510 EDUCATION

ADMINISTRATION
This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been

registered. All uses of pesticides must be registered by appropriate State and/or

Federal agencies before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, beneficial

insects, desirable plants, and fish or other wildlife—if they are not handled or

applied properly. Use all pesticides selectively and carefully. Follow recommended

practices for the disposal of surplus pesticides and pesticide containers.

FOLLOW THE LABEL

US, DffAIIMENT OF AC«ICULIU«E

Trade and company names are used in this publication solely to provide specific

information. Mention of a trade or company name does not constitute a warranty or

an endorsement by the U.S. Department of Agriculture to the exclusion of other

products or organizations not mentioned.

Library of Congress Catalog Card No. 78—600013

Washington, D. C. Issued March 1979

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

Washington, D.C. 20402

Stock Number 001-000- 03724- 2
CONTENTS

Chapter Page

1. Historical review 2

Early theories (1780-1880) 2

Bacterial origin and disease

transmission (1881-1900) 4

Nature, dissemination, and disease

control (1901-20) 6

2. Geographic distribution 10

North America 10

United States 10

Canada 11

Central and South America 11

Europe 12

Great Britain 12

Poland 13

The Netherlands 13

Denmark 14

West Germany 14

France 15

Belgium 15

Oceania and Africa 16

New Zealand 16

Egypt 17

Unconfirmed reports 17

3.

4.

Economic importance 19

Symptomatology

5.

6.

21

Pear and apple 21

Blossom blight 21

Twig and leaf blight 21

Fruit blight 23

Limb and trunk bhght 23

Collar and root blight 25

Quince and crab apple 25

Pyracantha and hawthorn 25

Cotoneaster, stranvaesia, and

mountainash 26

Diseases resembling fire blight 27

Host range 29

Cotoneaster 34

Hawthorn 34

Quince 35

Pyracantha 35

Chapter page

Mountainash 35

Miscellaneous plants 35

7. Causal organism 37

Taxonomy 37

Phenetic data 37

Serology 38

Molecular genetics 38

Morphology and cultural

characters 39

Generation time 45

Standard and selective

media 46

Growth and metaboHsm 47

Bacterial exúdate and toxin 51

Ooze formation 51

Strand development 51

Toxin production 53

Strains 54

Virulence and pathogenicity 55

Effect of temperature, pH,

and light 56

Longevity 58

Bacteriophagy 60

Effect of micro-organisms 61

Bacillus and Bdellovibrio 61

Erwinia herbicola 62

Phytobacteriological techniques — 63

8. Disease cycle 65

Primary infection 65

Overwintering cankers 65

Epiphytic Erwinia

amylovora 67

Secondary infection 70

Dissemination 70

Rain and wind 70

Insects 71

Birds 76

Man 76

9. Pathological anatomy 78

Host ultrastructure 80

Internal and resident bacteria 81

III
10.

11.

12.

Biochemistry of host tissues 84

Hypersensitive reaction 86

Induced host resistance 86

Conditions affecting disease

development 88

Host conditions 88

Tree growth and vigor 88

Tree nutrition 88

Intercellular humidity 91

Meteorological factors 91

Temperature 91

Rain, humidity, and hail 94

Edaphic factors 95

Soil types 95

Soil moisture and

temperature 95

Cultural practices 96

Control measures 97

Sanitation and quarantine 97

Pruning 97

Eradication 98

Tree nutrition 99

Chemical control 99

Copper compounds 99

Antibiotics 106

Carbamates 109

Miscellaneous compounds 109

Monitoring

Insect control

-109

-110

Biological control 110

13. Nature of resistance 112

Pear 113

Pyrus communis 113

Pyrus calleryana 126

Pyrus ussuriensis 128

Pyrus pyrifolia 128

Pyrus betulaefolia 128

Apple 129

Rootstocks 140

Pear 140

Apple 141

Artificial inoculation 142

Techniques 142

Inoculum 144

Disease evaluation 145

Breeding programs 145

Pear 145

Apple 152

Pyracantha 154

Hawthorn and cotoneaster 154

Literature cited 154

Theses and dissertations on fire blight 193

Survey of fire blight research in the

United States and Canada 195
FIRE BLIGHT

A BACTERIAL DISEASE OF ROSACEOUS PLANTS

During 1965-76, an intensive search was made for

all available literature on fire blight with the assistance of personnel at the Library of Congress and the

Computer Service of the National Agricultural Library. The amount and classification of this literature, pubUshed since 1870, are shown in figure 1.

Even though the number of pubhcations varied between years, some peaks prior to 1960 were attributed to certain discoveries or other important phases of fire blight research. The increase in publications from 1960 to 1976 was attributed to the follow-

' I ' I ' I ' I ' I ' I ' I '

NATURE OF RESISTANCE

NTERNAL AND EPIPHYTIC

RESIDENT BACTERIA

(MONITORING)

DISCOVERIES

BACTERIAL BEE

NATURE TRANSMISSION

(1880) (1891)

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980

ing factors: (1) First occurrence of fire blight in 1957

and its subsequent spread in Europe, (2) more widespread use of the electron microscope and scanning

electron microscope in research, (3) emphasis on

studies of internal and epiphytic populations of Erwinia amylovora in and on plant tissues, and (4)

increased investigations on the nature of fire blight

resistance in pear, apple, and ornamental rosaceous

plants. Only about 15 percent ofthe fire blight literature was published outside the United States, principally in Canada, New Zealand, England, Denmark, West Germany, and the Netherlands. The

literature search was terminated as of December 31,

1976. Since then we have included several articles

that were sent to us or were printed during 1977.

The historical review of fire bhght in this publication covers the period 1780-1920 and is followed by a

critical analysis of the literature from 1921 to 1976.

The section on chemical control is separated into two

parts, with the advent of antibiotics in 1950 as the

dividing line.

Since 1900 only a few brief reviews on fire blight

were published. In 1928, Groves {llOJ^y prepared a

master's thesis with 208 reference citations, equaling about 65 percent of the available literature at

that time. In 1930 and 1951, Elliott {266, 267) wrote

two manuals on bacterial plant pathogens, which

included E. amylovora. In 1974, three articles appeared in different parts ofthe world—in California

by Schroth et al. {8Jf7), in Great Britain by EdenGreen and Billing {257), and in West Germany by

Zeller {10J/.6) in German. Our review was prepared to

serve as a complete and comprehensive analysis of

all available literature in the world. Of approximately 2,000 known references, 63 percent are referred to in the text.

FIGURE 1. —Amount and classification ofliterature on fire blight,

1870-1976.

Italic numbers in parentheses refer to Literature Cited, p.

154.
CHAPTER 1

HISTORICAL REVIEW

Fire blight, caused hyErwinia amylovora (Burr.)

Winslow et al., is undoubtedly the oldest, most serious, and perplexing bacterial disease of pomaceous

fruit trees. It is very destructive to pear and less so

to apple, quince, and several other members of the

family Rosaceae. Even though the actual coining of

the name cannot be traced, the term "fire blight"

was well chosen. Trees with blighted branches and

persistent blackened leaves appear scorched. Although its original name is English, the disease is

known in many other languages.

Fire blight is apparently indigenous to North

America. It was first noticed in the late 18th century

in New York and was not reported from any foreign

country until over a century later. The disease probably occurred on native American plants, such as

crab apple, hawthorn, and mountainash. From these

native hosts the bacterium probably spread to the

susceptible cultivated pears and apples planted by

the early American settlers.

The three principal pome fruits, pear (Pyrus

communis L.), apple (Malus sylvestris Mill.), and

quince (Cydonia oblonga Mill.), are indigenous to

eastern Europe and southwest Asia between

Kashmir and the Caspian Sea (625), Many years

before the beginning ofthe Christian Era, cultivated

pears and apples were known to the Greeks. According to Magness (62J^), Theophrastus mentioned

wild and cultivated pears and described the art of

grafting. Pliny, of ancient Rome, named more than

30 pear and apple cultivars. With the migration of

the Romans, the pome fruits were distributed

throughout the temperate regions of Europe. During the 18th and 19th centuries, much interest developed in pear breeding in France and Belgium.

Van Mons (1765-1842), a physician and pharmacist

in Louvain, Belgium, at one time had 80,000 pear

seedlings in his gardens and originated over 400

cultivars, 40 of which have proved of lasting value.

The history of pome fruit growing in North

America began when the early settlers brought

seeds to the New World and possibly grafted trees of

European cultivars. George Menifie, who came to

Virginia in 1623 and settled along the James River,

2

had a famous orchard of apple, pear, and cherry

trees (307). Taylor (927) mentioned that grafted

apple trees were recorded in Virginia as early as

1647. In 1726, Dudley (in Fletcher, 307) stated that

''our apples are without doubt as good as those of

England, and much fairer to look to, and so are the

pears." By 1771, the Prince Nursery on Long Island

listed 42 pear cultivars in its catalog. Quince never

became as popular as pear and apple, and only a few

commercial plantings exist in America today.

Early Theories (1780— 1880)

The first printed notice on fire blight was a report

taken from a letter by William Denning, dated December 22, 1793, which was published in the "Transactions ofthe New York Society for the Promotion of

Agricultural Arts and Manufacturers" (218). Denning saw the ''disorder'' on apples, pears, and

quinces in the Highlands ofthe Hudson River Valley

as early as 1780 and suggested that it was caused by

a borer in the tree trunks, which he had found after

much labor.

He stated: ''In the first I discovered two worm

holes, running perpendicular from the tap-root up

through the heart; these holes were large enough to

admit a common pipe-stem, and reached about fourteen inches above the surface of the ground, and

from each hole I screwed out a worm." In the final

paragraph he stated: "As I am confident I have discovered it, the next step is to find a remedy; as I have

not yet succeeded, I submit it to the consideration of

the Agricultural Society, whether a publication of

the real cause of the disorder may not lead to a

discovery that may tend to stop the ravages of the

worm."

Not much was published on fire blight during the

next 25 years until 1817, when William Coxe (187)

mentioned it in his book "Cultivation of Fruit

Trees." He presumably gave it the name "fire

blight" and described it as a disease that "frequently

destroys trees in the fullest vigor and health, in a

few hours turning the leaves suddenly brown, as if

they had passed through a hot flame and causing a

morbid matter to exude from the pores of the bark.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS

of a black ferruginous appearance." He reported the

pear cultivar St. Germain as very susceptible and

Seckel as quite resistant.

In 1826, Lowell (593) suggested the 'Insect"

theory as the cause for fire blight and stated that

Xyleborus dispar (F.) (as Scolytus pyri Peck) was

killing the limbs. He suggested as the most satisfactory control measure the removal and burning of

diseased branches 30 cm below the lowest mark of

discoloration. A few years later Fessenden (300)

mentioned the same insect, followed in 1844 by reports of additional insects (79), as the cause of fire

blight. In 1837, the Pennsylvania Horticultural Society offered "a premium of five hundred dollars to

be paid to the person who shall discover and make

public an effective means of preventing the attack of

the disease usually termed pear blight" (306). This

offer brought forth many suggestions, such as soaking the ground with soapsuds, wrapping limbs with

rags sprinkled with brimstone, and driving rusty

nails into the tree to give it "an iron tonic."

Until this timeX. dispar (as S. pyri) was thought

to infest only pear trees, but the same insect was

soon also discovered in the limbs of apple trees.

Beecher (79), who considered early fall freezes as

the cause of pear blight, mentioned an unidentified

writer in the "Farmer's Advocate" in Jamestown,

N.C., who had traced the blight to "small, red, pellucid insects, briskly moving from place to place on

the branches." In addition, the ambrosia beetle

(Xyleborus dispar F.) was also beheved to cause fire

blight (69),

Even though the insect theory became rather well

accepted, there were many who could not believe it,

for no other reason than the lack of explanation for

the oozing of the branches. With this symptom in

mind, thoughts were turning more and more to early

freezes in the fall of the year, and thus frozen sap in

the branches was suggested.

In 1845, Downing (225) wrote in "Fruits and Fruit

Trees of America" that the problem of fire blight

should be considered as two distinct diseases. He

treated these individually under the headings "insect blight" and "frozen-sap blight." He described

the former as a summer blight caused by the insect

S. pyri in the young shoot growth. Symptoms of the

latter were (1) "the appearance, at the season of

winter or spring pruning, of a thick, clammy sap, of a

sticky nature, which exudes from the wounds made

by the knife, " (2) "the appearance, in the spring, on

the bark of the trunk or branches, often a considerable distance from the extremities, of black, shrivelled, dead patches of bark," and (3) "in early summer

months, the disease fully manifests itself by the extremities shrivelling, turning black, and decaying,

as if suddenly killed."

Downing explained his theory at great length by

saying that early autumn freezing would change and

poison the sap and that the poison, in turn, would be

distributed by the movement of the sap in the

spring. In support of his theory he also pointed out

that those cultivars that ripened their wood early in

the fall, like the Seckel, would be the least affected.

As preventive measures, he suggested avoiding the

use of wet land and unduly enriched soils, late summer pruning, and the culture of pear cultivars that

ripen their wood late.

Additional prooffor the frozen sap theory was that

1832 and 1844 were serious blight years in Indiana

that were preceded by early and severe autumn

freezing (79). In addition, an orchard favorably lo-'

cated to ripen its wood early in 1843 had no blight in

1844, whereas all neighboring trees were affected.

Spring frosts were not considered to cause blight,

for in 1844 they were severe in May, and no blight

followed. In Georgia, White (1022) did not accept the

frozen sap theory for the simple reason that sap

never froze in the warm climate of this State.

In 1846, Eaton (255) pubHshed a review of the

opinions on pear blight and considered, besides the

insect and frozen sap blight theories, the following

less publicized theories for the true pear blight: (1)

The effect of electricity and atmospheric changes,

since diseased trees were often noticed after a thunderstorm, (2) old age or long duration of pear cultivars, and (3) epidemics transmitted by the air, like

pneumonia or yellow fever. After careful consideration and examination of his own orchard, Eaton accepted Downing's frozen-sap theory "in order to give

the best accord with facts."

In 1846, Gookins (355) disagreed with Beecher

that freezes change the tree sap to poisonous juices,

but he considered the blight a "disease ofthe circulation." He had visited Mr. Ragan of Putnam County,

Ind., who had obtained fire blight symptoms after

introducing a small quantity of diseased sap into an

incision in the bark of a young pear tree in the nursery. This was the first known account of the experimental reproduction of fire blight without any

knowledge of the actual causal agent.

From 1848 to 1863 the theories as to the cause of

fire blight became more confusing. Ernst and
AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Downing (277) introduced a new ''sun scald" theory

and ascribed the disease to the heat of the sun,

assisted by raindrops acting as lenses. They described the effect as to "scald the sap, burst their

vessels, and produce precisely the same results that

a scorching fire would." One of the remedies they

suggested was to select cultivars with wood "of a

compact texture and slow of growth." One of the

main reasons in favor of the sun scald theory was

that summer temperatures were considerably

higher in the eastern and central United States as

compared with those in the pear-growing countries

of northwestern Europe.

In an editorial in the "Horticulturist," Downing

(226) suggested whitewashing the trees during the

winter to prevent scalding. In 1848, Wendell and

Downing (1013) reported washing trees with lime

the previous fall but still losing 15 trees from blight

affecting the trunks, although never before had they

lost a tree from this disease. In turn. Downing commented that pear blight might not have been due to

freezing but to the sun's rays acting in summer on

spots where the lime had been washed off by the

showers.

During the next several years the arguments as to

the cause of fire blight remained between the insect

S. pyriy the frozen sap theory, and the sun's rays,

but none seemed sufficiently satisfactory

(106, 276, Í71, Í72). In 1850, Kennicot (519a) reported the disease especially prevalent and severe

throughout Illinois, where the general opinion was

that the blight was caused by insects. He therefore

looked closely for insects and concluded that the

insects which killed the twigs did not produce true

blight and those which did not kill the twigs had

nothing to do with the disease.

By 1860 the knowledge about fungi causing plant

diseases was slowly becoming more widespread, in

particular that of the potato late blight epidemic in

Ireland, a disease with symptoms closely resembling

those of fire blight. The first known report suggesting a fungus as the causal organism of fire blight

appeared in 1863, when Salisbury and Salisbury

(829) described in detail several fungi belonging to

the Fungi Imperfecti. They referred all of them to

one species, named it Sphaerotheca pyrus Salisb.,

and spoke of it as a "discovery of the cause of fire

blight."

By 1867, Meehan (63i) theorized that cells were

weakened by frost or other injury and the presence

of fungi. He proposed a "fungal" theory as the main

cause of fire blight, even though he also considered

insect blight due to S. pyri and frozen sap blight. He

believed fire blight was caused by the inability ofthe

tree to maintain in a given plant part "heat enough to

maintain life." He commented that the parasitic fungus grew in the bark "causing fermentation," and

that, when it girdled the limb, it prevented the sap

from rising and thus killed the portion above the

girdled limb "as if cut away or thrown onto the fire."

In 1868, Hull (í55), a physician and fruit grower in

Alton, 111., spoke of seeing small "cells" under a

"powerful microscope" and referred to these as

fungi. He stated that "blight is induced by an extremely minute fungus" whose spores enter the tree

through pores of the bark. Later he grafted small

slices of bark with enough wood from diseased apple

shoots into succulent pear shoots. After 34 days all

were found blighted and the disease had extended up

to 5 cm above and 8-36 cm below the wounds. Thus

he also proved that fire bhght was a transmissible

disease and concluded that "the fungi causing pear

and apple blight are identical" (457). Hull also practiced root pruning as a remedy for blight control for

many years to force early setting of terminal buds

(455, 456).

During 1870-80, several investigators reported

on fire blight, some favoring the fungal theory but

others not (70, 390, 926). In 1875, Kirtland (530)

attempted to correlate certain human diseases with

this disorder to explain blight epidemics and believed that an atmospheric organism was a more

plausible cause for fire blight than any of the previous theories.

Bacterial Origin and Disease Transmission

(1881 — 1900)

Even though bacteria generally have been known

since about 1676, when van Leeuwenhoek first described the "very little animalcules," it was not until

the late 1800's that real emphasis was placed on their

importance by such researchers as Pasteur, Koch,

and Bijerinck. In April 1870, Thomas J. Burrill

(fig. 2) began his lifelong career at the University of

Illinois to study the devastating pear blight disease.

Six years later in his first report he still accepted the

fungal theory as a cause of fire blight (129). He

stated that "the cambium of the bhghted branch is

filled with very minute moving particles, very similar to those known as Spermatia in fungi and other

low plants." Atthis point Burrill was standing on the
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS

PN-6374

FIGURE2.—Thomas J. Burrill (1839-1916).

threshold ofdiscovering the true nature ofthe causal

organism of fire blight.

In 1878, he (129) stated that in "the mucilaginous

fluid from the browned tissues under our microscope, the field is seen to be alive with moving atoms

known in a general way as bacteria and . . . are

discovered in advance of the discolored portions of

the tissues." He concluded that "so far as I know, the

idea is an entirely new one - that bacteria cause

disease in plants - though abundantly proved in the

case of animals."

In June 1880, Burrill's attention was again drawn

to the blight problem by an unusually destructive

outbreak of pear blight in his neighborhood. That

summer he inoculated healthy pear and apple trees

with material taken from other pear trees affected

with blight. Sixty-three percent of the inoculated

trees became diseased, and typical characteristic

symptoms offire blight appeared 9 days later. Based

on these observations Burrill presented in 1880 his

epoch-making contribution in the field of plant

pathology to the annual meeting of the American

Association for the Advancement of Science (130).

Besides the name fire blight and apple twig blight,

he also called the disease "anthrax of fruit trees,"

since he considered the organism to resemble the

anthrax bacillus or the "vibrion butyrique of Pasteur

and the Bacillus amylobacter of van Tieghem." The

earliest pictures of the bacterium were published in

1881 (132). It was not until 1882, however, that

Burrill (133, 13Jf) published the description of the

bacterial species and named the blight pathogen

Micrococcus amylovorus.

In 1881, Peffer (733) in Wisconsin was the first to

report that fire blight started in the flowers, but he

did not accept Burrill's bacteria theory since his

inoculation experiments failed to induce the disease

without puncturing the shoot. He sent specimens of

these blighted blossoms and twigs to Burrill, who

accepted Peffer's theory that blight could start in the

blossoms. However, Peffer still believed in the frozen sap theory followed by fermentation of the tissues with the beginning of warm weather.

Even though Burrill discovered the bacterial origin of fire blight, named the causal organism, and

performed several inoculation experiments, he

never isolated the bacterium in pure culture nor

reinoculated it into healthy tissues to prove definitely thatAÍ. amylovorus was the real causal agent.

In 1884, Arthur U7) at Geneva, N.Y., performed

several cross-inoculation experiments vrith pears,

apples, quinces, hawthorns, and juneberries. He

isolated and grew the bacteria by means of a succession of artificial cultures in a sterilized infusion of

cornmeal. He then separated the bacteria from their

juices, filtered a strong infusion of blighted pear

through a porous earthenware vessel, and inoculated the bacteria and the filtrate into green Bartlett

fruit. After 1 week, fruits inoculated with the bacteria were thoroughly blighted, whereas those

treated with the filtrate showed no signs ofinfection.

Thus in 1885, Arthur (1081) presented the first

doctoral dissertation on the blight organism and concluded that the bacteria were the direct cause ofthe

disease Hi). During 2 years he published many reports on fire blight, including descriptions of blight

symptoms, growth and development of the bacterium in culture media, and the application of control measures through limb removal and the use of

lime and sulfur U5, i6, 4-8-52).

Up to this time the blight organism was known

and observations had shown that the disease started

in flowers and shoots, but no one had actually observed how the bacteria reached these plant parts.

As early as 1884, Forbes (317) observed blight lesions associated with feeding of the tarnished plant

bug (Lygus lineolaris (Palisot de Beauvois)) (as L.
6 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

pratensis L.), and with only observational evidence

he considered that this insect was acting as a vector

of the disease.

In 1890 at the U.S. Department of Agriculture

(USDA), Merton B. Waite (fig. 3) was the first to

undertake a research project on fire blight. He soon

discovered that the blight organism could enter

blossom nectaries without punctures and that bees

spread the bacterium from infected to healthy pear

flowers (986-988). He was able to start a small-scale

artificial epidemic by infecting the flowers of a few

pear trees on the edge of an orchard and allowing

free access of bees. When he protected some flowers

with mosquito netting, they remained free from

blight. In 1901, Waite {99i) stated, 'Ve have

scarcely been able to find a direct normal method of

introduction of the disease to twigs without the introduction of some mechanical or insect puncture."

He observed up to 40 species of insects visiting pear

blossoms, many of which were shown experimentally to be carriers. In 1904, he reported that insects

not only distributed the blight organism but that

they were active agents injecting the germs into the

tissues (995).

In 1895, Waite (989) stated that ''after prolonged

investigations the complete Hfe history of the microbe. Bacillus amylovorus (Burr.) Trevisan, has

been worked out" and that the removal of late infections in the fall is "the preventive remedy for pear

blight." In 1898, he published the earhest account of

some of the cultural characteristics of the blight

pathogen (993). Waite became the first to isolate a

"false yellow germ" ^ from blight-infected tissues,

and he was also the first to establish a pear-breeding

program for fire blight resistance.

Waiters (991) chapter on fire blight in the 1895

USDA "Yearbook of Agriculture" was the first detailed account of the disease at that time. He and

others during this period agreed that the only satisfactory method of controlling pear blight was to

prune the blighted parts of the trees far below visible blight symptoms, followed by burning all diseased branches (190, 896, 910, 992, 999). At this

time the succulent watery shoots were generally

known to be susceptible to blight infection. Wood

ashes and ground bone were being recommended to

induce early maturation of the wood.

In New Jersey, Halsted (377) conducted a detailed, long-range experiment to test the effect of

2 Unpub. data, Fruit Lab., U.S. Dept. Agr., Beltsville, Md.

summer or winter pruning on the Kieffer pear cultivar grown in cultivated or sodded rows with or

without barnyard manure or commercial fertilizers.

He found differences in tree yields not significant

and blight incidence inconsistent.

A more detailed description of the new concept of

phytobacteriology and the significance of the discoveries by Burrill, Arthur, and Waite was published in 1971 by Baker (61).

Nature, Dissemination, and Disease Control

(1901—20)

In 1901, Chester (171) in Delaware was the first to

report on a rather detailed inoculation experiment,

in which he tried to determine differences in susceptibility between leaves, fruit, buds, succulent

shoots, and older wood of pear. He concluded that

pears became infected only by direct inoculation if

the surface was injured. Only the more tender succulent tree tissues became infected.

In New York, Whetzel (1016, 1017) and Whetzel

and Stewart (1018) made detailed studies on the

nature and role of bhght cankers in apple trees.

These were the first and most extensive reports

ever pubHshed, with numerous photographs of limb,

crotch, trunk, and collar cankers.

In Colorado, Sackett (826) determined that 20

percent of blight cankers on small limbs and twigs

contained virulent bacteria at the time of tree blossoming.

In 1905, Chisholm (172) used the term ^^zymotic''

pear blight to distinguish the true bacterial blight

from insect, frozen sap, and summer blight. Control

of the disease was recommended through early maturation ofshoots in the growing season and rigorous

pruning of blighted branches with emphasis on disinfecting tools.

At the U.S. Department of Agriculture, Erwin F.

Smith (fig. 3) was beginning to attract international

fame with his extensive research in the newly established field of phytobacteriology (875). In 1905, he

was the first to publish detailed accounts of the bacteriological and cultural characteristics of many

bacterial plant pathogens, including B. amylovorus

(876). Of special interest are his detailed studies on

flagella stains and the detrimental effect of direct

sunlight on the growth of the blight pathogen in

vitro. In 1912, Smith (877) reported on a new plating

and identification technique to classify bacteria on

the basis of their Gram stain. This was the first

record of 5. amylovorus being rod shaped , motile.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS

PN-6375

FIGURE 3.—Pioneers in pathology and bacteriology of fire blight

at the U.S. Department of Agriculture about 1900: Left, MertonB. Waite(1865-1945);rií^feí, ErwinF. Smith(1854-1927).

and a Gram-negative bacterium. His first textbook

{878) on bacterial diseases of plants in 1920, in which

he devoted 30 pages to fire blight, established the

field of phytobacteriology.

In 1913, Bachmann (55) presented the first definite account with camera lucida drawings that fire

blight bacteria travel mainly through the intercellular spaces of the host tissue. In Oregon, McCormick (1116) made the first histological studies ofpear

tissues and indicated no differences in the cell wall

thickness and the size, arrangement, or position of

the intercellular spaces between resistant and susceptible pear species or cultivars.

In Washington orchards, Heald (393) observed

that blight bacteria were able to enter the leaves of

apple and pear without apparent injury to the tissues. He noted all stages ofleafinvasion, from slight

marginal infections to lesions that had advanced

throughout the entire leaf blade and down the

petiole. Several years later Heald (39^) confirmed

his observations through artificial inoculations.

Under Wisconsin conditions, Reinking (1127)

confirmed previous observations that B.

amylovorus is very sensitive to rapid drying conditions. All attempts to isolate the organism from

twigs blighted the previous season failed, and he

never observed any advance of the disease during

the winter or ooze production from cankers in the

spring. Fulton (318) also was unable to inoculate

healthy apple twigs with pieces of blighted tissues

from branches cut several months before. He concluded that the blight organism died rapidly in

pruned twigs left on the ground. In Washington,

Hotson (43i) observed that blight exúdate on

branches or fruit remained alive 10-13 days when

exposed to direct sunlight. When oozing branches

were left on the ground, partly shaded, or in a cover

crop, live bacteria were isolated about 30 days after

the limbs were cut.

During 1912-20, several interesting observations

were made on the epidemiology of fire blight. In

New York, Stewart (1135) wrote the second dissertation on fire blight, in which he covered nearly all

phases of the nature, life cycle, and control of the

disease. He added considerably to the knowledge of

bacteriology of ß. amylovorus, mentioned several

additional insects disseminating the disease, and

placed considerable emphasis on precautions to prevent disease spread through contaminated pruning

tools. In Oregon, O'Gara (70^) appears to be the first

to mention small bacterial strands (cirri) extending

from lenticels of infected pear fruit. He was also

among the earliest to prove that certain insects were

definite carriers ofthe blight organism by capturing

insects from blighted orchards and allowing them to

walk on culture media. Pathogenicity of the resulting cultures was proved by inoculation studies (703).

In 1915, Stewart (90i) was the first to mention the

effect of hail on severe outbreaks of fire blight. He

observed numerous infections on branches and

trunks of 2-year-old trees of the Bartlett pear cultivar in a nursery and on 4-year-old trees in an orchard 2 weeks after the trees were struck by a severe hailstorm. The following year, Hotson (^33)

confirmed similar observations in the Yakima Valley

of Washington. On many trees, few twig infections

occurred, but 50-60 percent ofthe fruit was severely

blighted.

In 1917, Gossard and Walton (360) reported that

rain proved to be an important carrier for fire blight.

Branches protected by cheesecloth, located below

artificially inoculated blossoms, showed a high percentage of infection. However, check trees, protected from rain drip by oilcloth or from insects by

cheesecloth, remained free from blight. The following year, Stevens et al. (898) reported their observations on possible wind dissemination of blight bacteria, but their evidence was inconclusive.

In addition to these meteorological factors, it became evident that rich soils, heavy manuring, use of

large quantities of commercial fertilizers containing

a great deal of nitrogenous material, irrigation, and

severe pruning tended to stimulate the growth of

tender, succulent shoots, which in turn increased the
AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

chance and degree of blight development

{Jf61, 825, 90Jf, 995, 1023).

Following the pioneer work of Waite on the insect

dissemination of fire blight, many investigators reported on different phases of the blight-insect relationship. In 1909, Whetzel and Stewart (1018) concluded on observational evidence that aphids and

leafhoppers were largely responsible for introducing

the blight germ into the tips of growing shoots.

These insects and the curculio were frequently observed inflicting wounds in the fruit.

According to the observations and experimental

evidence by Jones U8^) in Ontario, a large proportion of the new twig infections, including most of

those that occurred after the blooming period, appeared to be due to dissemination and inoculation of

the bacterium by aphids, particularly Ap/¿ís mali F.

and the woolly aphid Eriosoma lanigerum (Hausmann) (as Schizoneura lanígera Hausm.). He also

found aphids to be the principal means of spreading

the disease in apple nurseries and traced many cases

of blight in pear trees to wounds made by the

shothole borer (Scolytus rugulosus (Ratzeburg)).

These beetles were also found in apparently healthy

bark that later developed blight near their borings.

In 1911, Jones ií85) proved experimentally that this

beetle carried and injected the blight germ.

In 1913, Stewart (901, 902) observed that fire

blight was spread by the apple aphid (Aphis pomi De

Geer) in quince nurseries. However, he found the

tarnished plant bug much more important in the

dissemination of the blight organism in nursery

stock. Stewart showed that these insects visited

blighted tissues, became smeared with the gummy

exúdate, carried the bacteria to the tender succulent

shoot tips, and inadvertently inoculated the tissues

while sucking the sap, with the result that the twigs

soon developed typical fire blight.

Referring to the collar blight form offire blight on

apple trees, Orton and Adams (7H) stated that apple

tree borers were probably the most active agents in

spreading blight. They found in 90 percent of the

observations that these borers had been associated

with the disease at the collar. They did not determine whether these insects were direct carriers or

only provided points of entry for the blight germ. On

account of the almost universal presence of the

woolly aphid in collar blight cases in Pennsylvania,

this aphid was thought possibly to be the carrier of

the organism to the wounds made by the borers.

In 1914, Reed (770) attributed large numbers of

insects, particularly honey bees (Apis mellifera L.),

to an unusual outbreak of apple blossom blight. He

considered that this epidemic was due to the concomitant presence of large numbers of blossoms,

insects, and holdover cankers. In New York,

Stewart and Leonard (905, 906) proved by direct

experimentation in apple seedlings that the following insects could disseminate and inoculate the

blight pathogen: A leafhopper (Empoasca fabae

(Harris) (as E. mali (Le Baron)), a capsid (Plagiognathus politus Uhler), and four sucking insects

(Campylomma verbasci (Meyer), Orthotylus

flavosparsus (Sahlberg), Polymerus basalis (Reuter) (as Poeciloscytus basalis Reuter), and Adelphocoris rapidus (Say)). They considered the tarnished plant bug and C. verbasci particularly important because of their abundance in nurseries and

preference for succulent shoot tips. The pear psylla

(Psylla pyricola Foerster) and two species of fly

(Polleniaruáis (F.) andHomoneura bispina (Loew)

(as Sapromyza bispina Loew)) gave negative results.

In Wisconsin, Burrill (128) demonstrated experimentally that the aphid Macrosiphum avenae (F.)

(as Aphis avenae (¥.)) and leafhoppers carried the

blight organism and inoculated it into the wild crab

apple (Pyrus coronaria L.). In Kansas, Merrill

(64-1-61^3) observed a direct relationship between

aphid infestation and the amount of bhght infection

and demonstrated that aphids deposited their eggs

in blight cankers and other rough places in the bark.

Soon after hatching from the eggs in the spring, the

young crawled through the blight exúdate.

By 1916, Gossard (357, 358) in Ohio reported on

the role of beehives in the spread of fire blight. He

found that the blight organisms remained alive and

sufficiently virulent after 47 hours' incubation in

sterilized honey and 71 hours in aphid honeydew. A

few years later, Gossard and Walton (361) summarized their extensive work on bees, beehives, and

honey in relation to blight dissemination. They obtained blight infection by inoculation oftender twigs

with (1) apple pollen from the baskets of bees, (2)

mouth parts of bees, and (3) honey from different

beehives. They also found that the organism survived up to 72 hours in honey, up to 7 days in aphid

honeydew, and up to 5 days in peach, plum, and

cherry nectar.

The earliest methods of controlling fire blight included eliminating or pruning out diseased twigs,

branches, and water sprouts (20, 21, 172, Í63, Í69,
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS

560, 702-70Í, 928, 1012, 1022). In the larger limbs

and tree trunks, cankers were removed with cutting

and scraping tools (1012, 1018).

In 1918, Reimer {778) reported a new disinfectant

for wound dressings. Cyanide of mercury proved

very effective by inactivating all cankers treated.

However, cankers treated with bordeaux paste,

bichloride of mercury, cresol, lime sulfur, nicotine

sulfate (Black Leaf 40), and chlorozene remained

active. Cyanide of mercury was not effective for

disinfecting metal tools.

Chester {171 ) also made a rather extensive study

of the nature and control of blight cankers in trees.

After experimenting with a formaldehyde-glycerin

mixture, a copper-whale oil-soap mixture, and a

bordeaux-rosin-soap mixture for application to

scraped cankers, he found the formaldehyde mixture the most promising for wound treatment and

tree recovery. Formaldehyde and corrosive sublimate (bichloride of mercury) were the most extensively used disinfectants for canker blight control for

many years.

During the early years of blight control, considerable emphasis was placed on collar blight. With the

confusion that some collar blight was due to winter

injury, this form of fire blight started receiving attention about 1905(^70, 7U, 1016,1017). At first the

disease appeared to be most prevalent and more

serious on apples but soon after was also observed on

pears. Recommendations for the control of collar

blight were similar to those mentioned for cankers in

large limbs {1^60, 713, 7U, 997). If tree trunks were

more than half girdled and beyond surgical treatment, trees were removed at once.

An interesting attempt to control fíre blight was

tried by Bolley (100) in North Dakota. He strapped a

large bottle to a pear tree to feed it internally with

formalin and various nutrient solutions, but the effects on blight control were inconclusive and the

treatment required too much manipulation for the

average grower.

The earliest attempts to control fire blight by

spraying were started with lead arsenate and lime

sulfur, which were used to control fungus diseases,

the San Jose scale (Quadraspidiotus perniciosus

(Comstock)), and many other insect problems (323,

70i, 851). These chemicals were soon followed by

sprays of bordeaux mixture, which were also being

used to control apple scab (Venturia inaequalis

(Cke.) Wint.) (20, 606, 7J,6, 781). Stronger bordeaux was used successfully in a late dormant

spray in the spring to kill oozing bacteria on apple

and pear.

Soon after 1900 the emphasis was also on varietal

resistance as a means of controlling fire blight. With

the introduction of many pear and apple cultivars

from Europe, differences in degree of blight resistance were observed among cultivars and within

seedling populations (20, 396, 850, 918). In 1908,

Waite established the earliest pear-breeding program at the U.S. Department of Agriculture (990).

Soon after, Hansen (381) and Reimer (777, 780)

traveled to the Orientto collect blight-resistant pear

material. Cox (186) published an excellent early account of the oriental pears and their hybrids. By

1920, small breeding programs were underway at

several locations throughout the United States (5^,

63, 381, 731, 1035).
CHAPTER 2

GEOGRAPHIC DISTRIBUTION

In many reports and popular bulletins the presence of fire blight in certain countries has been erroneously mentioned. Based on published records

and detailed correspondence, we have attempted to

determine accurately the present distribution offire

blight in the world, including North America, Central and South America, Europe, and Oceania and

Africa.

If our knowledge and survey are complete, fire

blight has been officially recorded in 14 countries—8

in Europe including Egypt and 6 bordering the

Pacific Ocean including the United States (fig. 4).

This is the extent of the distribution according to

published records of the disease. Unavailable records or unpublished observations of fire blight in

other countries cannot be ruled out.

North America

United States

Since the earliest known observations in 1780 of

fire blight in New York (218), and as orchards became numerous and more contiguous, fire blight

gradually spread westward over the Allegheny

Mountains into the Mississippi River Valley. By

1817, it was recognized by Coxe (187) in the oldest

American book on fruit tree culture as one of the

important disease problems in fruit production. Severe blight epidemics were experienced in the Eastem States in 1826 and 1832. About 1840, fire blight

reached Ohio, Indiana, and Illinois. Beecher (79)

reported that one of the most widespread and destructive epidemics occurred in 1844. Many orchards

were completely ruined. During 1876-80, fire blight

^m WIDESPFlbAfJ

^ FIRST (X;CUHI-iE:MCE

SECONDAHY OCCURRENCE

10

FIGURE 4.—Worldwide distribution of fire blight.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 11

had become so destructive in the orchards of Ilhnois

and adjacent States that it became one of the chief

topics of discussion at the State and county horticultural meetings of that period.

Fire blight moved steadily westward with the settlement of the country. For many years the vast

expanse of the Great Plains, as well as the Rocky

Mountains, provided effective barriers for movement of the bacterium and prevented spread of fire

blightto the Pacific Coast States. After reaching the

west coast and where there were no plains and

mountains as barriers, fire blight rapidly moved into

all the Southern and Gulf Coast States.

Despite claims by growers in the fruit regions of

Cahfornia that the blight could not thrive there, it

suddenly appeared in the pear orchards about 1888

near Chico, Calif. By 1902, Pierce (7^0, 7U) first

confirmed the identity of the disease throughout the

State. Fresno and Kings Counties appeared to be

the center of the disease. Detailed accounts of the

early history of fire blight in California have been

published {61, 322).

After blight was found in California, it spread

rapidly northward and caused disastrous epidemics

in Oregon and Washington. Certain parts of the

Rocky Mountain region were the last to be invaded

by the disease. Fire blight was not reported in Montana until about 1905, when it first appeared near the

town of Hamilton (918), During the following few

years it spread to all parts ofthis State. By 1908, fire

blight had appeared in the Rogue River Valley of

Oregon and within the next few years it reached the

Umpqua and Hood River Valleys {702), It was not

until 1915, after the disease had been reported from

sections of Washington State and the Province of

British Columbia in Canada, that fire blight was

discovered in the Willamette Valley of Oregon.

About 135 years after it was first observed in New

York, fire blight had moved several thousand

kilometers across the North American continent and

to every corner of the United States.

Canada

Fire blight was first observed outside the United

States in the Niagara peninsula of Ontario, Canada.

When the disease reached this area is not precisely

known; it may well have been before fire blight

reached California. In 1904, Harrison {888) mentioned its presence in many counties of Ontario and

quoted several fruit growers who had seen the disease as early as 1840. After many years of comparative freedom from blight, an outbreak in 1943 in the

Niagara peninsula brought fire blight into prominence again {159).

On the west coast of Canada, Eastham {253) reported that the disease attracted attention in British

Columbia about 1911, but it undoubtedly had been

there for some time. By 1924, fire blight was still

confined to the interior of British Columbia, but a

few sporadic cases of twig blight had been observed

along the coast. Since 1924, fire blight has spread to

all pear and apple growing areas of Canada. In the

French-speaking Province of Quebec the disease is

known as "Brûlure bactérienne." In the early 1940's,

fire blight was very severe in several Quebec apple

orchards {766). Thatcher {930, 931) mentioned that

the disease was also catastrophic 10-12 years earlier

and referred to a ''blitzkrieg" by the causal organism.

For many years the Annapolis Valley in the isolated maritime Province of Nova Scotia was known

for the absence of fire blight. A report in 1914 mentioned the presence of blossom blight near Wolfville,

but the description may have inferred pear blast

{113). A survey in 1950 by the Canada Department of

Agriculture {H2) reported the presence of the disease in New Brunswick on apple and pear and in

Prince Edward Island on apple, pear, Crataegus,

andSorbus. Atthattime, fire blight was not found in

Nova Scotia. In 1966, however, Gourley et al. {362)

and Lockhart and Gourley {587) reported the first

confirmed identification of fire blight in 17 pear orchards in the Annapolis Valley. The finding of some

old cankers indicated the presence ofthe blight prior

to 1966. The disease was also observed on apples and

hawthorns adjacent to the pear orchards. Analysis

ofweather records indicated thatin 1966 the number

of days of consecutive temperatures exceeding the

minimum and optimum for blight development was

about double that of the preceding 4 years. This

condition, combined with heavy rains during late

May and early June, apparently provided ideal conditions for the spread and growth ofthe bacterium.

For a detailed account of the occurrence of fire

blight in each Province since the 1920's, refer to the

annual reports of the Canadian Plant Disease Survey {U2). A survey in 1972 in southern Ontario

revealed that damage of economic significance occurred in one-third of the orchards visited {239),

Central and South America

In 1921, Ramirez {768) included fire blight in a list

of plant diseases observed around Mexico City,
12 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Mexico, but this observation was not confirmed at

that time. In 1943, fire blight (tizon de fuego) was

seen on apples and pears in the Canatlán region of

Durango (797). In the fall of 1967, Ridley ^ observed

fire blight on Bartlett pear trees in Guanajuato,

Mexico.

In 1968, Schieber and Sanchez (8J^2) included fire

blight in a preliminary list of plant diseases in

Guatemala. They reported finding the disease on the

pear cultivars Bosc and Bartlett in Cantel, Departamento de Quezaltenango, and in San Bartolomé

Milpas Altas, Departamento de Sacatepéquez. In

Guatemala, pear plantings are ratherrecent and also

somewhat isolated at high elevations. Fire blight

was most likely introduced with the original plant

material.

In South America, fire bhght has only been reported from Chile by the U.S. Foreign Agricultural

Service. The disease was apparently first observed

on apples during the 1959-60 growing season in the

locality of Padre Hurtado near Santiago in the

Province of Santiago. Identification ofthe bacterium

through reinoculation was confirmed by Mrs.

Romoli "^ at the Ministry of Agriculture. Fire blight

was apparently confined to the Padre Hurtado area

and occurred only when climatic conditions were

favorable for disease development. In all three Latin

American countries the disease is considered oflittle

economic importance.

Europe

Great Britain

The most recent wide-range spread of fire blight

occurred with its introduction to England, probably

sometime during the late 1940's or early 1950's. Outbreaks were first reported in 1958 by Crosse et al.

(192y 193) on pear trees near Maidstone, Kent, in

southeastern England. Immediately the disease became very severe on the cultivar Laxton's Superb

because of its late blooming characteristic {796), According to the best available information, the bacterium was probably brought into the country either

on infected plant material or on contaminated fruit

boxes from overseas shipments. Fruit boxes were

used in these orchards in Kent where initial blight

was found about 1956-57 {570). By 1959, fire blight

was observed on pears, hawthorns, and whitebeams

{Sorbus aria (L.) Crantz) in the southern suburbs of

^ Pers. commun., Gerber Prod. Co., Fremont, Mich.

^ Pers. commun., U.S. Foreign Agr. Serv., Santiago, Chile.

London as well as in the borough of Southend-onSea, located across the Thames River from Kent

{J^29). These areas are both within 16 to 32 km of the

infected orchards in Kent.

By 1966, fire blight had spread east to Canterbury

and north and westward to Suffolk and Berkshire

Counties. This was considered the worst blight year

in England, with new infections observed in several

thousand pear trees and pyracantha and cotoneaster

plants {S66). In addition to the Kent area, isolated

outbreaks were reported in 1958 from Worcestershire on pear, in 1960 from Wisbech on pear, and in

1967 from the Merthyr Tydfil area in South Wales on

hawthorn {367).

Lelliott {571) and Glasscock {329) published detailed accounts of the history, present distribution,

and attempted eradication of fire blight in the

United Kingdom. Because of its presence in many

rosaceous hosts growing in parks and private gardens throughout London and its suburbs, attempts

at complete eradication of fire blight have failed. In

1965, Lelliott and Hayward {576) reported several

hosts ofErwinia amylovora, many of which had not

previously been observed with symptoms ofblightin

the United States or elsewhere. Of interest was

their observation that Swedish whitebeam {Sorbus

intermedia (Ehrh.) Pers.) and Cotoneaster horizontalis Decne. appeared to be immune under EngHsh

conditions, whereas Sorbus aria and other cotoneasters were severely infected. In 1971, however, Lelliott {574) and the European and Mediterranean

Plant Protection Organization {290) reported moderately severe natural shoot infection in containergrown plants of C. horizontalis in a nursery in

Bucks.

The first occurrence of apple bhght was recorded

in 1967 in two commercial apple orchards in Kent

{329, 573). The following year it appeared in more

trees there and elsewhere. In August 1969, the

Ministry of Agriculture {368) announced that severe

outbreaks of fire blight had occurred in 37 apple

orchards throughout Kent. By September 1969, the

number of blighted orchards had risen to 45, with

more than 1,700 infected trees. The outbreaks occurred mainly in the Teynham, Faversham, and

Canterbury areas but also in fruit-growing areas of

Essex and Suffolk. Fire blight was most severe on

the Q- to 10-year-old Cox's Orange Pippin cultivar

trees on Mailing 2 and 9 rootstocks. Many infections

were also noticed on the cultivars Egremont Russet,

Crawley Beauty, and Miller's Seedling.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 13

In September 1971, fire blight was found on hawthorn in and around the city of Bristol (283). Apart

from this and other outlying occurrences in Somerset, Worcestershire, and Wales, the disease is generally present in the entire area east of a line from

Southampton over Oxford to The Wash. Although

present knowledge ofthe distribution offire blight is

restricted to the areas indicated here, infection possibly, if not likely, has occurred in other areas of

England and Wales.

Poland

Following a questioning note in 1959 by Pieniazek

(739), fire blight was first observed in continental

Europe in July 1966 (103). The disease, known as

'zaraza ogniowa,' was seen in experimental orchards

of the Research Institute of Pomology at Milobadz,

about 25 km south of the port of Gdansk (lOJ^). The

disease occurred in an orchard of 8-year-old pear

cultivars and later spread to two other sections of

the orchard. By the end of the summer, fire blight

was also observed on 8-year-old apple trees growing

near the infected pears. Some apple trees and more

than 50 percent of the pear trees were destroyed by

the disease in one season. Borecki and Lyskanowska

(lOi) made many isolations and positive identification of the causal organism.

In June 1968, Burkowicz (12J^) observed a mild

case offire blight in the pear cultivar collection ofthe

main Research Institute of Pomology at Skierniewice. The disease was seen in the current season's growth of a single tree, which had been propagated in the spring of 1968 with grafts ofthe cultivar

Conference obtained from abroad. Careful examination ofthe other trees in this pear collection revealed

no other cases of fire blight, and the infected tree

was removed and destroyed. In spite of many surveys, the disease was not observed in Poland in 1969.

In September 1970, new outbreaks were recorded

in young apple trees and some old hawthorn bushes

in a nursery in Dworek in northwestern Poland

(12J^). In September 1971, fire blight was detected in

two pear orchards, 6 km apart and about 40 km south

of Gdansk (287). In one orchard only one tree

showed disease symptoms, and in the second orchard six trees were infected. Since that time, surveys have been conducted throughout the country,

and the northern coastal region has been strictly

quarantined to prevent spread of blight to central

Poland (12Í).

The Netherlands

In August 1966, fire blight (perevuur, bacterievuur) was observed for the first time in the

Netherlands in a few pear trees and several hawthorn shrubs on the island of Noord Beveland in the

Province of Zeeland (679). The following spring,

Roosje (800) and Roosje and Meijneke (801) prepared warning articles for Dutch fruit growers to

acquaint them with the symptoms of this dangerous

disease. This blight, recorded in the far southwest

corner of the Netherlands, was not near a seaport

but occurred in the Dutch pear-growing area 100 km

from the orchards in Kent, England. This suggested

the possible dissemination of the bacteria through

air currents by long-range flying insects or migratory birds. Severe eradication procedures were

started during 1967, including the destruction of 10

km of hawthorn hedges and nearly 175,000 individual bushes and also the strict enforcement of the

removal of beehives and secondary blooms from the

remaining orchards (635). No recurrences of fire

blight were observed in 1967 or during the following

3 years (313, 678).

In 1971, however, new occurrences of fire blight

were discovered on the neighboring island of

Schouwen-Duiveland and in a second region, 100 km

to the north, near the towns of Den Helder and

Wieringen in the Province of North Holland (287,

635a, 636, 637). Infection was found in several pear

orchards, but primarily on hawthorns along dikes

and roads, on farms, and in dunes and private gardens. In 1972 and 1973, infected Cotoneaster

salicifolius Franch., Sorbus aria, and one

pyracantha were also found (635a, 639, 680). All

infected plant material was destroyed by fire and the

trunks were covered with a mixture of soluble

2,4,5-T ester and oil.

In 1974, an intensive survey resulted in the discovery of three new foci of fire blight in addition to

the two existing ones (635a). They were observed in

and around the towns of Castricum (North Holland),

Rolde (Drente), and Apeldoorn (Gelderland). The

last two were the first areas located away from previous recordings along the coast ofthe Netherlands.

Except for a few infected hawthorns, all blight recordings were made on cotoneaster, mainly as blossom blight. In 1975, Meijneke ^ mentioned additional blight observations on cotoneaster in the cen-

^ Pers. commun., Plant Protect. Serv., Wageningen, the

Netherlands.
14 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

tral and and northeastern regions of the country. C.

dammeri Schneid., C. salicifolius, and C. ivatereri

Exell and their cultivars proved to be very susceptible. In all nurseries, parks, and garden centers

combined, more than 2 million cotoneasters were

destroyed (635a). Since 1975, the name of the disease has been changed to 'bacterievuur' in order to

cover all rosaceous host plants (680a).

Cooperative work is in progress among investigators from Denmark, West Germany, and the

Netherlands to establish field plots with various

species, cultivars, and clonal selections of rosaceous

host plants in a search for resistant germplasm. In

the meantime, intensive surveys are continuing in

the Netherlands to detect new outbreaks, and a

policy of rapid, thorough blight eradication is in effect.

Denmark

In late August 1968, fire blight was reported for

the first time in Denmark (217, Í88). The disease,

known as 'ildsot,' was noticed on approximately 100

fruit trees in 8 orchards within an area of 40 hectares

between the towns of Norre-Alslev and Stubbekobing in the northern part of the island of Falster.

During a survey by the State Plant Protection Institute, fire blight was found on several pear and apple

cultivars, a single bush ofCotoneasterwatereri, and

many hawthorns. The pear cultivar Conference

seemed the most seriously infected.

Klarup (531 ) published a detailed account of the

initial outbreaks in Denmark, including eradication

measures. These followed the strict procedures used

in the Netherlands in 1966, including complete

eradication of all host plants in the infected area,

strict enforcement of the removal of beehives and

secondary blossoms from the fruit orchards, and

intensive inspection of nurseries and orchards to

locate new outbreaks. Despite the enforcement of

these intensive eradication procedures, fire blight

recurred in Denmark in 1969.

Nilsson (690) reported that the disease was not

observed until early August 1969, but then at 80

locations in the northern part of the islands of Falster and Lolland, as well as on the small islands of

Femo, Fejo, and Asko between these two larger

islands. By the middle of November, fire blight had

been recorded in more than 300 locations, but predominantly in hawthorn hedges surrounding the orchards (56). The disease was reported in an area of

about 260 km^, restricted to a narrow band 25 km

wide and 130 km in length. Migratory starlings have

been suspected of having a role in disseminating the

blight pathogen from England or northern Poland

(690). Bech-Andersen (77, 77a) stated thatthe oldest

infections were on hawthorns located on migration

routes of starlings (Sturnus vulgaris L.) and willow

warblers (Phylloscopus trochilus (L.)).

In 1970, fire blight was observed in new areas on

Falster and Lolland and during that summer also on

the neighboring islands of Langeland and Sjaelland

(282, Í88). Infections were observed primarily on

hawthorn (7Jf) but also on pears and apples. Klarup

(532) reported that the first symptoms in 1970 were

noticed in early June with definite evidence of blossom infection in pear. All infected and suspicious

plant material was destroyed and additional emphasis was placed on insect control programs in pear

and apple orchards. Hellmers (397) and Nilsson (691,

692) reported in more detail on the Danish hosts

infected with fire blight and methods of control.

Bech-Andersen (75) listed 16 pear cultivars and 25

apple cultivars with their degree of blight resistance.

In 1971, fire blight recurred again on all four islands and again mainly on hawthorn but also on

pears and apples (282). After more extensive surveys, many infected hawthorn hedges were also discovered along the west coast of southern Jutland

between the towns ofTonder and Ribe. According to

the amount of damage caused by blight, the original

infections were considered to be 4-5 years old. Following eradication of infected hawthorns surrounding fruit orchards, the amount of fire blight was

significantly reduced when hawthorns were kept at

least 25 meters away from the fruit trees (76).

In 1976, the Plant Protection Service (216a) reported the presence of fire blight in 72 localities

throughout southern Denmark, principally on hawthorn and cotoneaster. Currently about 2,500 plants

belonging to 40 species in the Rosaceae are being

tested for their degree of blight resistance in 2

localities in Denmark where natural infection on

hawthorn is extensive (1^89).

West Germany

More than 10 years before fire blight (feuerbrand)

was first observed in West Germany, several

lengthy reports appeared in various journals emphasizing the danger of this disease to the German

pome fruit industry (101, 107, 127, 308, 5U, 888-

890). Since the discovery offire blight in Denmark in
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 15

1968 just north ofthe Danish-German frontier (south

Jutland), the Government of West Germany undertook Wight inspection surveys in northern Schleswig-Holstein, particularly on islands in the North

and Baltic Seas. In August 1971, fire blight was

discovered in hawthorns on the islands of Sylt, Fohr,

and Nordstrand, located nearest to Denmark along

the upper North Sea coast of West Germany {282,

302, 6^6), Immediate action by the Plant Protection

Service resulted in the uprooting and destruction of

18,000 plants. Of these, 11,349 were infected (88

percent hawthorn and 11 percent pears) {302).

In late May 1972, numerous new infections were

noted, many of which were approximately 100

meters from infected shrubs in the previous year

{6^7). In that year, 19,000 plants were destroyed, 50

percent ofwhich were infected (97 percent hawthorn

and 3 percent pear). Besides these two hosts, fire

blight was also observed on 20 quince trees and 2

Sorbus aucuparia L. plants {92, 309, 6^5, 6^7). The

careful survey of this area ofWest Germany and the

detection and destruction of host plants have been

extremely intensive. No consistent spreading of fire

blight was found during 1973 except a few infected

Cotoneaster salicifolius plants in nurseries northwest of Hamburg {293). More inspections in this area

did not reveal any other foci. Additional surveys are

being continued throughout northern Germany

every year to detect blight outbreaks farther south.

In 1974, Zeller {10Í6) published the first German

review on fire blight. Since then, he has established

large test plots in Schleswig-Holstein to study the

possible existence of resistant plant material among

numerous ornamental host plants in the rosaceous

family {lOJ^Sb, 1046c).

Symptoms offire blight reportedly were observed

by Müller ^ on pear and hawthorn in East Germany,

but detailed information on location and degree of

damage was not available.

France

In August 1972, fire blight was detected in the

northernmost part of France {291, 832). The disease,

known as 'feu bactérien,' was found mainly in hawthorn hedges near the Belgian border in an area 20

km long and 10-15 km from the coast, including the

city of Dunkirk. This region, which is located on the

routes of migratory birds, has no fruit orchards or

ornamental nurseries {831). The blight organism

was isolated, identified, and its pathogenicity

proved on pear fruit {83Jf.).

During 1973, only a few additional blighted hawthorns in the same focal area were found, and all

infected and suspected hedges were uprooted and

destroyed {291, 29^, 831). A study of several cultures ofE. amylovora recovered from the FrancoBelgian region revealed a close similarity to isolates

from Great Britain, Canada, and the United States

{732).

Belgium

Within 1 month of the discovery of fire blight in

France, the disease was also reported immediately

across the Franco-Belgian frontier around the city of

Adinkerke {292, 976). In addition, blight symptoms

were also observed 5-10 km farther north along the

Belgian coast near the towns of Wulpen and Nieuwpoort and 20 km farther south along the French

frontier near the town of Haringe. Findings mainly

concerned hawthorn hedges, but blight was also detected in two pear trees and one bush ofCotoneaster

salicifolius. Veldeman and Porreye {977) positively

identified the blight organism and reported that a

blight detection and destruction program was being

followed similar to the one undertaken earlier in

France and the Netherlands.

Gautier {325), Ghio {326a), Melckebeke {6i0),

Ride {787), Ride et al. {788), and Veldeman {976a)

published brief reviews of fire bhght in French.

^ Pers. commun., Inst. f. Phytopath., Aschersleben, East

Germany.

During 1958-70, fire blight was reported in

Europe on about 24 pear and 35 apple cultivars

{1055). Several other cultivars have been added

since. In addition, the disease has been observed on

at least 14 species of Cotoneaster and on several

species of Crataegus, Sorbus, Pyracantha, and

other genera in the Rosaceae {329, 369, 571, 576).

Hawthorns in particular seem to have a very important role in the spread of fire blight. In all seven

European countries mentioned here, hawthorns

either became infected first or were directly responsible for spreading blight to nearby apple or pear

orchards {330, Í88, 635).

The initial outbreaks offire blight in Great Britain

and in Poland most likely resulted from shipments of

infected fruit, contaminated propagating wood, or

infected nursery stock through the respective seaports of London and Gdansk. Once blight was established on pears, secondary infections were easily

transmitted by wind, rain, birds, or insects to other
16 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

hosts. The blight observed in 1968 at Skierniewice,

Poland, apparently resulted from the introduction of

infected propagating wood {12Jf).

The numerous occurrences of blight in hawthorns

along the coastline from England and France to

northern Poland, particularly on the clusters of islands in Denmark and on those along the coast of

West Germany and the Netherlands, are very

strong evidence that migratory birds were responsible for spreading the bacterium. For more information, see chapter 8.

The introduction of fire blight to northern Europe

has caused increased concern among the central and

southern countries. Articles concerning the

threatening danger of the disease to apple and pear

industries have appeared in Austria {983), Czechoslovakia {5Jp5a), East Germany {669, 1036), France

{1,62, 786), Italy {llf3a), Portugal {221, 222), South

Africa {280a), Sweden {289, 710, 710a), and Switzerland {99, 99a, 123, 7^7, 8^8, 921). Considering

the extensive list of hosts and the fact that in Europe

the disease has spread and maintained itself considerably well under less favorable environmental conditions than in the United States, it may only be a

matter of time before fire blight will be present

throughout the European continent.

It is not possible to predict precisely how the

disease would develop in the various European regions. When comparing the experience gained in

America with that in England and northwestern

Europe, it is apparent that climatic conditions have a

decisive effect on the time at which the disease becomes active and on the general way in which infection occurs. In turn, these factors largely determine

which host plants will be attacked first as well as the

rate of infection by the pathogen. Environmental

conditions vary greatly throughout Europe, but average summer temperatures in Bavaria, southern

France, and northern Italy are at least at minimum

levels for blight development {106i). For several

hours during the day they should be well within the

optimum range. In addition, unusually high spring

or summer temperatures with sufficient moisture

could provide optimum conditions for serious outbreaks of fire blight in any country.

For those European countries where fire blight

has not yet been observed, the most important control measure is to make all possible attempts to keep

the disease out of the country. Rigid quarantine of

plant material from areas where the disease is

known may prevent or at least delay introduction of

fire blight. Many countries do not enforce such

quarantine restrictions, but Norway {288), Sweden

{286, 289), and Switzerland {285) have adopted very

strict regulations against importation of plant material from the 10 most widespread genera in the family Rosaceae.

Oceania and Africa

New Zealand

In 1919, fire blight first appeared in New Zealand.

Cockayne {178) and Campbell {lH) reported the initial outbreaks on apple, pear, quince, and hawthorn

in Auckland Province on the North Island. Waters

{1003, lOOJf) was the first to isolate and identify the

causal organism. Fire blight is beheved to have been

imported on infected nursery stock. Apparently

hawthorn was very important in the spread and

overwintering of blight {179, 681). In the early years

the disease destroyed many hectares of pear trees in

a single season, but within 2 or 3 years damage was

limited to the death of a few branches and a small

proportion of spurs {53, 205). Eight years after its

intoduction, the disease caused severe losses to most

of the apple cultivars in the Hawke's Bay area {6).

Curtis {206) reported that rootstocks of Pyrus calleryana Decne. and P. ussuriensis Maxim, were

distinctly resistant to root and body bhght.

Despite several quarantine regulations concerning shipments of bees or plant material from the

infected area, fire blight reached the South Island in

1929 {772). It was not detected in the Otago region

until 1936. Within a few years, however, after several outbreaks ofthe disease in other new areas, fire

blight seemed to become less serious {205). In 1963,

the disease appeared to be increasing in incidence,

but excellent control was obtained with streptomycin {682).

Phillips {737) pointed out that considerable daily

variations in temperature and humidity might partially account for fluctuations in disease appearance.

Even though the use of streptomycin is not allowed

in New Zealand, recent work has indicated that excellent control can be obtained by applying

bordeaux-streptomycin sprays {250, 1026).

Austraha has managed to remain free of fire

blight. As early as 1924, a quarantine proclamation

prohibited the importation of all deciduous fruit

trees and other plants in the Rosaceae family, including fruits and seeds, that were grown in any country
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 17

in which fire bhght existed (181). That same year,

Noble (697) mentioned an additional restriction that

honey produced in New Zealand should be held in

containers for 14 days prior to export. This strict

quarantine enforcement is adhered to today (1).

Egypt

In 1964, El Helaly et al. (26^) reported the first

localized occurrence offire blight (el-lafha el-naryah)

in northern Egypt near the port city of Alexandria.

Blight had been observed during 1962 and 1963 in

several localities of Alexandria, Behera, Sharkia,

and Dakahlia Provinces. Bacterial isolates were

tested for pathogenicity by artificial inoculation of

potted 3-year-old Le Conte cultivar pear trees and

young, green fruit. A 1966-72 survey of pear orchards and laboratory examination ofsuspected tree

tissues indicated that trees in Egypt are free of the

disease (263).

Unconfirmed Reports

During this literature survey we found many unconfirmed reports of fire bhght on pear, apple, and

other rosaceous hosts (table 1). The causal organisms were reported as Bacillus amylovorus,

Bacterium amylovorum, or Envinia amylovora.

For nearly three-quarters of a century fire blight

has been assumed to occur in Japan (10^9). As early

as 1903 the disease was reported by Uyeda (97i) and

was believed to have been brought over on nursery

stock from America. Fire blight was observed in

both apple and pear orchards in Akita prefecture on

the island of Honshu and in Ehime prefecture on

Shikoku island. The disease was apparently rather

prevalent during the 1920's and 1930's (Í97, 86J,). In

1955, Okabe and Goto (708) published a hst of bacterial diseases and their pathogens in Japan. Even

though fire blight was included in the list, the authors also listed it and a pear bacteriosis (Erwinia

sp.) in a separate list with 13 other bacterial diseases

because of unconvincing proof of pathogenesis. In

1974, these investigators reported that all previous

reports on fire blight in Japan appear to have been

based on misidentification and that the country has

never been infected with the blight pathogen (295).

Several articles have been published in the

U.S.S.R. and Italy concerning the possible presence

of fire blight in these countries. In 1960, Verderevskii (978) described the general fire blight

(ozhog plodovykh derer'ev) symptoms, but he did

not consider the disease to be present in the

U.S.S.R. Izrailskii et al. (1,67, 1,68), Shklyar (865),

and Shklyar and Orlova (866) compared many E.

amylovora-\ike isolates with Canadian and English

isolates of E. amylovora in agglutination and fruit

inoculation tests. They found E. amylovora to be

absent in all fruit tree specimens and concluded that

previous reports of fire blight in the U.S.S.R. were

unfounded.

Additional reports for the U.S.S.R. indicated that

several blights, resulting in dieback of twigs on apple, pear, peach, and other fruit trees bearing symptoms similar to fire blight, also were proved not to be

fire blight (^7^, i66, 523, ^^Oa). Studies of biochemical and serological properties of the pathogens reportedly show similarities to Pseudomonas species

(J,90, 711a, 712).

In 1963, Luchetti (59i) isolated a yellow and white

bacterium from cankers of 2-year-old apple trees of

the cultivar delicious in the Province of Ferrara,

Italy. Based on bacteriological tests and comparative morphology studies, he identified the white bacterium 2isE. amylovora. The following year, Ercolani (275) studied similar cankers on the same apple

cultivar in several locations of the Province and concluded that the organism was a species of

Pseudomonas. Mazzucchi (632) surveyed pear orchards in the Provinces of Ferrara, Modena, and

Rovigo and found that blossom blight was most frequently caused hyPseudomonas syringae Van Hall.

However, he reportedly isolated Erwinia-like bacteria from large cankers on branches and trunks of

Bartlett and Conference pear.

In 1972, the Ministry of Agriculture in Turkey

reported through the European Plant Protection

Organization the occurrence of fire blight on pear

from several locations along the coasts of the Black

Sea and Aegean Sea, i.e., in the Province of Zonguldak near the towns of Zobran and Karabuk and in

the Province of Amasya near the town of Caferh and

Vakfikebir (28J,). To date no other reports of its

distribution or confirmation of the causal organism

have been made.

Without any additional proof or further confirmation, we consider all records of fire blight in table 1

synonymous with those of the well-known blossom

blast of pears caused by P. syringae (1105). In

America, pear blast was first observed about 1932

and has since been found in all fruit-growing regions.

Today blossom blast has been reported from many

other countries (321,, 1,77, 617, 719,101,0). The symptoms of blast and other diseases resembling fire

blight are described in chapter 5.
18 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 1. — Unconfirmed reports of bacterial diseases in fruit trees attributed to fire blight organism

Location Year Causal organism Host Reference

Bermuda 1938

China:

Eastern China -

Bacillus amylovorus Loquat1926

Southern China 1933

Hopeh, Sikang 1952

Kwangtung 1955

Northwestern China 1959

Bacillus

Bacillus

Bacillus

Erwinia

Erwinia

amylovorusamylovorusamylovorusamylovora —

amylovora —

Waterston (1005).

France: Toulouse - 1932

Germany:

Baden 1919

East Prussia 1926

Italy:

Catania 1912

Torino 1922

Naples 1923

Sicily 1925

Po Valley 1931

1963

1969

Bacillus amylovorusPear Porter (751 ).

Sand pear Tu (957).

Pear Siang (868).

do Ciferri (173).

Apple Sun (913).

Loquat Nicholas and Aggery (

Bacillus amylovorusBacillus amylovorusPear-

-doMuUer (670).

Pape (721).

Bacillus amylovorus

Bacillus amylovorus

Bacterium amylovorum -

Bacillus amylovorus

Bacillus amylovorus

Erwinia amylovora

Erwinia spp.

-doJapan: Honshu, Shikoku- 1903-30 Envinia amylovora -

JordanRumania:

Bessarabia -

Bukovina —

1954

1938

1940

Erwinia amylovora -

-— Savastano (838).

do Ferraris and Ciferri (299).

Apricot Savastano (839).

Loquat Passalacqua (729).

Pear, apple Montemartini (659).

Apple Luchetti (59J^).

Pear Mazzucchi (

Pear, appleApple

Kazui UP7), Shiraishi (864),

Uyeda (97^).

Vestal (980).

Pear, apple—

do -

— Savulescu et al. (8M.

— Veresciaghin (979).

Southern Rhodesia: Fort

Victoria 1927

South Vietnam 1965

1967

Erwinia amylovora —

Erwinia amylovora —

Bacillus amylovorus Apple Hopkins (430).

Erwinia amylovora -

Erwinia amylovora -

Plum --

Apple -

Switzerland- 1939

Turkey: Black Sea coast 1972 Erwinia amylovora -

My (676).

USAID (962).

Pear Osterwalder (715).

do EPPO (284).

U.S.S.R.: Kursk, Crimea,

Transcaucasia 1915 Bacterium amylovorum — Pear, apple- Serbinov (852).
CHAPTER 3

ECONOMIC IMPORTANCE

The injuries caused by the bhght pathogen make

fire bhght not only destructive to the current year's

crops but extremely dangerous to the pear or apple

industry in a region or country. Blossom infection

means a certain reduction in the current crop and

that of the next year through the killing of fruit

spurs. Twig blight destroys wood that would often

bear fruit spurs the following season. In pears and

quinces as well as in certain cultivars of apples, twig

blight and the blighting ofsuckers often result in the

death oflarge limbs or even the entire tree. Blighted

nursery trees are usually rendered unsalable, even if

not completely killed. Trunk, collar, and root blight

strike the tree in its most vital parts and their destruction constitutes a heavy drain on orchard

stands. The blighting offruit after it is partly grown

or its death from girdling ofthe branches on which it

is produced often causes severe losses, especially in

pears.

Accurate estimates are difficult to obtain of the

annual losses from fire blight for given localities or

for the country as a whole. Fire blight is an epiphytotic disease and occurs very erratically. Moreover

its destructiveness varies with the seasons. With no

specific methods to measure its devastation, only the

most gross losses can be approximated. Some conception of its destruction may be obtained from the

following facts and observations.

During 1860-85, thousands of pear trees, mostly

the cultivars Duchess and Bartlett, were grown in

the Tidewater area between Richmond and Norfolk,

Va. (307), Fruit shipped by steamer to New York

brought $12 per bushel (ca. 35 kg), and net profits of

$600-$ 1,000 per acre (about $2,000 per hectare)

were realized for several years. In 1873, the Old

Dominion Fruit Growing Company planted 20,000

Bartlett trees and paid 20 percent dividend on capital stock in 1880 and 60 percent in 1881. However,

the pear boom was soon snuffed out by fire blight,

resulting in a great loss to the stockholders.

Atthe 1860 meeting ofthe Fruit Growers Association of Eastern Pennsylvania, the undisputed

statement was made that ^^pears do better

everywhere in Pennsylvania than apples" (306), One

grower in Jenkintown, Pa., had 7,000 trees of about

500 pear cultivars and obtained $8-$12 per barrel

(ca. 116 kg) at markets in Philadelphia. The inevitable reaction from 25 years of horticultural inflation

came in 1872. One of the four main causes for the

depression at that time was the heavy loss in pear

trees infected by fire blight.

Soon after fire blight had reached the west coast,

the disease wrought such havoc in California between 1901 and 1910 as has seldom been known in a

fruit-growing country. In 1902, the State Board of

Horticulture reported that Fresno County had

125,000 pear trees and Kings County 43,700 trees.

By 1904, fire blight had reduced these numbers to

1,500 and 0, respectively (702). By 1906, considerable blight was observed in the Holhster and Santa

Clara Valley areas, and by 1908, two-thirds of the

Bartlett trees in the State had been destroyed by

blight. One estimate claimed a loss of more than $5

milHon worth of Bartlett pear orchards during

1903-8 (1038). In four counties of the southern San

Joaquin Valley 95 percent of the pear trees were

wiped out and a major pear industry has never been

reestablished there.

In 1930, Milbrath (6^8) reported a loss of 1-35

percent of the pear acreage in 33 counties of California from the previous year. The total expenditures

for labor and materials to control blight in 19 ofthese

counties amounted to $825,000. Figure 5 shows the

losses to the California pear industry for 1890-1960.

Between 1900 and 1910 there was a 28-percent decline in the total number of pear trees. The small

decline in production in 1920-24 as well as the severe decline in 1930-35 was almost entirely due to

blight epidemics in the Sacramento area (61).

Except for a few areas near the Great Lakes in

New York and Michigan, fire blight has also been

very destructive throughoutthe Eastern, Southern,

and Midwestern States. In 1914 there was an estimated loss of $1,500,000 to the apple and pear crop in

Illinois (738). In 1930, blight was also so severe on

the apple cultivar Yellow Transparent in southern

19
20 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

z

o

Û

O

1920 1930

FIGURE 5. — Estimated loss of pear production and orchard trees

caused by fire blight in CaHfornia during 1890-1960 (after

Baker, 61 ).

Illinois that it reduced the crop at least 30 percent

and caused a direct loss of $50,000 {22). Tullis {961)

mentioned an average annual loss of 4.6 percent to

the apple crop in Michigan due to fire blight during

1921-25. In Pennsylvania this loss amounted to 2.5

percent in 1958 and an average 73 percent of the

blossoms blighted in 1944 in nine orchards in the

central part of the State {529, 10J,7).

In the United States alone in 1936 the total production of pears was reduced 14 percent by fire

blight, representing an approximate reduction in

yield of 2,890,000 bushels (ca. 23,000 tons) or a loss of

more than $4 milhon {UÍ3). In 1938, fire blight was

very severe in young apple trees in Pennsylvania

and northern IlHnois {23, 2U, 529) and in 1942 as

blossom blight in southern Ohio {18). In 1944 an

orchardist near Greenwich, Conn., experienced his

third severe outbreak (first in 1922 and second in

1934) in 14 of 65 acres (6 of28 ha) ofpears {U9). Many

of the trees were reduced to mere stumps. Based on

pruning cost and crop loss the operator estimated

that the disease cost him at least $1,000 per week

since early June.

For additional estimates of crop losses in America

due to fire blight during 1920-40, refer to the data

collected by the USDA Plant Disease Survey {965).

During 1951-60 the overall average annual loss was

estimated at $iy2 milhon for the pear crop and nearly

$2y2 million for the apple crop {970). In the California

pear industry with an income of $20 million, the

estimated loss for 1963 due to fire blight was about

14 percent {971). For the apple crop with an income

of $13 million, the loss was estimated at 5 percent.

For more information on fire blight in many States

during 1915-40, see the Plant Disease Reporter

{963, 96Jf, 966-968). In the nursery business, Boyd

{1085) reported a 1955 loss to fire blight of about

200,000 apple and pear trees for one nursery in

southeast Iowa.

Losses due to fire bhght were not limited to North

America. Soon after the disease appeared in New

Zealand and Great Britain, orchardists were

perplexed at the damage it caused. In New Zealand,

fire blight destroyed in a single season many hectares of pear trees, but within 2 or 3 years after the

initial outbreaks it ceased to cause more damage

than the death of occasional branches {205). During

1927-28, bud infection was so severe in apples that

the Sturner and Ballarat cultivars suffered almost

100-percent loss {6).

Once fire blight became estabhshed throughout

southeast England, the disease caused severe infection and serious losses. In 1966, considered the

worst blight year in England, nearly 12,000 infected

trees were found from April 1 to November 5, most

ofwhich were on farms and in nurseries {366). Ofthis

total, 5,800 were Laxton Superb and 1,600 Bartlett

pear trees. During 1958-69, total losses in England

amounted to 20,000 pear trees, or about 100 acres (45

ha), and approximately 20,000 hawthorn, 15,000

cotoneaster, and 2,000 pyracantha shrubs {369).

Not until the 1970's, when fire blight became really prevalent on the most popular ornamental host

plants, especially cotoneaster, did the disease affect

the ornamental nursery business. In 1975, for example, more than 2 million cotoneaster, 13,000

pyracantha, 8,700 stranvaesia, and 4,500 mountainash were destroyed in nurseries and garden centers in the Netherlands {635a). Exporttrade ofthese

plants to other countries was reduced, and losses to

the nursery industry were tremendous.
CHAPTER 4

SYMPTOMATOLOGY

The original name 'fire blight' is descriptive of the

most characteristic symptom of this disease—a

blackening of twigs, flowers, and foliage as though

they had been swept by fire. Depending on the plant

part affected, many names, such as blossom, twig,

fruit, trunk, and collar blight, have frequently been

used.

Pear and Apple

Blossom Blight

This is usually the first symptom of blight and is

found in early spring. A single flower or an entire

cluster may be affected (pi. 1, A). Blossoms first

appear water-soaked, then wilt, shrivel, and turn

brownish to black. The blight progresses into the

peduncle, which also may appear water-soaked, become dark green, and finally turn dark. During

warm humid weather, ooze droplets sometimes

exude from the peduncle. Young fruitlets often become infected. They turn black, appear dried and

shriveled, and usually remain attached to the tree.

The disease spreads rapidly and the bacteria invade

the neighboring spur leaves through the midrib and

main veins. A small canker is frequently seen on the

supporting branch. The leaves wilt and the entire

spur turns brown in apples or dark brown to black in

pears. Infected blossoms may fall or remain attached

to the tree, a symptom useful in detecting blighted

trees from a distance. Not uncommonly some of the

fruitlets of a diseased cluster may at first escape

infection but later become invaded through the

pedicel from the affected cluster base (pi. 1, B).

Pear and apple cultivars differ widely in their

susceptibility to blossom blight (1057). In England

the pear cultivar Laxton's Superb is especially susceptible to blossom blight, attributed partly to its

tendency to produce abundant secondary blossoms

(194). In Denmark, on the other hand, primary blossom infection is more common, whereas infection in

secondary blossoms is only sporadically observed

(254). Differences in climatic factors are assumed to

be responsible.

Twig and Leaf Blight

After the blossoms, the succulent twigs or shoots

and water sprouts or suckers are the next most

susceptible part of the plant. In Beltsville, Md., a

severe blight area, about 50 percent of the blight

observed is twig blight. During some seasons twig

blight may be the only blight seen. Twig symptoms

are similar to those found in blossoms except that

infection usually progresses more rapidly, especially

under optimum weather conditions for blight development. In a few days, infection can move 15-30

cm or more into the twig. Infected shoots, bark, and

leaves usually appear Hght to dark brown in apples

and dark brown to black in pears (pi. 2, A and D).

Symptom expression in apple shoots inoculated with

E. amylovora has been due to production of ammonia (592).

Blighted twigs and water sprouts often form a

canelike or shepherd's crook at their tips, a characteristic symptom of the disease (pi. 1, D). During

wet conditions, drops of bacterial ooze frequently

appear on the blighted shoots (fig. 6, A). The border

tissue between healthy and diseased parts oftwigs is

filled with gum (í07). Twig blight may also result

from girdling below the shoot tip after invasion

through the spurs or previously blighted twigs or

leaves. Numerous blighted twigs with attached dead

leaves appear as though scorched by fire (pi. 2),

hence the common name of fire blight.

A common practice in some parts of the United

States is to break or prune water sprouts or suckers

to prevent fire blight infection. However, sucker

removal increased blight infection in California

(973). In Illinois, Tehon et al. (929) studied trends of

blight occurrence during 1922-28 and reported that

the intensity of blight attack varied much more

proportionately than its prevalence. Failure to

prune blighted twigs and branches will, however,

subject the fruit to attack by such rot organisms as

Botryosphaeria ribis Gross, and Dug. and

Physalospora obtusa (Schw.) Cke., which invade

the dead twigs.

21
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PN-6376

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FIGURE 6.—Characteristic symptoms of fire blight in Bartlett pear shoots: A, Severe oozing ofsucculent terminal shoot; B, spread of blight infection from petioles into leaf

midribs and shoot; injury caused by simulated hail; C, infection ofleafspurs, accompanied by ooze production of petioles, originating from blighted stem; note necrosis at

base of leaves.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 23

Leaves may become infected after bacteria enter

directly through stomata, trichomes, and

hydathodes but more frequently through wounds

caused by hail and wind whipping. Ifinfection occurs

in the blade, a necrotic section appears. This part of

the leaf may dry out, but infection frequently

spreads through the secondary veins into the midrib, then into the petiole and the stem. Petioles

appear to be very susceptible to infection, and

characteristic blackening of the petiole and leaf midrib often occurs (fig. 6, B). Ooze droplets frequently

are observed (fig. 6, C). In some infected leaves

only a small necrotic part extends inward from the

margin 0.6 to L2 cm (V4-V2 in), whereas in others the

affected area includes the midrib, and in still others

the affected area spreads nearly or completely over

the entire leaf. Under certain weather conditions

bacterial strands are sometimes produced on the

blighted planttissues but most commonly on petioles

(pi. 1, C).

Fruit Blight

Fruit blight generally is found in immature fruit,

although symptoms in fruit after harvesting and

packing for shipment occur occasionally (102^ 183).

Infection spreads directly through lenticels in the

skin, through wounds, or from an infected spur into

the fruit. The infected part of the fruit may at first

appear oily or water-soaked, with the diseased part

becoming brown to black. Pears often show a premature, dark-green, water-soaked edge along the necrotic area, whereas apples commonly produce a

premature redness in advance of the rotted area

(pi. 3, C and D), A sticky, milky to amber-colored

fluid collects at the core and sometimes oozes from

the lenticels {603). Masses of bacterial tendrils or

strands have been observed on pear fruit in Washington State (fig. 7, A and B). Infected apple and

pear fruits turn brown and black, respectively,

shrivel, and remain attached to the spur, taking on a

mummified appearance. Fruit blight is most common following a severe summer hailstorm

{596a, 1062, 1070).

Limb and Trunk Blight

In blight-susceptible hosts the disease may advance downward from the blossoms, shoots, or fruit

through the larger twigs to older branches causing

localized stem cankers. It may continue into the

scaffold limbs and main body of the tree, often accompanied by ooze running along the bark

(pi. 4, A and 5).

Cankers formed atthe base of blighted fruit spurs,

water sprouts, or small limbs serve as sites where

the bacteria may hve over the winter. Cankers develop in the bark when progress of the infection is

slowing down. They may be shghtly sunken, varying

in size and surrounded by irregular cracks in the

bark (pi. 3, A andß). Active fire blight cankers have

a dark, water-soaked appearance. The margin is

indefinite, raised, or blistered. Later it may become

marked by a definite crevice or crack (pi. 3, A).

The surface of the affected bark becomes sunken

and remains smooth. Sometimes cankers, not identified by cracks or blisters, can be recognized by an

outward discoloration of the bark, often purplish.

Cankers formed early in the season, especially the

small ones, usually are surrounded by a callus. They

may girdle entire limbs and thus kill that part of the

limb above the girdle; if in the trunk, they may kill

the entire tree. Characteristic reddish-brown

streaks are often found in the sapwood when the

bark is peeled or cut away from the infected limb or

twig. In England the overwintered cankers are

distinguished from newly established summer cankers in that ''the tissues become 'foxy-red,' merging

through a diffuse mottled red-green area into the

healthy bark" {IH).

Fire blight in the main part of a tree trunk is

referred to as trunk or body blight. One ofits earliest

symptoms is usually the presence of ooze running

along the bark, sometimes accompanied by small

cracks visible in the bark tissue (pi. 4, C). In trees

with trunks susceptible to fire blight, such as the

Magness pear cultivar, infection usually spreads

rapidly from the trunk into scaffold limbs within a

few months, frequently resulting in death of the

tree. In this cultivar a distinct purplish coloration of

the diseased bark tissue aids in identifying the disease (pi. 4, D).

Active fire blight cankers in the trunk have a dark

water-soaked appearance. Their margins are indefinite or raised and blistered at first but later become

definite and marked by a crack or crevice. The surface of the affected bark finally becomes sunken and

usually remains smooth. The affected area may show

streaking caused by an amber-colored exúdate running down the trunk (pi. 4, A). Trunk blight may

occur in trees that show no other evidence of blight
24 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

PN-6377

FIGURE 7.—Bacterial strands on pear fruit and hawthorn shoots: A, Numerous cirri-like strands on blighted Bartlett fruit (natural size);

B, section of fruit in A magnified five times; C, thin, uniformly coiled strand on hawthorn shoot; D, short, unevenly coiled strand

originating from havrthom shoot. {A and B, Courtesy Tree Fruit Res. Cent., Wenatchee, Wash.; C andD, courtesy East Mailing

Res. Sta., Maidstone, Kent, England.)
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 25

infection. This has been particularly evident in Magness trees at Beltsville (1060, 1063).

Collar and Root Blight

These two types offire blight can be most destructive and frequently cause the immediate death ofthe

tree. Fire blight cankers atthe base (collar or crown)

of the tree trunk or at ground level usually are referred to as collar blight. It may spread from the

collar into the roots or sometimes from the roots into

the collar. Detailed descriptions of symptoms and

pictures of collar and rootstock blight have been

published {69JÍ, 71U, 1016).

Bark on the roots is killed in much the same manner as that on the trunk. Invasion of the crown and

roots may occur in one of several ways: (1) Through

infected suckers or water sprouts, (2) washing of

bacteria from infected twigs and fruit down the

trunk into the soil containing the roots, and (3) internal translocation of the fire blight bacteria from infected plant parts above ground to the roots. In

recent years considerable collar and root blight have

been found in apple rootstocks used for dwarfing

purposes (513).

In Oregon, Coyier (188) frequently observed poor

growth of apple and pear trees associated with fire

blight infection in their root system. In prehminary

experiments, apple seedlings were root pruned and

either dipped in a suspension of E. amylovora or

planted in soil infested with the blight pathogen. He'^

found a marked difference in growth between the

inoculated and noninoculated plants. He concluded

that in commercial nursery plantings the trees may

be accidentally inoculated by fire blight bacteria

during planting from using contaminated pruning

tools.

Quince and Crab Apple

Symptoms of fire blight in quince and crab apple

are similar to those described for apple. Succulent

shoots often produce ooze. In early summer the

leaves turn light to dark brown and remain attached

to the branches. The fruit of crab apple oozes as

much as that of regular apples; however, oozing has

not been observed as frequently on quince fruit.

Pyraeantha and Hawthorn

With few exceptions, blight symptoms on

pyraeantha or firethorn {Pyraeantha sp.), hawthorn

"^ Unpub. data, Ornamental Plants Res. Lab., U.S. Dept.

Agr., Corvallis, Oreg.

{Crataegus sp.), and cotoneaster (Cotoneaster sp.)

are generally similar to those described on other

hosts.

In pyraeantha the blossom clusters are generally

infected first. They turn brown and die. Although

infection of soft shoots is reported infrequently and

infection of branches is rare in England (369)^ the

reverse is usually true in the United States.

Characteristic symptoms of fire blight on soft

pyraeantha shoots are reddish-brown leaves,

whereas the shoot tip often shows the typical

shepherd's crook (pi. 5, A). Infection frequently

penetrates from the infected blossoms and the young

shoots into the larger branches, sometimes killing a

large part of the plant (pi. 5, B). In a later stage of

infection, the leafturns a characteristic light brown.

Ooze droplets are rarely observed on young succulent shoots but more frequently on older branches.

In susceptible hawthorn shrubs the disease produces many infected shoots with Hght- to dark-brown

clinging leaves (pi. 5, C). However, a closer observation may reveal thatthese branches are dead from

winter damage, Monilinia canker, or salt-wind injury when plants are growing near the ocean. More

positive identification can be obtained, however, if

cankers can be located on the larger limbs. Meijneke

(635) reported a characteristic purplish discoloration

without cracks along the edge of the canker

(pi. 5, D). Upon slicing the bark in this discolored

area and immediately below, the typical watery,

red-brown discoloration of the inner bark appears

(87a, Jf62, 680). With slight pressure, droplets of

bacterial exúdate may be produced. Usually they

are first milky white, later changing to orange and

brown. In addition to ooze, bacterial strands have

also been observed on hawthorn (fig. 7, C and D)

(256, 635).

Meijneke (639) described an interesting blight

symptom on hawthorn during field inspection in the

Netherlands in 1972. He noticed bacterial ooze in the

form of many tiny ghstening droplets on petioles of

leaves and fruit as well as shoot tips. The droplets

did not become discolored but dried out and remained as a silvery film on the plant surface. These

symptoms reportedly occurred only under special,

yet unknown weather conditions.

Clusters of black shriveled blossoms have been

reported on hawthorn, but opinions vary as to

whether the flowers became infected directly or indirectly from nearby shoots (7^ 76, 788). Meijneke

(636, 638) reported from the Netherlands that twig
Agriculture Handbook 510, U.S. Dept. of Agriculture PLATE 1

A, Severe blossom blight of pear, showing light- to dark-brown flower clusters; B, closeup of blighted pear blossoms and young fruitlets,

with characteristic water-soaking and light-brown to black discoloration of stem tissues; C, aerial bacterial strands of Erwinia

amylovora, forming a cottony mass around leaf petiole; note pale-yellowish ooze exúdate on infected stem; D, blighted pear shoots,

originating in terminal blossoms, forming characteristic shepherd's crook and clinging brown leaves.
Agriculture Handbook 510, U.S. Dept. of Agriculture PLATE 2

Viifl^m j»vJ.iA^'^A¿ii

A, Twig blight in apple tree, showing typical brown leaves adhering to branches; B, severe blight in apple trees on dwarfrootstock; C,

pear orchard showing many dead trees due to fire blight; D, severe twig blight in Bartlett pear tree, with characteristic necrotic,

dark-brown to black leaves attached to branches.
Agriculture Handbook 510, U.S. Dept. of Agriculture PLATE 3

A, Advanced blight canker in main leader of young pear tree, showing typical cracking along upper margin of cankered area; note

characteristic blackening toward base of canker and brown clinging leaves; B, several young bhght cankers on branch ofGolden Delicious

apple tree, with characteristic orange-brown discoloration within cankered area; infection apparently started in young shoots and spread

into main limb; C, advanced symptom of blight in Moonglow pear fruit, exhibiting typical water-soaked margin and green ring along

border of blackened necrotic area; note numerous droplets of bacterial ooze clinging to blighted fruit; D, advanced blight in Jonathan

apple fruit, showing characteristic reddish margin along blotchy necrotic area; note ooze droplet on left of fruit.
Agriculture Handbook 510, U.S. Dept. of Agriculture PLATE 4

A, Characteristic brown streaking on central leader of pear tree resulting from profuse production of bacterial ooze; note typical orange

color of dried ooze and brown leaves on recently infected branch; B, closeup of bacterial ooze on tree trunk of pear tree, exhibiting white

to light-brown exúdate starting to flow; C, first sign of blight in Magness pear tree trunk, manifested as cracking.of bark and yellowish

ooze running on surface; D, canker in trunk and base of scaffold limbs in Magness pear tree due to blight infection originating in trunk;

note characteristic purplish to black discoloration of bark in this cultivar.
Agriculture Handbook 510, U.S. Dept. of Agriculture PLATE 5

A, Twig blight in young pyracantha shoot, showing characteristic reddish-brown leaves and typical curving of shoot tip; B, advanced

blight in pyracantha; note retention of dark-brown leaves on shoots invaded by blight organism and light-brown leaves on limbs

apparently girdled at their base; C, numerous blighted shoots on large hawthorn bush in background, with brown discolored cHnging

leaves; D, closeup of blighted spur on hawthorn, showing darkened necrotic canker in main limb. (A and ß, Courtesy U.S. Nati.

Arboretum, Washington, D.C.; C and D, courtesy Plant Protect. Serv., Wageningen, the Netherlands.)
Agriculture Handbook 510, U.S. Dept. of Agriculture PLATE 6

A, Severely infected cotoneaster shrub, showing characteristic brown-black leaves clinging to shoots; B, typical shepherd's crooks on

cotoneastertwigs with hght-brown leaves and dark midribs; note constriction at base ofnecrotic portion on two shoots at left; C, blighted

blossom cluster and light-brown leaves on stranvae.sia shrub; D, masses of clinging brown leaves on blighted mountainash tree. (A,

Courtesy Biol. Bundesanstalt, Kitzeberg, West Germany; B,C, and D, courtesy Plant Protect. Serv., Wageningen, the Netherlands.)
26 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

blight is much more important than blossom blight

and that hawthorn fruit blight has not been observed. Twig blight of hawthorn can best be recognized at first by the yellowing of the leaves, which

later turn brown and remain attached to the tree,

but some investigators report infected branches

without clinging leaves (77^ 369), This phenomenon

has also been observed on pear and may be a result of

the blight organism moving slowly through its host

tissue and thus enabling the plant to produce abscission substances.

Cotoneaster, Stranvaesia,

and Mountainash

Symptoms offire blight in cotoneaster are usually

similar to those in pyracantha, but the affected

internal tissue is often a lighter brown and the reddish-brown discoloration ofsoft shoots is less apparent. On largershrubs with older woody branchesthe

leaves turn dark brown following infection and usually cling to the shoots (pi. 6, A). Meijneke^ reported

a remarkable symptom on cotoneaster in the

Netherlands, i.e., the rapid girdling around the infection in blossom trusses and in shoots and twigs

(pi. 6, B), As a result, these trusses partially contained healthy flowers and the shoots often showed

the well-known shepherd^s crook.

On stranvaesia, the drooping of large brown terminal blossom clusters with light-brown leaves underneath is the most characteristic symptom of

fireblight on this shrub (pi. 6, C). On infected

cotoneaster and stranvaesia leaves, Meijneke® also

observed ooze droplets in rows on both sides of the

midrib on the undersurface of the brown-black

leaves. These droplets were grayish white and later

changed to light brown.

On mountainash {Sorbus), fire blight is characterized by masses of light-brown leaves clinging to

the branches (pi. 6, D).

Symptoms of fire blight on other rosaceous hosts

generally agree with one or more of the symptoms

described here {90, 311, 823, 93JÍ, 938).

^Pers. commun., Plant Protect. Inst., Wageningen, the

Netherlands.
CHAPTER 5

DISEASES RESEMBLING FIRE BLIGHT

Blight or blast, similar to fire blight, especially in

buds and blossoms, sometimes is caused by organisms other than E. amylovora. The following

causal agents of blossom blast have been reported:

Pseudomonas nectarophila (Doidge) Burkholder (as

Bacterium nectarophilum Doidge) (220, ^^i),

Pseudomonas barkeri (Berridge) Clara {6Jf,, 65, 267),

P. prunicola Wormald (810, lOJ^O), P. utiformica

Clara (175), and P. syringae Van Hall (91, S2J,, 1^77,

617, 633, 711, 719, 789, 835, 837, 1027). The symptoms described under these organisms, and the records of the unconfirmed reports of fire blight from

various European and other countries since about

1910, are thought to be similar to those of one single

disease known as blossom blast, caused by P. syringae {138, 331, 719, 1105).

To diagnose the cause of blast symptoms is often

very difficult. During the bloom period the blossom

and bud blast may be the only symptom seen. Discolored water-soaked spots first appear and sometimes increase in size. They may spread until the

flower receptacle becomes diseased. In some cases

the peduncle becomes infected and the entire flower

blackens and withers. The infection may stop in the

flower ot it may extend 2-5 cm into the stem and

cause darkening and cracking of the bark, and it

produces definite cankers. Blast cankers are usually

light brown to tan and when older the outer bark

becomes scaly. Leaf infections appear as dry,

localized lesions.

Confusion between fire blight and blossom blast is

most likely to occur with blossom infection because

at this time the two diseases cannot be separated

based on symptoms alone. Usually fire blight infection in blossoms, unlike that caused by P. syringae,

originates through nectariferous tissues in the calyx

cup and spreads rapidly into the receptacle and then

through the peduncle into the branches. Such differentiation is rarely possible in the field because origins ofinfection are quickly obscured by their extension. Even though blossom blast rarely progresses

beyond the base of the peduncles or distal parts of

the spurs, there are exceptions where extensive

cankering into branches may occur (572, 1027). It is

these exceptions that require accurate diagnostic

methods to separate the two diseases.

Brief but detailed descriptions of the comparative

symptoms of fire blight and blossom blast and their

respective causal organisms have been pubUshed

{391, 392). The first and simplest method to identify

the respective causal organisms is to use the potato

and pear test {88, 572). From 48 to 72 hours after

placing the suspected organisms on a surfacesterilized potato slice or injured pear fruitlet, P.

syringae should cause a rotting of the entire potato

slice but should only produce localized, dry, black

lesions on the fruitlets. In contrast, E. amylovora

should not cause a potato rot but should produce

typical drops of milky bacterial exúdate on the pear

surface. About 1970, Kleinhempel et al. {532a) modified the pear fruitlet test by implanting infected

tissue directly into the fruit.

Confirmatory diagnosis of P. syringae and E.

amylovora can be made with serological tests {268,

712, 822, 828, 10J^6a, 1128a), phage sensitivity tests

{195, 3i0, 353, 718), and slide or tube agglutination

tests {88, 326, Í67, ^68, 533, 572).

There seems to be no doubt that bud or blossom

blast (P. syringae) usually occurs when trees are

predisposed to frost or cold periods during the

spring {719). Several investigators mentioned serious outbreaks of blast following such weather conditions {65, lOiO). Panagopoulos and Crosse {720)

found that pear blossoms exposed immediately before inoculation to temperatures from -V to -2° C

(30°-28.5° F) increased their susceptibility to blast

infection. They furthermore showed that the causal

organism (P. syringae) in the surface microflora can

induce infection when conditions are provided, i.e.,

sprayed with water after frost treatment {719).

There are, however, records where blossom blast

has occurred without mention of frost or after wet

cold periods in the spring {65, J/.77, lOJ^O).

Until recently, bacterial ooze on blasted pear buds

was associated only with infection caused by E.

amylovora. In fact, the absence of ooze was one of

27
28 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

the features used to separate the two diseases. In

the spring of 1968, however, we observed ooze produced by an unidentified Pseudomonas species in or

on pear blossom and leaf buds (fig. 8). This organism

caused a severe loss offlower buds in Magness pear,

and temperatures preceding the first observations

were higher than those normally recorded for infection by P. syringae {506).

In the U.S.S.R., D'yakova {in Gorlenko, 356,

1096) described a bacterial blight on pear caused by

P. pyñ (Djakowa) Gorl. This disease is also characterized by a twisted shoot tip but differs from fire

blight and blast in that the bark cracks and peels off

and no infection of unripe fruit occurs. This organism

reportedly is a nonspore-forming, lophotrichous rod,

which does not produce ooze.

Certain fungus diseases have been reported associated with fire blight. Black rot {Physalospora

obtusa) and bitter rot {Glomerella cingulata (Ston.)

Spauld. and Schrenk) usually follow infection byE.

amylovora {235, 555, 713, 92^). Both of these diseases may cause difficulty in identifying fire blight

the following season because of holdover cankers.

Fire blight damage sometimes may be confused

with the periodic cicada {Magicicada septendecim

(L.)) and winter injury. Soon after the cicada or

17-year locust lays its eggs, the leaves on infested

apple and pear branches start to wilt. The leaves

become shriveled and dry and many remain attached

to the branches, but ooze is not produced as it sometimes is in branches showing fire blight. Fire

blight-Iike symptoms have also been observed on

hawthorn in Denmark caused by larvae of the gall

midge Resseliella crataegi (Barnes) (as

Thomasiniana crataegi (Barnes)) {952).

Limbs showing winter damage usually shrivel and

dry without the presence of attached leaves, which

may occur on blighted limbs during the final stages

of the disease. On the other hand, dead branches

with attached leaves have been observed on hawthorn near the ocean, caused by salt in the air blown

by severe winds. In all cases regardless ofthe cause.

PN-6378

FIGURE 8.—Cluster of flower buds from Magness pear, showing

ooze drops produced by unidentified Pseudomonas species.

such damaged limbs should be pruned for they serve

only as a haven for secondary organisms.

Two bacterial diseases should be mentioned that

are unrelated to fire blight of apple and pear. They

are referred to by the same name but caused by

different species ofErwinia. They are fire blight of

cosmos {Cosmos bipinatus Cav.) caused by E. cosm,ovora Prasad {763) and fire blight (vlamsiekte) of

grape (Vitis sp.) caused by£'. vitivora (Baccarini) du

Plessis {272). Both diseases are characterized by

necrotic lesions on the leaves and by cankers on the

shoots and woody twigs.
CHAPTER 6

HOST RANGE

To determine the types and numbers of host

plants susceptible to fire blight, many natural blight

observations and artificial inoculations of plants

were made during 1925-35 {726, 7^2, SOU, 828, 98J^,

987, 988), Table 2 is a listing of the genera and

species reported susceptible to fire blight except

Malus and Pyrus, which are included in chapter 13.

Besides these 2 genera, 129 species in 37 genera of

the family Rosaceae have been reported susceptible

to fire blight. Of these genera, those that are most

important economically and show the most severe

blight are Cotoneaster, Crataegus, Cydonia,

Pyracantha, and Sorbus. The genera and species in

the rosaceous family that reportedly showed no infection following artificial inoculation are listed after

table 2. Among this group are 21 genera containing

39 species (859, 984, 988), One wonders if additional

investigations might find many or all of these plants

susceptible to fire blight.

TABLE 2. — Genera and species in family Rosaceae susceptible to fire blight

Scientific name Common name

Blight

index ^

Location Reference ^

Amelanchier:

alnifolia (Nutt.) Nutt.-

canadensis (L.) Medic.

Serviceberry-

-doKs)

Kb)

laevis Wieg.-

Aronia:

Juneberry 1 (b)

arbutifolia (L.) Pers. Red chokeberry 1

melanocarpa (Michx.) Ell. 1

Aruncus Sylvester Kostel. Goatsbeard 2

Chaenomeles:

japónica (Thunb.) Lindl. Flowering Japanese 2-3 (b)

ex Spach. quince.

lagenaria (Loisel.) Koidz. do 1-2

Cotoneaster:

acuminatus Lindl. Sharpleaf cotoneaster 1-4

adpressus Bois Creeping cotoneaster 1-4

affinis Lindl.

ambiguus Rehd. and Wils. -

. 1.4 (s)

_ 1_4

apiculatus Rehd. and Wils. Cranberry cotoneaster 1-2

ascendens Flinck and Hylmö 4

bullatus Bois 1-4 (b)

bullatus f. floribunda (Stapf)

Rehd. and Wils.

buxifolius Wall, ex Lindl.

buxifolius f. vellaea Franch. ~

commixtus (Schneid.) Flinck

and Hylmö.

congestus Bakerconspicuus Marquand1

4

2

4

3-4

dammeri Schneid.

See footnotes at end of table.

Necklace cotoneaster 1-2

Bearberry cotoneaster 2-4

Calif., Wash. 702, 93Jf

N.Y., Wis., West J^5,U7,859,10J^6b,10J^6c

Germany.

Ark., Wis., Denmark— 80Jf, 859, PC

Calif., Wis. 859, 93J^

Denmark PC

Calif. 93^

Calif., D.C., N.J., 93J^, 10J,6b, 10^6c, PC

West Germany.

Ark., Calif., N.Y. 7J^2, 823, 934

D.C., N.J. 207a, 261, 1021

Calif., D.C., N.J., 207a, 261, 93^, 938,

West Germany. 1021, 1046b, 1046c

D.C., Gt. Brit. 261, 576

Calif., D.C. 261, 934

Calif., D.C, N.J. 207a, 261, 934

D.C. 261

D.C, Denmark, 261, 1046b, 1046c, PC

West Germany.

Calif. 934

D.C 261

Calif. 934

D.C. 261

D.C, West GermanyD.C, N.J., West

Germany.

D.C, N.J., West

Germany.

261, 1046b, 1046c

207a, 261, 1046b, 1046c

261, 1021, 1046b, 1046c

29
30 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 2.—Genera and species in family Rosaceae susceptible to fire blight ^—Continued

Scientific name Common name

Blight

index ^

Location Reference "

dielsianus Pritz.

divaricatus Rehd. and Wils.

elegans (Rehd. and Wils.)

Flinck and Hylmö (syn.

C. dielsiana var. elegans).

ßoccosus (Rehd. and Wils.)

Flinck and Hylmö.

foveolatus Rehd. and Wils.

franchetii Bois

frigidus Wall, ex Lindl.

glabratus Rehd. and Wils.

glaucophyllus Franch.

harrysmithii Flinck and

Hylmö.

henryanus (Schneid.) Rehd.

and Wils.

hissaricus Pojark.

horizontalis Decne.

ignavus Wolfinsignis Pojark. (syn.

C. lindleyana).

khasiensis Klotz

lacteus W. W. Smith

laxiflorus Jacq. (syn.

C. melanocarpa var.

laxißora (Jacq.) Schneid.,

C. polyanthema E. Wolf.).

lucidus Schlecht.

melanocarpus Lodd.

microphyllus Wall, ex Lindl. —

moupinensis Franch.

multiflorus Bge.

nanshan Mottet

nitens Rehd. and Wils.

obscurus Rehd. and Wils.

obtusus Wall, ex Lindl.

pannosus Franch.

perpusillus (Schneid.) Flinck

and Hylmö.

polyanthemus E. Wolf.

prostratus Baker

racemiflorus (Desf.) K. Koch —

radicans (Dammer ex

Schneid.) Klotz (syn. C.

dammeri var. radicans).

rhytidophyllus Rehd. and Wils.

See footnotes at end of table.

Diels cotoneaster 1-2

Spreading cotoneaster 1-3 (b)

1

2 4

Glossy cotoneaster 1

Franchet cotoneaster 1-4

Himalayan cotoneaster 1-4 (s)

1

1 (s)

4

D.C., N.J., West

Germany.

D.C., N.J., Denmark,

West Germany.

Calif.

207a, 261, 1021,

10^6b, 10^6c

207a, 261, 10J^6b,

10J^6c, PC

93J^, 938

Calif., D.C. 261, 938

D.C., N.J.

D.C, N.J.

Calif., D.C, Gt. Brit.

Calif.

D.C, Gt. Brit.

D.C

207a, 261

207a, 261, 1021

261, 576, 93J^, 938

93Jf

261, 576

261

3(s) Calif., Gt. Brit. 576, 938

1 DC.

Rockspray cotoneaster 2-4 (b, s) Calif., D.C, N.J.,

Denmark, Gt. Brit.,

West Germany.

1 DC.

__„ i_2 Calif., D.C.

1 D.C

1 Calif. -—

1 (s) Gt. Brit.-

Hedge cotoneaster

Rockspray cotoneaster

Large flowering cotoneaster

Ko Ko Nor cotoneasterBloodberry cotoneasterSilverleaf cotoneasterSungari rockspray

cotoneaster.

2-4 (s)

1

1-4

4

1-4 (b)

4

2-4

1-4 (s)

1

3-4 (s)

1

1

2

3-4

2-3

(s)

D.C, N.J., Denmark—

D.C

Calif., D.C, N.J.,

West Germany.

D.C

D.C, N.J., West

Germany.

D.C

Calif., D.C, N.J., Wis.-

D.C, N.J., Gt. Brit.—

D.C

Ark., Calif., D.C, N.J.-

D.C

Gt. Brit.-

Calif. —-

D.C, N.J.

Calif., West Germany —

261

207a,261,57J^,93J^,938,

1021, 1046b, 1046c,

PC

261

261, 934

261

934, 938

90

207a, 261, PC

261

207a, 261, 934, 938,

1021, 1046b, 1046c

261

207a, 261, 1046b, 1046c

261

207a, 261, 859, 934

207a, 261, 576

261

261, 809, 873, 934,

938, 1021

261

90

934, 938

207a, 261

934, 938, 1046b, 1046c

Gt. Brit. 576
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 31

TABLE 2.—Genera and species in family Rosaceae susceptible to fire blight ^—Continued

Scientific name Common name

Blight

index ^

Location Reference ^

roseus Edgew.

rotundifolius Wall, ex Lindl.

rubens W. W. Smith

salicifolius Franch.

simonsii Bakersoongoricus (Regel) Popov

(syn. C. racemiflora),

splendens Flinck and Hylmö —

stemianus (Turrill) Boom

tenuipes Rehd. and Wils.

tomentosus (Ait.) Lindl.

veitchii (Rehd. and Wils.) Klotzvillosulus (Rehd. and Wils.)

Flinck and Hylmö (syn.

C. acutifolia).

wardii W. W. SmithX watereri Exell

zabelii Schneid.

4

Redbox cotoneaster 1-2

1

Willowleaf cotoneaster 3-4 (b, s)

Simons cotoneaster 1 (s)

Redbead cotoneaster 4

4

1_4

4

2-4

4

Peking cotoneaster 1-4 (b)

D.C.- 261

Calif., D.C. 261, , D.C.

Calif., D.C, N.J.,

Belgium, Denmark,

Gt. Brit., Netherlands,

West Germany.

Calif., D.C, N.J.,

Gt. Brit.

Calif., D.C.

261

90, 261, 293, Jf29, 576,

639, 93Jf, 976, 977,

1021, 10^60, 10Jf6c,

PC

261, 576, 938, 1021

D.C

D.C, West Germany—

D.C

Calif., D.C.

D.C

Calif., D.C, N.J.,

Denmark, West

Germany.

D.C, Gt. Brit.

261

261, 10Jf6h, 10Jf6c

261

261, 93^

261

207a, 261, 93^, 1046b,

1046c, PC

Cowania stansburiana Torrey

(syn. C. mexicana).

Crataegomespilus dardarii

Simon-Louis.

Crataegus:

amoldiana Sarg.

crusgalli L.

1_4 (b, s, f) D.C, Denmark, Gt.

Brit., West Germany.

Cherryberry cotoneaster— 1-4 Calif., D.C, N.J.

Cliff rose 1 Calif.

1 Gt. Brit. 576

261, 571, 576

261, 571, 1046b, 1046c,

PC

207a, 261, 934

1

Cockspur thorn -

N.Y., Wis.- 859, 937

576, 770, 859, 937

douglasii Lindl.

flabellata var. grayana

(Egglest.) Palmer (syn.

C. grayana).

mollis (Torrey and Gray) Scheele

monogyna Jacq.

Western black hawthornEnglish hawthorn -

1-2 (b,s) Mc, N.Y., Wis.,

Gt. Brit.

1 Wash., Netherlands 639a, 702

1 Wis. 859

N.Y.-

oxyacantha L.- -dopedicellata Sarg. (syn.

C. coccinea).

phaenopyrum (L.) f. Medic.

(syn. C. cordata).

punctata Jacq.

succulenta Link

uniflora Muenchh. (syn.

C. tomentosa).

sp.

1

1-3 (b, s,f) Calif., N.Y., Wis.,

Denmark, Gt. Brit.,

West Germany.

1-3 (b, s) Calif., N.Y., Wis.,

Denmark, Gt. Brit.,

West Germany.

1 Wis.

2(s)

2(s)

1

Oriental crataegus 3 (s)

Wis.

N.Y., Wis.-

N.Y.

Wis.

Ark.

937

74y 87a, 282, 308, 330,

571, 576, 645, 692,

859, 934, 937, 1046b,

1046c, PC

45,47,74,87a,282,571,

576, 645, 692, 742,

859, 934, 937, PC

859

859

859, 937

937

859

804

See footnotes at end of table.
32 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 2.—Genera and species in family Rosaceae susceptible to fire blight ^—Continued

Scientific name Common name

Blight

index ^

Location Reference ^

Cydonia:

oblonga MilL- Quince 1-2 (s)

sinensis (Dumont de Cour.)

Thouin.

Dichotomanthes tristaniaecarpa

Kurz.

Docynia delavayi (Franch.)

Schneid.

Dryas sp.

Eriobotrya japónica (Thunb.)

Lindl.

Exochorda sp.

Fragaria:

X ananassa Duch. (syn.

F. chiloensis).

virginiana Duch.

Geum sp.

Heteromeles arbutifolia M. Roem.

(syn. Photinia arbutifolia).

Holodiscus discolor (Pursh.)

Maxim.

Kageneckia oblonga Ruiz and

Pavon.

Kerria japónica (L.) DC.

Mespilus germánica L.

Osteomeles anthyllidifolia Lind.

Peraphyllum ramossissimum Nutt.

Photinia:

deflexa Hemsl.

glabra (Thunb.) Maxim.

villosa (Thunb.) DC.

Physocarpus sp.

Potentilla sp.

Prinsepia sp.

Prunus:

alleghaniensis Port.

armeniaca L.

avium (L.) L.

besseyi Bailey

cerasifera Ehrh.

dasycarpa Ehrh.

domestica L.

fremontii Wats.

ilicifolia (Nutt.) Walp.

lusitanica L.

mume Sieb, and Zuce.

nigra Ait. (as P. am^ericana)

salicina Lindl.

simonii Carr.

Chinese quince4

1(f)

1 (s)

2

Mountain avens (s)

Loquat 1 (s)

Pearlbush (s)

Strawberry 2

1-2

Avens (s)

Toyon (Calif, holly) or 1-2 (s, f)

Christmasberry.

Creambush 1 (s)

1

Kerria (s)

Medlar (Mespil) (s)

2

1

1

2

3

Ninebark (s)

Cinquefoil (s)

(s)

Allegheny plum 1

Apricot 1

Sweet cherry (f)

Western sand cherry 1

Myrobalan plum 1

Purple apricot 1

Agen prune, Italian prune, 4 (s)

Spaulding plum. 3 (s)

l(s)

Native desert apricot 1

HoUyleaf cherry 1

Portugal-laurel 1 (s)

Japanese apricot 1

Cheney plum 1 (s)

Japanese plum (s)

Apricot plum 1 (s)

Calif., 111., N.Y.,

Gt. Brit., West

Germany.

Denmark

Calif.

Gt. Brit.-

Calif.

N.Y.

Calif., D.C., Fla.,

Ga., N.Y., New Zeal.

NY.

Calif., N.Y.

131, 302, Jf2Jf, 576, 6^5,

901, 93J^, 938

PC

93Jl^, 938

576

93J^

726

206, Jf2U, 722, 93^, 996,

1003

726

671, 7U2

N.Y.- 7h2

N. Y.- 726

Calif., D.C., Wash. 702, 93U, 938, 996

Calif., N.Y., Denmark ~ 726, 93Uy PC

Calif. 93U

N.Y. 726

N.Y., New Zeal. 206, U2U, 726, 1003

Calif. 93U

Calif. 93U

Calif. 93U

Calif. 93U

Calif., N.Y. 7U2, 1021

N.Y. 726

N.Y. 726

N.Y. 726

Calif. 938

Calif. 938

Wash. U32

Calif. 938

Calif., Denmark 938, PC

Calif. 938

Oreg. U70

N.Y., Oreg., Wash. U32, U70, 1018

Conn. 911

Calif. 93k

Calif. 93k, 938

Calif. 93h

Calif. 938

Vt. U87

Ark., Conn. 823, 911

Calif., Colo. 717, 938

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 33

TABLE 2.—Genera and species in family Rosaceae susceptible to fire blight ^—Continued

Scientific name Common name

Blight

index ^

Location Reference ^

spinosa L. -

triloba Lindl.

Pyracantha (fire thorn):

angustifolia Schneid.

atalantioides (Hance) Stapf. -

coccínea Roem.

Sloe plum 1

Flowering almond (s)

Denmark —

N.Y., Wis.-

PC

^2J^, 881

crenulata (D. Don) Roem.

crenulata var. kansuensis Rehd.

fortuneana (Maxim.) Li (syn.

P. gibbsii yannanensis),

koidzumii (Hayata) Rehd. (syn.

P. formosiana).

rogersiana (A.B. Jacks.) Bean —

Raphiolepis:

indica (L.) Lindl.

umbellata (Thunb.) Mak.

Rhodotypos scandens (Thunb.) Mak.-

Rosa:

blanda Aitón

multiflora Thunb.

rubiginosa L.

rubrifolia Vill.

sp.

Rubus:

idaeus L.

sp.

Sorbaria sp.

Sorbus:

americana Marsh

aria (L.) Crantz

4 (s)

4

1-3 (b,

1-3 (s)

1

1-3

1-3

1-3 (s)

Indian hawthorn 2

1 (s)

Jetbead (s)

Sweetbriar eglantine 1

1

Cultivated rose 1-3 (b,

Red raspberry - 2 (b, s,

Thornless blackberry 1 (s, f)

False-spirea (s)

American mountainash 2

aucuparia L. (syn.S. laciniata) —

mougeotii Soyer-Will. andGodr.-

occidentalis (Watson) Greene —

tianshanica Rupr.

Spiraea:

cantoniensis Lour.

densiflora Nutt. ex Torr.

and Gray.

vanhouttei (Briot.) Zabel

Whitebeam 1-2

European mountainash 1 (s)

Western mountainashNative spirea

Vanhoutte spireaStranvaesia davidiana Decne. -

(s)

2

l(s)

I

1

1-2 (b,

Calif., D.C., New Zeal.- 206, 262, 772, 93J,, 938

D.C. 262

f, s) Calif., D.C, N.Y., Den- ^5, J^7, 262, J,29, 93Jf,

mark, Gt. Brit., 938,10^6b, 10J^6c, PC

West Germany.

Calif., D.C. 262, 938

D.C. 262

Calif., D.C, N.Y. 262, J^2^, 93U, 938

Calif., D.C. 262, 93Jf, 938

Calif., D.C, Denmark- 262, 938, VC

Calif. 93Jf, 938

Calif., N.Y. Jf2J^, 93^, 938

N.Y. 726

N.Y. 7U2, 998

Calif., West Germany— 93J^, m6b, 1046c

N.Y., West Germany— 7J^2, 1046b, 1046c

N. Y. 742

s) Ark., D.C, N.Y., 742,823,998

Wash.

f) Maine, N.C 310, 569, 892

111. 789a

N.Y. 726

N.Y., Wis. 45, 859, 984, 1039

Gt. Brit., Netherlands— 90, 576, 639

Calif., N.Y., Wis., 90, 302, 645, 859, 867,

Denmark, Gt. Brit., 934, 937, 1046b,

West Germany. 1046c, PC

Wis. 859

Wash. 702

Gt. Brit. 576

Calif., Va. 375, 938

Calif. 934

Ark., Calif., West

Germany.

s) Calif., Denmark, Gt.

Brit., West Germany.

923, 938, 1046b, 1046c

576, 867, 934, 1021,

1046b, 1046c, PC

^ Not included are Malus and Pyrus, which are listed in chapter 13.

2 Based on artificial inoculation in shoot: 1 = 1-25, 2 = 26-50, 3 = 51-75, and 4 = 76-100 percent ofshoot blighted; letter in parenthesis

indicates reported natural blossom (b), shoot (s), or fruit (f) infection; single scores indicate most severe blight recording obtained from

multiple entries; blanks indicate no mention made of degree of infection.

^ PC=pers. commun., Jorgensen, H. A., and Jensen, A., State Plant Path. Inst., Lyngby, Denmark (489).
AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

The following genera and species in the family

Rosaceae showed no blight infection after artificial

inoculation (859, 9Si, 938):

Acaena microphylla Hooker f.

Adenostoma sparsifoliuyn Torrey

Cercocarpiis betuloides Nutt. ex Torrey and Gray

Chamaebatia foliolosa Benth.

Cotoneaster bacillaris Wall, ex Lindl.

Cotoneaster disticha Lange (as C. decora)

Cotoneaster harroviana Wils.

Cotoneaster hebephylla Diels

Cotoneaster newryensis Lemoine

Cotoneaster silvestri Pamp. (syn. C. hupehensis)

Crataegus prunifolia Pers.

Dryas X suendermannii Kell, ex Sundermann

Exochorda racemosa (Lindl.) Rehd.

Fallugia paradoxa (D. Don) Torrey

Fragaria chiloensis (L.) Duch.

Geum chiloense Balbis

Lyoyiothamnus floribundus Gray

Margyricarpus setosus Ruiz and Pavon

Osmaronia cerasiformis (Torrey and Gray) Greene

Photinia serrulata Lindl.

Physocarpus capitatus (Pursh) Kuntze

Potentilla fruticosa L.

Prinsepia s'inensis (Oliv.) Oliv, ex Bean.

Prunus arnericana Marsh.

Prunus bokhariensis Royle

Prunus cerasus L.

Prunus dulces (Mill.) D. A. Webb (as P. amygdalus)

Prunus hortulana Bailey

Prunus insititia L.

Prunus mahaleb L.

Prunus mira Koehne

Prunus pérsica (L.) Batsch

Prunus tomentosa Thunb.

Rosa californica Cham, and Schlecht.

Rosa gymnocarpa Nutt. ex Torrey

Rosa spithamea Watson

Spirea prunifolia Sieb, and Zuce.

Cotoneaster

The earhest known report of this ornamental

genus as a host for fire blight came from Cahfornia in

1930 (873). During the following 5 years several

species of Cotoneaster were reported susceptible to

the bhght organism under natural conditions or following artificial inoculation (809, 859, 938). As observed with other hosts, individual clones of a

species may differ in their degree of blight resistance. C. microphylliis was reported as only slightly

susceptible after artificial inoculation in the field in

California (93i), whereas the cultivar Emerald

Spray ofthis species proved to be rather susceptible

following inoculation under optimum Wight conditions in the greenhouse (30i). Egolf (261) reported

C. microphyllus very susceptible following artificial

inoculation in the greenhouse.

A few years after fire blight was observed in England, Cotoneaster was found to be an important host

ofthe blight organism. Lelhott (571) and Lelliott and

Hayward (576) found 11 species of Cotoneaster susceptible to fire blight that had never been reported

previously. At first C. horizontalis seemed immune

under field conditions, but later severe blight development was observed in a collection of young

container-grown plants at a nursery (290, 574).

Eight species of Cotoneaster reportedly were unaffected following artificial inoculation, as shown in

the preceding Hst.

At the U.S. National Arboretum in Washington,

D.C., Egolf (^^i) has inoculated and screened large

numbers of cotoneasters for blight resistance. Of 52

species evaluated, 15 percent did not show blight

symptoms after repeated inoculations (table 2). An

additional 10 percent of the plants showed no more

than 6 percent of the plant blighted. Of all resistant

cotoneaster species, C. glaucophyllus appeared to

be most tolerant to fire blight. In New Jersey, Davis

and Peterson (207a) reported 8 of 25 cotoneaster

species as fairly resistant to fire blight following a

severe blight year.

In West Germany, Zeller (10J^6b) evaluated 25

species and hybrids of cotoneaster against natural

and artificial inoculation of the blight organism. To

date, more than half ofthe plants appear susceptible

to the disease.

Hawthorn

The occurrence of fire blight in hawthorn

(Crataegus) dates back to the early 1900's (770). In

1929, Rosen (80^) made the first report of fire blight

severely injuring large limbs of an oriental species of

hawthorn in a short time. Shaw (859) published detailed records on artificial inoculation of 10 species of

Crataegus and stated that differences in degree of

resistance were very marked, ranging from no

blight in C. prunifolia to severe in C. oxyacantha.

Soon after the introduction of fire blight into

Europe about 1957, hawthorn obviously became an

important secondary host. This was especially true

in England, Denmark, West Germany, and the

Netherlands, where the plant is used in hedgerows

to serve as windbreaks or property boundaries. A

complete issue of the "Netherlands Forestry Journal" was dedicated to the taxonomy of Crataegus

and to the importance of hawthorn as an essential
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 35

and beautifying plant on the Dutch landscape (5^5).

Meijneke (638) published a detailed account of the

role of hawthorn in maintaining and disseminating

the fire blight organism.

Shortly after the introduction offire blight in New

Zealand, hawthorn proved very instrumental as a

host for the overwintering bacterium (6, 179, 205).

This, in turn, made satisfactory control very difficult. Since recent occurrences of fire blight in

Europe, infested hawthorn hedges have served as a

definite source of inoculum in disseminating the bacterium to apple and pear orchards {76,330, 638, 639).

Quince

Many references can be found regarding natural

and artificial infections in the cultivated quince

(Cydonia oblonga Mill.) (131, 823, 93i, 938). Certain

quince cultivars are as susceptible to fire blight as

the most susceptible pears and apples. Stewart (901 )

listed six quince cultivars equally susceptible following artificial inoculation of shoots on 2-year-old

trees.

The flowering Japanese quince (Chaenomeles

japónica (Thunb.) Lindl. ex Spach) is a closely related ornamental shrub, commonly planted in the

United States. Artificial inoculations of this plant

have shown that it is very susceptible to blossom

blight and moderately susceptible to twig blight

(823),

Pyracantha

One of the earnest records of fire blight on this

ornamental host dates back to the late 19th century

(^7). Since then pyracantha has proved to be an

important host for the blight pathogen (934, 938).

Following repeated inoculations, Egolf (262) reported 25 percent of eight pyracantha species tested

as highly resistant to fire blight (table 2). Two

blight-resistant cultivars — Shawnee and Mojave —

are commercially available (259, 260).

Mountainash

Shaw (859) was among the first to perform artificial inoculation on mountainash (Sorbus species).

Today the important ornamental tree S. aria (L.)

Crantz, and its cultivars S. lutescens Hartw. andS.

majestica Zab., as well as S. tianshanica Rupr.,

have been severely affected in England (90, 576).

Miscellaneous Plants

In addition to these 5 important genera there are

33 additional genera in the family Rosaceae with

plants found susceptible to fire blight. Most of them

are ornamental shrubs or trees. Of the few remaining fruit crops, the genera Fragaria, Prunus, and

Rubus deserve mention. Records of fire blight on

these three fruit crops have been rare, and all those

on strawberry and stone fruits were made prior to

1935 (J^32, U87, 671, 717, 881, 93J,, 938). Three observations on raspberry have been made since 1947, and

the blight organism was reisolated from the infected

plant material (310, 569, 892). In 1976, Ries and

Otterbacher (789a) reported rather severe fruit

blight on thornless blackberries in Illinois.

Crosse ^ reported definite isolation and proof of

virulence of E. amylovora from blighted apricot in

Missouri. Ten species of Prunus reportedly were

unaffected with fire blight following artificial inoculation (p. 34). In addition to these and the 8

Cotoneaster species, there were 21 species in 19

other genera reported without blight symptoms following artificial inoculation.

In the spring of 1972 an extensive host plant experiment was conducted in Denmark to evaluate

many rosaceous hosts, mainly ornamental shrubs

and trees, for fire blight resistance (J^89). This evaluation planting consisted of more than 2,300 plants in

35 species of 14 genera. Available results are incorporated in table 2.

In addition, two extensive trial plantings were

established in Schleswig-Holstein, the northernmost section of West Germany (10Jf6b). Besides

several apple and pear cultivars, these plantings

contained 59 different ornamental plants belonging

to 15 genera in the family Rosaceae. At one location

more than 5,000 cotoneaster seedlings comprising 15

cultivars were planted and artificially inoculated.

Preliminary data indicated a high percentage of very

susceptible seedlings.

Some plants outside the family Rosaceae have

been inoculated and tested for susceptibility to the

blight organism. Black necrotic lesions were obtained on shoots and nuts of several species of Juglans (walnut) in California (87^). Inoculations on

shoots ofHicoria pecan Brit, gave negative results.

In Canada, Layne (562) obtained definite sunken

^ Pers. commun., East Mailing Res. Sta., East Mailing, England.
36 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

lesions on stems of cowpea (Vigna sinensis (Torner)

Savi) 3 days after needle inoculation. However,

some isolates of E. amylovora failed to produce a

positive reaction (5^6). Additional plants producing

negative results following artificial inoculation ofthe

shoots withi/. amylovora are avocado (Persea sp.)

U31), persimmon (Diospyros sp.) (938)^ and poplar

(Populus sp.) (52),
CHAPTER 7

CAUSAL ORGANISM

Taxonomy

In 1882, Burrill (133) published the first technical

description of the fire blight pathogen and named it

Micrococcus amylovorus under the erroneous assumption that the bacterium destroyed starch. In

the description published in the ''American

Naturalist" the following year, the organism was

accidentally misspelled as M. amylivorus, which

caused some confusion in the literature until 1914,

when it was corrected (135, 903),

In 1889, Trevisan (956) published his "I Generi e le

Specie delle Batteriacee," in which he changed the

generic name of the blight organism from Micrococcus toBacillus amylovorus. Eight yearslater,

Chester (170) in Delaware changed the name to

Bacterium amylovorus, By 1915, Serbinoff(^5^) described a bacterial necrosis of the bark of fruit trees

in several regions of the southern U.S.S.R. He considered the disease synonymous with fire blight but

described the causal organism as Bacterium

amylovorum. In 1920, Winslow et al. (1030) established the germsErwinia, within, amylovora as its

type species. Three years later, this name was officially accepted by the Society of American Bacteriologists and has been maintained until today.

The changes in the nomenclature of the fire blight

bacterium are summarized as follows:

1882 Micrococcus amylovorus Burrill

1889 Bacillus amylovorus (Burr.) Trevisan

1897 Bacterium amylovorus (Burr.) Chester

1915 Bacterium amylovorum (Burr.) Serbinoff

1920 Erwinia amylovora (Burr.) Winslow et al.

1923 Erwinia amylovora (Burr.) Com. Soc.

Amer. Bact.

The genus Erwinia, named after the famous

American bacteriologist Erwin F. Smith (fig. 3.),

was established to combine all the peritrichous plant

pathogens into one group. Erwinia amylovora

(Burr.) Winslow et al., the first bacterium proved to

cause a plant disease, is today the type species ofthe

genus Erwinia (575a).

The tribe Erwineae, containing Erwinia as the

only genus, was established within the family Bacteriaceae (1030). In 1923, an additional genus,

Phytomonas, was created to accommodate the

phytopathogenic bacteria other than Erwinia, and a

few years later the family Enterobacteriaceae was

established with the germs Enterobacter (893). Waldee (1000) and Waldee et al. (1002) proposed three

well-defined groups of genera for the phytopathogenic species 0Î Bacillus, one of which would

contain the type species E. amylovora. It was proposed that the genus Erwinia be restricted to the

nonpectolytic, phytopathogenic enterobacteria, and

a new genus Pectobacterium be established for the

pectolytic ones (893, 1001).

On the basis of phenotypic similarity, Martinec

and Kocur (630) proposed that all phytopathogenic

enterobacteria be grouped into only two species of

the genus Erwinia, i.e., E. amylovora and E.

carotovora (L. R. Jones) Bergey et al. They found

that 40 strains of E. amylovora agreed with the

description of the type specimen. They also proposed i/. vitivora as a synonym ofE. amylovora and

E. cosmovora as a synonym of £'. amylovora var.

salicis (see chap. 5). In Japan, Komagata (5^0) and

Komagata et al. (5^^) recognized three species in

adding E. herbicola to E. amylovora and E.

carotovora. They found E. amylovora and E. herbicola homogeneous for the tested characteristics,

whereas E. carotovora was taxonomically

heterogeneous.

Phenetic Data

The most detailed taxonomic studies on the genus

Erwinia were completed in 1968-69 by Dye (2J^6-

2Í9) in New Zealand. Based on biochemical characteristics, he recognized four groups within the

genus, i.e., amylovora (246), carotovora (2^7), herbicola (248), and a group consisting of atypical Erwinia species (249). In the amylovora group. Dye

(246) found no main differences in biochemical

characters or carbohydrate utilization to suggest

species different from E. amylovora and proposed

37
38 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

the following five subspecies under E. amylovora:

Var. tracheiphila, var. salicis, var. nigrifluens,

var. quercina, and var. rubrifaciens.

Lockhart and Koenig (588) made a numerical

taxonomic analysis of the genus Erwinia, including

three isolates of E. amylovora. They showed that

the blight pathogen was separable from the other

Erwinia species and that there are apparently no

distinctions within the homogeneous group consisting OÎE. carotovora, E. aroideae (Town.) Bergey et

al., E. atroseptica (Van Hall) Jennison, and E.

ananas Serrano. They obtained similar results when

such ''key" characters as pathogenicity and pectinase production were omitted from the numerical

analysis. In a cross-inoculation study Spalding and

Smale (884) observed limited necrosis without ooze

production when E. carotovora was injected into

succulent Bartlett shoot tissue but no symptoms

whenE. amylovora was inoculated into leaves and

petioles of potato plants.

Comparative gel-electrophoresis studies by

Gardner and Kado (320) and Kado et al. (J^92) showed

that proteins clustered into two major electrophoretic groups of six to nine bands each. They concluded therefore that Erwinia is a heterogeneous

and somewhat artificial genus whose members

should be distributed within the general family of

Enterobacteriaceae (320).

In England, Lund (598a) studied the formation of

reducing sugars from sucrose as a possible tool to

distinguish i/ru^ima species. She found glucose and

fructose as the main reducing sugars and an unidentified compound, possibly oligosaccharide. Results

were inconclusive and further tests are needed to

separate £'. amylovora from otherErwinia species.

Serology

Burkholder (122) discussed in detail the taxonomic

position of the genus Phytomonas in the order

Eubacteriales and its relation to other genera and

families. Rosen and Bleecker (822) made a detailed

comparative serological and pathological study ofE.

amylovora and several species of Phytomonas^

especially P. syringae. They emphasized the value

of serological investigations for determining relationships of bacterial plant pathogens. Elrod

(268y 1099) made a detailed serological study oîE.

amylovora and could not detect any antigenic differences among the isolates tested. Four isolates oíE.

amylovora failed to reduce trimethylamine and produced no growth with the Eijkman reaction test

(269). He concluded that serologically £'. amylovora

was an exceedingly homogeneous species (268).

Hildebrand (Í09) demonstrated the failure oíE.

amylovora to form pits on polypectate gels, whereas

all other tested Erwinia and Xanthomonas species

did so. This trait was suggested as a useful

taxonomic character. In another study of immunological techniques. Lazar (567) concluded that

there was a close serological relationship among the

three principal species included in the genus. Slide

agglutination tests by Le Minor et al. (577, 578) in

France revealed that the species belonging to these

three Erwinia groups all possessed the common

antigen (CA). However, E. amylovora agglutinated, whereas £'. carotovora did not (578). Samson

(830) analyzed the antigenic structure of 25 isolates

ofE. amylovora by the agglutination method. Apart

from a common antigen in each isolate, three more

were detected and the isolates were classified into

five serotypes. Immunodiffusion tests also showed

that serotypes 0 are due to different antigens in the

lipopolysaccharide of the cell wall (833).

Molecular Genetics

In California, Starr and his associates (166, 168,

5U8, 672, 893, 895) have made extensive and detailed

studies of the taxonomy, biochemistry, metabolism,

and genetics ofErwinia. Certain groups ofErwinia

species were found related on the basis of the ''GC

content" (percentages of guanine -h cytosine) of the

base ratios of deoxyribonucleic acid (DNA) (895). Of

A^ Erwinia isolates tested from animals, 16 were as

injurious to certain test plants as some Erwinia

strains that were generally considered as

phytopathogens (5Jf8). Murata and Starr (672) found

that all of Dye's phenotypic groups in the genus

Erwinia were hardly separable on the basis of DNA

segmental homology. In-depth studies on gene

transmission of certain factors elucidated some connections of the taxonomic relationship of Erwinia

species to other enterobacteria (165-169).

Chatterjee and Starr (168) were able to obtain

stable donor strains of E. amylovora from strain

EA178Ri by selection for clones resistant to curing

by acridine orange. They observed integration of

plasmids (F' factors) into ÜieE. amylovora chromosome, including the fertility plasmid E (169).

In 1972-73 studies of DNA relatedness among

Erwinia species, Brenner et al. (108-111) conñrmeá

ideas by others thati/. amylovora and the soft rotting species belong to the same genus in the family of
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 39

the enterobacteria. They also supported the view,

however, that the phytopathogenic soft rot organisms be placed in the genus Pectobacterium, as

proposed by Waldee (1000, 1001).

In Canada, Wu and Middleton {10Jf2a) produced

phenotypically stable Trp"^ hybrids between E.

amylovora and Salmonella typhimurium (Loeffler)

Castellani and Chalmers. These hybrids had high

genetic homology values in the tryptophan region.

In a 1974 review of the overall taxonomic position

of E. amylovora, Fonnesbech (312) concluded that

species within Erwinia show marked differences

and should not belong to the same genus. She proposed that this genus be amended so as to comprise

only such organisms that do not reduce nitrate to

nitrite but which otherwise behave as enterobacteria with regard to other metabolic characters

(1100).

This brief summary on the phenetic, serological,

and molecular investigations of the family Enterobacteriaceae and the genus Erwinia shows that

more research is needed to elucidate the real

taxonomic position ofthe different species as well as

the genetic mechanism controlling virulence among

the species to both plants and animals.

Morphology and Cultural Characters

Since the first brief description ofthe morphological characters ofMicrococcus amylovorus by Burrill

(133, 13Í) in 1882 and 1883, several investigators

have studied the causal organism of fire blight in

more detail. The morphological characters of Erwinia amylovora are summarized in table 3. The

wild- or normal-type cell ranges from 0.6/im to

2.5/im in length and from Q.b\xm to 1.2^m in width

(av. l.ljim-1.6/im x 0.6jLim-0.9jim). These bacterial cells are usually reportd as Gram-negative short

rods, with rounded ends and motile by many peritrichous flagella. In 1926, Bryan (120) published the

earliest good photographs showing clearly the peritrichous flagella of the bacterium (fig. 9, A). Rosen

(802), however, disagreed after finding only single

polar flagella and believed that the organism should

be placed in the genus Phytomonas. Several years

later, Rosen (819) show^ed that the bacteria derived

from exúdate were enveloped in slimy capsules of

stainless material, which was nonproteinaceous (fig.

9, B and C). An extreme outer layer of staining

matter surrounded the stainless material.

TABLE 3.—Morphological characters o/Erwinia amylovora

Year Cell size

Length (fim) Width (jim) Cell shape Motility and flagella Staining reaction Reference

1883 1.0-1.4

1886 1.0-1.25

1898 1.0-1.6

1901 1.5-2.0

1901 1.0-2.5

1904 1.0-1.8

1913 1.6-1.8

0.7

0.5-0.75

.6-.8

.7-1.0

.5-.9

.7-.9

Singly or united in

pairs, never in

elongated chains.

Bacillus singly,

pairs, chains, or

masses.

Short rods, rounded

ends; singly, in

pairs or chains,

oval rodlike.

Rounded ends;

mostly singly,

sometimes in 2's.

Rods with rounded

ends; singly in

pairs, occasional

chains of 3 or 4.

Motile; not motile

in acid or alkaline

media.

Actively motile;

scattered flagella.

Many cells motile

but not all.

Cells nonmotile;

no flagella.

Cells motile; 1-3

flagella at 1 pole.

Cells motile; 2-3

peritrichous

flagella.

Stains well with

usual dyes.

Stains readily

with aqueous

aniline dyes.

Stains uniformly

with usual dyes.

Gram-negative —

Burrill {13J^).

Arthur (50).

Waite {993).

Whetzel {1016).

Chester a 70).

Jones {JfSJf).

-do Stewart {901).
40 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 3.—Morphological characters o/Erwinia amylovora—Continued

Year Cell size

Length (/im) Width (|jtm) Cell shape Motility and flagella Staining reaction Reference

1913- 1.0

1961—1.2-1.8; .8-1.2

1964 .6-1.2

1964 .9-1.8

1965 1.0-2.0

1965 2.0-7.0

1965 7.0-35.0

1969 1.0-2.5

1969 7.0-35.0

0.5-.75

1922 1.0-1.9 .5-.7

1926

1927

1937— .93-1.40 (strong);

1.50-1.71 (weak).

1948 Short, 1.5

diameters.

1954 1.42-1.61 .67-.72

.8 (live); .5

(heat

fixed).

.6-.9

.7-.8

.8-1.2

.8-1.2

.8-1.2

.8-1.2

Mostly singly,

sometimes short

chains (2-4).

Rods —

-doRods with rounded

ends.

Ovoid or rod shaped;

singly or in pairs

or groups; occasional

long rods or in

filaments.

Rods singly or in

clumps; short rods.

Cells motile.

Single, polar

flagella.

Peritrichous; some

cells with single

polar flagella.

Cells motile;

peritrichous

flagella.

Actively motile;

peritrichous.

1-7 flagella; shortest

cells with most

numerous and

longest flagella.

Motile; 1-5

peritrichous

flagella.

Hewitt U04).

Snow (881),

Rosen stain Rosen (803).

Casares-Gil Bryan (120).

stain.

Ark (30).

Short rods; singly

or in pairs.

Short

Most strains

nonmotile;

peritrichous

flagella.

Motile

Intermediate Nonmotile

Filamentous Actively motileWild type Peritrichous

flagella.

.8-1.2 Filamentous- -doGram-negative— Waldee (1001).

Hildebrand (Jf2Jf).

Gram-negative— Billing et al. (87).

-do Martinec and

Kocur (630).

-do El Helaly et al.

(26J^).

Voros and

Goodman (982).

Do.

Do.

Huang and Goodman

(Jt51), Huang

(1109).

Do.

In Great Britain, Billing {88) examined 150 isolates ofE. amylovora and distinguished between the

"typical" dominant type of colony displaying characteristic markings on Yeastrel peptone agar plates

incubated at 30° C (86° F) and the "atypical" colonies,

which were relatively featureless and less opaque.

Phase-contrast microscopy with the use of india ink

demonstrated that the cells of typical strains had

small capsules, some atypical strains had none, and

the remainder had varying proportions.

At the University of Missouri, Huang and Goodman {JÍ51, JÍ52) and Huang {1109, 1110) studied in

great detail the morphology of E, amylovora. By

using electron microscopy they showed the characteristic rounded ends and numerous peritrichous

flagella of the cells (flg. 9, D and E), Ultrathin sections of the wild-type virulent E. amylovora cells

prepared at room temperature revealed two separate, electron-dense layers in their walls (flg. 10,

A-C). When cells were flxed at 4° C (39° F), three

layers were visible. Silva and Sousa {869) found that

these cell walls were almost indistinguishable when

uranyl acetate and calcium were omitted in their

procedure.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 41

PN-6379

FIGURE 9.—Bacterial cells oîErwinia amylovora: A, Isolated from pear, apple, and hawthorn and stained by Casares-Gil's flagella stain

(after Bryan, 120); B, derived from fresh exúdate, dried rapidly, and stained in basic fuchsin (x 1,725) (after Rosen, 819); C,

derived from fresh exúdate kept at 50 percent relative humidity for 4 months and stained as inß; note unstained envelope around

cells followed by extreme outer layer of staining matter (x 1,820) (after Rosen, 819); D, single virulent cell with abundant

peritrichous flagella (x 18,000) (after Huang, 1109); E, scanning electron micrograph of wild-type virulent cells (x 19,500) (after

Huang, 1109).
42 AGRICULTURE HANDBOOK 51O, U.S. DEPT. OF AGRICULTURE

PN-6380

FIGURE 10.—Ultrastructure of Erwinia amylovora: A, Single virulent cell (x 54,000); B, early stage of cell division (x 55,900); C,

enlarged part of cell wall inS, showing double-track structure ( x 118,250); D, advanced stage of cell division, with cell membranes

of daughter cells entirely formed before completion of cell wall constriction ( x 57,000); E, filamentous cell with minicell attached

(X 36,000); (after Huang and Goodman, Í51, Í52).

In Canada, Gibbins et al. {327á) made a study of

the ultrastructure of the cell envelope oîE. amylovora NCPPB595, using the freeze-fracture technique. This technique exposed the outer regions of

the cell envelope at the level of the plasma membrane (fig. 11, A). Thus they were able to expose
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 43

PN-6381

FIGURE 11.—A, Freeze-fractured exponential phase cell of Envinia amylovora NCPPB595: a, Convex surface exposed by probable

cleavage of outer membrane; 6, outer surface of cytoplasmic membrane (particulate); c, outer surface of cytoplasmic membrane

(nonparticulate); d, surface revealed by cleavage of cytoplasmic membrane; arrow indicates direction of metal deposition and bar

represents 0.1 nm (courtesy L. N. Gibbins, Dept. Microbiol., Univ. Guelph, Canada). B, Colony of avirulent B. amylovora on

crystal violet medium, showing only few craters (after Huang, 1110). C, Single avirulent cell oîE. amylovora with few peritrichous

flagella ( x 18,000) (after Huang, 1110). D, Plaque morphology of phage PEal halo and nonhalo (arrow) after 48 hours' incubation at

27° C (°81 F) with E. amylovora 110; scale bar equals 1.0 cm (after Ritchie and Klos, 795b). E, E. amylovora cell parasitized by

Bdellovibrio bacteriovorus (x 26,200) (after Stolp and Starr, 907).
44 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

four planes in this region and concluded that the

plasma membrane at the site of the fracture must be

devoid of included particles.

In addition to the small wild-type cells, E.

amylovora also produces long filamentous cells.

Voros and Goodman (982) reported that the

filamentous cells were 7.0^m-35.0jim long with a

width similar to that of the wild-type cell. Intermediate cells were 2.0fxm-7.0^m in length and reportedly were nonmotile. Filamentous cells were

equally virulent and phage sensitive (982) as wildtype cells, whereas many produced minicells U51)

(fig. 10, E), Huang (1109) reported the size of

minicells as 0.3jim-0.8fxm in diameter, but serial

sections showed no evidence of any nuclear material

(i51 ). Both types of cells divided similarly (1109) and

were preceded by the invagination of cell membranes

followed by cell wall constriction (fig. 10, B and D).

Besides the wild- and filamentous-type cells, E,

amylovora may also have avirulent cells and thus

produce avirulent isolates. They usually appear

rather rough in culture and are often referred to as

rough isolates (SO, 35JÍ). In California, Ark (30,1080)

observed the dissociation oîE. amylovora in culture

from the normal smooth (S) colony to the rough (R)

forms upon aging of the cultures. He found the R

type avirulent on some susceptible shrubs and only

slightly virulent on green pear fruit and succulent

tips of pear seedlings. In contrast to the virulent

strains ofE. amylovora, Huang (1110) reported only

a few well-separated craters on the surface of colonies of an avirulent strain (fig. 11, B). Single cells of

an avirulent strain showed only a few peritrichous

flagella (fig. 11, C) as compared with a virulent

strain. She observed no remarkable differences in

external morphology and ultrastructure between

the two types except the precocious flagellai development ofthe virulent strain. After 26-48 hours'

incubation the avirulent strain formed small

butyraceous colonies with dark-red centers,

whereas the virulent ones formed fluidal white colonies with small bright-pink centers.

WhenE*. amylovora is isolated from blighted host

tissue, nearly pure cultures may be obtained when

the surface is properly sterilized and laboratory conditions are aseptic (511), Bacterial growth from

small sections of host tissue is a characteristic

smooth creamy white (fig. 12, A). When this growth

is streaked on standard culture media, single colonies should be small, round, and white, with a typical glistening shine (fig. 12, B).

Miller and Schroth (653, 654) developed a selective

medium for isolating E. amylovora and described

the colony morphology as having characteristic

dark-orange centers, smooth peripheries, and

translucent margins (fig. 12, D), Goldberg and Morgan (332) observed lumps in the cytoplasm of E.

amylovora cells treated with noninhibitory concentrations ofstreptomycin. These lumps were thought

to be any solid constituent of the cytoplasm, including nuclei, lipoids, or chromatin material.

At the University of Missouri, Crosse and Goodman (196) observed characteristic craters when colonies grown on a high sucrose medium were examined under oblique light at maximum 30 magnification. Later, Huang (1110) reported more numerous

craters, especially on colonies of virulent isolates

(fig. 12, C). She observed that several of these cratersfused and formed irregular bowl-shaped depressions. In Canada, Dueck and Quamme (239) reported

that E. amylovora colonies on the high sucrose

medium appeared convex and "uniquely striated"

when viewed with transmitted incandescent light

after 48-72 hours' incubation at 28° C (82.5° F).

Some of the colonies from pear reportedly were

more cone shaped than convex, had darkerstriations

in the center, and were not striated at the margins

(fig. 12, E). They observed the cratered appearance

only in very young colonies.

Moore and Hildebrand (661) subjected bacterial

cells from ooze to electron microscopy and found that

they were essentially nonvacuolated, whereas vacuoles were common in cultured cells. They

suggested that differences in cellular morphology

may be manifestations of physiological changes

operating in the survival mechanisms of the blight

pathogen.

Besides the morphological characteristics, the

principal cultural characteristics of E. amylovora

are confinement ofliquefaction in gelatin stabsto the

upper layers, a thin turbidity in nutrient broth, no

odor or pigmentation on potato, coagulation of litmus milk after 3-4 days, no production ofindole, and

no nitrites produced from nitrates. The positive effect of nicotinic acid for optimum growth of E.

amylovora has been well documented (265, 89Jf).

Lewis and Tolbert (581) and Lewis (1113) found that

E, amylovora readily used aspartic acid, glutamic

acid, asparagine, glutamine, beta-alanine, and

gamma-amino butyric acid as sources of nitrogen in

synthetic culture media.

Standard selective culture media generally used
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 45

for E. amylovora are mentioned at the end of this

section. The effect oftemperature, pH, and Hght, as

well as the effect of other micro-organisms, including ß. herbicola, is discussed later in this chapter.

Generation Time

Naturally the rate of growth and generation time

forE. amylovora vary with the culture medium and

incubation temperature. In 1938, Hildebrand Ul9)

determined a generation time of 71 -94 minutes (av.

82 min) for seven strains ofE. amylovora grown in

nutrient broth at 30° C (86° F). Adding glucose to the

basic nutrient medium increased the generation

time by 13 minutes, whereas the deviation between

the original strain and single-cell isolates amounted

to 17 minutes in the extreme. Later he U2i) reported an apparent relationship between generation

time and degree of pathogenicity. For an apparent

avirulent isolate the generation time was 72-77 minutes and for a virulent isolate 80-82 minutes.

In England, Billing et al. (87) determined a generation time of 72-75 minutes for an E. amylovora

isolate obtained from a canker on a Laxton's Superb

pear. In vitro growth experiments with shaken

broth cultures indicated that temperatures of 27° C

(80.5° F) or more were not much more favorable for

bacterial multiplication than those between 21° (70°)

and 27° (,84a, 85). Below 18° (64.5°) the doubling time

FIGURE 12.—Colony characteristics of virulent isolates of Erwinia amylovora: A, Growth surrounding sections from infected pear

shoots plated on nutrient-yeast-dextrose agar (NYDA); B, small, round, white, glistening colonies on NYDA isolated from oozing

pear canker; C, single colony on crystal violet medium, showing numerous characteristic craters on surface (after Huang, 1110); D,

smooth colonies with dark-orange centers and translucent margins on selective medium (after Miller and Schroth, 65.i ); E, colony

isolated from pear, with dark striations in center and smooth margin (after Dueck and Quamme, 2S9).
46 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

increased rapidly, requiring more than 36 hours at 3°

(37.5°) {8Jta). Doubling times of streptomycin-resistant cultures were greater than those of the parent culture both in shake broth culture and in apple

seedlings {81g). In a study of virulent and avirulent

isolates of E. amylovora in Missouri, Burkowicz

(1088) determined mean generation times at 28° C

(82.5° F) of 3.7 hours for the avirulent isolate and 4

hours for the virulent one. For a saprophytic yellow

bacterium (Erwinia sp.), the generation time was

about 10 hours.

Standard and Selective Media

The following standard media have generally been

used by many investigators for culturing E.

amylovora in fire blight research: Nutrient agar

(NA), containing 23 g of nutrient agar per liter of

distilled water; nutrient yeast dextrose agar

(NYDA or NYGA), containing, in addition to the

nutrient agar, 10 g of dextrose (glucose) and 5 g of

yeast extract per liter of distilled water; and nutrient yeast dextrose (glucose) broth (NYDB or

NYGB), in which 8 g of nutrient broth is substituted

for the nutrient agar in NYDA.

In addition to these media, some workers have

used a partial or completely synthetic medium for

special studies ofE. amylovora (30, 265, U27, 572,

581, 8H, 1080,1113). This synthetic medium usually

contains per liter of distilled water varying quantities of the following ingredients: Ammonium

chloride, monobasic and dibasic potassium phosphate, magnesium sulfate, ferric sulfate, zinc sulfate, nicotinic acid, and phenol red. The nicotinic acid

is usually dissolved first before adding all the other

ingredients and the final mixture is well shaken. For

special studies of amino acid utilization hyE. amylovora, Lewis (1113) used several liquid and soHd

media for growth of the organism.

The phosphate buffer solution sometimes is used

to prepare a suspension oîE. amylovora for inoculation experiments (353a), This buffer supposedly

prevents inactivation of the bacterial cells and is

made from two stock solutions—(A) 27.2 g per liter

of 0.2 M monobasic potassium phosphate and (B)

34.8 g per liter of 0.2 M dibasic potassium phosphate.

The 0.05 M buffer solution (pH 6.5) is prepared by

adding 70 ml ofstock A and 30 ml ofstock B to 300 ml

of distilled water.

Certain media have been prepared for the selective isolation of E. amylovora. In 1970, Kado and

Heskett (^1) reported five selective plating media

for plant pathogenic bacteria. They obtained a plating efficiency of 77.7 percent for E. amylovora on

medium D3. This medium contains the following ingredients per liter:

G

Agar 15.0

Sucrose 10.0

Arabinose 10.0

Lithium chloride 7.0

Casein hydrolysate 5.0

Sodium chloride 5.0

Glycine 3.0

Magnesium sulfate .3

Acid fuchsin . 1

Bromothymol blue .06

Sodium dodecyl sulfate .05

It is adjusted to pH 8.2 with sodium hydroxide before autoclaving at 12F C (250° F) for 15 minutes and

has a pH of 6.9-7.1 after autoclaving. Following

plating, the medium produces a characteristic red,

the intensity depending on the species. The amylovora group produces a Hghter tint of red than the

group that causes soft rots.

Another selective medium, developed by Miller

and Schroth (652-654), contains the following ingredients in 970 ml of distilled water:

G

Agar 20.0

Mannitol 10.0

L-Asparagine 3.0

Sodium taurocholate 2.5

Dibasic potassium phosphate 2.0

Nicotinic acid .5

Magnesium sulfate .2

Nitrilotriacetic acid ^ .2

Sodium heptadecyl sulfate . 1 (ml)

Bromothymol blue ^ .04

Neutral red ^ .01

lAs 10 ml of 2 percent aqueous solution neutralized with 0.73 g

of potassium hydroxide per gram of NTA.

2As 9 ml of a 0.5 percent aqueous solution.

3As 2.5 ml of a 0.5 percent solution.

The pH is adjusted to 7.3 with N sodium hydroxide

(ca. 5 ml), and after autoclaving the following ingredients are added: 50 mg of cycloheximide as 1 percent aqueous solution and 17.5 mg ofthallium nitrate

as 1.75 ml of a 1 percent aqueous solution. Reddishorange colonies were indicative of the Erwinia

genus. These authors also found that substitution of

10 g ofsorbitol for mannitol restricted the growth of

E. herbicola.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 47

Crosse and Goodman (196) developed a high sucrose medium to isolate E. amylovora living

epiphytically on apple leaf surfaces. This medium

contains in 380 ml of distilled water 160 g ofsucrose,

0.8 ml of crystal violet (0.1 percent in absolute alcohol), 20 ml of 0.1 percent cycloheximide, and 12 g

of agar. The medium had a high plating efficiency

and colonies examined under oblique light at 15-30

magnification showed characteristic craters for E.

amylovora, as reported by Huang {1109),

In 1976, Ritchie and Klos {795c) reported a less

complex selective medium containing some of the

same ingredients as the Miller-Schroth medium.

This medium produced moist, chalky-white colonies

with a cleared center and gave the colony an "eye"

appearance. It was used successfully for 3 years in

detecting E. amylovora in cankers, blossoms, and

other apple and pear tissues.

In West Germany, Zeller {10J^6a) used Lelliott's

{572) sucrose nutrient agar (SNA) and observed

raised, hemispherelike colonies of E. amylovora

after 2-3 days' incubation at 27° C (81° F).

These are not the only media used by fire blight

investigators. Special media are required for specific

studies on isolation and identification of E. amylovora {88, 26i, 617, 6Í9, 783, 877, 887, lOiÔa), growth

and metabolism {3, 29, 30, 265, 35i, ^21, i2i, Í58,

521, 581, 585, 836, 855, 89Jf), and detection of streptomycin resistance {853, 9Í9).

Growth and Metabolism

In addition to the cultural characteristics of E.

amylovora mentioned previously, the more complex

phases of its growth are covered under metabolism.

The principal references on amino acid, carbohydrate, and organic acid utilization as well as enzyme

production have been summarized in tables 4 and 5

according to positive, negative, or variable results in

utilization of the compounds by E. amylovora.

TABLE 4.—Amino acid, carbohydrate, and organic acid utilization

by Erwinia amylovora based on literature cited

Compound Utilization

Positive Negative Variable

AMINO ACID

Acetamide 581, 1113

Alanine

alpha-Alanine 581,1113

beta-Alanine 1113 581

alpha-Amino butyric acid 581, 1113

30, m

gamma-Amino butyric 1113

acid.

Arginine

Asparagine 30, UU, 1027

D-Asparagine 581, 1113

DL-Asparagine 581, 1113

L-Asparagine 581, 1113

Alanylasparagine 581, 1113

D-Aspartic acid 581, 1113

DL-Aspartic acid 581, 1113

L-Aspartic acid 581, 1113

Chloracetamide

DL-Citrulline

Cysteine

L-Cysteine

Cystine J^2J^

DL-Glutamic acid 581, 1113

L-Glutamic acid 581, 1113

DL-Glutamine 581, 1113

L-Glutamine 581, 1113

Glycine

Glycineamide 1113

Glycylglycineamide

581

1113

581, 1113

581

30, UU, 1113

581

30, 581, 1113

581

581, 1113
48 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 4.—Amino acid, carbohydrate, and organic acid utilization

by Erwinia amylovora based on literature cited—Continued

r^ 1 Utilization Compound

Positive Negative Variable

AMINO ACID—continued

Histidine 1113

L-Histidine 581

L-Homoserine 581

Isoleucine 30

Leucine 1113 30,^24-

L-Leucine 581

L-Lysine 581

Nicotinamide 581, 1113

DL-Ornithine 581

Phenylalanine 1113

DL-Phenylalanine 581

Proline 30, Jf2Jf

L-Proline 581

Serine 1113

DL-Serine 581

DL-Threonine 581

Tryptophane —' 30, U2U, 1113

DL-Tryptophane 581

Tyrosine 30 J^2U

Valine 30, U2U, HIS

DL-Valine 581

CARBOHYDRATE

Amygdalin 30, h2h

Arabinose 30, 87, 267, U2U, 1027 630

Arbutin 30, 1^24-

Cellobiose 30 630, 1001 87, J^2J^.

Cellulose J^2Jf, 630, 1001

Dextrin 30, J^2Jf 87, 1001

Dextrose 30, 87, 265, J^2J^, J^38,

Jf8J^, 630, 1001, 1027

Erythritol J^2J^, 1001

Fructose 30, 87, 265, J^2J^, J^38,

630, 1001, 1027

Galactitol (as dulcitol) — 30, Jf2Jf, 630, 1001

Galactose 87, 265, U2U, 630, 1027 30

Glycerol 265

Glycogen

Inositol 87

Inulin 30

Lactose

Maltose 30, U8Uy 1027

Mannitol 87, U2U, 630

Mannose 30, 87, 265

Melezitose

Phloridzin 30, h2h

Raffinose 30, 265, 1027

Rhamnose

Ribitol (as adonitol)

Salicin 30, 265, k2U

Sorbitol 87, 630

Sorbose

87, 630, 1027 30, U2U

87 U2J,

630, 1001 U2U

87, 630, 1001 h2h

87, 265, 630, 1001, 30, U2J,, US8

1027

265, 630, 1001 87, },2U, Jf38

1001, 1027 30

1001 h2h, 630

lot

^^J4.

1001 h2J,, 630

30, 630, 1027 87, U2U

630

87, 1001 630

1001 k2h

87, 1001
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 49

TABLE 4.—Amino acid, carbohydrate, and organic acid utilization

by Erwinia amylovora based on literature cited—Continued

Compound

Utilization

Positive Negative Variable

Starch—

Sucrose -

Trehalose -

Xylose —

Acetic —

Benzoic -

Citric—

Formic--

Glycolic ~

HippuricLactic —

Maleic-—

Malic

Malonic

A/'-Methylglycine (as

sarcosine).

Oxalic

Propionic

Salicylic

Succinic

Tartaric

Valeric

CARBOHYDRATE—continued

30, 87, 265, J^2J^,

630, 1001

30, 87, 265, U2Uy U38,

U8U, 630, 1001, 1027

87, 630, 1027

30, h2U, 1027

ORGANIC ACID

30, 87, h2U

630

30, 87, U2U

87, U2U, 630

87, U2U, 1027

30, h2U, 630

1027

87, 1027

87, 630

630

630

87, 1027

30, 87, U2U

1027

30, 87, U2J!f, 630

30, 87, U2U, 1027

87, U2h

30, U2U

1027

30, 87, 630, 1027

30, Jf2U, 630

30

U2U

TABLE 5.—Enzyme production by Erwinia amylovora based on literature cited

Enzyme Positive

Production

Negative Variable

Amylase

Arginine dihydrolase —

Catalase 87, 630

Cytochrome oxidase

beta-Glucosidase

Glutamic acid

decarboxylase.

Lecithinase C

Lipase

Lysine decarboxylase —

Omithine decarboxylase

Pectase

Pectinase

Phenylalanine

deaminase.

Protopectinase

Tyrosinase

Urease

7U2, 1001

630

87, 630

630

87, 630

630

630

7U2

7U2

87, 630

87, 630

87, 630

U3, 846

630
50 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

The nutritional requirements of the

phytopathogenic Erwinia species, including E.

amylovora, were determined by Starr and Mandel

(89^). They reported that with few exceptions Erwinia species grew in the glucose-salts basal

medium and that E. amylovora required nicotinic

acid obligately for its growth. This has been confirmed by El-Helaly et al. (265) in Egyptian isolates

of £'. amylovora.

In 1940, Kent (520) and Kent and Melhus (521)

showed that E. amylovora could utilize only complex organic forms of nitrogen after successive

transfers in liquid synthetic media at 48-hour intervals. In comparison, various soft rot bacteria were

capable of using nitrates or ammonium salts as well

as simple amino or amide forms.

Sutton and Starr (91i) determined that the major

products of glucose dissimilation by E. amylovora

were lactic acid, ethanol, and carbon dioxide and

that lactic acid and ethanol were produced in greater

quantities than reported for other enterobacteria.

They suggested thatthe high yield of ethanol may be

the result of a possibly unique enzyme reduction of

pyruvate inE. amylovora. This idea has since been

substantiated by Haq and Dawes (385) based on the

thiamine pyrophosphate-dependent pyruvate decarboxylase of E. amylovora. Later Sutton and

Starr (915) showed that some extract ofE. amylovora contained the enzymes necessary for cyclic operation of the hexose monophosphate shunt in

aerobic metabolism of glucose. Tracer evidence substantiated the effective operation of the EmbdenMeyerhof pathway of glycolysis in anaerobic

metabolism of glucose by cells of the blight organism.

White and Starr (1020) studied the glucose fermentation end products by Erwinia species and

other enterobacteria. They observed five different

fermentation patterns in eight isolates of E.

amylovora and concluded that members of this and

several other genera were heterogeneous in fermentation end product patterns, and this may account

for the very slow fermentation. The production of

acetoin (3-hydroxy-2-butanone), a product related to

the production of 2,3-butanediol, has been reported

as variable in this species (630).

Nitrate reduction, as shown inE. carotovora and

E. herbicola (5^0)^ has not been detected in E.

amylovora. In the conventional peptone-nitrate

media, E. amylovora and related Erwinia species

do not form detectable nitrite (893). This inability to

reduce nitrate in a complex medium, but to do so

readily in a minimal medium supplemented with

nicotinic acid and nitrate as the sole source of nitrogen, might result from regulation of enzyme activity, enzyme synthesis, or both in the presence of a

readily utilizable source of organic nitrogen (893).

In Canada, Katznelson (493-495) found that E.

amylovora utilized 2-oxogluconate (ketogluconate)

in the oxidation of glucose and produced carbon

dioxide anaerobically from glucose. Studies with

sonic preparations of the organism demonstrated

the presence of certain key enzymes involved in the

glycolytic and shunt pathways of glucose

metabolism. He suggested that species of Erwinia

may use either the glycolytic or the oxidative route

(i95). The latter route was confirmed by Suzuki and

Uchida (916, 917). Farago and Gibbins (296a, 297)

found that dissolved oxygen tension and rate of

growth affected the metabolism of glucose-limited

chemostat cultures oíE. amylovora.

Lewis and Tolbert (581) and Lewis (1113) determined the nitrogenous compounds used by E.

amylovora in synthetic media. They found that the

bacterium grew when aspartic acid, asparagine,

alanylasparagine, glutamic acid, glutamine, or ammonia was the only source of nitrogen available to

the bacterium (table A).E. amylovora did not grow

well on 24 other amino acids. Grou (371^) made a

comparative study on the changes in amino acids and

electrophoretic characters of cytoplasmic components by species of Erwinia belonging to the

amylovora, carotovora, and herbicola groups.

Casida (H8) found that E. amylovora (strain

P-182) utilized phosphite phosphorus (orthophosphite) for heterotrophic growth but did not accumulate phosphate in the oxidation product.

The enzyme production hy E. amylovora reportedly has been entirely negative for the production of

catalase (table 5). Martinec and Kocur (630) observed that 49 strains of the bacterium reacted

positively.

Negative results are recorded in the literature for

the following miscellaneous biochemical tests:

AcetyImethyIcarbinol (3-hydroxy-2-butanone),

ammonia production, Braun's test, casein hydrolysis, hemolysis, hydrogen sulfide production, Índole, methyl red, méthylène blue reduction, sodium

chloride tolerance (5 percent), and nitrate and nitrite

reduction (30, 87, J^8i, 630, 1001, 1095). The only

reported positive test is that for gelatin hydrolysis

(87, J^8Jf, 1001).
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 51

There are some reports ofthe detrimental effect of

certain conditions on the growth of E. amylovora.

Perry and Weinberg (735) showed that rate of death

of many bacterial strains, including the blight organism, was accelerated by iron deprivation and

slowed by storage at low temperatures. Stewart

(900) found growth of E. amylovora retarded by

tributyl (2,4-dichlorobenzyl)phosphonium chloride

(Phosfon) at concentrations from 10~^ to 10~^ M.

Chantano (1093) reported a significant reduction in

the number of colonies and viable cells oîE. amylovora following 15- to 25-minute exposure to 1.3-2.0

ppm of chlorine.

Basic studies on growth and metabolism of E.

amylovora should be expanded and the results correlated with research on the infection process between host and pathogen.

Details on E. herbicola and its morphology,

metabolism, and possible relation to E. amylovora

are discussed at the end ofthis chapter under Effect

of Micro-Organisms.

Bacterial Exúdate and Toxin

Ooze Formation

The nature of the bacterial exúdate or ooze ofE,

amylovora has been studied through the years (llJ^y

U6, Jf21, 629, 651, 7Jf2, 813, 819). Ooze is most commonly known in the liquid form as single droplets on

succulent shoots or blighted fruit (fíg. 6, A; pi. 3, C

and D ) or running along the bark of a severely blighted tree (pi. 4, A and B), Bacteria in such ooze are

generally virulent. However, Rosen (80Jf) reported

that cultures obtained from ooze in Arkansas failed

to produce infections in vigorous Bartlett shoots

inoculated in the greenhouse. Bacteria in dried ooze

stored at room temperature as long as 2 years or

more reportedly have retained their viability and

pathogenicity (27, 7U2, 819, 1123).

When Rosen (819) subjected natural exúdate to a

controlled temperature of 16° C (61° F) in Arkansas,

the bacteria remained viable and infectious for over

a year at relative humidities of 0-45 percent. At

controlled temperatures of 25°, 30°, 35°, and 40°

(77°-104°), the bacteria remained viable for long

periods when the relative humidity was low, but

they died when it reached 45 percent. When exúdate

was exposed to fluctuating outdoor temperatures,

the bacteria remained viable for almost a year at 0

percent relative humidity and for over 9 months at

50 percent, but they died rapidly at relative

humidities of 45- 90 percent. Bacteria within blighted host tissue lived for approximately the same

length of time as those in the natural exúdate at low

relative humidities, whereas bacteria in artificial

cultures were very short lived under similar conditions. Morphological studies ofthe bacteria obtained

from exúdate indicated that they were enveloped in

slimy capsules, which were mainly nonproteinaceous (fig. 9, B and C).

The most detailed study of the nature of ooze was

made by Hildebrand (U2í)m New York. He actually

tasted blight exúdate and considered it relatively

free from sugar but with a flat, starchy, acid taste.

Chemical analysis, however, indicated that ooze

contained 31 percent dextrose. The thermal death

point of the bacteria in ooze was 5° C (9° F) lower

than when grown on agar or in broth. In its natural

matrix £". amylovora was also more sensitive to the

action of bactéricides than on culture media. Cultures of the organism on the synthetic carbohydrate

medium could utilize the dilute sterile exúdate as a

source of carbon.

Ooze can vary from pure white to deep red, with

various shades of brown, yellow, and orange in between (U21,1051). Martin (629) reported the production of a dark-green ooze on the twig of a loquat tree

(Eriobotrya japónica (Thunb.) Lindl.) artificially inoculated with a pure culture ofE. amylovora.

Strand Development

In 1937, Ivanoff and Keitt (^6^) reported the first

observations of abundant aerial bacterial strands of

E. amylovora on potted pear trees inoculated in the

greenhouse. Since that time strands have been reported from Maryland (509, 512), Iowa (72), Washington (885), and Great Britain (89, 256) (fig. 7; pi. 1,

C).

The length of bacterial strands reportedly varies

from a fraction of a millimeterto several centimeters

and the width from 6/Lim to 300ptm (256, Í6JÍ, 512). All

investigators have reported that strands could easily blow in the wind, were instantly dispersible in

water, and could thus have a significant role in the

dissemination offire blight (chap. 8). One wondersif

the appearance in certain orchards ofblight infection

attributed to streptomycin-resistant Í?. amylovora

may be due to strands blown by wind into these

orchards from long distances.

In 1971 we made a detailed study of bacterial

strands by means of a scanning electron microscope

(510, 512). Two principal types of strands were observed—smooth and beaded (fig. 13, A). Smooth
52 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

FIGURE 13. —Scanning electron micrographs of aerial strands ofErwinia amylovora: A, Smooth and beaded strands among trichomes

on petiole surface ( x 120); B, ridged strand extruded from lenticel ( x 1,000); C, pieces ofbroken smooth strands, showing irregular

breaks and cracks ( x 1,300); D, enlarged portion ofstrand end shown in upper center ofC, ( x 13,000); E, magnified part of cross

section in D, showing bacterial cells (rounded structures) and cavities (depressions) in cementing material ( x 26,000); F, clumps

and single cells connected by cobweblike network resulting from strands dissolved in water and liquid evaporated under vacuum ( x

2,600); G, individual bacterial cells resulting from strands dissolved in water and resuspended in ethyl alcohol ( x 2,600); insert

shows single cell oîE. amylovora (x 26,000).
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 53

strands appeared to be formed by a uniform quantity

of matrix extruded through natural openings of the

epidermis. Beaded strands, however, seemed to be

formed by extrusion of matrix in spurts. Figure 13,

B, shows a ridged strand originating from a lenticel.

Time-lapse photography in England showed that

fine strands were produced gradually over periods of

several hours (25A).

Strands are very firm and rigid, become brittle

with age, and when crushed they break or shatter

like glass into many fragments with irregular ends

(fig. 13, C and/)). Broken ends ofstrands suggested

thatthey are-composed largely ofmatrix rather than

bacteria. A magnified part of a broken strand clearly

showed rounded ends of bacterial cells and pockets

where cells were missing (fig. 13, E), Preliminary

analyses indicated a composition of 80 percent matrix and 20 percent bacterial cells, which recently has

been confirmed by Eden-Green and Knee (258),

Their studies showed that material similar to sorbitol accounted for 28 percent of the dry weight of

the exúdate.

When strands were dissolved in water and later

evaporated under vacuum, the bacterial cells remained connected by a thin cobweblike network (fig.

13, F). When strands were dissolved in water, the

suspension was diluted with 95 percent ethyl alcohol

or acetone, and the liquid was then evaporated

under vacuum; the network disappeared and the

bacteria were observed as individual cells (fig. 13,

G). Collapsed sections ofthe cell wall were observed

due to prolonged exposure to the electron beam.

Flagella were not observed and may have been detached during preparaton of the samples (fig. 13, G,

insert).

Toxin Production

With the presence of the bacteria in the intercellular spaces, some toxic action seemed necessary to

cause plasmolysis ofthe protoplasts in the host cells.

In 1886, Arthur ÍÍ9) performed repeated tests to

find the poisons or ''ptomaines" produced by E.

amylovora. Sterilized, filtered solutions in which

the bacteria were grown did not cause rotting of

green pear fruit, however. In Canada, Jones iíSí)

made extensive cultural studies ofthe organism, but

found no toxin produced in culture. By 1913,

Bachmann (55) stated that "all ofthe first changes in

the cells which result from infection may be attributed wholly to a loss of water." Stewart (901)

disagreed with her and thought that a toxic or enzymic substance caused the killing of the host cells.

All his extractions to produce diastase or the cell

wall dissolving enzymes, pectinase and cellulase,

from bouillon cultures proved negative, however.

By 1927, Nixon (695) observed an apparent toxic

plasmolysis followed by a complete collapse of the

protoplast and the formation of schizogenous

cavities. However, he never observed a complete

dissolution of the cell walls or cell contents.

In 1931, Pierstorff (7J^2, 112S) made the earliest

recording of the presence of a toxic substance in the

bacterial exúdate of E. amylovora on green pear

fruit. He reported that this substance contained

many of the characteristics of a true bacterial toxin

but that it was not thermolabile and was not inactivated when exposed to air for 14 hours. He concluded it to be an endotoxin or a decomposition product of the host cells. Eight years later Hildebrand

(Í21 ) observed wilting in cut pear shoots immersed

in sterile exúdate matrix and thus pointed to the

presence of a true toxin. The wilting was accompanied by necrosis of the cut ends and plasmolysis of

the cells in the lower parts of the stem. The toxic

substance was thermostable, withstanding steam

heat for several hours and a dry air oven at 100° C

(212° F) for 3 months.

In Missouri, Shaffer (1129) observed that one of

the rough isolates of £'. amylovora grown on solid

medium produced an endotoxic substance, whereas

a smooth isolate evoked a response in test shoots

indicative of an exotoxic principle. It was not until

1974 that Goodman and his coworkers (8^7^ 3^8)

announced the isolation of a true host-specific toxin

and named it amylovorin. This toxin, isolated from

previously inoculated green apple fruit, consisted of

98 percent galactose in polymeric form and 0.375

percent protein, and it had an average molecular

weight of about 165,000. Tip cuttings from susceptible pear and apple cultivars showed wilting within

1-3 hours, whereas those from resistant cultivars

showed no visible symptoms until 12-24 hours after

they were placed in the toxin solution at a concentration of lOOjLtg per milliliter. The polysaccharide was

found to resist the enzymatic degradation of betagalactosidase but was partially hydrolyzed by this

enzyme prepared from a fungus (W). Later, Stoffl

et al. (906a) separated amylovorin into three fractions on a Dow 1-X8 column in the carbohydrate

form. Each fraction contained a carbohydrate and a

protein component. Tests with apple shoots revealed that only the carbohydrate induced wilt. No
54 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

toxin activity was detected at pH 4.0-5.0, but a

maximum was noted at pH 7.0-8.0.

In 1976, Hsu and Goodman (^^^a) reported that

the host-specific toxin could not be produced in an

artificial culture medium, in the filtrate from control

suspension culture, nor in such a filtrate inoculated

with an avirulent strain of £'. amylovora. In host

ultrastructural studies with amylovorin, Goodman

and White {S5Ua) found that some xylem parenchyma cells appeared plasmolyzed and neighboring

vessels became occluded with a loose gellike substance. Under high magnification this substance appeared to be composed of a network of fibrils and

granules.

In New York, Sjulin {1132) studied the effect of a

wilt-inducing polysaccharide (PS) from oozing, immature pear fruit on succulent shoots ofCotoneaster

pannosus Franch. He found the mean water potential of PS-wilted shoots to be significantly lower than

that of nonwilted control shoots but not from that of

shoots wilted to a similar degree by sealing shoot

bases with wax. Sjulin and Beer (871a) concluded

that PS induced wilting by water stress resulting

from occlusion of vascular elements. They (871by

871c) also indicated that infection altered cell permeability, whereas amylovorin caused wilt by

nonspecific restriction of water movement in the

xylem. Thus their results did not agree with the

host-specific toxin theory.

The production oflarge amounts oftoxin may have

a significant impact on future studies of fire blight,

especially in attempts to understand the nature of

resistance. Preliminary studies by Beer and

Aldwinckle (81 ) indicated no significant relationship

in several apple cultivars between blight susceptibility and sensitivity to amylovorin.

Mclntyre et al. (613, 614) protected etiolated pear

seedlings with cell-free sonicates of virulent and

avirulent £'. amylovora. These sonicates, however,

did not inhibit reproduction or affect the virulence of

E. amylovora in vitro, and this finding suggested

that induced resistance occurred in Bartlett pear.

Samples of these extracts, supphed by the Indiana

investigators, failed in tests conducted at Beltsville

to produce any significant practical change in the

resistance of Bartlett pear trees grown under either

field or greenhouse conditions.

Strains

Arthur (50) was among the earhest fire bhght

investigators who attempted to prove the existence

of different strains ofE. amylovora. He observed

some differences in blight infections when Bartlett

and Seckel trees were inoculated with the bacterium. Variations in cultures ofthe blight organism

were also noticed by Stewart (901 ). He used the term

"strain" to indicate different isolations and stated

that an isolate from Colorado always grew more

slowly than other isolates, whereas an apple isolate

from New York was usually the most rapid grower.

Pierstorff (7^2) noted extreme variations in the

ability of different cultures to cause blight infections. His use of pear seedling material probably

accounted for some of the differences. Howard (1^38)

studied in detail E. amylovora cultures collected

from various parts of the United States and New

Zealand. He subjected the isolates to several

pathogenicity and cultural tests and concluded that

the slight differences between the cultures were

insufficient to justify establishing distinct strains.

He considered E. amylovora an exceptionally constant species.

In California, Ark (28, 30) studied 10 different

isolates of £". amylovora from 6 localities and 8 suscepts. Morphological studies showed the isolates

varied in cell size and in size and form of the colony.

Marked variability in virulence was also noted and

correlated with some morphological and physiological characters (1080). Variation was found among

the isolates in utilizing sugars, alcohols, glucosides,

amino acids, proteins, fatty acids, and amides. It is

doubted that the differences he found are stable

enough to separate the isolates into strains.

Hildebrand (i22, i2i) made a 5-year study of 136

isolates from Canada, New Zealand, and the United

States, including numerous morphological,

physiological, and pathogenicity tests. The effect on

virulence of successive biweekly transfers to nutrient broth demonstrated more variability than stability. The physiological experiments failed to find a

more reliable criterion for evaluating strains of the

organism than pathogenicity. Although numerous

small differences were obtained between individual

strains or isolates, he concluded that no strain was

outstanding enough to be used in any breeding program for disease resistance.

At Beltsville, 15 different cultures of E. amylovora were isolated from as many different pear cultivars. They were compared morphologically in the

laboratory and inoculated into succulent shoots of

clonal Bartlett trees in the greenhouse. All isolates

appeared morphologically similar and typical of the
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 55

type. Aqueous cell suspensions were equally

pathogenic on immature pear fruit slices and nearly

so in the Bartlett shoots. We concluded that the

small differences in virulence were inadequate to

consider any strains among these 15 isolates (1056,

1057). Additional unpubhshed data^^ failed to show

any significant difference in pathogenicity or degree

of virulence inE. amylovora isolates from Magness

and Bartlett pear when cross-inoculated into pear

and apple.

Studies in Great Britain using phage sensitivity,

interactivity with common epiphytes, and virulence

in pear sHces have all failed to demonstrate differences between isolates ofE'. amylovora from different hosts {25Jf.). In general, therefore, all these

studies on various isolates of E. amylovora have

failed to show any good evidence that might be used

to separate the isolates into true strains.

As far as we know, the only record of a form

species OÎE. amylovora was published in 1951, when

Starr et al. (892) suggested the nameE. amylovora'

f. sp. rubi for a bacterial isolate obtained from red

raspberry in Maine. Upon artificial inoculation, this

pathogen produced no infection in Mclntosh apple

shoots in the greenhouse but did so in raspberry. On

the other hand, an apple isolate produced infection in

apples but not in raspberry.

Virulence and Pathogenicity

Even though there is no definite indication for the

existence of strains, isolates ofE. amylovora have

been known for a long time to differ in their degree of

virulence and pathogenicity. Jackson U70) found

cultures of E. amylovora from prune to be more

virulent than pear cultures. In New Zealand,

Waters (1003) repeatedly isolated fire blight cultures from medlar (Mespilus germánica L.), which

showed ''wavy radiations" in the colonies in addition

to being very virulent. Ark (28, 30, 1080) observed

smooth, virulent, and rough avirulent isolates ofE.

amylovora and found a reversion ofthe rough to the

smooth form. He also found life of the bhght pathogen prolonged and degree of virulence diminished

after adding different reducing substances to the

culture medium (31). Virulence returned to normal,

however, even after the first transfer to a medium

without any reducing agent. Dalzell (1095) distinguished between smooth and mucoid phases of E.

amylovora and their degree of resistance to ultraviolet radiation.

10 Fruit Lab., U.S. Dept. Agr., Beltsville, Md.

In Illinois, Powell (756) and Reinhardt (1126) did

not observe any noticeable changes in pathogenicity

OÎE. amylovora due to storage at subfreezing temperatures but found a direct relationship between

survival and concentration of the bacterial cells.

Frampton and Hildebrand (315) were unable to find

a correlation between pathogenicity or virulence and

electrophoretic velocity of the bacterial cells. In

New York, Hildebrand (^22, i2J^) observed the following relationship between degree of virulence in

63 cultures and the mean length ofthe bacterial cells:

Very virulent (1.42^m), moderately virulent

(1.57^m), slightly virulent (1.61/j,m), and nonvh'ulent (1.59jim). Length of the flagella also seemed to

be related in some way to virulence; the proportion

of cells with short flagella increased directly with a

decrease in pathogenicity. However, there was no

correlation between virulence and the physiological

behavior of cultures.

At the University of Missouri, Shaffer (1129) and

Shaffer and Goodman (853, 855) studied in detail

virulent and avirulent isolates of E. amylovora.

They confirmed Ark's observation of the avirulent

growth pattern in vitro of the rough colony form and

the virulent pattern of the smooth type. Avirulent

isolates reached maximum growth after 11 hours

and the virulent isolates in 50 hours. They also obtained some avirulent isolates resistantto 1,000 ppm

of streptomycin after three to four transfers when

exposed to increasing concentrations of the antibiotic (853). At first, however, virulent forms could

not be made resistant to concentrations of more than

5 ppm, but later studies indicated the use of virulent

E. amylovora resistant to 1,000 ppm (582, IIH).

Similar results were obtained by Bennett and Billing (81g) in England in 3 out of 16 virulent strains of

E. amylovora. They also demonstrated that streptomycin resistance was more readily developed in

strains ofE. herbicola than £'. am.ylovora. In one

case, resistance was associated with a complete loss

of virulence.

The occurrence ofstreptomycin resistance in fruit

orchards in the United States and its impHcation in

the fire bhght control program are discussed under

Chemical Control in chapter 12.

Goodman and Shaffer (353) obtained more than

1,300 bacterial isolates, 80 percent of which were

avirulent, yellowish mucoid, and phage sensitive.

They also isolated phage-negative virulent and

avirulent cultures (855). Rough avirulent isolates

from apparently healthy apple buds or from known
56 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

virulent cultures of E. amylovora reverted repeatedly to the smooth virulent form in a minimal

broth containing a water extract of homogenized

aphids (35J^, 1082), Goodman {3U2, SU) studied 240

isolates of E. amylovora obtained from several

rosaceous hosts around the world and concluded that

differences in degree of virulence occurred. Computer analysis of data on 196 responses to biochemical and cultural tests revealed no clear ''clustering"

of isolates to permit associating specific responses

with virulence. A study of population trends of

selected virulent and avirulent isolates in susceptible host tissue suggested virulence to be ''an all or

nothing phenomenon for this pathogen" {3U).

In California, Pugashetti and Starr {76Jf) demonstrated that the genes which determined plant virulence in E. amylovora appear to be transferred

readily and completely from the donor strain (Hfr

99) to avirulent recipient strains (EA 178-M64S and

EA 178-M173-M173S) during the first 15 minutes of

a 3-hour mating period. This appears to be the first

report showing that such genetic determinants oíE.

amylovora virulence can thus be transferred to avirulent recipient strains. Chatterjee and Starr {165)

found that the episomic element F Tac"*" (lactose) was

transferred, probably by conjugation, from Escherichia coli to Lac~ strains of Erwinia herbicola

(Geilinger) Dye, E. amylovora^ and E. chrysanthemi Burkholder et al. In further studies they demonstrated that antibiotic resistance carried on R factors was transferred by conjugation íromE. coli and

Shigella flexneri Castellani and Chalmers to E.

amylovora as well as to otherErwinia species {166).

Transfer of multiple antibiotic resistance from Erwinia exconjugants harboring a repressed plasmid,

SRI, was not obtained in preliminary trials with an

E. coli F~ strain as the recipient culture.

In England, Bennett and Billing {81g) reported in

1975 that streptomycin resistance carried on an R

factor was transferred by conjugation fvomE. coli to

E, amylovora and E. herbicola. They suggested

that avirulent cells oîE. amylovora might presist or

even progress together with virulent ones in natural

infections. These findings could have serious implications in the nature of resistance and control of fire

blight.

Effect of Temperature, pH, and light

The principal references on the temperatures and

hydrogen ion concentrations (pH) needed for growth

OÍE. amylovora are summarized in table 6. With

few exceptions nearly all investigators agreed that

the optimum temperature for growth in vitro occurs

between 2r and 28° C (70°-82.5° F) {SO, 85, 87, 1,81,,

71,2, 901). Minimum temperatures vary from 3° to 12°

(38°-54°) {SO, 87, 6SO, 1115) and maximum temperatures from 35° to 37° (95°-99°) {SO, 87, 6S0, 901).

Lipman {586) reported high resistance to low temperatures of liquid air. The thermal death points

reportedly range from 45° to 50° (113°-122°).

These summary data, however, are very brief.

Each bacteriologist conducted many additional experiments in which E. amylovora was tested in different soHd and Uquid media and was incubated

under various conditions. Pierstorff {71,2) determined that the bhght organism withstood -183° C

(-297° F) for 10 minutes and that heating to 48°

(118.5°) did not always inhibit its growth. Stewart

{901) observed growth ofE. amylovora in bouillon at

23° (73.5°) following previous exposures of -14° to

-28° (7° to -18.5°). Jones {1^81^) reported that bacteria

in freshly inoculated bouillon cultures were not killed after 10 minutes' exposure at 45° (113°) but were

killed after similar exposure at 50° (122°). Bacterial

growth also occurred when freshly seeded agar

plates were placed for 30 minutes to 20 hours in an

ice and salt freezing mixture at 0° to -10° (32°-14°)

and then incubated at 25° (77°).

In Illinois, Reinhardt and Powell {78S) and

Reinhardt {1126) found that anaerobically grown £'.

amylovora was more sensitive to freezing and that

bacterial ooze protected the cells from death by

freezing. The organism could not stand suspension in

phosphate buffer or Emerson broth at -4.5° C

(24° F) in a supercooled state. Reinhardt also found

repeated freezings more lethal than a single freezing. In 1976, Ritchie and Klos {795b) kept excised

Jonathan apple cankers, 1.6-3.0 cm in diameter, at

four temperatures from 20° to -28° (68° to -18.5°) for 3

months. E. amylovora was detected only in the

cankers kept at -28° (-18.5°).

E. amylovora has little resistance to drying. The

organism reportedly survived on cover glasses for

24-36 hours irrespective of the presence or absence

of moisture and from 4 to 10 days in gauze strips {SO).

Jones {1^81,) observed growth of a glass smear culture

from broth dried in the dark at room temperature for

5 days and then resuspended in bouillon. Similar

smears exposed to sunhght for 30 minutes and kept

dry in the laboratory for 6 days showed no growth in

bouillon. However, Stewart {901) did observe

growth after 9 days. In Egypt, El-Helaly et al. {261,)
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 57

TABLE 6. — Temperatures and pH needed for growth of Erwinia amylovora

Year Temperature {°Cy Thermal death pH

Minimum Optimum Maximum point (°C)^ Minimum Optimum Maximum Reference

1911-.. 23-25

(73.5-77)

1913... 22-25

(7L5-77)

1929-- 25

(77)

1931...

1937... 3_8

(37.5-46.5)

1960-

1961—

1964-

1964-

12

(54)

3-5

(37.5-41)

10

(50)

3-8

(37.5-46.5)

1972—

1974—

28

(82.5)

21-27

(70-81)

25-27.5

(77-81.5)

30

(86)

30

(86)

21-30

(70-86)

18-28

(64.5-82.5)

37

(99)

37

(99)

37

(99)

35

(95)

37

(99)

35

(95)

37

(99)

6.6

5.9

4.6

4.6

4.0-4.4

45-50

(113-122)

47

(117)

49

(120)

45.1-48.3

(113-118.5)

48.3-49.5

(118.5-121)

47.5-50.0 5.2

(117.5-122)

4.3

4.0-4.4

7.0

6.6-7.5

6.8

6.0

6.6-7.5

6.8

7.6

7.6

8.7

8.8

Jones U8J^).

Stewart (901).

Howard U38).

Pierstorff (7^^).

Ark (30).

Luepschen {1115).

8.1 Billing et al. {87).

8.5 El-Helaly et al. {26Jlf).

8.8 Martinec and Kocur {630).

Eden-Green {1097).

Billing {85).

^Approximate °F in parentheses.

found thatJ?. amylovora did not survive more than

48 hours when it was exposed to 27°-29° C (81°-84°

F) on glass rods dipped in a concentrated sahne

suspension and transferred to tubes of sterile sahne

solution at various intervals.

The most recent temperature studies were performed by Billing {85) and Eden-Green {1097) at

East Mailing. They showed a linear relationship between the doubling rate of the bacterium and temperatures from 9° to 18° C (48°-64.5° F). The doubling time decreased sharply at 18°-28° (64.5°-

82.5°). They found this change at 18° of special interest, because this temperature has been cited by

many as the minimum for the occurrence of blossom

blight {598, 656, 758).

The optimum hydrogen ion concentration (pH) on

the growth oîE. amylovora varies from 6.0 to 7.5

{30, 87, 26Jf, Jf8i, 630, 901). These and other investigators reported the minimum range at pH 4.0-5.9

and a maximum at 7.6-8.8. Again the differences

were based on such factors as variation in culture

media and incubation temperatures. Reinhardt

{1126) observed thati?. amylovora was killed when

suspended in juice of pH 2.8 obtained from green

apples. The organism was able to grow in this juice,

however, when the pH was raised to between 4.0

and 4.4.

The earhest evidence of the detrimental effect of

direct sunlight on the growth of E. amylovora in

culture was pubHshed in 1905 by Smith {876). He

exposed a gelatin culture of the bacterium in a petri

dish for 4 hours to direct sunhght after covering

parts of the plate with pasteboard numbers of the

year 1896 {Tig. 14). The culture was then incubated

for 5 days at about 24° C (75° F). The bacteria grew

exclusively under the protected parts. In similar

tests Stewart {901) confirmed these observations,

whereas Jones {í8i) in Canada reported 15-percent

loss of colonies after 30 minutes' exposure of plates

to sunhght.

In field observations, Hotson (i^^) determined

that E. amylovora bacteria on pruned blighted

branches survived for 10-13 days when they were

exposed to direct sunhght. When leftin part sunhght

and part shade the bacteria remained ahve for 27

days, and when left under a cover crop providing
58 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

/^-

'* * , >.* -5V

PN-6384

FIGURE 14.—Inhibitory effect of sunlight on growth ofErwinia

amylovora; bacteria grew only under protected parts of petri

dish covered with pasteboard numbers (after Smith, 876).

protection they lived for 29 days. Luepschen (596a)

reported survival of the bhght pathogen in the orchard under high light intensity conditions (1,650-

meter elevation) in western Colorado following artificial inoculation and exposure for 47 days to high

temperature and low humidity. In Michigan, Sobiczewski and Klos (881b) exposed inoculated apple

leaves to ultraviolet Ught and observed that most

bacteria were killed after a 5-minute exposure.

However, some cells survived even after 120 minutes.

From Wisconsin, Brooks (lli) reported that exposure of small drops of exúdate to direct sunlight

killed the bacteria after 22 hours. In drops of exúdate kept in the laboratory in wax paper bags open

to the air but protected from direct sunlight, they

remained viable for 2-9 months. In small twigs cut

from the trees and exposed to sunlight for 2 hours

each day during June and July, the organism

remained viable for 15 days but was dead after 25

days.

Longevity

The longevity ofErwinia amylovora may be different in various media and may be affected by different circumstances. Apart from such conditions as

culture media, temperature, pH, light, and drought

previously mentioned, additional factors that have

been studied are nectar, honey, soil, and age of tree

branches as they affect survival of the organism.

In 1935, Thomas and Ark (935) reported that the

nectar of fruit tree blossoms grown in a dry atmosphere contained sugars in concentrations greater

than those permitting growth ofthe blight organism

in culture solutions. In Wisconsin, Ivanoff and Keitt

(465, 515) concluded beyond any doubt that nectar

concentration could limit blossom blight infection.

Growth OÎE. amylovora rapidly decreased with increased sugar concentration, and none occurred at

30 percent. The bacteria survived for 48 hours in

nectar drops containing 20 percent sugar, 24 hours in

30 percent, and less than 24 hours in 40-50 percent

sugar. However, when tubes of artificial nectar containing 40 percent sugar were heavily seeded, the

bacteria survived for 72 hours and incited infection

when placed in pear blossoms. Infection of unwounded pear or apple blossoms, inoculated by

placing small droplets of bacterial suspension in the

nectar, occurred freely only when sugar concentrations were lower than those in natural nectar.

In detailed experiments and observations in Ohio,

Gossard (357) and Gossard and Walton (359, 361)

found that E. amylovora survived in honey for a

maximum of 72 hours. When contaminated honey,

incubated for 4,28, and 47 hours, was inoculated into

succulent apple shoots, it resulted in 84, 64, and 52

percent infection, respectively. When drops of aphid

honeydew were contaminated with the blight pathogen and then inoculated after incubation of 20, 43, or

71 hours, 67, 83, and 100 percent of the shoots become blighted, respectively. They also found the

bacterium to be infectious following survival for 5

days in peach, plum, and cherry nectar. Pierstorff

and Lamb (7Í5) showed that the longevity of E.

amylovora in pure honey varied from 5 to 11 days,

and McLarty (618) found live bacteria after 6 weeks.

Thomas (933) reported isolation of the bacterium

from artificially contaminated honeycomb cells,

wood of the frame, and waxy surface of the comb up

to 15, 20, and 55 days, respectively.

In East Germany, however, Beyme et al. (82a) did

not detect the bacterium on wooden frames after 3

days or on combs after 21 days. On the other hand,

they isolated i/. amylovora from the digestive tract

ofthe bee up to 24 hours after feeding the insect a 50

percent sugar solution containing 1 x 10® cells per

milliliter of the pathogen.

In New York, Hildebrand and Phillips (Jf27) made

the most extensive study of the longevity of E.

amylovora in various sugar-containing media (fig.

15). When they used a synthetic culture solution as a

base, the maximum sugar concentrations at which

the blight pathogen grew in dextrose, lévulose, artificial nectar, and sucrose were 30, 30, 35, and 58

percent, respectively. When introduced through the

food of the bees, E. amylovora was not reisolated
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 59

70 •

1 I 240C ' ^

60 /

— Dextrose

— Sucrose + Dextrose -

50 k

—-— Lévulose

-

40

30 :

20

1

" ^^^

-

10

n 1 *^y \i 1

10 20 30

WEEKS

40 50

24° C

• Sucrose

"T-q

— —. ... Dextrose

—-•—— Lévulose

Synthetic Nectar

——— Honey

FIGURE 15. — Effect ofseveral sugar solutions, nectar, and honey at various temperatures on longevity

of Erwinia amylovora: A, Single culture; B, mixture of 30 cultures. (After Hildebrand and

Phillips, J^27.)
60 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

after 3 days from honey, comb, frame, and bees. The

only exceptions were from pollen after 13 days and

from frame scrapings after 12 days. Because the

longevity ofthe fíre bhght bacterium varied with the

incubation temperatures, the sugar concentration,

and the various sugars used, they concluded that

survival of the organism in beehives was highly improbable at the temperatures and sugar concentrations normally found in the apiary.

In California, Ark (30, 1080) studied numerous

isolates OÍE. amylovora and found all grew in 50

percent sucrose. Two cultures from Crataegus crusgalli L. and one from pear tolerated 60 percent sucrose. However, the pear strain failed to grow in 14

percent glucose, whereas an isolate from C. crusgalli in a California collection developed in 20 percent glucose.

Using the insect Aphis pomi DeGeer, usually associated with blight dissemination, Plurad {112JÍ)

andPluradetal. {7^9, 750)8howedth^tE. amylovora

persisted for at least 72 hours in the body of the

insect. However, aphids feeding on plants were unable regularly to introduce 70 or more bacterial cells

into apple leaves. Blight infection did not occur even

though the aphids were maintained for 72 hour^ on

the host plants.

Ark (30) obtained E. amylovora from orchard

soils and found that the organism persisted for 54

days in sterilized soil and for 38 days in unsterilized

soil in the laboratory. He used a simple technique of

immersing green, immature pear fruit in a water

suspension of soil collected from beneath a blighted

tree. The blight organism appeared in the form of

characteristic droplets of bacterial exúdate. Rosen

(818), however, was unable to isolate the organism

from 67 soil samples collected from beneath severely

blighted pear and apple trees.

The blight organism is also reported to survive on

the surface of tree branches for a certain length of

time. Hotson (i^^)reported a longevity of 10-29 days

on pruned branches depending on their sunny or

shady exposure in the orchard. In Arkansas, Rosen

(815y 818) found thatE. amylovora in ooze attached

to blighted pear twigs and kept in the laboratory for

varying periods was dead after 1 year. On the other

hand, cells were still viable and infectious after that

length of time after being placed over concentrated

sulfuric acid in an atmosphere approaching 0 percent

relative humidity. Fire blight exúdate keptin corked

vials suspended from an apple Hmb in an orchard

yielded no viable bacteria after less than 3 months.

In our longevity studies of the blight pathogen at

Beltsville, young Bartlett trees in the greenhouse

were sprayed with a bacterial suspension and allowed to dry. When these trees were sandblasted 4

weeks later to simulate a severe hailstorm, 67 percent became infected, indicating that the bacterium

could survive in this dry state for at least 1 month

(500y 501). In followup experiments, Bartlett trees

were spray-inoculated with a different isolate ofE.

amylovora and sandblasted 4 weeks after storage at

32° C (90° F). One isolate caused infection, whereas

the other did not, thus indicating differences in ability of E. amylovora to survive on tissue surface

(1052, 1062).

Bacteriophagy

Several reports have described the isolation of

bacteria other than E. amylovora from hosts infected with fire blight and their separation through

using bacteriophagy. Coons and Kotila (182) in 1925

isolated a lytic principle (phage) from soil, river

water, and diseased tissue showing fire blight symptoms. Phage titer was increased by repeated association with susceptible bacteria. Its activity varied

according to the strains of the organism tested.

These investigators pointed out that phages may be

important in the infection phenomena as well as in

plant disease control.

It was not until 1947, when Thomas (H2, H3)

showed that phages could be used to identify and

differentiate E. amylovora isolates from pear,

apple, mountainash, cotoneaster, and hawthorn.

Since then several articles have been published on

bacteriophagy. In Missouri, Baldwin and Goodman

(62) and Goodman and Shaffer (353) obtained 1,324

avirulent bacterial isolates from 758 dormant apple

buds in orchards with fire blight. Forty-eight percent ofthe buds harbored bacteria sensitive to one or

more E. amylovora phages and thus accounted for

40 percent of the isolates. Yellowish-white mucoid

isolates were the most frequent of six colony types

identified. Later two yellowish-white mucoid isolates were shown to be mixtures of a virulent white

E. amylovora and a nonvirulent yellow bacterium

(3W). The component cultures were considered related because they were lysed by some of the same

phages.

In England, Billing (83) used phages to identify

and separate bacterial isolates oíE. amylovora from

saprophytic Erwina-Wke organisms and Pseudomonas species. Billing et al. (88) also used phages to
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 61

show the relationship of typical E. amylovora

strains with capsules and atypical strains without

capsules when grown on Yeastrel peptone agar.

Hendry et al. UOl) studied 616 Gram-negative

bacterial isolates from trees and orchards with fire

blight. Serologically 194 E. amylovora isolates were

distinguished from 57 nonpigmented isolates of miscellaneous coloquial types (1106). All the E,

amylovora isolates, 26 miscellaneous white isolates,

and 94 yellow isolates were lysed by 1 or more of 7

phages. Two phages lysed many isolates from all

three groups indicating a relationship.

Neal et al. (677) attempted to determine whether

aphid eggs are possible overwintering sites of E.

amylovora by challenging bacterialisolates from the

eggs with phages specific for the fire blight organism. Of 156 isolates, 14 of 46 white rough isolates

were sensitive to the phage but avirulent. None of

the yellow mucoids were sensitive, although one

proved virulentin apple tissue and one white smooth

isolate was sensitive and virulent.

Erskine (279) went a step farther than other investigators to show the possible role of E. amylovora phage in the epidemiology of fire bhght. Bacteriophage isolated from soil beneath peartrees with

fire blight lysogenized yellow saprophytic bacterial

isolates associated with E. amylovora but did not

lysogenize a virulent strain (PR 1) of£'. amylovora.

These studies suggested thatthe yellow saprophyte,

which is invariably isolated from fruit trees withi/.

amylovora, may frequently occur in its lysogenic

form in nature and serve as a reservoir of phage,

which may affect the occurrence and severity of fire

blight.

Erskine (278, 280) demonstrated indirectly the

possible role oftoxin in the etiology offire blight. He

found that phage lysis of highly virulent forms oîE.

amylovora led to the release of a highly toxic agent

when injected into rabbits. The toxic factor was

liberated to a less extent from less virulent forms of

the pathogen but not from related nonpathogenic

saprophytes. These results suggested that toxin

production by the pathogen may be determined by

the activity ofthe phage and the possible association

of the toxin with virulence characteristics of the

bacterium in plant tissues.

In Canada, Harrison and Gibbins (387a) studied

the morphology and activity ofa temperate phage isolated fromi/. herbicolaYAß. Theyfound thatthe range

of bacterial strains to this phage was limited to two

isolates ofE. herbicola and none ofE. amylovora.

In Michigan, Ritchie and Klos (795b) demonstrated the presence of high titers ofE. amylovora

bacteriophage (10^ plaque forming units per terminal) from diseased and symptomless aerial parts of

apple and pear trees. Plaque morphology of phage

PEal halo and nonhalo types occurred after 48 hours'

incubation at 27° C (81° F) (fig. 11, D). Three phage

isolates were found specific to E. amylovora

and did not lyse E. herbicola, Agrobacterium

tumefaciens (Smith and Townsend) Conn, or

Pseudomonas syringae.

When the scientist learns how to manipulate and

maintain high titers ofE. amylovora bacteriophage

on plants in the field, this bacteriophage may become

potentially useful in biological control.

Effect of Micro-Organisms

Some investigators have referred to the effect of

certain saprophytes on E. amylovora. Other organisms in the soil and plant microflora also seem to

affect the etiology and epidemiology of E. amylovora.

Baciüus and BdeUovibrio

In California, Ark and Hunt (38) studied two soil

bacteria that showed a strong antagonism to E.

amylovora as well as certain other bacteria. The

bactéricide produced by these two organisms withstood boiling for 60 minutes. However, when the

bactericide-containing medium ofthe yellow species

was autoclaved, the bactéricide was inactivated

after 15 minutes at a pressure of 10 pounds per

square inch (0.7 g/cm^), whereas that of Bacillus

vulgatus Trevisan was still active after 10 minutes'

sterilization at 20 pounds per square inch (1.4 kg/cm^).

In Egypt, Abo-El-Dahab and El-Goorani (2)

showed that Bacillus subtilis (Ehrenberg) Cohn

produced an antibacterial substance in sucrose nutrient broth in sufficient concentration to inhibit the

growth ofE. amylovora. They found the inhibitory

effect of the Bacillus equivalent and sometimes

superior to that of certain antibiotics, particularly

streptomycin.

In 1963, Stolp and Starr (907) isolated from soil

and sewage an interesting new group of bacteria

(BdeUovibrio bacteriovoms Stolp and Starr) that

was parasitic on other bacteria (fig. 11, E). These

bacteria caused reactions similar in their outward

manifestations to bacteriophage-induced lysis. All

isolates studied possessed lytic activity only against

Gram-negative bacteria, including i/. amylovora.
62 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Further studies by Starr and Baigent (891)

showed that B. bacteriovorus, after attaching to the

host cell, formed a pore in the cell wall followed by

disorganization ofthe host nucleus and ofthe murein

layer of the wall. The parasite completely invaded

the host cell, and the cell contents were digested.

When the host protoplast was entirely lysed, the

parasites left the disintegrated 'ghosted' cell envelope and were ready to reinitiate the parasitic

cycle.

Erwinia herbicola

Since the earliest available report of a "yellow,

non-parasitic schizomycete" by Waite (chap. 1) in

relation to fire blight, many investigators mentioned

such a saprophyte association with E, amylovora

after conducting isolation studies from cankers and

other blighted host tissues (62, 280, 281, 296, 3i0,

353, 790, 791, 879, 1051, 1130). Other yellow-pigmented bacteria, possibly similar to those described

here, have been isolated from plants, animals, and

miscellaneous sources (86, 364, 5^1). In 1920, Smith

(878) referred to Waite's observation and added that

"Dr. Arthur must also have had it in some of his

cultures for in one place he described the growth of

Bacillus amylovorus as 'yellowish.'"

By 1929, Shaw (1130) made the first detailed study

atthe University ofArkansas relative to the identity

of a yellow organism associated with fire blight. He

described it as a Gram-negative rod, usually occurring singly, motile by means of peritrichous flagella,

not acid fast, and producing a yellow color on most

ordinary culture media. After 48 hours on potato

dextrose agar plates at pH 6,8, colonies were described as "circular, smooth, raised entire to very

finely lobed, amorphous, yellow in center and averaging 2.5 mm. in diameter." As the colonies grew,

Shaw mentioned a shghtly irregular colony margin,

a distinct buff yellow to apricot yellow in the center,

and a very thin (1-4 mm width) grayish precipitate

developed around the colonies. He concluded that

the organism was a species oíFlavohacterium, possibly F. aquatilis (Frankland and Frankland) Bergey et al. Laboratory tests and spray inoculation

experiments on young Bartlett shoots revealed that

the yellow organism had a definite inhibitory effect

on the fire blight pathogen.

In 1964, Dye (2^5) proposed the name ''Erwinia

herbicola (Duggeli) nov. comb." for the yellow organism usually associated withE. amylovora. Two

years later the bacteriology committee for "Bergey's

Manual" recognized Pseudomonas herbicola

(Geilinger) as the first valid publication ofthis name

and thus accepted the name as E. herbicola

(Geilinger) Dye. Following a major taxonomic study

of the gemxs Erwinia, Dye (2J^8) recognized the herbicola group as one of four groups of Erwinia

species, E. herbicola as the type species. He found

that E. amylovora and E. herbicola shared many

cultural and biochemical characters, with the principal exception of an acid reaction in purple milk,

growth at 3T C (99° F), and a yellow waterinsoluble pigment for the latter. In general. Lazar

(567) agreed with the separation of Dye's E.

amylovora, E. carotovora, anàE. herbicola groups

on the basis of cross-agglutination and gel-diffusion

studies. Descriptions of other yellow-pigmented

bacteria isolated from various plant materials

reportedly fit the one for E. herbicola (5^1).

In Illinois, Smith and Powell (879, 880) and Smith

(113Jf) compared disc electrophoretic protein patterns OÎE. amylovora with several yellow isolates

collected from blighted apple or pear trees. They

found the yellow isolates more comparable to Xanthomonas pruni (Smith) Dowson than to E. amylovora. In a study of 20 yellow bacterial isolates from

Jonathan apple trees, all were Gram-negative,

peritrichously flagellated, and with rods 1.1/im2.9jim long and 0.6jim-0.9)im wide; their temperature requirements were almost similar to those oíE.

amylovora (1133).

Chatterjee and Gibbins (161 -163) studied in detail

the metabolic activities of E. herbicola and found

that it produced phloretin when grown on a defined

medium containing phlorizin as the sole source of

carbon. They also observed that several strains ofE.

herbicola produced white variants at high frequency

when grown in yeast broth at 37° C (99° F). These

variants did not reverse to the parent strain, however, but varied in their extracellular diffusible

antigens among themselves and with the parent

strain (327). A variable but minor degree of crossreaction with an isolate of E. amylovora was observed.

Chatterjee (1094) also presented evidence that all

yellow isolates utiHzed arbutin, whereas only three

isolates grew with phlorizin as sole source of carbon.

These results suggested that the yellow organisms

could tolerate higher concentrations of hydroquinone in the microenvironment than E. amylovora. The findings supported the hypothesis thatthe

yellow isolates in vivo could act on arbutin with the
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 63

consequent release of hydroquinone^ which might

inhibit E. amylovora, whereas the yellow isolates

could multiply to degrade more arbutin. This phenomenon has been suggested as having a role in the

epidemiology of fire blight (16i).

In Indiana, Mclntyre et al. (612) observed an apparent relationship between several isolates oí E.

herbicola and virulent as well as avirulent isolates of

E. amylovora. They obtained profiles from the hydrolysis of 27 naphthyl phosphoramides, sulfonamides, and acetamides and concluded that

aminopeptidase profiles offer a rapid reproducible

means of microbial identification to supplement

morphological and cultural criteria.

During 1956-76, severalinvestigators studied the

nature and possible relationship between E. herbicola and E. amylovora in the overall fire blight

syndrome. Farabee and Lockwood (296) frequently

isolated a nonpathogenic yellow species of Bacterium from fire blight cankers on apple and pear

treesin Ohio. Most ofthe isolates inhibited growth of

E. amylovora by increasing the acidity ofthe culture

media to a degree unfavorable for growth of the fire

blight pathogen.

Baldwin and Goodman (62) isolated among other

types a dominant yellowish-mucoid bacterium from

dormant apple buds. They suggested that it was

analogous with the "y^How" organism associated

with fire blight for many years by Rosen (803),

Goodman (339) later conducted studies with an avirulent yellow bacterium ''closely related" to E.

amylovora, which suppressed the growth ofvirulent

isolates ofE. amylovora in vitro. Rapidly growing

apple shoots were protected against infection by a

virulent strain of £'. amylovora by prior inoculation

with a yellow Erwinia-like isolate (3il). In further

studies, Goodman (3i0) mixed the yellow avirulent

form (35 A-Y) with a white virulent form (35 A-W),

both isolated from apparently healthy dormant

apple buds. He then found that the longer they were

incubated before introduction into the host, the less

infectious the white virulent form became.

In Michigan, Riggle and Klos (790, 791 ) and Riggle

(1128) conducted several studies on E. herbicola.

They found large numbers ofthis organism on leaves

of apple, cherry, and apricot but smaller numbers on

pear. Pear blossoms inoculated wWhE. herbicola 24

hours before inoculation with virulenta/, amylovora

partially controlled fire blight (791). Laboratory

studies indicated that£'. herbicola reduced the pH of

the substrate, which was inhibitory to E. amylovora. Further studies supporting this hypothesis

showed that filtration of a simulated nectar medium

in which E. herbicola had been growing would not

support growth oíE. amylovora (790). However,

growth ofthe latter resumed when amino acids were

supplied and the pH was adjusted to neutrality.

In Canada, Erskine (279) showed by pathogenicity studies on pear slices that symptom development

was delayed when mixtures of either E". amylovora

plusE'. amylovora bacteriophage ovE. amylovora

plus a yellow saprophytic bacterium was used as

inoculum. Symptoms did not appear when E.

amylovora and a lysogenic form of the yellow organism were inoculated together. The yellow organism may appear frequently in its lysogenic form

in nature and serve as a reservoir of phage, which

may affectthe occurrence and severity offire blight.

In other laboratory studies, Erskine and

Lopatecki (281) showed by inoculated pear slices

that variation of temperatures and sucrose concentration determined independently the equilibrium of

a readily reversible alternation of predominance of

the yellow bacterium and E. amylovora. These investigators suggested thatJÏ. amylovora may sometimes exist as an avirulent resident on the surface or

within healthy host plants when environmental conditions favor growth ofthe yellow saprophyte rather

than the pathogen. Furthermore, such conditions

are more likely to occur in midsummer and the fall,

with decreased humidity or diminution of sap flow

and with increased sugar contentin the hosttissues.

These studies indicate that the yellow organism,

reportedly associated withi?. amylovora in the fire

blight disease syndrome, may possibly be one and

the same asE". herbicola. When fully understood it

may be used in biological control of the blight pathogen.

Phytobacteriological Techniques

Apart from the materials and methods previously

discussed here for culture media, inoculation techniques, and taxonomy and separation of pathogens,

many other procedures and techniques have been

used in fire blight research. For more information,

see Goodman (3^Iia).

Descriptions of these techniques by various researchers include among others those for lyophilization of bacterial pathogens, electron microscopy,

histological studies of plant material, methods for

flagella stain, bacteriophage studies, starch-gel zone

electrophoresis, studies of glucoside metabolism in
64 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

diseased plant tissues, methods for the separation of

closely related bacteria by gel diffusion test, singlecell isolation of bacterial cultures, and procedures

for pathogen identification, including rapid detection through use of the hypersensitive reaction of

plants. Additional phytobacteriological literature is

listed with most articles. Methods for bioassay of

streptomycin in pear and apple tissue have been published by Coyier(188a) and Grove and Randall (87^a).

For specific materials and techniques used in recent studies for advanced degrees at various universities relative to the fire blight organism and its

host-parasite relationship, see the theses and dissertations grouped by the following subjects: (1)

Taxonomy and serology—Ark {1080), Chatterjee

{lOH), Dalzell {1095), El-Goorani {1098), Elrod

{1099), Hendry {1106), Huang {1109), Schroder

{1128a), and Shaffer {1129)', (2) etiology and

pathogenesis—Addy {1078), Burkowicz {1088),

Gowda {llOS), Huang {1110), Lewis {IIU), Mclntyre {1118), Pierstorff {112S), Plurad {112J,),

Reinhardt {1126), Sjulin {1132), and Wrather {1139)',

(3) biochemistry—Ahn {1079) and Hildebrand

{1107)', (4) bacteriophagy—Baldwin {1082)', and (5)

Erwinia herbicola—Higgle {1128), Shaw {1130),

and Smith {1133, 1134), For more specific and detailed fire blight studies, more sophisticated

methods and techniques may have to be developed.
CHAPTER 8

DISEASE CYCLE

As far as we know, Erwinia amylovora passes its

entire active life (primary and secondary cycles) in

direct association with the living host. In 1898,

Waite (993) made the first brief description of the

organism's life cycle. Since then numerous life cycles

have been proposed or sketched by various investigators. Modifications, additions, and improvements have been made by many.

For this handbook we prepared a new diagram of

the life cycle of £". amylovora (fig. 16), based on all

available information about the disease and its

pathogen. The description starts with the primary

infection in the spring (blossom period) and continues clockwise in the diagram through the summer

ending with the formation of cankers in the fall

(dormant period). Special emphasis is placed on

resident bacteria present internally and externally

in host tissues, as well as on all known means of

dissemination and overwintering.

Primary Infection

The origin of primary infection in the spring has

been the subject of much controversy through the

years. Since some holdover cankers from the previous season were found to exude bacterial ooze in the

spring, ooze was believed to be the principal source

of primary inoculum, based on the assumption that

insects visited the ooze and carried the organism to

the blossoms and twigs where infection originated in

the spring U16-JÍ18, 651). This theory was often

discredited because (1) oozing frequently occurred

after the first infections appeared; (2) insects only

accidentally came in contact with ooze; (3) only flies

were attracted by ooze but were not observed to

visit flowers; and (4) ooze did not seem to be attractive to bees and other pollinating insects.

Overwintering Cankers

Sackett (826) was the first to distinguish between

regular fire blight cankers and holdover cankers on

pear, in which bacteria overwintered from one season to the next. Miller (651) in Wisconsin observed

that cankers on apples showing no line of demarcation between diseased and healthy tissue were responsible for carrying the organism through the

winter, whereas cankers on apples with a definite

Hne of deUneation (crack) at the margin of the cankered area were usually found to be inactive.

In New York, Specht and Beer (SSUa) used a novel

method to detect smooth-margined cankers caused

by E. amylovora. When infected pear and apple

trees were photographed (through filters that removed radiation of less than 700 nm) with film sensitive to infrared radiation, cankers were readily detected. Conceivably such a technique might be modified for practical use in the field.

In other studies. Beer and Norelh (81c) recovered

E. amylovora from intact canker margin surfaces of

15 percent of 130 cankers examined. The organism

was recovered more frequently from cankers with

indeterminate-type margins than those with determinate margins. Inoculation of nursery-grown apple

trees early in the growing season produced cankers

with determinate-type margins, whereas trees inoculated later produced cankers with indeterminate

margins.

Viable infectious bacteria have been isolated from

cankers on pear (7JÍ2, 795a, 795c, 1050,1051), apple

(lU, JÍ16, 791, 795a, 80J^, 813), and hawthorn (256,

635, 638, 1097). Reinking (1127) reported all negative isolations from apple cankers in Wisconsin,

whereas Rosen (80J^) reported the collection of

"sterile" noninfectious ooze from apple cankers in

Arkansas. Many of these investigators observed

ooze production from no more than 12 percent ofthe

cankers in any one orchard (IH, 7JÍ2, 804, 1051). In

addition to cankers, the blight bacterium reportedly

overwinters in diseased fruit (25, 335, 619), small

twigs, and occasionally in twigs and branches left on

the ground after pruning (My 318).

Fire blight cankers vary from only 1 or 2 mm in the

current season's shoots to 15-20 cm orlarger in limbs

and tree trunks. In a study of 100 cankers on 10-

year-old seedling pear trees in Maryland, we found

that 65 percent ofthe cankers were formed following

invasion through the top of the tree downward and

only 35 percent in the main trunk after invasion

through a lateral branch (1050, 1051, 1075). In this

65
66 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

SUMMER FALL-WINTER

Entrance through stomata, hydathodes, and wounds followed by

invasion of intercellular spaces.

\

Secondary infection of shoots, leaves,

and fruit by bacteria from ooze or

strands, spread by insects, wind, or

rain.

Canker produced directly from blighted flower spur

Primary infection of blossoms and shoots by resident

bacteria from apparently healthy buds or from oozing,

overwintered cankers, spread by insects, wind, or rain.

SEPAL ANTHER

Invasion of flower parts.

LATE SPRING

Dissemination from blighted to healthy flowers

FIGURE 16.—Life cycle oiErwinia amylovora (sketches after Hildebrand,.420).

EARLY SPRING

PN-6385

study nearly all the oozing cankers were in large

scaffold limbs and tree trunks. The percentage of

live tree tissue above the canker was correlated with

the positive isolation of E. amylovora from these

cankers. In Michigan, however, Ritchie and Klos

(795a) isolated the pathogen from pear and apple

cankers as small as 0.4 cm in diameter. They concluded that small cankers may be as important as

large ones in overwintering £. amylovora.

In the spring some fire blight bacteria are carried

by wind, rain, and insects from winter cankers to

blossoms or young tender tissues of shoots where

infection may start. The bacteria enter unwounded

blossoms through natural openings (651, 80U, 816).

Hildebrand ÍA17) reported that the bacteria enter

the specialized nectar-secreting stomata of the

flower's nectary, uncutinized stigmas, undehisced

anthers, and stomata on the sepals. The bacteria
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 67

multiply and advance into the intercellular spaces.

After a few days the cells begin to collapse owing to

plasmolysis, at which time discoloration manifested

as necrosis sets in. After a few more days the flowers

blight completely.

In pear flowers, invasion occurred more rapidly

through nectaries and pistils, whereas in apple flowers the stigmas and the anthers were first invaded

{Jfl7, 816). The morphological differences of open

receptacles in pears versus closed ones in apples

appear to account for this variation. Hildebrand

Hl8) reported that he was able to initiate infection in

apple blossoms with a single bacterial cell but failed

to do so in pear blossoms.

Rosen (^iö) undoubtedly made the most detailed

study of the penetration and invasion oíE. amylovora into apple and pear blossoms. He concluded that

(1) the nectarial surface is the most vulnerable to

invasion; (2) nectar is an excellent medium for bacterial multiplication; (3) masses of bacteria can be

traced through several layers of cells below the nectarthode chamber soon after blossom inoculation; (4)

the stigmas of the apple pistils are receptive structures for invasion; and (5) bacteria travel through

intercellular spaces, often actually penetrating into

protoplasts. Ivanoff and Keitt U65) reported a correlation between the nectar concentration of apple

and pear flowers and the growth of £'. amylovora.

Infection occurred freely when sugar concentrations

were low but not at medium or high concentrations.

These observations have been confirmed (196y U27,

Jf65, 515, 935).

After killing the blossom, blight infection moves

into the flower stem and then into the peduncle and

spur, where it invades the leaves and finally the

branch. At this time the infection, if walled off, produces a canker or it penetrates farther into the

branch and then into the trunk, where it moves up

and down and results sometimes in death ofthe tree.

In some instances, a few to many limbs may be

killed, with the infection penetrating into and girdling the trunk ofthe tree. As a result, those parts of

the tree above the girdle usually die {1051).

At the time of or following flower infection, leaves

and tender succulent shoots not associated with the

flowers may also be invaded {197, 39i, ¿33, 560).

Insects, wind, and rain may carry the fire blight

organism to these tissues, where by chance they are

deposited, or the contaminated insects inadvertently introduce the bacteria into the host cells during feeding. In fact, twig or shoot infection may

occur in orchards where little or no blossom infection

is found. At Beltsville, Md., where a heavy concentration of natural inoculum is usually found, records

show that about 50 percent of natural infection occurs after the spring bloom {505).

Based on this information, combined with our experience and observations, we have concluded that

some fire blight cankers are instrumental in the primary infection process but to a less extent than

generally believed in the past.

Epiphytic Erwinia amylovora

It does not seem logical that the small number of

oozing cankers supply sufficient inoculum to cause

the great number of infections often observed in the

spring, especially when little or no active blight was

observed during the previous season. In addition to

ooze from overwintered cankers, some other source

of inoculum, occurring both in the spring and at

other times during the growing season, has been

suspected for a long time. This we refer to as resident bacteria in tree tissues and is shown schematically in the center of figure 16.

As early as 1929, Rosen {80J^) reported from careful histological studies that "the pathogen may actually pass upward or downward in some of the ducts

without calling forth any disease symptom." He

showed very conclusively that the xylem vessels

were occupied by masses of bacteria, even though

there was no definite proof of identity or

pathogenesis. In 1933, Rosen {813) concluded that

primary blossom blight may originate as (1) internal

extensions of the previous year's blight; (2) bud infections in which the buds, though infected the year

before by internal blight extensions, remain alive

through the winter; (3) infections resulting from

bacterial exudates produced the previous growing

season; or (4) the well-known twig blight induced by

inoculum from overwintered cankers. These masses

of bacteria in the xylem appeared to be similar to

cysts reported by Nixon {695) a few years earher.

The fact thatE'. amylovora could spend part ofits

life cycle ostensively as a resident organism in the

tissues of its hosts apparently remained unnoticed

for about 30 years. In 1962-63, Baldwin {1082) and

Baldwin and Goodman {62) reported the isolation of

E. amylovora from apple buds in Missouri. About 40

percent of the bacterial isolates from more than 750

apple buds were sensitive to 1 or more of 5 typing

phages. Similar isolations from apparently healthy
68 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

apple and pear buds were confirmed by Dueck and

Morand (2S8a) in Ontario, Canada.

We (508y 511) discovered thati/. amylovora may

live for long periods as residents in or on apparently

healthy pear and apple tissues without producing

blight symptoms. Bacteria were isolated from symptomless (1) side shoots developing from axillary buds

below the base of cankers on apple and pear trees in

the greenhouse, (2) suckers on blighted Bartlett

trees in the field, and (3) dormant branches of three

other P. communis cultivars without records of

visible fire blight. All cultures of E. amylovora

proved virulent upon reinoculation into young

shoots of Bartlett trees in the greenhouse. E,

amylovora also has been recovered after more than

70 days from symptomless stems of apple seedlings

in Great Britain {575).

In California, Miller and Schroth {653, 65Jf) and

Miller et al. {655) developed a selective medium for

E. amylovora in order to monitor the epiphytic

populations on insects and pear tissues. They have

shown conclusively that the blight organism can live

as an epiphyte on flowers, fruits, and leaves of apparently healthy pear trees, as well as on the surface

of inactive cankers and on Pegomya and Minettia

insect species. The epiphytic population varied

among orchards and trees and was detected from 2

to 4 weeks prior to the appearance of any blight

symptoms.

Studies have revealed through use ofthe scanning

electron microscope and photography the actual

presence of £". amylovora on leaf and blossom surfaces of pear and apple. In Michigan, Sobiczewski

and Klos {881a, 881b) showed that apple leafwounds

afforded both entry points and protective covers for

the blight bacteria following artificial inoculation

(fig. 17, A andß). Pear blossoms held at 25° C (77° F)

had the greatest increase of E, amylovora population after 48 hours when epiphytic microflora became evident. At 10° (50°) and 15° (59°), population

increase was slower and survival time longer {881b),

Natural epiphytic populations ofE. amylovora on

shoots, leaves, and buds were detected in the early

1970's in Michigan {915a) and Ontario {2S8a), However, the blight pathogen usually was seen only at

the time of symptom initiation or following visible

blight in the orchard. Monitoring during the 1973

growing season in Michigan showed that bacteria

first appeared in late bloom {538). Under these conditions the prediction of blight epidemics would be

impossible. However, an improved monitoring

technique possibly could be used to advantage in a

pest management program such as is employed in

California.

In California, Thomson ^^ studied the epiphytic

colonization of pear flowers collected from the field

and found J?. amylovora to be present almost exclusively on the stigmatic surface ofthe pistil (fig. 17, C

and D). Under high magnification (x 15,000),

numerous bacterial cells, some dividing, were readily visible (fig. 17, E). He reported no sign of blight

infection even though bacterial populations sometimes exceeded 10^ cells per healthy pistil. In 1975

only 23 percent of Bartlett pear flowers that were

open for 1 day were colonized with E. amylovora,

whereas 70 and 100 percent of the flowers open for 3

and 5 days, respectively, were colonized. All of the

flowers open for 7 days (petal fall) and 89 percent of

the young fruits were colonized epiphytically.

Thomson et al. {951a) suggested that this random

occurrence was congruous with insect dissemination

of the bacterium. Simulated rain in the field caused

movement ofE. amylovora from the stigmatic surface to other flower parts but without a resultant

increase in infections {9i8a). Through the use of the

immunofluorescent staining technique, Thomson

and Schroth {9i8b) demonstrated epiphytic E.

amylovora in pear blossoms in 3-4 hours, compared

to 3-4 days with the Miller-Schroth selective

medium. In pear orchards monitored during

1972-76, Zoller and Sisevich {1046d) determined

that at least 110 degree hours above 18.3° C (65° F)

were necessary before blossom populations developed to the point of epidemic blight.

In New York, Beer and Norelli(^la) studied inoculated blossoms on potted pears at various temperatures. They found that the time required for development of fire blight symptoms was negatively

correlated with temperature and inoculum dose. The

amount ofinfection was proportional to the inoculum

dose but independent ofincubation temperature and

pear cultivar with an inoculum dose of about ICH cells

per blossom. However, incubation temperature was

positively correlated with a dose of 10^ cells per

blossom.

In the mid-1970's, Ritchie and Klos {795b) were

the first to report the isolation of E. amylovora

bacteriophage in aerial parts of apple and pear trees.

In what numbersthese phages are generally present

and what role they have in nature and in the fire

blight syndrome have yet to be determined.

^^ Pers. commun., Dept. Plant Path., Univ. Calif., Berkeley.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 69

PN-6386

FIGURE 17. —Scanning electron micrographs of apple and pear, showing presence of epiphytic Srwiwia amylovora: A, Bacterial cells on

uppersurface of apple leaf48 hours after inoculation ( x 900) (after Sobiczewski and Klos, 881a and 881b); B, bacterial cells on lower

surface of apple leaf( x 5,000); note size ofbacteria in relation to stomatal pore (after Sobiczewski and Klos, 881a and 881b); C, pistil

of Bartlett pear flower from field, showing terminal part of style (Se) and stigma (Sa) ( x 150); D, part of pistil shown in C with

bacterial cells (arrows) evident only on stigma ( x 250); E, magnified part ofstigma shown in D with individual bacterial cells ( x

15,000); note some cells undergoing binary fission. (C, D, and E, Courtesy S. V. Thomson, Dept. Plant Path., Univ. Calif.,

Berkeley.)
70 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

We consider that these facts and observations

support the argument that the presence of resident

epiphytic bacteria in and on the host tissues may be

the most significant segment of the Hfe cycle oíE.

amylovora. Even though epiphytic populations ofE.

amylovora may vary between trees and orchards,

the resident bacteria afford a logical explanation for

blight epidemics in new orchards without any oozing

cankers or any previous record of fire blight (362^

1066).

Secondary Infection

Once primary infection occurs and the disease is

advancing through the tissues, secondary infections

may continue throughout the growing season.

Sources ofinoculum may be bacterial ooze or strands

produced on shoots, leaves, fruit, or larger

branches. They can be disseminated by rain, wind,

insects, or birds. In addition, man may also spread

the pathogen by means of contaminated pruning

tools. The secondary infections are usually far more

numerous than the primary ones and generally cause

serious injury to the trees.

When leaves and succulent shoots become infected, the fire blight bacteria may enter hosttissues

directly or through wounds. Lewis and Goodman

{582, 583) showed that the bacterium can enter

through excretory glandular trichomes and

hydathodes on the upper surface of Jonathan apple

leaves and through lenticels on the stem. They

stated that once the pathogen entered the leaf, it

migrated rapidly in the stem via the phloem.

Numerous experiments and field observations

also indicated that wounds offer definite avenues for

the bacterium to enter the host. Types of injuries

vary from small insect punctures and stem abrasions

to large wounds caused by severe wind or hail.

Young pear fruit especially is susceptible to blight

infection during hailstorms (see chap. 11).

Secondary cycles may continue throughout the

remainder of the growing season and each infection

may terminate as a small or large canker.

Toward the end ofthe growing season the bacteria

remain abundant in the edge of advancing infection,

but as soon as the bark tissues die, most of the

bacteria die also. Progress of the bacteria slows

down and usually ceases with the formation of a

crevice at the edge ofthe invaded tissue. Sometimes

brown streaks may be found in the live bark tissue

beyond the crevice. In these tissues the bacteria

may be abundant throughout the following winter

and spring, and thus the life cycle ofE. amylovora is

concluded.

Dissemination

Dissemination ofthe fire blight organism is essential for distributing the pathogen and completing the

disease cycle. E. amylovora is usually spread by

rain, wind, insects, birds, or man.

Rain and Wind

Of the meteoric conditions, rain is probably the

major factor in disseminating the pathogen from

holdover or fresh inoculum {360, 516, 651), Gossard

and Walton {360,361 ) were the firstto emphasize the

importance of rain drip and windblown rain as a

means of disseminating secondary inoculum, but

they did not appreciate its possible significance in

primary infection. They proved conclusively the effectiveness of rain in spreading the bacteria from

blossom to blossom. Proof of the significance of

water in primary infection was demonstrated by

Brooks {lU, 1086), Miller {651), Tullis {961, 1138),

and Keitt et al. {516). They maintained that the

greater part of primary infection resulted from inoculum that was rain splashed or blown from holdover cankers to blossoms.

In a 3-year study on apples. Miller {651) showed

that if a source of inoculum was present in the upper

part of a tree, the area of secondary infection below

it was cone shaped, caused by the downward spread

of bacteria by spattering rain. Such a pattern would

not result when blight inoculum was randomly

spread by bees. On the other hand, Pierstorff (7^^,

1123) and Parker {722,1122) concluded that meteoric

water did not appear to spread the bacterium from

flower to flower. They stated that the bacteria were

spread by wind and rain throughout the orchard, no

matter what their source, landing in many sites,

some favorable and some unfavorable for infection.

The favorable sites were usually those with young

actively growing tissues.

Indirect effects of rainfall on the epidemiology of

fire blight have also been reported. Thomas and Ark

{935) and Ivanoff and Keitt {Jf65, 515) pointed out

that during dry weather the nectar in blossoms was

too concentrated for bacterial growth, but rain diluted the nectar so that bacteria multiplied and

caused more infection. In Arkansas, Shaw {860)

demonstrated that apple and pear shoots were more

susceptible during wet weather because ofincreased

intercellular humidity, which favored infection and

disease development.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 71

Since the earliest observation by Stevens et al.

(898) that wind was important in spreading the bacterium, many observers have noted that fire blight

seems to spread in the direction of prevailing winds.

In Iowa, Bauske (71, 78) studied in detail the wind

dissemination oîE. amylovora. He observed a relationship between the severity of blight epiphytotics in nursery rows and the exposure ofpeartrees

to prevailing southerly winds, and he noted that

spread of fire bhght was sharply reduced through

using wind barriers in the rows. He reported that

water droplets containing the bacteria were easily

transferred distances of up to 1 meter at a wind

velocity near 14 miles per hour (22 km/h). Pear

foUage injured by wind also facilitated infection

(1083). Boyd (1085) determined that southerly winds

were most frequent during May, June, and July in

areas ofsouthwest Iowa, when fire blight developed

rapidly on pear and apple nursery trees.

Nearly identical observations were made in England by Glasscock (329, 330), where bhght epidemics

occurred on apples following a hailstorm with strong

northwesterly winds from the direction of diseased

hawthorn hedges. In one orchard with more than 700

infected trees, strong northwest winds accompanied

by light hail occurred in July 1969, blowing from the

direction of three hawthorn hedges suspected as

sources of blight. Humid weather with temperatures of 22°-25° C (72°-77° F) followed the hail and

continued until the blight symptoms were first

noticed. In all cases, infection started in young succulent shoots. The highest number ofshootinfections

(90-140) was observed in trees nearest the infected

hawthorn hedges. This number rapidly decreased to

only a few infections a short distance away. We have

made similar observations in our extensive pear cultivar and seedling plantings in Maryland aftersevere

summer rainstorms without any hail.

With wind dissemination, the organism is usually

carried in drops of dew or rainwater. However, E.

amylovora may be produced as bacterial strands

(72, JÍ6Í, 509, 512), which can be blown for long

distances by the wind. Strands have been reported

in apple orchards in Illinois by Powell, ^^ on pear in

Washington (885), and on hawthorn in Great Britain

and in the Netherlands (89, 25Í, 256, 637, 638) (fig.

7). They have been found on leaves, stems, fruit, and

even in the spring on overwintered cankers. Strands

vary from a fraction of a millimeter to several cen12 Pers. commun., Hort. Field Lab., Univ. 111., Urbana.

timeters in length and normally from 6jim to 35um in

width. Short, thick (lOOfxm-SOO/Lim) strands have

also been observed (fig. 7, D). Short fragments of

strands, less than 1 mm long, are not too noticeable

with the unaided eye but are readily seen with magnification. Bacterial strands are thought to be either

wafted into wind currents and transported in the dry

state over long distances or carried upward by wind

currents into nearby clouds where they are dispersed in moisture and subsequently fall in raindrops at a distant location.

In Great Britain, Southy and Harper (883) demonstrated the survival of E. amylovora on small airborne particles in the laboratory at relative

humidities between 40 and 90 percent. Significant

numbers of organisms were viable after 2 hours'

exposure to the open air. In preliminary wind-tunnel

dissemination studies at East Mailing, strands were

produced over a range of constant environmental

conditions when plants were incubated in a constant

airflow of 0.78 meter per second, but their integrity

was lost under constant humidity (25i). When plants

with strands were exposed to bursts of turbulent

wind at 12.5 meters per second, 3 shoot infections

were observed on a total of 30 target shoots.

Insects

In general, insects are probably the most important agents for spreading the blight pathogen. Since

1891, when Waite (986) first observed that bees and

wasps spread the bacteria from flower to flower,

these pollinating insects are frequently considered

the main disseminators of the pathogen. After reviewing the extensive fire blight literature, however, we concluded that bees have a role in dissemination but not to the extent generally believed during the 1920's. Numerous observations and experiments since then revealed that (1) pollinating insects

rarely were seen in contact with ooze produced by

overwintering cankers; (2) flies, ants, and other

crawling insects were frequently found in contact

with or feeding on ooze; and (3) bees may be instrumental in disseminating the blight pathogen from

flower to flower.

During this literature review we have listed the

insects reportedly associated with the dissemination

of fire blight (table 7). They represent 77 genera.

During the 1930's the role of bees in disseminating

fire blight became extremely controversial. Many

investigators were convinced that bees spread the

organism from blighted to healthy flowers (358, 369,
72 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 7.—Insects associated with dissemination offire blight

Scientific name^ Common name Host2 Location Reference

Acalymma spp. (as subgenus

Diabrotica).

*Adelphocoris rapidus (Say)—

*Alsophüa pometaria (Harris) -

Andrena varians (Rossi)

Cucumber beetles Pear (b, s)- Calif.- ^83

Rapid plant bug ~

Fall cankerworm -

Apple (s)-

Pear

Anthrenus sp.

Aphis mali F.

*Aphis pomi DeGeerPear (b) —-

Apple (b) —

Dermestid beetle Pear (b) —

Apple

Apple aphid Pear (s) —

Apple (s)—

ELR

Aphis sp. - Aphid- Pear(b)

Apple (b, s)-

Mich., N.Y., Ohio - 358, 361, 905, 961

Calif. ^83

Gt. Brit. 271

Gt. Brit. 271

Calif. 93J^

Ontario ^84^

Calif., N.Y. J^81, m, 722, 901, 1018

Kans., Mich., Ohio, lU, 128, 358, 361, 961

Wis.

7ii>, 750, 1121, 1124

271, 358, 934

*Apis mellifera L. -

Green apple aphid

Honey bee Pear (b)

*Attagenus megatoma (F.)

(as A. piceus).

Bibio albipennis Say

Bombus hortoTum (L.)

Bombus terrestris (L.)

Bombyliidae

^Byrobia rubrioculus (Scheut.)

(as B. arbórea).

Campylomma verbasci (Meyer) -

Black carpet beetleELRPear (b)-

March fly —

Bumblebee -

-do-

-do-

-do-

-do-

-doMo.

Calif., Ohio,

Gt. Brit.

Kans., Mich.,

Mont., Ontario.

Calif., N.Y., Wis.,

Gt. Brit., Ontario.

East Germany -

Calif.

485, 641-643, 919, 920, 961

40, 271, 357, 417, 426,

483, 515, 628, 644,

805-808, 939

82a

934

*Chlamydatus associatus

(Uhler).

Cosmopepla bimaculata (Thomas)

(as C. camifex).

Cynomyopsis cadaverina (R.D.)

(as Cynomyia cadaverina).

Dasyneura crataegi Winn.

Bee fly

Brown mite ELR

Mirid Apple (s)-

Ragweed plant bug do—

Pentatomid do—

Fly Pear (s) --

N.Y. 722

Gt. Brit. 271

Gt. Brit. 271

Calif. 483

Ohio 361

Mich., N.Y., OhioN.Y.-

N.Y.-

N.Y.-

358, 361, 579, 901, 902,

905, 961

902

902

722

Diabrotica soror LeConte

Didea sp.

Drosophila funebris (F.)

Drosophila melanogaster Mg. ~

*Dysaphis plantaginea (Pass.)

(äs Anuraphis roseus and

Aphis sorbi).

*Edwardsiana rosae (L.)

Elateridae

Gall midge Hawthorn (s).

Beetle Pear (b)

Syrphid larva ELR

Vinegar fly Pear (s)

do ELR

Denmark 952

Calif.-

Ohio~

N.Y. -

Calif.-

361

722

40

Rosy apple aphid Apple (s) Mich., Ohio 361,961

*Empoasca fabae (Harris)

(as E. mali).

Empoasca sp.

"^Eriosoma lanigerum (Hausm.)

(as E. lanigera and

Schizoneura lanigera).

*Eristalis tenax (L.)

Rose leafhopper Apple N.Y.-

Click beetles Pear (s) —

Potato leafhopper Apple (s)-

901, 902

Leafhopper

Woolly apple aphid -

-do-

-doCalif. 483

Mich., N.Y., Ohio, 128, 358, 361, 901,

Wis. 906, 961

Wis. 114

Ohio, Pa., Ontario- 361, 485, 486, 714

Drone fly - Pear(b) Gt. Brit.

Apple (b) — Gt. Brit.

271

271

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 73

TABLE 7.—Insects associated with dissemination offire blight—Continued

Scientific name^ Common name Host2 Location Reference

^Formica fusca L.

Formica pallidefulva

nitidiventris Emery (as F.

pallidi-fulva subsp. schanfussi

var. incerta).

Formica sp.

Silky ant Pear (s) -

Ant do--

do Pear (b) —

Apple (b) -

Glischrochilus fasciatus (Oliv.). Sap beetle Pear (s)—

"^Hemerocampa leucostigma

(J. E. Sm.).

Heterocordylus malinus Reuter —

^Hippodamia convergens Guer.

Homoneura bispina (Lw.)

(as Sapromyza bispina).

"^Hoplocampa testudinea (Klug)

Whitemarked tussock

moth.

True bug

Convergent lady beetle —

Fly

European apple sawfly —

Pear (b, s)-

Apple (s)-

Pear (b)--

Apple (s)-

Pear (b) —

Apple (b) -

*Hylemya antigua (Mg.) Onion maggot Pear (s) —

(as Helemyia antigua).

Ilnacora malina (Uhler) True bug Apple (s)

"^Lasius alienus (Foerst.) Cornfield ant Pear (b)-

(as L. niger var. americanus).

"^Laspeyresia pomonella (L.)

(as Carpocapsa pomonella).

Lithophane sp.

Lucilia sp.

Lygidia mendax Reuter

Lygocoris invitus (Say)

(as Lygus invitus).

Lygocoris pabulinus (L.) Common green capsid Pear (s)

Codling moth Pear (b, s)—

Apple (b, s)-

Fruitworm Pear (b, s)—

Blow fly Apple (b) —-

True bug Apple (s)

do Pear (f)

Pear (f)

Tarnished plant bug Pear (b, s)—

Pear (s)

Apple (s)

English grain aphid do

-do- Apple -

*Lygus elisus Van Duzee

*Lygus lifieolaris (P. de B.)

(as L. pratensis (L.)).

*Macrosiphum avenae (F.)

(as Aphis avenae).

"^Macrosiphum avenae (F.)

(as Siphocoryne avenae).

Melanotrichus flavosparsus

(Sahib.) (as Orthotylus

flavosparsus).

"^Melanotus oregonensis Oregon wireworm Apple (b,s) -

(LeConte). Pear(b,s)—-

Meligethes sp. Sap beetle Pear (b)

Minettia sp. Fly ELR

*Musca domestica L. Housefly Apple

Pear

ELR

Mirid Apple (s)-

Musca sp. - Fly Pear (b, s, f)

Muscina assimilis (Fall.) Root maggot -—

*Muscina stabulans (Fall.) False stable flyNabis ferus (L.) True bug

(as Reduviolus ferus).

Apple —

Pear

--doApple —

N.Y. 722

N.Y. 722

Calif., Ohio 358, U83, 722, 93J^

Calif. J^83

N.Y. 722

Calif. m

N.Y. 905

Calif. 93J^

N.Y. 906

Gt. Brit. 271

Gt. Brit. 271

N.Y. 722

Ohio 361

N.Y. 722

Calif. U83

Calif. 93J^

Calif. m

Gt. Brit. 271

N.Y. 905

N.Y. 905

Gt. Brit. 271

Colo. 113J^a

N.Y., Ohio 358, ^81, 902

Calif., Mich., N.Y. 905, 906, 93U

N.Y., Ohio 358, 361, 901, 902, 961

Ohio, Wis. lU, 128, 358

N.Y. 902

Mich., N.Y., Ohio - 358, 361, 902, 905, 961

Calif. 93Jf

Calif. 93J4.

Gt. Brit. 271

Calif. 65Jf

Quebec 767

Calif. W

Calif., Ohio, JfO, 358, J^83, h8h

Ontario.

Calif., N.Y., m, h86, 722, 93h

Ontario.

N.Y. 905

N.Y. 722

N.Y. 722

N.Y. 901,902

See footnotes at end of table.
74 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 7.—Insects associated with dissemination offire blight—Continued

Scientific name^ Common name Host2 Location Reference

Neoascia sp.

Orsodacne atra (Ahrens)

Orthotylus marginalis Reut. -

"^Paleacrita vemata (Peck)—-

"^Panonychus ulmi (Koch)

Pegomya lipsia (Walker)

(as Helemya lipsia).

Pegomya sp.

(as P. calyptrata).

Pentatomidae

Phaenicia sericata (Mg.)

(as Lucilia seriata).

Phaonia variegata (Mg.)

"^Philaenus spumarius (L.) —

Plagiognathus politus Uhler -

Polistes sp.

Syrphid fly

Chrysomelid beetle

Dark green apple capsid -

Spring cankerworm

European red mite

Fly

do

Pear (b) Calif., Gt. Brit.

do Calif.

Pear (s) Gt. Brit.

Pear (b, s)— Calif.

Ohio

Pear (s) N. Y.

Stink bugs -

Blow fly—

do N.Y.-

ELR Calif.-

Pear (b, s)— Calif.-

ELR Calif.-

Muscid fly

Meadow spittlebug -

Mirid

Paper wasp

'^Pollenia rudis (F.) Cluster fly

Polymerus basalis (Reuter)

(as Poecilloscytus basalis).

Prenolepis imparis (Say)

"^Psylla pyricola Foerst.

(as P. simulans Foerst.).

Rhopalosiphum sp.

(as R. prunifolium).

Rhynchaenus canus Horn

(as Orchestes canus).

*Rhynchaenus pallicomis

(Say) (as Orchestes

pallicomis).

Scaphytopius acutus (Say)

(as Platymetopius acutus).

Scatophaga stercoraria (L.)

(as Scopeuma stercorarium (L.)).

' ^'•Scolytus rugulosus (Ratz.)

(as Ecoptorgaster rugulosus).

Pear Gt. Brit.

Pear (s) Gt. Brit.

Apple (s) Mich., N.Y.

Apple (b, s)-- Calif.

Pear (b, s)— Calif.

Apple, pear, N.Y., Quebec

quince.

Apple (s)-— Mich., N.Y., Ohio

Ant

Pear psyllaAphid

Apple flea weevil

do—-

Calif.

Pear (s) 1 N.Y., Gt. Brit.,

Apple (s) { Ontario.

ELR Ontario

Apple (s) Ohio

271, J^83

93J^

271

^83

361

722

722

65Jf

J^83

UO

270

271

901, 902, 906, 961

93J^

931^

767, 906

358, 905, 961

93J^

271, Jf81, 906

102Jlf

361

-do- Ohio-

-do- -do 358, 361

Leafhopper

Anthomyiid flyShothole borerSephena cinérea Kirkaldy

"^Stictocephala bubalus (F.)-—

Sylvicola fenestralis (Scopoli)

(as Anisopusfenestralis).

^Tachypterellus quadrigibbus

(Say) (as Anthonomus

quadrigibbus).

Taedia colon (Say)

(as Paracalocoris colon).

Taeniocampa hibisci Guenee

(as Orthosia hibisci).

"^Taeniothrips inconsequens

(Unzel).

Tetranychus sp.

Slate gray plant hopperBuffalo treehopper

False crane fly

Apple N.Y. 901, 902

Pear(b) Gt. Brit. 271

Apple (s) Mich., Ohio, 358, J^85, J^86, 961

Ontario.

ELR Ontario J^8J^

New Zeal. 179

Apple (s) Ohio 361

Pear Quebec 767

Apple curculio -

True bug —

Fruitworm -

Apple (s) Mich.

ELR- Ohio961

361

Pear thripsSpider mite -

Pear (b, s)— Calif. 4^83

Pear (b, s, f) Calif. 1^83

ELR Ohio 361

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 75

TABLE 7.—Insects associated with dissemination offire blight—Continued

Scientific name^ Common name Host2 Location Reference

Vespula sp. - Wasp Apple (b, s)- Calif.—

Pear (b, s)— Ontario93J^

Jf86

^Scientific insect names were verified by the Systematic Entomology Laboratory, Agricultural Research Service. Names with an

asterisk (*) are in the list of "Common Names of Insects Approved by the Entomological Society of America," December 1970. Names

originally used in the literature are in parentheses.

^Recordings include experimental laboratory research (ELR) and host observations with special emphasis on blossom (b), shoot (s), or

finiit (f).

Jf27, JÍ83, 515, 628, 7JfS, 7^5, 806-808, 867, 939),

whereas others disagreed with this view {212, 6^^,

7SJÍ, 7JÍ3, 801a), The latter were mainly those whose

livelihood depended on bees. Since apparently

healthy flowers may harbor fire blight bacteria, we

believe that bees may also be instrumental in disseminating the pathogen fi:-om "infested" but apparently healthy flowers to healthy flowers. The extent

ofthis spread depends largely on environmental factors that control bee activity and bacterial multiplication.

Rosen {808) reported successful isolation of E,

amylovora from beehive material collected

throughoutthe summer, winter, and early spring, as

well as from bees taken from the hives in early

spring before development of the disease. This observation was confirmed by Hildebrand and Phillips

{J^27) by using contaminated food. This theory was

opposed by Thomas and Vansell {989), Pierstorff

{71^3), Pierstorff and Lamb (7^5), and Thomas and

Ark {93JÍ), Gossard and Walton {361) stated that

fertilized blossoms became resistant to infection

sooner than did unfertilized ones. They concluded

that blossoms pollinated for 72 hours were not likely

to be infected and that susceptibility to infection did

not exist in blossoms 144 hours after pollination.

However, Miller (ö5i ) disagreed and suggested that

flowers remained susceptible to infection much

longer.

In contrastto Rosen's {808) findings, the longevity

of £'. amylovora in pure honey was found to range

from 3 to 11 days {361, 7^5), but the organism could

not be detected on the combs, frames, or in the

honey taken from beehives 24 hours afterinoculation

{7^5), Bacteria remained viable in the viscera of

honey bees for 48 hours but were not recovered from

their heads 12 hours after inoculation iiO), Gossard

and Walton {361) reported survival of bacteria in

aphid honeydew after 7 days and after 3 more days

when moisture was added.

Apart from the fact that bees may carry the blight

organism from flower to flower, the nectar concentration appears to be very important in that it may

affect the multiplication of the bacteria deposited

there. However, other factors may also limit blossom blight transmission by bees. The reports of

Stewart {902) and Stewart and Leonard {906) on

aphids and other sucking insects spreading fire

blight have also been controversial. Many early investigators (chap. 1) considered piercing and sucking insects as important disseminating agents offire

blight. However, in thorough studies of these insects, Miller {651) and Tulhs {961, 1138) concluded

that aphids were rarely involved in either primary or

secondary infections and therefore seemed to have a

minor role in disseminating the blight organism.

In New York, Hildebrand {Í20), Parker {722,

1122 ), and Parker et al. {726 ) emphasized insect control as a means of effectively controlling fire bUght.

They claimed that aphids and leafhoppers had a relatively important role as inoculating agents of succulent terminals. In Missouri, Plurad et al. {7i9)

proved through artificial feeding experiments that

the blight pathogen could be found in the apple aphid

within 5 minutes after feeding and remained there

for at least 72 hours. Later they demonstrated that

at least 70 bacterial cells had to be injected into the

host plant for systemic infection of young apple

shoots. The inability of the apple aphid to regularly

introduce 70 or more cells into the host seemed to

preclude its effectiveness as a vector. Similar results

were reported by Thygesen et al. {952) from Denmark.

In Michigan, Jones (^^i ) demonstrated the association of the pear psylla {Psylla pyricola Foerster)

with fire blight infections in leaf axils and the base of

flower clusters. He also noted feeding of the tarnished plant bug {Lygus lineolaris) on flower parts

and opening buds. He confirmed observations by

Stewart {901) that this insect may be important in
76 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

starting primary infections. Luepschen ^^ mentioned that unusual occurrences of Bartlett fruit

blight near harvesttime in western Colorado appeared to be associated with large populations ofthe

tarnished plant bug on weeds under the trees. In

further studies, Stahl (llSJ^a) isolated from contaminated Lygus species equal numbers of E.

amylovora from external and internal parts of the

insect. He showed thatthese insects can successfully

transmit the blight pathogen within a short time

after acquiring the bacteria and suggested that they

have a high potential for wounding pear fruit, which

results in fruit blight if inoculum is present.

In 1965, Leonard (579) reported the first known

record of Atractotomus mali (Meyer) in Connecticut. He considered it distinct from A. crataegi

Knight known on hawthorn in Iowa and did not

report either insect associated with fire blight.

Whitcomb et al. (1019) isolated anErwinia-like bacterium from internal parts of leafhoppers. Since

leafhoppers have often been reported in the dissemination offire blight, they conceivably could also

harbors', amylovora.

Hildebrand (^20) was the only investigator to

publish a series of unique cartoons depicting the life

cycle ofthe causal organism and its dissemination by

insects.

In Denmark, Bech-Andersen (77a) distinguished

the following types of dissemination: (1) Short distance (0-100 m), such as rain splashing or certain

insects that spread bacterial ooze from blighted

hawthorns to fruit trees; (2) middle distance (100-

5,000 m), accomplished by such vectors as pollinating insects between trees and nearby orchards; and

(3) long distance (>5 km), most likely caused by

migratory birds.

Birds

Without any experimental proof, birds have been

mentioned as instrumental in disseminating fire

blight (77, 369, 639, 690). Since the first outbreaks of

fire blight along the coastal regions of northwestern

Europe, migratory starlings were suspected as carriers of the pathogen. Reports from Kent indicated

that branches were broken by large numbers of

roosting birds (571), Bech-Andersen (77a) reported

from Denmark in 1974 that (1) hawthorns serve as

shelter and feeding places for such birds as starlings

(Sturnus vulgaris L.) and warblers (Phylloscopus

' Pers. commun., Colo. Agr. Expt. Sta., Grand Junction.

trochilus (L.)); (2) viable E. amylovora bacteria

were isolated from starling excrement and from

their feet 8 days after these birds were artificially

infested with the organism; and (3) starlings and

warblers often complete their migration from England to Denmark in 2 to 3 days before continuing to

Poland and the U.S.S.R. Considering that starlings

are berry and fruit feeders and that large numbers

have been observed in hawthorn shrubs and fruit

orchards, these birds appear to have an important

role in disseminating the blight organism.

Man

The blight pathogen may be spread by man himself on orchard equipment, fruit, and budwood.

Pruning tools are the most important means of

spreading bacteria from blighted to healthy

branches. Pruning shears and saws used to remove

blighted shoots and limbs can spread the organism if

they are not properly decontaminated between cuts

(507, 778, 989, 992). The organism may also be

spread by hands, clothing, shoes, and wheels of orchard equipment if they have been in contact with

the inoculum.

The expression "one rotten apple spoils the barrel" can certainly apply to pears in the spread of fire

blight. Since internal inoculum has been reported in

pear fruit, such infected fruit can be instrumental in

short- or long-range dissemination of fire blight.

Without definite proof, contaminated fruit boxes

have been suggested as introducing the blight

pathogen into Great Britain prior to the observed

outbreak in 1957 (369, 571). Fire blight was first

observed in a pear orchard near the farm gate where

the assumed infested crates were stacked for future

use.

Whether the blight organism can be transported

on or in pome fruit has often been questioned. In

Canada, McLarty (618) recovered viable £'. amylovora bacteria from mature fruit ofseveral artificially

inoculated apple cultivars 5 months after ordinary

storage. On the other hand, Dueck (237) found that

the organism failed to survive for a 24-hour period on

the surface of artificially inoculated apples. However, survival in the laboratory was excellent on the

fruit surface when the bacterium was applied as

natural ooze or in a water suspension.

Man may also spread the bacterium by shipping

contaminated budwood, especially from trees with a

history of fire blight. Many such observations have

been reported, though few are documented in the
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 77

literature. In Michigan, Klos^^ reported a local nursery had a severe outbreak of fire blight in

rootstocks budded the previous summer. In 1972,

Burkowiez {12Jf) stated that the localized occurrence

of fire blight at the research station in Skierniewice,

1"* Pers. commun,, Dept. Plant Path., Mich. State Univ., East

Lansing.

Poland, was probably due to using imported infected

propagating material.

Evidently the fire blight bacterium can be disseminated in many different ways and its movement

is difficult to contain. Therefore grow resistant plant

material, ehminate existing sources ofinfection, and

apply eradicative and protective control measures as

thoroughly as possible.
CHAPTER 9

PATHOLOGICAL ANATOMY

In the earliest histological studies by Burrill (133)

and Waite (991), slight mention was made of intercellular migration of the bacterium. Jones USi),

Bachmann (55), Smith (878), Nixon (^^5), and Miller

(651) showed thatE. amylovora, following infection

and early penetration, makes its way intercellularly

by dissolving cell walls and middle lamellas. In 1927,

Nixon (695) and Gibbons (1102) reported two phases

in the life cycle of £'. amylovora—the vegetative

stage, including the intercellular migration in the

form of zooglea, causing lysigenous cavities, and the

pseudofructification stage, comprising intracellular

migration with ultimately the formation of cysts in

schizogenous cavities. Nixon's early drawings are in

striking agreement with recent observations under

the electron microscope. Similar observations were

made by Haber (376) on apple leaf tissue and Wahl

(985) on quince.

The most extensive histological studies on the

mode of penetration and invasion of£'. amylovora in

apple and pear tissues were by Rosen (80^, 816) at

the University of Arkansas. He found that the nectarial tissues of apple blossoms consisted essentially

of the same structures as those in pear blossoms.

Also, the mode of bacterial invasion through nectarial tissue was similar in both types of flowers. However, he (816) observed the following differences in

his studies:

(1) Pear blossoms, unlike apples, have a fully exposed nectarial region, forming a shallow, open,

greenish, saucerlike dish. In apple blossoms the nectarial region is almost completely hidden by the enlarged, hairy base of the stamens and the funnel- or

cup-shaped form.

(2) In pear blossoms the nectar forms an excellent

medium for multiplication of the fire blight bacteria.

Nectarthodes, free of any cuticular covering, serve

as definite avenues of bacterial invasion.

(3) In apple blossoms the bacteria penetrate more

frequently in stigmas, anthers, outer receptacle

walls, and calyx lobes than in nectarial tissue. The

outermost layer ofstigma cells has no cuticle and the

thin walls are easily infected by the pathogen. Open

78

anthers serve as excellent avenues of bacterial invasion.

(4) In both pear and apple, progressive penetration occurs in the subnectarthode chambers and

stigmatic or stylar tissues mainly by means of intercellular spaces (fig. 18, C) and the localized dissolution of middle lamellae and delicate cell walls.

Rosen (816) illustrated these observations (fig. 18,

A and B) and concluded that pear and apple flowers

should be fully covered with a protective spray to

control blossom blight.

Rosen (80i) also made the most detailed cytological and histological study of infected pear petioles

and pear and apple stems. In these tissues the cell

walls of cortical tissues were noted to undergo a

distinct swelling and lamellation, indicating the production of cell wall-destroying enzymes acting partly

in advance of bacterial invasion.

He emphasized that cortical invasions are most

common in infected stems of pear and apple but that

the most serious invasions destroy the phloem and

cambium. Rosen (80Í) published the earliest pictures of masses oí E. amylovora bacteria in xylem

vessels of apparently healthy Kieffer pear shoots

(fig. 18, D). He also described masses of normal

appearing, disintegrating, and finely granular

gummy E. amylovora material.

Hildebrand (il 7) confirmed these observations on

apple, pear, and quince flowers as to obtaining a

more effective blossom blight control program.

Greenhouse tests on dwarf pear trees showed that

the blossoms were most susceptible to infection

during the 2 days after opening. Hildebrand also

showed details of the progressive invasion of the

blight pathogen through stomata, substomatal

chambers, and intercellular spaces during the first 3

days following inoculation.

In New York, Aldwinckle and Preczewski (16a)

observed in many apple cultivars discolored streaks

in the xylem extending in advance of externally visible lesions following artificial inoculation. They isolated pathogenic £'. amylovora from the streaks and

concluded that xylem streaking may indicate the
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 79

PN-6387

FIGURE 18. — Mode of penetration and movement oiErwinia amylovora in pear and apple tissues: A, Longitudinal section of nectarial

disk of Bartlett pear blossom, showing penetration of bacteria through nectarthode ( x 950) (after Rosen, 816); B, movement of

bacteria from nectarthode into subnectarthodal chamber ( x 950) (after Rosen, 816); C, bacteria in intercellular spaces of Duchess

pear petal ( x 800) (after Rosen, 80^ ); D, bacteria in xylem vessel of apparently healthy Kieffer pear shoot ( x 500) (after Rosen,

80i ); E, presence of virulent cells in intercellular spaces of cortical tissue of apple shoot ( x 9,000) (after Huang, 1110); F, presence

of virulent cells in xylem vessel of apple shoot (x 19,000) (after Huang, 1110).
80 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

potential for rare severe infections in usually less

susceptible apple cultivars.

Host Ultrastructure

Nearly all research on the ultrastructure of host

tissues has been done at the University of Missouri.

In 1968, Burkowicz and Goodman (125) reported a

rapid increase in permeability of cellular membranes

when immature pear fruit slices were inoculated

with either virulent or avirulent strains of E.

amylovora. However, avirulent bacteria did not

spread beyond inoculated surface layers and their

capacity to induce permeability changes was limited.

The rate of pathological alterations in Jonathan

apple leaves was correlated with the length of the

bacterial generation time (126), Inoculum concentration and age of leaves were the primary factors

that affected the rate and extent of the membrane

permeability alterations. They concluded that the

hypersensitive reaction was due to membrane damage, which permits rapid water loss from tissues,

which in turn precludes the growth of bacteria in the

intercellular spaces (3J^5).

Addy and Goodman (7) and Addy (1078) found that

polyphenol oxidase and peroxidase activity in apple

leaves, infiltrated with either the virulent or the

avirulent strain of E. amylovora, increased with

time. Maximum difference in activity between the

control and infected leaves was noted between 12

and 18 hours. Addy (6a) observed a linear relationship of leakage of electrolytes and total phenols

when the leaves were infiltrated with either strain.

This leakage occurred well in advance of visible

browning symptoms, which were evident about 12

hours after infiltration. In California, Pitman and

Cruess (7i8) found thati/. amylovora had little or no

pectinase activity in pectin hydrolysis studies.

Huang and Goodman 0,53, i5i) and Huang (1110)

made the most extensive electron microscopy

studies on the modifications of the ultrastructure in

Jonathan apples following infection by E. amylovora. One day after inoculation with virulent bacteria, no noticeable structural changes were found,

but bacterial cells were observed in xylem vessels

and in intercellular spaces (fig. 18, E and F). Two

days after inoculation, ultrastructural modification

included plasmolysis, aggregation of cytoplasm, disruption of chloroplast envelopes, and disorganization oflamellar structures. Virulent bacteria formed

a protective layer in the host tissue, consisting of an

electron-lucent zone and five filaments. They also

observed degradation of subcellular structures beyond recognition and the formation of lysigenous

cavities as reported by Nixon (695) and Crosse et al.

(197). In their 1976 report, Huang and Goodman

(J^5J^a) concluded that ultrastructural changes induced by the fire blight toxin were much the same as

those induced by the pathogen per se.

After inoculation with avirulent bacteria, Huang

et al. (i5J^b) and Huang (iiiO) observed the following

types of host defense reactions in apple petioles: (1)

Bacteria in intercellular spaces of cortical tissue

were arrested and localized by the hypersensitive

reaction; (2) long-distance translocation of bacteria

in xylem vessels was stopped by an agglutination

reaction accomplished by the aggregation of avirulent cells into clumps by host-formed granules; and

(3) bacteria observed in living xylem parenchyma

cells were digested within the vacuoles. These findings suggest that avirulent cells ofE. amylovora are

localized in petioles and do not translocate into stem

tissue. The hypersensitive reaction of apple tissue to

avirulent cells does not explain this phenomenon

satisfactorily, as this reaction reportedly is a host

defense mechanism that is operative only in living

cells (53Í, 536). Huang (1110) concluded that ultrastructurally the effects of avirulent bacteria on cortical parenchyma cells were degeneration of subcellular organelles but without plasmolysis. This observation is similar to the ultrastructural modification ofhypersensitive tobacco leaftissuesinduced by

E. amylovora (350, 351).

In other studies, structural protein (SP) was prepared from chloroplast membranes of tobacco leaf

tissues infiltrated with 10® cells per milliliter oiE.

amylovora and mixed with chloroplast lipid. It did

not reaggregate as well to form membrane structures similar to the original chloroplast membrane as

did SP from water-infiltrated tissue. Huang et al.

(JfJt'9) suggested these changes may be responsible

for the alteration in membrane permeability and

integrity in bacterially induced hypersensitivity in

plant tissues.

In studies on Cotoneaster pannosus Franch.,

Seemuller and Beer (8J^) concluded that cell wall

degradation may not be an important factor in the

development offire blight. In Denmark, HockenhuU

(Í28, 1107a) made a detailed anatomical study of

healthy and diseased hawthorn following natural

infection by E. amylovora. He found that the most

common type of blight lesion involved the formation

of a defense periderm, which isolated the diseased
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 81

tissues, causing them to shrink and dry up. Two

types of cankers were distinguished, closed and

open. In closed cankers, originating from shallow

infections, the defense periderm formed a continuous barrier. In deeper infections in open cankers,

cortical and phylotic tissues are involved in the formation of the xylem or cambium, but the latter is

destroyed and a typical wound healing reaction is

initiated by healthy cambial cells.

Internal and Resident Bacteria

Following the initial studies by Miller (651), Nixon

(695), and Rosen (80J^, 816) on resident fire blight

bacteria in pear and apple tissues, additional research was undertaken during the 1960's on the

prevalence oíE. amylovora in these tissues and its

recovery from symptomless stems and shoots on

selective media. In 1963, Baldwin and Goodman (62)

reported the isolation of E. amylovora from dormant Jonathan apple buds in Missouri. Forty percent of all isolates obtained were sensitive to one or

more ofthe five typing phages. The 523 phage-sensitive isolates fell predominantly into 5 phage typing

patterns, whereas 1 ofthe patterns, a combination of

2 phages isolated in England, typed 80 percent of all

phage-sensitive isolates.

In morphological and anatomical studies of

Jonathan apple tissues, Lewis and Goodman (582,

583) showed that hydathodes, glandular trichomes

on leaves, and lenticels on the stems were natural

openings for infection. They also found that the bacterium migrated very rapidly in the leaf and stem,

and in the stem it moved via the phloem.

A few years later, Gowda and Goodman (363) detected the downward movement of E. amylovora

exclusively in phloem 70 cm from inoculation in the

stem apex, whereas visible disease symptoms were

restricted to only 16 cm from this point. The

maximum upward movement from inoculated roots

into the stem was 40 cm. Such recoveries were confirmed in our studies at Beltsville with Bartlett pear

and Jonathan apple, both in the greenhouse and the

field (508, 511). Following surface sterilization ofthe

shoots with 0.5 percent sodium hypochlorite, pure

cultures of £^. amylovora were obtained from sections 3 mm thick plated on nutrient-yeast-dextrose

agar (fig. 12, A). The bacterium survived in both

types of tissues for up to 6 months.

Figure 19 shows a comparison of the presence of

E. amylovora in the tissues mentioned in the previous three studies. All platings were done on nutrient-yeast-dextrose agar (NYDA). Lewis and Goodman (582) and Lewis (lllJ^) spread a drop of inoculum over 6 mm of the serrated margin of uninjured upper surface of the sixth leaf from the shoot

apex. Free hand sections were prepared from leaf

and stem tissues, and microtome sections were made

following routine histological procedures. The bacteria invaded the entire length of the shoot in approximately 7 hours, and 90 percent of the actively

growing shoots developed fire blight symptoms (fig.

19, A).

Gowda and Goodman (363) and Gowda (iiO^) immersed about 6 cm of a decapitated root tip of a

young Jonathan apple tree in a culture tube containing 5 ml of an aqueous suspension ofE. amylovora (10^ cells per milliliter). Two weeks after inoculation, the bacterium had spread a maximum of 40

cm into the symptomless aerial part ofthe shoot (fig.

19, C). Migration from the roots into aerial sections

of the stem seemed to be limited to the second week

following inoculation. In separate tests in which the

inoculum was applied to the severed apex, typical

blight symptoms were observed to a maximum

length of 15-16 cm in 16 days, although bacteria had

moved downward as much as 70 cm.

In our studies at Beltsville, virulentE'. amylovora

was readily isolated from the internal tissues of

symptomless side shoots (75-90 cm) of Jonathan

apple and Bartlett pear trees in the greenhouse (fig.

19, B). These shoots developed from axillary buds

immediately below the base of the cankers when

blight progress ceased (511). In addition, blight bacteria were recovered from many apparently healthy

suckers on blighted Bartlett trees and from symptomless shoots ofother pear cultivarsin the orchard.

Covey and Fischer (18^) obtained more bacteria

from inoculated apple than pear shoots when sand

was used to facilitate grinding of tissues. About 32

hours after injury the host-pathogen balance apparently shifted in favor of the pathogen, which then

multiplied sufficiently to cause symptoms. It was of

interest to note that the number of bacterial cells

decreased within the first 24 hours. Similar results

were reported in 1975 in recovery studies on pear

blossoms in New York (81a).

Lewis and Goodman (582) reported that fire blight

bacteria passed from the vein parenchyma into the

petiole, where they seemed to multiply rapidly in the

phloem and then moved into the xylem parenchyma,

where bacteria were evidentin conspicuous strands.

This phenomenon was later confirmed by Crosse et
SCALE ■■BLIGHTED STEM

g^^^^^ BACTERIA (PLANT A)

BACTERIA (PLANT B)

SOIL LINE

B

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FIGURE 19. —Schematic diagrams showing movement ofErmnia amylovora in apparently healthy pear and apple tissues: A, From leaf surface through petiole and stem to

shoot apex (after Lewis and Goodman, 582); B, from base of blighted shoot into side shoots developed from axillary buds after visible blight ceased to spread in main

stem (after Keil and van der Zwet, 511); C, from root tip through stem into upper part of shoot (after Gowda and Goodman, 363).

>

O

B

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a

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FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 83

al. (197). Gowda and Goodman (363) concluded that

the pathogen moved downward from the stem apex

via the phloem. Upward movement from the root

was not determined but was believed to occur

through the phloem.

In studies on overwintering ofthe blight pathogen

inside living host tissue in Utah, Morrill (odJ^y 1120)

concluded that holdover bacteria were present in

apparently healthy as well as diseased buds of apple

and pear trees, pyracantha, chokecherry, and mountainash. Using a selective medium, Miller and

Schroth {653, 65i ), Miller et al. {655 ), and Thomson et

al. {950, 951) recovered E. amylovora from pear

flowers in populations commonly ranging from 10^ to

10^ cells per flower. Up to 33 percent of the flowers

were colonized by E. amylovora without any saprophytic bacteria, and the insects Pegomya sp. and

Minettia sp. carried surface populations of blight

bacteria ranging from 10^ to 10^ cells per insect. In

Great Britain, Lelliott {575) readily recovered E.

amylovora bacteria from symptomless stems as long

as 70 days after inoculation. We emphasize again

that fire blight bacteria have been isolated from

dormant buds {62) and that infected buds have resulted in great losses of nursery trees {901, 999).

In 1943, Borden and Thomas {102) reported from

California that Bartlett trees sprayed with a summer oil had 17 percent of the fruit infected with E.

amylovora, whereas trees not sprayed with oil had

only 0.5 percent of the fruit infected. Moreover a

shipment of apparently healthy pears, originating

from orchards sprayed in the summer with oil-lead

arsenate to control red spider mites, had 30-50 percent of the fruit infected when it arrived in Hawaii

{972). It is possible that the oil sprays affected the

epiphytic bacterial population and thus had a significant role in the spread of fire blight in these

orchards. Internal fruit infections also have been

observed in Great Britain {369). All reports appear

to agree that the small external symptoms on the

fruit surface are connected by a thin thread of infected tissue to large (up to 2 cm) pockets ofinfection

in the center of the fruit.

We believe that there is sufficient evidence today

that E. amylovora can enter its host through

nectaries, hydathodes, lenticels, and other avenues

and spread through the trees systemically as resident bacteria in or on shoots, roots, flowers, fruit,

and other tissues (figs. 17-19). According to the

laws of nature under which the healthy rather than

the diseased state predominates, numbers of bacteria apparently remain low. Increased numbers of

bacterial cells and the subsequent infection process

depend on and are determined by many factors,

such as degree of innate resistance, percent intercellular humidity, tree nutrition, environmental

conditions, and injury caused by wind, hail, farm

equipment, and so forth.
CHAPTER 10

BIOCHEMISTRY OF HOST TISSUES

Studies on the biochemical nature of pear and

apple tissues in relation to fire blight resistance were

started in the 1960^s. Keil and Wilson (502) were the

first to observe that leaf disks and stem cross sections from a blight-resistant cultivar ofPyrus communis L. were more inhibitory of E. amylovora

than were comparable tissues from a susceptible

cultivar. Significantly larger inhibition zones were

produced by leaf disks and cross sections from Magness than from Bartlett leaves of the same age (fig.

20, A and B). Leaf ash was also demonstrated to be

active, and the substances responsible for the bacterial inhibition were water soluble but insoluble in

absolute methyl and ethyl alcohol.

Hildebrand and Schroth U12, iU, Í15) and Hildebrand (HOT) reported the first evidence of a disease-resistant compound, the glucoside 4-hydroxyphenyl beta-D-glucopyranoside (arbutin), in

pear tissue, showing antibacterial activity against

E. amylovora. This glucoside was hydrolyzed by

glucosidase, splitting off the glucose and producing

hydroquinone (quinol), which becomes antimicrobially active when oxidized to the semiquinone. Direct evidence for the antibacterial activity of hydroquinone was obtained by placing pear blossom sections and leaf disks on nutrient agar seeded withE".

amylovora (fig. 20, C and D). Growth of the bacterium was not suppressed, however, near the cut

end of major veins. The enzyme beta-glucosidase

was also produced by Pseudomonas syringae in a

medium containing large amounts of glucose (^13).

In later tests Schroth and Hildebrand (84^6) found

that all isolates of P. syringae synthesized large

amounts of beta-glucosidase, whereas E'. amylovora

showed only sHght activity. The maximum hydroquinone concentration in nutrient broth at which

detectable growth oîE. amylovora occurred ranged

from 400 to 800 ppm, whereas P. syringae isolates

grew at 1,200-1,800 ppm. They postulated that the

low amount of beta-glucosidase in E. amylovora

would probably not significantly affect pathogenesis

in pear, but that the high amount in P. syringae

would enable it to hydrolyze arbutin and thus possi84

bly inhibit further advance in pear tissue (8^6).

A comparative study between fire blight-resistant

and susceptible pear species revealed some striking

differences in the arbutin-hydroquinone complex

and associated enzymes in comparable tree tissues

(ilO). A reagent composed of 0.2 percent pphenylenediamine in 2 N ammonium hydroxide was

used for the cytochemical demonstration of arbutin

(dark blue to dark purple) in the tissues (8J^5).

In 1967, Powell and Hildebrand (752) discovered a

correlation between beta-glucosidase content of tissues and their antibiotic activity against £'. amylovora. Nectarial tissue of the susceptible pear cultivar Forelle had a large amount of beta-glucosidase

as compared with that of the resistant cultivar Old

Home, but neither tissue exhibited antibiosis.

In addition, Hildebrand (J^80) found that aqueous

extracts of pear leaf blade and woody tissues exhibited greater antibiotic activity than did extracts

of petioles plus leaf midribs and bark. He noticed

largest increases in antibiotic activity in extracts of

tissues that showed the greatest amount of antibiosis in the tissue bioassay test. Also, arbutin was

found in greater amounts in blight-resistant than in

susceptible cultivars (HO). The usual invasion

routes of nectaries, lenticels, and cortex contained

little arbutin regardless of the cultivar. From all

these data it was concluded that other pathways may

exist in pear leading to the formation of antibiotic

substances from arbutin, including the formation of

toxic substances and an interaction with the hydrolytic pathway (ilO, 753).

Smale and Keil (872) extensively studied the

mechanism responsible for fire blight resistance in

six cultivars of P. communis. They associated the

following factors with mechanically injured leaves of

highly resistant cultivars: (1) Presence of large

amounts of arbutin and free hydroquinone in unaltered leaf homogenates, (2) accumulation and persistence of antibacterial concentrations of hydroquinone enzymatically released from arbutin during

oxidation ofleafhomogenates, and (3) disappearance

of an unidentified antagonist ofhydroquinone follow-
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 85

PN-6388

FIGURE 20.—Bioassay of pear tissues on culture media seeded with Erwinia amylovora, showing varying zones of inhibition: A, Leaf

disks cut from young pear leaves: Large zones from Magness and small zones from Bartlett; note dislis were removed prior to

photography (after Keil and Wilson, 502); ß, leaf tips-Ze/i, Magness; rifif/ii, Bartlett (after Keil and Wilson, 502); C, longitudinal

section of pear blossom; note absence ofinhibition zone around nectarial region (N) (after Hildebrand and Schroth,^J2 ); D, leaf disk

with well-defined inhibition zone except for bacterial growth near cut end of major veins ( x 3.4) (after Hildebrand and Schroth,

W5).

ing cell disruption. They concluded that the balance

between the rapid appearance and disappearance of

antibacterial hydroquinone in mechanically damaged leaf tissue is a major determinant of relative

blight resistance.

Preliminary studies ^^ on leaves and stems of

Bartlett, Magness, Pyrus ussuriensis 76, and P.

calleryana cv. Bradford indicated no relationship

between bioactivity and hydroquinone concentration within 36 minutes of oxidation.

Plurad Í112Í.) sought to estabhsh a relationship

1» Unpub. data. Fruit Lab., U.S. Dept. Agr., Beltsville, Md.

between phloretin, the aglucon of phlorizin, and resistance of several apple cultivars to infection by£'.

amylovora. Although the level of phloretin in floral

tissue was lower than that in stem tissue, comparable amounts were detected in blossoms of both

resistant and susceptible cultivars. The antibacterial activity of phloretin and phlorizin was nearly

comparable to that of arbutin.

Challice (151, 1092) and Challice and Westwood

(i 5.4) studied in detail the genus Pyrus and reported

no relationship between phenolic compounds in leaf

and bark tissues resistant to fire blight, crown gall,

and the woolly pear aphid. However, their results
86 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

did indicate that under certain circumstances the

phenolics may be used to differentiate between resistant and susceptible individuals.

Challice {152,153,1092) and Challice and Williams

(156-158) reported many details on the nature of

phenolic compounds in Pyrus species. They concluded that not all phenolics were distributed

equally throughout buds, cork, green cortex, white

phloem, and xylem. Some phenolics tended to be

restricted to particular tissues at particular times.

In Minnesota, Ahn and Stushnoff {9) measured

phenolic compounds in several apple cultivars from

flowering to leaf fall and found greater amounts of

catecholases and cresolases in resistant versus susceptible cultivars. They also found lower levels of

enzyme and phenolic compounds in flowers than in

stem tissue.

Hypersensitive Reaction

The hypersensitive reaction (HR) in plants is a

resistance mechanism to pathogens that normally do

not parasitize them. It is characterized by a rapid

necrosis of tissue occurring usually within 24 hours

after introduction of large numbers of micro-organisms into the intercellular spaces ofthe host {31^3,

ill). In 1966, Element and Goodman (53^) showed

that virulent strains oîE. amylovora produced fire

blight symptoms on inoculated apple shoots,

whereas avirulent strains induced only small brown

discolorations at the inoculation sites. Both types of

bacteria started to multiply in the shoots, but the

multiplication of the avirulent bacterium stopped 24

hours after infection.

Both types of E. amylovora and Pseudomonas

syringae also induced the HR necrosis in tobacco

leaves.Conductometric measurements of tissue

leakage indicated that the two pathogens also induced significant changes in the selective permeability of the host cells {31^3, 535). Electron microscopy ofthe tobacco leaftissue revealed a progressive

degeneration of bounding membranes of chloroplasts, mitochondria, and cytosomes {351 ). Exposing

healthy leaves to ammonia gas caused similar tissue

necrosis as when inoculated with E. amylovora or

three Pseudomonas species {352, 591). It was

suggested that ammonia may be produced by these

bacteria as a necrotoxin, which altered the tertiary

structure of the membrane protein.

In studies of tobacco leaf tissue changes following

the hypersensitive reaction, Huang and Goodman

{H5, H6) in 1970-72 reported the alteration and

reaggregation ofstructural protein from chloroplast

membranes during development ofthe HR reaction.

E. amylovora did not produce phosphatidase in vitro

but did stimulate the synthesis of host phosphatidase D. Host phosphatidase activity was most

stimulated by 10^ cells per milliliter, a concentration

that did not induce HR. Structural proteins (SP)

were isolated from chloroplast membranes and all

SP preparations migrated as a single boundary during ultracentrifugation {J^i6, J^50). In dialysis the SP

from water-infiltrated tissues formed spherical

aggregates, whereas the SP from bacteria-infiltrated tissues aggregated into irregular clumps.

Since the lipids used in the experiments were identical, the inability to form membranelike structures

was attributed to the alteration in SP properties by

HR-inducing bacteria. The capacity of bacteria to

induce HR on tobacco leaftissue was detected within

2 minutes by measuring the oxygen consumption of

mixtures of separated leaf cells and bacteria U^7).

This finding suggested the existence of a well-defined recognition system between the reactants.

In the mid-1970's, Goodman et al. {3i9) and Huang

and Goodman {Í5i) showed that in vivo agglutination of incompatible bacteria in plant tissue is a general defense mechanism but distinct from the hypersensitive defense reaction. They postulated that the

agglutinating factor may migrate from the living

xylem parenchyma cells into dead xylem vessels.

Induced Host Resistance

Following preliminary investigations during the

1930's on the antagonistic effect of various micro-organisms on E. amylovora {38, 722, 93i), Goodman

{339) reported in 1964 on the protection ofJonathan

apple stem tissue against fire blight infection by

introducing an avirulent yellow bacterium prior to

inoculation with the blight pathogen. In vitro the

virulenta?, amylovora (35 A-W) and the yellow organism (35 A-Y) grew at the same rate, but the

growth of 35 A-W was reduced significantly when

the two cultures were inoculated together {31^.0). The

inhibitory effect of 35 A-Y on 35 A-W was also demonstrated in vivo and appeared to be due to acid

production by the former. The initial inoculum (protecting strains) or inducer (IN), provided by either

an avirulent isolate of E. amylovora, a yellowish

Erwinia-\i]ie isolate, or Pseudomonas tabaci (Wolf

and Foster) Stevens, protected the apple tissue from

subsequent infection by a virulent strain of E.

amylovora or challenge inoculum (CI) when inocu-
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 87

lated 30 minutes later {SJ^l). The duration of the

protective effect reportedly was dependent on the

number of "protecting" bacteria present in the apple

tissue atthe time the virulentinoculum was appUed.

The association of a yellow saprophytic bacterium

with E. amylovora has been known for many years

(62, 86, 296, iOl, 1051).Xanthomonaspruni (Smith)

Dowson, X. campestris (Pammel) Dowson, Bacillus

subtilis (Prazmowski) Cohn, and B. cereus Frankland and Frankland did not provide any protection

against £'. amylovora (339, 3il, 613).

Mclntyre (1118) studied the protection of Bartlett

pear tissue by avirulentE'. amylovora, E. herbicola,

or P. tabaci at various intervals before inoculation

with virulent £'. amylovora. Fire blight symptoms

were not delayed when CI was apphed 0.5 hour after

IN. Using young etiolated pear seedlings, Mclntyre

et al. (613, 61U, 616) were able to determine that

symptom expression could be delayed from 1 to 14

days, with maximum delay occurring when CI was

inoculated 24 hours after IN. They also obtained a

delay in symptom expression when cell-free sonicates of both avirulent and virulent E. amylovora

were used as inducers. Deoxyribonucleic acid

(DNA) from P. tabaci and E. herbicola, bacteria

that protected against fíre bhght, provided no protection, whereas DNA from virulent ¿Z. amylovora

did protect (61Uj 615). The possibility of an interaction between host and bacterial DNA as a factor in

protection is being explored. Wrather et al. (lOJ^l,

1139) found these same bacteria protected fruit of

mature Anjou and immature Bartlett pear and

Jonathan and Red Dehcious apple cultivars against

infection by £'. amylovora. In these fruits, however,

protection appeared permanent.

The host-parasite relationship in fire bhght appears to be an intricate, complicated balance between £'. amylovora and the various tissues of the

host plants. The numerous biochemical interactions

appear to have potentially great merit and in the

future could lead to the production of plants possessing prolonged or permanent fire blight resistance.
CHAPTER 11

CONDITIONS AFFECTING DISEASE DEVELOPMENT

Fire blight development depends on the interaction between pathogen and host and is affected by

the environment. Rather than a simple process, it is

a comphcated balance involving the bacterium with

many host conditions and meteorological and

edaphic factors. Each ofthem has to be optimum and

synchronized for maximum blight development.

Host Conditions

Tree Growth and Vigor

Succulent shoot growth of pear, apple,

pyracantha, hawthorn, or any other host plant usually is very susceptible to blight during an outbreak

of the disease. Pear and apple trees alike should

produce a slow, steady, uniform, and fairly vigorous

growth. Such growth will usually develop stockier,

hardier twigs, which are much less susceptible to

blight infection.

Succulent shoot growth is usually low in carbohydrates. Blake {9Jf) reported such conditions in apple

trees growing in clay loam during hot, dry summers

in New Jersey. Trees with rather thick, medium

dark-green leaves that cease their shoot growth by

July 1 in New Jersey reportedly were high in carbohydrates and were the most resistant to fire blight.

In preliminary work, Hewitt UOi) found a positive

relationship between high starch content in host tissues and susceptibility to fire blight. Miller {651)

described a peripheral cork barrier delineating the

superficial cankers on resistant apple cultivars. He

concluded that "resistance is probably due, in part at

least, to the laying down of a cork barrier by the

host."

The various tissues of pear and apple trees differ

markedly in their degree of blight susceptibility.

Extensive inoculations in different tissues of 10 cultivars ofPyrus communis and a clonal selection No.

76 of P. ussuriensis at Beltsville showed that three

cultivars were most susceptible to blossom blight,

three to shoot blight, two others and the clonal selection to blight in 2-year-old branches, and one to

trunk blight (1058, 1057). Green, immature fruit on

all trees blighted within 2 weeks after inoculation.

Succulent shoots and woody trunk tissue in the

resistant Magness pear differ considerably in their

degree of blight resistance {1060, 1063), Reimer

{780) was among the first to report this phenomenon

in the pear cultivars Douglas, Orel, and Surprise

following artificial inoculation.

Mature fruits of pear and apple are much more

resistant to blight infection than young fruits of the

same cultivar {936,1062). A partial parallel between

susceptibility and the anthocyanin pigment in the

bark of pear seedlings has been reported {932).

Thomas and Ark {936) did not observe any marked

differences in the average extent of blight infections

between two lots of pyracantha seedlings and

bench-grafted pears, one showing the least and the

other with the most red pigment.

Studies reported in 1972 on shoot infection revealed that low levels of inoculum can infect expanding leaves when placed on the exposed vascular

system of a cut leaf {197). Also, young leaves on

undamaged apple and hawthorn shoots were readily

infected in early morning when thoroughly wetted

before inoculation. Such wet treatment was effective in the evening with hawthorn but not with apple

shoots {25Í). Billing {8J^a) concluded that both vascular damage and wetting in early morning favor

blight infection.

Tree Nutrition

For many years nitrogen has been associated with

vigorous growth and, in turn, has been correlated

with fire blight infection {80b, 9Jf, 303, U25, 581^, 727,

899,936). In addition, certain other elements usually

applied as fertilizers may also affect blight susceptibility {303, 522, 580, 689, 727, 899, 1113). Nightingale {689) found that the relative concentration of

carbohydrate and nitrogenous compounds within

apple tissues and the balance of these one with the

other were of much greater importance in determining the development of the fire blight organism

within the tissue than the relative water content or

the amount of resistance offered by the physical

nature of the host cells. A relatively low carbohydrate, high organic nitrogen content correlated with
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 89

susceptibility and the reverse with resistance to fire

blight. In addition, these studies showed that E,

amylovora grew well on agar containing extracts

from succulent twigs, low in carbohydrates and high

in organic nitrogen, but grew poorly or failed to

grow on that made from hard twigs, high in carbohydrates and low in organic nitrogen.

In a nitrogen-phosphorus-potassium factorial fertilizer test conducted over several years in the field

on pears, Fisher et al. (303) and Parker et al. (727)

found that adding potassium with nitrogen caused no

increase in blight severity over nitrogen alone.

However, adding phosphorus or phosphorus and

potassium with nitrogen reportedly produced more

succulent shoots and substantially increased susceptibility. Furthermore, inoculation of apple trees

growing in sand culture with nutrient differentials

supported the field results on pears.

In other nutrient solution studies on Bartlett

pear, Kenworthy (522) and Lewis and Kenworthy

(580) showed that the relationship between absorption of an element as measured by leaf analysis and

concentration of nutrient solution can be significantly changed by interaction of the elements. Deficiencies of nitrogen, phosphorus, or calcium resulted in a significant increase in leaf potassium. On

the other hand, excess nitrogen, phosphorus, or iron

significantly decreased potassium in the leaf,

whereas plants receiving no copper contained less

potassium than check trees. High levels of potassium decreased the concentration ofleaf calcium and

magnesium by the same magnitude as withholding

these two elements. In these studies the susceptibility of inoculated Bartlett pear trees appeared to

be affected by the nutritional environment. Lowest

susceptibility was found in trees with a high supply

of calcium. A low supply of boron also appeared to

reduce susceptibility. All other treatments, except

minus phosphorus and minus iron, resulted in a

slight or significant increase in susceptibility as

compared with the check. Thus they concluded that

withholding fertilizer applications would not necessarily increase resistance to fire blight.

In Virginia, Hickey (JfOJfO) confirmed that high

rates of potassium increased the susceptibility of

Jonathan apple trees on MM 106 rootstocks to fire

blight when applying modified Hoagland solutions to

trees in greenhouse sand culture. A 10 x copper

treatment appeared to increase susceptibility compared with the basic Hoagland solution, but the effect of high calcium (4 x) was not significantly different.

In a 2-year study on Bartlett pear trees grown

outdoors in washed quartz sand at Beltsville, less

fire blight was observed in inoculated trees that

received low nitrogen and high potassium plus an

adequate supply of other macroelements and microelements, and greatest blight was seen in trees

receiving high nitrogen and low potassium (Jí99).

Analysis of leaves from trees with intermediate

potassium levels showed intermediate blight development (fig. 21). Fisher et al. (303) also indicated

that Bartlett trees growing in New York State on

poorly drained sites were low in potassium and

showed more blight infection compared with trees

growing in well-drained areas and containing higher

potassium levels.

280 —

o

ce

240 —

200

<

\-

Q

LU

d

<

>

<

160 —

120 —

80 —

FIGURE 21.—Effect of nitrogen and potassium (K) amendments

on fire blight infection in Bartlett pear trees in sand culture.
90 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

In other studies ^^ at Beltsville, periodic leaf pH

readings were made on several field-grown pear cultivars and Jonathan apple for two seasons to see

whether any significant changes took place during

the season and how pH could be assimilated into the

fire blight syndrome. One gram of either old or

young leaves was added to 50 ml of demineralized

water and thoroughly mixed with a Waring blender.

The pH readings were made on the resulting leafwater suspension. Young leaves up to three-fourths

grown of all seven pear cultivars — Bartlett,

Stewart Bartlett, Dawn, DeVoe, Kieffer, Magness,

and Moonglow—showed a striking increase in acidity from April 24 to June 13. On April 24 (full bloom

to petal fall) all cultivars showed relatively high pH

values (5.3-6.0). Such values are known to support

optimum growth of E. amylovora in vitro (30, 87,

630y 901 ). On subsequent test dates (May 8 and June

13) the pH values of young leaves progressively

decreased. Young leaves of Bartlett and Stewart

Bartlett, both highly susceptible cultivars, never

had pH values below 5.2 at any time during the

study, whereas Magness, Kieffer, and Moonglow

had pH values of 4.6, 4.9, and 4.9, respectively. No

such increase in acidity was observed in Jonathan

apple. As pear leaves became older, the pH appeared to stabilize and showed values less acid than

those of young leaves sampled on the same date.

Based on these results, one may argue that the old

leaves support growth of the fire blight organism

more than young leaves. However, this is probably

unlikely because other factors, such as morphology

and phenolic potential, take over as leaves mature.

It is therefore suspected that the pH of the sap of

pear cultivars may in part affect the resistance of

susceptible young leaves of a given cultivar. Further

studies comparing young leaves and their petioles

demonstrated that the latter always had higher pH

values than the former. This appeared to correlate

with our field observations and greenhouse inoculation studies (fig. 6, B and C). Studies also indicated

lower pH values for leaves and petioles on resistant

Magness and Moonglow cultivars compared with

higher values for those on the susceptible Bartlett.

Because some of the cultivars reacted similarly in

the field, these data indicate that additional studies

should be conducted to resolve the effect of sap pH

on fire blight resistance.

Sands and Mclntyre (835a) reported in 1975 on the

16 Unpub. data, Fruit Lab., U.S. Dept. Agr., Beltsville, Md.

effect ofpotassium citrate and tartaric acid sprays in

lowering leaf pH ofthe pear cultivar Clapp Favorite

from 6.5 to about 4.0. Plants were treated with these

acids and then spray-inoculated withE". amytovora 1

to 2 days later. Leaves were assayed with a selective

medium 2 to 5 days after inoculation. Those from the

water controls (pH 5.5 -6.5) showed an average 3 x

10^ cells per leaf, whereas the acid-treated leaves

(pH 4.0-4.5) had approximately 50 cells per leaf.

Other greenhouse studies ^^ at Beltsville indicated that leaf ash from young tender leaves, vulnerable to fire blight attack, can inhibit growth oîE.

amylovora. Spectrographic analysis of these leaves

from plants growing in aluminum sulfate-amended

soil showed no difference from leaves of plants

growing in nonamended soils when analyzed for

aluminum, boron, calcium, copper, iron, manganese,

phosphorus, sodium, and zinc. However, an increase

in potassium appeared to be positively correlated

withJ?. amylovora inhibition. On a dry weight basis,

ifthe potassium level ofyoung tender leaves is below

1 percent, the inhibition zones appear to be materially reduced. We suspect that potassium is associated in some way with certain unidentified complex compounds responsible for resistance, because

tests with potassium chloride and potassium sulfate

at higher potassium ion concentrations than those

found in leaves demonstrated no E. amylovora inhibition. One might also suspect that adding

aluminum sulfate makes the soil more acid and allows roots to take up more potassium. However,

sulfur lowers soil pH more than aluminum sulfate,

but plants growing in soil amended with the latter

contained more potassium.

At the University of Illinois, Reinhardt (1125)

unsuccessfully attempted to increase sufficiently the

copper content of young Jonathan apple trees so as

to reduce the degree of blight severity following

artificial inoculation. In later studies Bushong et al.

(137) obtained a marked reduction in blight incidence

in trees injected with various copper compounds in

the trunk and foliar bordeaux spray treatments.

Generally copper levels in succulent shoots were

inversely proportional to the amount of fire blight.

Marked differences in amino acid content occurred

mainly in shoots of trunk-injected trees. However,

neither protein nor nitrogen levels were affected by

the copper treatments (1089),

In the mid-1970's, Aldwinckle and Beer (14a)

studied the degree of artificial blossom blight in

Golden Delicious apple trees on M 7 rootstocks under
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 91

nitrogen-phosphorus-potassium nutrient regimes in

greenhouse sand culture. They observed widest

differences between nutrient treatments when

about 10 bacterial cells were applied per blossom.

Trees with high nitrogen and phosphorus contents

had most infections, whereas in trees with low phosphorus content, those with higher potassium appeared more susceptible. One wonders whether nutrient measurements in leaves should be used to

determine differences in degree of blossom blight.

Intercellular Humidity

Following his extensive studies of the effect of

atmospheric and soil moisture on blight development in apples and pears, Shaw (860) conducted

detailed studies on the amount of intercellular

humidity (IH) in host plants, calculated by measuring the degree of turgor deficit of plant cells. He

prepared tables showing the relationship between

osmotic pressure and vapor pressure and between

vapor pressure and relative humidity.

Figure 22 summarizes the data of this significant

work on fire blight. Potted pear and apple plants at

different atmospheric humidities and different soil

moistures showed high IH and were very susceptible to fire blight when atmospheric humidity was

high. When moisture content of the environment

was low, both IH and degree of susceptibility were

low. When the average was 97-98.5 percent, the

plants were only slightly susceptible or were completely resistant to the pathogen; when IH was 99.5

percent, the plants were very susceptible. Plants

with intermediate IH were intermediate in their

degree of blight susceptibility.

We consider that the relationship between atmospheric humidity and IH is obvious and that it significantly affects the overall expression ofthe blight

pathogen in its host tissues. IH probably is a major

factor in causing differences in susceptibility in comparable groups of plants in environments varying in

amount of moisture.

Meteorological Factors

The amount of fire blight on individual trees or in

certain orchards in one season does not appear to be

correlated with that of the preceding or succeeding

season. The degree of disease is regulated by many

factors, such as optimal temperatures, excessive

rainfall, high relative humidity, and prevalence of

insects. Weather is probably the most important

factor in the epiphytology of fire blight, both in its

effect on the host and on disseminating the pathogen. In the host the bacteria develop best in rapidly

growing meristematic tissues. Warm, rainy weather

followed by periods of high humidity induces such

growth not only in shoots but also in fruit and bark.

Once the bacterium in introduced into such tissues,

especially in a susceptible cultivar, the rapid and

destructive development of the disease is assured

(321).

Temperature

Development of blight infection is favored by

moderately high temperatures, and often during

bloom the temperature is low enough to retard infection considerably. The exact temperature limits

have never been defined, but the following have

been generally accepted: Minimum 18.5° C (65° F),

optimum 21°-27° (70°^1°), and maximum 32°-35°

(90°-95°).

Under controlled environmental conditions,

Brooks (m) found that optimal temperatures for

blight development in apple twigs were between 21°

and 28° C (70°-82.5° F). This does not mean, however, that the disease does not develop below 21°

(70°). Miller (651) stated that at 16° (61°) blight development was so slow that practically no differences could be detected between the degree of

resistance in shoots of the apple cultivars Northwestern Greening and Wealthy. However, at 24°

(75°) and 28° (82.5°) Northwestern Greening was

distinctly more resistant than Wealthy.

In Wisconsin, Shaw (859y 1131) made detailed inoculation studies at various controlled temperatures. Apple cultivars Northwestern Greening and

Yellow Transparent were grown at 16° C (61° F),

20° (68°), 24° (75°), and 28° (82.5°) and were placed at

24° following inoculation. Results showed that plants

were most resistant at 16°, intermediate at 20° and

24°, and least resistant at 28°. Shaw concluded that

fire blight resistance in apple was related to (1) the

inherent characters of the cultivar, (2) the

physiological and morphological maturity of the tissues, and (3) the environmental conditions to which

the plants were subjected.

Several investigators reported the optimum temperatuie for the bacterium in culture to be 25°-28° C

(77°-82.5° ¥)(30, 85, 87, 901 ). The bacteria in blighting shoots 2-6 mm in diameter reportedly withstood

air temperatures for 4 hours at 48° (118.5°) or 30

minutes at 60° (140°) (93J^). In Colorado, Luepschen

(596a) observed survival of E. amylovora at tem-
92 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

100

< 100

98

97 —

96 —

95

6 AM

DELICIOUS

A Moisture Treatment

B No Moisture Treatment

—High Soil Moisture

■■■Low Soil Moisture

2 PM 6 PM 10 PM

TIME OF DAY

6 AM

100

99

98

97

96

95'

6 AM

6 AM

MclNTOSH

A Moisture Treatment

B No Moisture Treatment

All Plants at a

Low Soil Moisture

10AM 2 PM 6 PM 10 PM

TIME OF DAY

6 AM

100

_ KIEFFER

80 —

60 —

X

40

20 —

100

LU 80

IO

O

^ 60

100

20

_ DE LICIOUS

A

A

A Moisture Treatment

B No Moisture Treatment

—High Soil Moisture

■■ Low Soil Moisture

/

# •

/

/

/ 0

/

y

B

-*^^ B

1 1 1 II M 1

4 6 8 10 12 14

DAYS FOLLOWING INOCULATION

16 18

100

_ MclNTOSH

A Moisture Treatment

B No Moisture Treatment

All Plants at a

Low Soil Moisture

4 6 8 10 12 14

DAYS FOLLOWING INOCULATION

FIGURE 22. — Relationship between intercellular relative humidity and fire blight susceptibility in shoots of pear and apple at different

atmospheric humidities and soil moistures (after Shaw, 860).
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 93

peratures up to 34° (93°) when Bartlett trees were

spray-inoculated with 500 cc of 8-day-old broth cultures suspended in 500 gallons (1,890 liters) of water

with 2 cc of Triton surfactant. There is probably no

upper temperature Hmit for survival under natural

conditions.

On the other hand, low temperatures retard or

arrest the development of infections {756, 1126). In

areas with mild winters, temperature is a Hmiting

factor in the enlargement of cankers. Cankers entirely girdling the branches often increase several

centimeters more during the fall and winter on the

side exposed to the sun than on the opposite side.

Thomas and Ark {936) demonstrated a relationship

of low temperature to initiation of blight infection in

California by inoculating alternate rows of pear

seedlings on the north and south sides of the trunks

in February. By early April, 15 percent of the trees

inoculated on the north side and 29 percent inoculated on the south side were infected.

In 1975, Thomson et al. {950) reported detailed

studies of spring temperatures in several California

counties during full and late season bloom in pear.

Even though the correlation of temperature to the

incidence of blossom blight has as yet been inconclusive, epiphytic populations of E. amylovora are

monitored to improve the timing of bactéricide applications for fire blight control.

In New York, Mills (^5^) claimed successful forecasting of blossom bhght epiphytotics in apple using

degree days above 18° C (65° F). He found positive

correlations between degree days plus precipitation

during bloom and severity of fire bhght in the Lake

Ontario fruit counties during 1918-54 (fig. 23). A

few years later, Luepschen {1115)found in this area

that, on days during bloom with maximum tempera120

o 100

o

D

Q

Z

O

>-

<

Q

CC

80

60

40

20

• Degree days over 65° F (18.5°C)

Degree days over 70° F (21 °C)

Precipitation

20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

YEAR

VERY

SEVERE

SEVERE

MODERATE

SLIGHT

VERY SLIGHT

FIGURE 23. — Correlation among temperature degree days plus precipitation during bloom and fire blight severity in apple in New York

during 1918-54 (after Mills, 656).
94 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

tures of 18.5° (65°) without rainfall but following

rainy days, the mean relative humidity was as high

or higher than on days when precipitation occurred.

In controlled experiments, temperatures after blossom inoculation had a more significant role than

those at the time blossom sprays were apphed (598).

Observations over a 5-year period indicated that

serious blossom blight infections required a

minimum of 2 favorable days for infection.

In Illinois, Powell (757, 758, 760, 761 ) worked out a

rather unique method for predicting fire bhght infections based on prebloom freezing temperatures and

the number of ''degree days" above 18° C (65° F)

required for the organism to become active again.

He concluded that 18 degree days Celsius (30 degree

days Fahrenheit) between the last freeze and early

bloom, combined with maximum temperatures of

21°-27° (70°-80.5°) and a light rain or high humidity, were adequate for fire bhght infections. Principal factors reportedly interfering with blossom or

twig infection are (1 ) inadequate number of degree

days between latest prebloom freeze and early

bloom, (2 ) maximum temperatures either lower than

18° (65° ) or higher than 30° {^Q° ) during early bloom,

(3) drought preceding and during early bloom or

during June 1-July 15, and (4) excessive rains during early bloom.

In Pennsylvania, Petersen {736) suggested using

60-100 ppm of streptomycin for blossom blight control based on Powell's temperature studies. In Arkansas, Rosen (817) showed an apparent correlation

between blossom production regulated by frosts and

prevalence of disease. Investigations in California

by Thomson et al. {950) in 1975 showed no correlation between Mills' weather criteria and the presence OÍE. amylovora in blossoms.

In England, Billing {8Í, 8Ua) observed severe outbreaks of fire blight in apple and hawthorn in 1968

and 1969 when temperatures of 27° C (81° F) or

higher followed heavy rainstorms in early July. In

1976, Billing {85a) proposed a method for assessing

potential fire blight activity in the field in southeast

England. It is based on potential doubling derived

from in vitro growth rates of£'. amylovora at different temperatures combined with a rain score derived from daily precipitation values. She found

good agreement between accumulated potential

doubling values and accumulated degree days, using

both English and American temperature records.

Highest potential fire blight activity occurred in

warm, wet periods rather than in cool, dry ones.

In West Germany, Schroder {1128a) studied temperature-humidity limitations for blight development on 10 species of cotoneaster, crataegus, and

pyracantha. Following artificial inoculation of the

plants and incubation in a growth chamber, he was

able to obtain blight symptoms at 15° C (59° F)and 55

percent relative humidity during the day and 10°

(50° ) and 70 percent relative humidity at night.

Rain, Humidity, and Hail

The relation of weather to dissemination of the

blight pathogen is both direct and indirect {8It,). Observations and investigations by many workers,

especially those by Miller {651), Brooks {IH), and

TulUs {961 ), demonstrated conclusively that rain is a

very common and effective agent in disseminating

the bacterium (chap. 8). Rainy weather followed by

warm, cloudy weather, especially during blossoming

and ultimate shoot growth, is very favorable to

epiphytotic outbreaks of fire blight.

Periods of relatively high atmospheric humidity

have been reported by many investigators to be

frequently accompanied or immediately followed by

rapid development of fire blight in apple and pear

trees in the orchard. Brooks {IH) observed the

greatest amount of resistance in apple trees of the

cultivar Fameuse when they were kept at 50 rather

than 80 and 95 percent relative humidity after inoculation. Eighty percent appeared most favorable

for infection, whereas 95 percent was slightly less

favorable. Miller {651 ) reported no more blight

resistance in young Northwestern Greening than in

Wealthy apple shoots when they were placed in a

saturated atmosphere at 26° C (79° F) following inoculation. Shaw {859) confirmed this observation

when these two cultivars were subjected to high

humidity for 72 or 96 hours after inoculation.

A high relative humidity reduces the sugar concentration in pear nectaries to permit growth oíE.

amylovora. Under natural conditions, sugar concentration in the nectaries is generally higher than

lower (chap. 8).

For a long time, injury has been known to have an

important role in the infection process {112, lU, 500,

575, 596a, 599, 651, 7J^2). In 1966, Keil et al. {501)

proved that considerably more infections occurred

on artificially inoculated young Bartlett trees in the

greenhouse when they were lightly sandblasted

than when left uninjured. When trees were inoculated at various intervals from 0 to 64 days after

injury, the disease was more severe when they were
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 95

inoculated within 6 hours after injury than at longer

intervals (1052). More recently we showed that

blight development was more severe within 24 hours

after injury when trees were kept in a moist

chamber than on the greenhouse bench (1062). Infection also developed on all injured pear fruit

dipped in inoculum at the time of wounding (nail

puncture) and on 70 percent of fruit bruised 2 days

before or after inoculation. We found that infection

took place mainly through leaf petioles whether the

trees were injured or not (fig. 6, B). Crosse and

Shaffer (198) reported that severe shoot blight resulted if the vascular system of Jonathan apple

leaves was severed near the apex immediately before inoculation.

These facts indicate therefore that tissue injury

providing access to the vascular system predisposes

the development of fire blight. In the summer of

1967 our attention was directed to an epiphytotic of

fire blight in an isolated pear orchard in northern

Arkansas. A combination of heavy fertilization,

abundant rainfall, and a severe hailstorm had caused

blight in 80 percent of the trees in two orchards

(1066). Without definite proof, the blight organism

seemed to be present inside or on the surface of the

pear branches.

Stewart (90J^) reported unusually severe outbreaks in two separate orchards in New York during

the summer of 1914. The trees in these orchards

were struck by a severe hailstorm and many blight

infections were prevalent 2 weeks after the storm,

originating in the wounds of the bark made by the

hailstones. Similar observations were made in Colorado following a severe thunderstorm with hail

(596a).

From California, Wilson (1029) reported that increased humidity due to sprinkler irrigation caused

greater severity of fire blight in pear. In 1976,

Spotts et al. (88ib) reported measuring a ninefold

increase in twig blight on water-misted trees of the

cultivar Golden Delicious in Ohio over adjacent

nonmisted trees.

The effect of wind on blight development primarily pertains to dissemination of the pathogen in the

form of ooze droplets or bacterial strands (chap. 8).

Edaphic Factors

Soil Types

Usually blighted trees are more common in rich

than in poor soil, in which tree growth is severely

reduced. Rich soils, especially if manuring is practiced, result late in the season in extra succulence in

branches, which in turn are susceptible to fire blight

(899, 901). Trees in sod usually grow slower and the

new shoots become hardy and woody earlier in the

season (78Í, 955).

Blake (9J^) reported that trees were susceptible to

blight when grown in dense soils where the water

table was high at times or where they had limited

root systems because of lack of aeration. He listed

the following soil conditions in New Jersey that usually resulted in severe blight in apples and pears: (1)

Very light, sandy soils that are subject to marked

variations in water content; (2) shallow soils of any

type as a result of being underlaid by ledges of rock

or impervious clay; and (3) low areas where moisture

is rather near the surface, aeration is somewhat

unfavorable, or the trees continue to produce twig

and shoot growth after the first ofJuly, even during

dry, hot days.

Before establishing a new orchard, select a fertile,

well-drained site. Tree growth stops earlier in the

season on dry soil than on poorly drained soil (78i,

785). On well-drained soils, alfalfa or other deeprooted legumes may be used as a permanent cover

crop. Grass or alfalfa sod should be well mowed early

in the season and then usually allowed to grow in

midsummer to check tree growth. In New York,

Hildebrand and Heinicke (í25) observed in the same

orchard that apple trees under alfalfa culture were

less severely blighted than those in grass sod plus

nitrogen or under cultivation. The number of infections did not differ measurably, but the extent of

blight invasion from each infection was reduced

under alfalfa culture. These results and observations

were later confirmed by Toenjes (955) in Michigan.

Soil Moisture and Temperature

A relationship exists among soil moisture, tree

growth, and percent intercellular humidity in the

trees. Shaw (859) found the least blight in apple

shoots where soil moisture was low. Soil temperatures of 12° and 32° C (54° and 90° F) were less

favorable for blight development in Fameuse apple

shoots than intermediate temperatures. He observed a positive relationship between blight resistance and lowered vigor in the shoots.

There is comparatively little information on the

relationship oîE. amylovora to the soil. Arthur (52)

reported that the organism grew moderately well in

soil extract. In 1895 after careful investigation,

Waite (991 ) was unable to isolate the blight organism
96 AGRICULTURE HANDBOOK 51O, U.S. DEPT. OF AGRICULTURE

from soils. From California, however, Ark {27) reported that E. amylovora could persist in the laboratory for 54 days in sterilized soil and for 38 days in

unsterilized soil. He employed a special technique

and used green, immature pear fruit dipped in soil

suspensions to recover the blight organism (fig. 24).

The bacterium was obtained from different orchards

where it persisted for at least several weeks in dry or

moderately moist soils.

Thomson (1137) in 1969 showed in detailed isolation studies that E. amylovora overwintered not

only in soil but also on and within dead tissues, such

as blighted stems, twigs, and fruit mummies. He

recovered both virulent and avirulent£'. amylovora

isolates from apple or pear material and considered

soil to be ideal for isolating the organism in March

and April.

Cultural Practices

Cultivation and fertilization may markedly affect

the amount and control offire blight, particularly on

pear trees. If cultivation and fertilization cause a

rank, succulent growth, these trees are more likely

to be affected by fire blight. Therefore never cultivate pear and apple orchards late in the season because it is likely to cause new growth, which may

continue to spread fire blight. Severe pruning should

also be avoided on susceptible cultivars, because it

tends to produce rank, tender growth.

Growers should attempt to develop extensive,

deep root systems by providing a favorable soil reaction and uniform soil fertility. Applications of nitrogen should be based on the need of the individual

orchard and should be sufficient to provide only

moderate growth. Foliar application of nitrogen may

be used following the blossom period as a source of

supplemental nitrogen. Usually sufficient nutrients

will be obtained by the tree with applications of a

5-10-5 fertilizer at 1 pound per 100 square feet (0.5 kg

per 25 m^). In general, a grower should study his

trees and regulate growth in such a way that they

will produce moderate shoot growth without unduly

subjecting the trees to attack by fire blight.

PN-6389

FIGURE 24. — Isolation oí Erwinia amylovora from orchard soil

beneath blighted pear tree after immersion of immature pear

fruit in soil suspension (after Ark, 27).

There is considerable evidence that orchards

planted on well-drained soils are much less subject to

fire blight than those on wet soils. Fire blight problems have been reduced and tree productivity has

been increased when poorly drained sites were improved by tiling. A new orchard site should be sufficiently limed to maintain a soil reaction of pH

5.5-6.5. The Mme should be well worked into the

soil, preferably by deep plowing or subsoiling.

During late winter or early spring remove all root

sprouts and suckers from the main framework and

trunk of the tree. Their removal may prevent summer infections that might destroy large branches or

even entire trees. The removal of suckers from the

main trunk and scaffold limbs should be repeated

during the growing season. Wherever possible, also

remove late season or "rattail" blooms. For further

details on pruning, canker removal, and orchard

sanitation, see chapter 12.
CHAPTER 12

CONTROL MEASURES

Fire blight cannot be controlled by any one measure alone. Every attempt must be made to keep fire

blight out ofthe orchard. Ifthis serious disease gets

into the orchard, make every effort to eliminate or

contain it so that trees are not lost and production is

not impaired. Experience shows that the best control results when the grower follows an integrated

program of chemical control combined with sanitation, pruning, eradication, tree nutrition, and insect

control (8, 80b, 112, 188a, 23JÍ, 236, 298, 398, 627,

723, 72U, 10Í8, 106ia).

Sanitation and Quarantine

Nurserymen or anyone propagating plants susceptible to fire blight should be responsible for using

clean clonal or seedhng stocks and budwood taken

from plants free of the disease. Tracing fire blight

infection back to the propagator is difficult. Sufficient evidence seems to indicate that fire blight can

be spread from this source into an area where the

disease has not previously been found (12Jf,). The

grower, if possible, should be familiar with sanitary

conditions of the nurseryman supplying his trees.

Trees from a well-managed nursery may cost

slightly more, but to obtain healthy plants is worth

the price difference.

As a modified form of quarantine in some areas, a

common practice is to avoid planting both apple and

pear trees in the same orchard. In the early 1920's,

laws were passed in America to prevent pear trees

from being planted within 1.5-2.5 km of an apple

orchard (78), Where this was done, less fire blight

occurred in the apple orchards. Similarly the proximity of hawthorn to pear and apple has increased

the incidence and severity of fire blight i87a, 179,

330, 638, 639, 681), Removal of hawthorn hedges in

or near orchards has been suggested.

The most effective quarantine regulation is strict

enforcement to prohibit importation of all fruit,

seed, budwood, and other plant parts of all rosaceous

plants from any country with fire blight. Soon after

the introduction of fire blight into New Zealand

about 1919, the Commonwealth of Australia passed

a law prohibiting the importation of any plant parts

in the family Rosaceae from any country where fire

blight was present (181). This enforcement has been

continued ever since (1 ) and, together with the country's isolated location, undoubtedly has contributed

to the absence of fire bight in Australia today. Since

the introduction of fire blight into Western Europe,

strict quarantine regulations have been adopted by

Norway (288), Sweden (286, 289), and Switzerland

(285),

Pruning

Sometimes if fire blight is not too severe, especially when only a few twigs are blighted in an orchard, the blighted parts can be pruned out without

further spread of the disease. Often diseased

branches and apparently healthy "rattail" blooms

are regularly removed during the summer in orchards where a chemical control schedule is followed. Some of the largest pear orchards in the

Pacific Coast States follow this practice (2J,.0). If

pruning is done as soon as blight infection is observed, it will usually eliminate the diseased tissues

as well as the inoculum that provides the bacteria for

new infection (159, Jf.98), It also destroys the dead

tissue, which is vulnerable to invasion by rot organisms that affect fruit the following season (235).

Several years ago we experimentally demonstrated at Beltsville, Md., under severe disease conditions that fire blight could be contained by pruning

alone (503), Ten-year-old fruiting Bartlett peartrees

about 4 meters tall, in a virorous growing condition,

were inspected once or twice each week between

bloom and harvest. Each infection was pruned out as

soon as it was observed. Cuts were made 15-45 cm

below the visibly diseased part depending on the

amount of apparently healthy tissue available for

cutting. During the season, 135 pruning cuts, on an

average, were made on each unsprayed tree, and

although apparently free of blight, the trees were

only one-third their original size at the end of the

season. Similar trees not pruned died from blight

infection. This simple experiment indicated that fire

blight can be controlled by pruning even under severe disease conditions. Although following such a

97
98 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

procedure under similar conditions in large orchards

is impractical, pruning can help control the disease.

On the other hand, Lake {1112) and Lake et al.

{5Í7) demonstrated in Minnesota that summer

pruning of apples in early June markedly enhanced

infection of fire blight. From Indiana, Gardner

{S20a) reported that ringing limbs of certain apple

cultivars to induce early bearing resulted in considerable blight infection of the knife-cut girdle

wounds.

In some fruit areas, pruning out blighted branches

is particularly helpful when it supplements a chemical control schedule. Pruning tools should be dipped

in a disinfectant between cuts to prevent spread of

the bacteria. Several chemicals, including solutions

ofthe corrosive sublimate, mercury cyanide, various

denatured alcohols, and the common household

sodium hypochloride, have been used successfully to

decontaminate pruning tools U98, 507, 55Jf, 726, 778,

992). Sodium hypochloride has been widely used in

recent years because it is economical, easily accessible to the average household, and killsJî. amylovora

bacteria when the tool is dipped in a 10 percent

solution for about 2 seconds {8, 507). Unfortunately it

corrodes tool metal, but this can be prevented by

rinsing in running water and oiling each day after use.

The fact that pruning tools can transmit the blight

pathogen can best be demonstrated by an extreme

example in 1906, when Waite and Smith (999) observed a nursery block of 10,000 pear trees completely destroyed by fire blight. Holdover blight

cankers were present in the stock trees and pruning

tools apparently became contaminated and transmitted the disease to nearly every tree.

Aldwinckle and Preczewski il6a) reported in 1976

a discolored streaking in xylem vessels in advance of

externally visible blight symptoms in both field- and

greenhouse-grown apple cultivars. Pathogenic E.

amylovora was isolated from these vessels, and

length of xylem streaking in advance of external

lesions correlated with the standard error of lesion

length. These facts again emphasize the importance

of pruning far in advance of visible blight symptoms

and the use of disinfectants between cuts.

Eradication

In addition to blighted twigs and branches, serious body blight, manifested as cankers in large limbs

and the trunk, sometimes develops. Such blight infection often causes loss of the tree unless the bacteria in these cankers are eliminated or inactivated

before the blossoms open. Several methods of

eradicating the fire blight organism from cankers

have been used. In the Pacific Northwest, cankers

are scraped or scarified with a sharp-edged tool to

allow them to dry out. Usually they are also painted

with poisonous solutions, which kill the blight bacteria (199, 208, Í27, Í98).

Sometimes careful surgery is practiced, whereby

the diseased bark is removed followed by proper

disinfection. With this method the cut should be

made several centimeters into the apparently unaffected wood of the canker margin. In some parts of

the country, disinfectants are painted over the surface ofthe canker without any surgery. This method

requires the least time and has proved effective in

some orchards. Of the many chemicals tried in

canker paint formulas, only a few have proved effective and relatively noninjurious to healthy wood.

Combinations prepared by dissolving zinc chloride in

denatured alcohol, water, and hydrochloric acid

have been used extensively (150, 208-211, 228, 55i,

602, 60U, 608, 620, 9U).

Both mercuric chloride and cyanide of mercury

have been used alone but more successfully in combination, when they are referred to as Reimer's solution (66, lU, 199, 208, 209, ^27, ^69, 101^5). This

formula was improved by Day (208, 209) when he

added glycerin, which allowed better penetration

and prevented quick evaporation. Cardinell (IJ^i) of

Michigan added fuchsin red to the Reimer-Day mercury and glycerin solution, which allowed one to tell

at a glance which cankers had been treated. A third

canker paint based on cadmium sulfate was developed by Parker et al. (726). The formula includes

five parts cadmium sulfate solution plus two parts

glycerin, two parts muriatic acid, and five parts

denatured alcohol. This paint is applied to intact

bark, and the dead spurs or small twigs are removed

after treatment, not before (HI, 1^98, 527). All these

canker paints are poisonous and should not be used

unless approved by your State government officials.

Experimental studies by Gardiner (219) using

paints containing 550 ppm ofstreptomycin indicated

that weekly applications on cankers in the spring and

early summer may stop further development of the

cankers and keep them inactive until they finally dry

out. Coyier (188) also reported limited control of fire

blight infection in cankers with 2,4-xylenol plus mcresol (Bacticin) developed for treatment of crown

gall in various fruit and ornamental trees.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 99

Often itis difficult to detect all cankers on a tree. A

technique based on infrared sensing has been devised by Specht and Beer (884a) in New York State

and should alleviate this problem (chap. 8).

Eradication measures should also include

ehminating all susceptible wild host plants adjacent

to apple and pear orchards. Any susceptible escaped

wild plants harboring the fire bhght organism may

serve as a source of inoculum for orchard trees. This

has been shown conclusively in New Zealand (179,

681) and Great Britain (330), where bhght spread

from hawthorn into pear and apple orchards. The

best and only control is to remove these plants.

Tree Nutrition

Any cultural practice that overstimulates growth

usually makes the plant or tree vulnerable to serious

fire blight infection. One such factor is the production of vigorous growth through applying too much

nitrogen. Numerous investigators have associated

fire blight infection with nitrogen (80h, H, 188b, 303,

Í25, 58J,, 727, 899, 936), Blake (9^) found that trees

which became weak or rank in growth and high in

nitrogen also became susceptible to fire blight. A

relatively low carbohydrate and high nitrogen content appears to be correlated with fire blight susceptibility and the reverse with resistance (95, 689).

Shoot and body blight in four apple cultivars—

Delicious, Northern Spy, Mclntosh, and Rhode Island Greening—were markedly increased by application of nitrogen (i25). Cultivation, which makes

more nitrogen available to the plant, may also increase blight infection. Lewis and Kenworthy (580)

found that trees supplied with high nitrogen contained the largest concentration ofleaf nitrogen, and

several other workers (303, J^25, 859, 936) noted that

high nitrogen levels enhanced fire blight infection.

However, Lewis and Kenworthy also found that the

level of nitrogen in other susceptible trees was no

different from the level in the least susceptible trees.

The excessive or deficient supply of one element

may have an important effect on the balance of elements in the composition of a plant and may affect its

susceptibility to fire blight. Therefore the grower

should supply his apple and pear trees with a balanced nutrition that will produce maximum yield of

high-quality fruit without increasing blight. Such a

nutrition program will have to be supplemented by

other control measures, such as chemical sprays.

For more detailed information on the effect of tree

nutrition and soil fertility on blight development, see

chapter IL

Chemical Control

Much has been published about fire blight control

with chemicals. Data since 1920 have been summarized in table 8. Current chemical programs for

fire blight control are based principally on protective

schedules because available compounds possess poor

eradicative properties (405, 503, 650). Periodic applications are required to keep the new growth protected, especially when environmental conditions

are optimum for infection. To supplement this program, apphcations should also be made within 1 day

following storms that cause tree injury (1062).

Two groups of chemicals, copper compounds and

antibiotics, have had the most important role in controlling fire blight of apples and pears since the

1930's. Copper has been used since the 1900's and

antibiotics since the mid-1950's.

Copper Compounds

As shown in table 8, copper compounds gave variable control ranging from poor to excellent (8, 176,

319, 423, 518, 597, 606, 607, 675, 746, 775, 781, 811,

812,814, 862, 863, 975,1117). Even in those orchards

with satisfactory control, the growers usually complained about fruit russet caused by copper (519,

773, 815). Probably because of this injury, copper is

not used more often. However, in some orchards

where fruit is processed and russeting is not of great

concern, copper is still widely used during the dormant and blossom periods, probably because it costs

less than alternate treatments (756).

Copper sulfate plus lime (bordeaux mixture) has

been used more often than any other form of copper.

Fruit russet is directly proportionate to the copper

content of the formula. A 2-6-100 ratio (2 pounds

copper sulfate plus 6 pounds hydrated lime per 100

gallons of water) was used most frequently, usually

applied one to three times only during the dormant

and bloom periods because of injury to the fruit.

Spray tests on pears in New Zealand by Dye (251)

over a 5-year period failed to show any control offire

blight with bordeaux mixture, but adding streptomycin to this spray gave very effective control.

Other forms of copper have been tried with some

success, especially in the Western States (240, 514,

525, 597, 762). More applications are usually applied

with these copper compounds, but fruit russeting

still limits their use. California investigators (59, 60,
TABLE 8.—Summary of chemical compounds used to control fire blight on apple and pear

Chemical compound Tree Concentration^ Number of Disease Disease Research

appHcations severity^ control^ location

Year Reference

O

O

COPPER COMPOUNDS

Bioquin^-

Bordeaux^-

Apple -

Pear ~

Apple

1 lb/100

1 lb/100

.5-1-100

1.5-6-100

2-6-80

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100

2-6-100; 6-12-100

Citcop^

COCS^

Coposil®

Copper chelate

Copper hydroxide (Kocide)-

Copper-lime dustPear 1-1.5-100

2-6-100

2-6-100

2-6-100

2-6-100

6-12-100

10-10-100

do 1-2 gal/100

do 1 lb/100

Apple 4 lb/100

Pear .5 lb/100

do .25 lb/100

.25-2 lb/100

4-8 oz/100

do

10-90%

1-4

1-4

14

3

2

5

1

2

1-2

2

2

1-3

1-2

6

1-5

L-M

L-M

M

S

M

M

M-S* 0-3

M

L

S

L-M

M

M

M-S

M

M

Copper oxalate

Copper oxychloride -

Apple -

Pear —

20-80%

4 lb/100

1 lb/100

7 M

2 L-M

2 M

1-2 M

2 L-M

3 S

2 M

6-8 L-M

8 L-M

6 L-M

3 M

6 M-S

6-8 L-M

3 L-M*

L

L

L-M

7 M

6 L-M

3 M

2 m. 1947

2 111. 1947

2-3 111. 1959-60

0-1 N.Y. 1958-59

2 N.Y. 1943

3-4 Ark. 1932-33

Ind. 1927-32

2 Iowa 1938

1 Mo. 1954

2 N.C. 1934

2-3 N.Y. 1949

1-2 N.Y. 1960

1-3 S.C. 1933

1-2 Tenn. 1931-34

3 Wis. 1936

1_3 Wis. 1926-37

4 Canada 1951

2-3 N.Y. 1949

2 N.Y. 1954

2-3 N.Y. 1960

1 New Zeal.— 1964-69

1-2 Oreg. 1923-25

2-3 Calif. 1943

1 Calif. 1973

2-3 Calif.—---— 1973

3 Wis. 1936

1 Oreg. 1957

2 Calif. 1968-69

2-3 Calif. 1973

2-3 Colo. 1973

2-3 Calif. 1956

2 Calif. 1956

2-3 Calif. 1956

4 Canada 1951

3 Wis. 1936

3 Oreg. 1957

753a

753a

755, 756

598

U23

811, 812,

81Jp, 815

607, 608,

1117

176

33Jf

693

72U

725

675

862, 863

51U

518, 519,

7U6

319

72U

378

725

251

781

975

775

775

51J^

525

59

775

597

35

36

39

319

51Jf

525

>

O

o

a

a

>

a

td

o

o

w

Ö

O

>

O

O
Copper phosphate

+ lime+ bentonite-

Copper sulfate

Copper sulfate (tribasic)

Cuprioleat^

Cuprocide^^

ANTIBIOTICS

Aureomycin

Neomycin

Oxytetracycline (Terramycin)

Penicillin

Rimocidin

Streptomycin nitrate-

Streptomycin sulfate-

Streptomycin (15%)+ oxytetracyline (1.5%)

(Agrimycin 100).

See footnotes at end of table.

Apple 4+8+4/100

do4 lb/100

Pear8 oz.a00 1 lb/100

do .5%

Apple 1.5 lb/100

Pear 25 ppm

Apple 100 ppm

Pear 10-50 ppm

Apple 30-120 ppm

60-120 ppm

60-120 ppm

100 ppm

Pear 10 oz/100

do 1,000 ppm

Apple 30-120 ppm

do 50-100 ppm

100 ppm

Pear 100 ppm

100 ppm

200 ppm

Apple 60-120 ppm

60-120 ppm

100 ppm

do 30-120 ppm

50 ppm

50-100 ppm

50-100 ppm

50-100 ppm

50-200 ppm

60-120 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100-250 ppm

500 ppm

Pear 10-100 ppm

25-100 ppm

50 ppm

50 ppm

50 ppm

50-100 ppm

50-100 ppm

6 L-M3 Wis. 1936 51J,

1 (dormant)L2 111. 1954 762

2 L-M*1 Colo. 1973 597

5-7M2 Calif. 1954 2W

2M3 Denmark— 1972 38U

6 L-M3 Wis. 1936 51J,

4 L-M0 Calif. 1956 39

4S1 111. 1954 75Jf

3 L-M2 Calif. 1956 39

5 L-M 0-1 Del. 1952 W3 3 L-M 2-3 Iowa 1953 863a 3 M* 2-4 Ohio 1953 1032 3 M* 1 Ohio 1954 1031 8 L-M 2-3 Calif. 1973 775 1 M* 0 Calif. 1946 82U 3 M 0-1 Del. 1952 WS 3 S 2-4 N.C. 1954 177 2 M 1 N.Y. 1960 725 1 M* 4 N.Y. 1963 1111 5 M 4 Oreg. 1954 526 5 M 3 Conn. 1956 650 3 L-M 4 Ohio 1953-55 1031, 1032 3 S 2-3 Iowa 1953 863a 2 S 4 Pa. 1954 527 5 L-M 2 Del. 1952 WS 9 M 3-4 111. 1967 759

3-4S 2-3 Mo. 1954 336

2-10M 3-4 Mo. 1969 856

3S 2-4 N.C. 1954 177

8-9 L-S* 2-4 Md. 1964-65 505 3 M* 4 Ohio 1953 1032 6 L 3 Calif. 1956 39 4 S 2-3 m. 1954 75Jf

4-10M 2-3 Mo. 1970 857

3 M-S4 Ohio 1954 1033

2L 3-4 New Zeal— 1963 682

4-7L4 Mo. 1953 333

2M 1-2 N.Y. 1964 J,80, 1111

2L 1-4 Calif. 1956 39 L 1-2 Calif. 1956 35

17L? CaHf. 1968-69 59

3 L-M2 Colo. 1964 596

2-4S4 Mich. 1962 537

1 M*1 N.Y. 1961 380

1M 3-4 N.Y. 1965 lOU

oX>Q>>oom>OO

cl

Ul >

Ul
TABLE 8.—Summary of chemical compounds used to control fire Might on apple and pear—Continued o

Chemical compound Tree Concentration^

Number of Disease Disease Research

apphcations severity^ controP location Year Reference

ANTIBIOTICS—continued

Streptomycin -I- oxytetracycline- Apple -

PearStreptomycin-Copper A ^^-pyrophyllite dust ^^ — doStreptomycin-oxytetracycline dust do -

Streptomycin-oxytetracycline-copper dust doStreptomycin-pyrophyllite dust do -

Streptomycin-sulfur dust do -

Streptomycin + captan ^^ Apple

Streptomycin + dichlone ^^ doStreptomycin + dimethyl sulfoxide (DMSO) Pear

Streptomycin + glycerol- Apple

Pear ~

50-100 ppm

50-100 ppm

60 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100 ppm

100-200 ppm

200 ppm

10 oz/100

.04%

25 ppm + 50 ppm

100 ppm

125-250 ppm + 250 ppm

125-250 ppm + 250 ppm

10 ppm + 100 ppm

30 ppm + 3 ppm

50 ppm -f- 100 ppm

60 ppm

60 ppm -I- 6 ppm

100 ppm -I- 10 ppm

100 ppm -I- 10 ppm

100 ppm -I- 10 ppm

200 ppm

1,000 ppm; 10% copper

1,000 ppm; 50 lb/A

1,000 ppm; 50 lb/A

2,000 ppm; 30-50 lb/A

240-1,000 ppm

1,000 ppm

1,000 ppm

50 ppm -f- 2 lb

100 ppm + .5 lb

50 ppm + 1-2%

50 ppm -I- 1%

100 ppm + .5 gal/100

50 ppm -I- .5%

1 M* 3-4

6 M-S 3-4

3 S 3

3 L-M* 2-3

3 S 3-4

2 M 2-3

3 M 2

4 S 4

1 S* 2-3

5 M 4

2 L 3-4

4 L-M 3-4

2 M 4

5 M 3

8 L-M 2-3

2 M 3

4-6 S 1

3 M-S 3-4

4-7 L 3-4

4-7 M 3-4

L

M

0-3

3-7 2-3

1 M 1-3

10 M 4

3-7 M 2-4

3-7 M 3-4

2-4 S 2-3

5 M 4

5 M 3

6 L 3

10 M 2

10 M 2

10 M 2-3

4-14 L 2-3

6 L 2

6 L 2

3 L-M 2-3

3 L-M 0

1 M 2

1 L* 1

2 M 4

3 M* 2-3

N.Y. 1968

Calif. 1970

Oreg. 1955

Colo. 1973

N.Y. 1958-59

N.Y. 1960

N.Y. 1962

Md. 1959-61

Md. 1971

Oreg. 1954

New Zeal.— 1963

New Zeal.— 1964-69

N.Y. 1954

Conn. 1956

Calif. 1971-73

Denmark — 1972

Mo. 1955

Ohio 1954

Mo. 1954

Mo. 1953

Calif. 1956

Calif. 1954

N.Y. 1955

Calif. 1958

Calif. 1954

Calif. 1954

Oreg. 1954

Oreg. 1954

Conn. 1956

Calif. 1956

Calif. 1958

Calif. 1958

Calif. 1958

Cahf. 1953-55

Calif. 1956

Calif. 1956

Canada 1955

—do 1956

N.Y. 1965

N.Y. 1968

N.Y. 1960

N.Y. 1962

10J^3

59

0Z4

> 597 O

598 WH—(

1115 O

Jf82

503 H

1061

526 w

682 ffi

251 >

378 "Z

650

775, 8U o

n

38U P^

337 Oi

1033 o

33U

333 a

35 Ul

2U0, 2U1 Ö

922 w

3U ^

H

2W, 2hl O

2W, 2A1

W5 >

52Jf, 526 O

650

36 o

a

3h r^

3U

3U P^

32, 33 W

36

36

17A

17U

10u

10^3, 11W

725
streptomycin

+ glyodin

Streptomycin+ maneb^

Streptomycin+ oil

Streptomycin-oxytetracycline+ copper

Streptomycin+ oxytetracycline+ zineb ^''-

Streptomycin+ Polyram ^^

Streptomycin STA (Merck)-

Streptomycin STB (10%) (Merck) insoluble —-

Streptomycin STC (Merck) insoluble

Streptomycin STD (10%) (Merck) insoluble —-

Streptomycin STS (54%) (Merck) soluble

STS+ oxytetracycline -I- calcium nitrate

STS+ oxytetracycline+ glycerol

STS+ oxytetracycline+ kerosene

Thiolutin 19 Apple

CARBAMATES

100 ppm+ .5 gal/100

Apple 100 ppm+1 pt

Pear 50 ppm -I1 lb

do 50 ppm+ 1%

do 60 ppm+ 10-42% copper

do 30 ppm+3 ppm -I2 lb

do 50 ppm+1 lb

Apple 100 ppm

do 100 ppm

Pear 100 ppm

do 100 ppm

do 100 ppm

do 100 ppm

100 ppm

— 75 ppm+ 5(X) ppm

calcium nitrate.

— 75 ppm -I- 500 ppm

glycerol.

do 75 ppm+ 500 ppm

kerosene.

19 g/ioo

120-300 ppm

do

do

Ferbam 20 (Fermate) do1 lb/100

Pear1 lb/100

Mancozeb 21 (Dithane M-45) do .2%

Maneb ^^ (Manzate, Dithane M-22) Apple 1-2 lb/100 2 lb/100

Pear 1,000 ppm 1 lb/100 1 lb/100

Nabam 22 (Dithane D-14) do 1,000 ppm

Polyram i« do1 lb/100

Zineb ^^ (Parzate, Dithane Z-78) Apple2 lb/100 2 lb/100 2 lb/100 2 lb/100 2 lb/100 2 lb/100 2 lb/100

Pear .5-1.5 lb/100 2 lb/100 2 lb/100

do2 lb/100

2313

10

3-41432233255515

1-4

1-4222

Ziram ^^ (Zerlate)-

1-3

5-8433133

1-25

1-3

M

L-MMMMMMSSMMSSMMMMM

L-M

L-M

L-MMMM

M*

M*

M*

M*

M* L

L-MS

L-M* M

L*LLLLM

3-4

2-321

1-41222334342222

1-20031222120

2-3

1-212

0-1

0-12

2-4

2-402

N.Y. 1960

Canada 1956

N.Y. 1965

N.Y. 1962

Calif. 1958

Calif. 1954

N.Y. 1965

111. 1954

N.C. 1954

N.Y. 1954

N.Y. 1954

N.C. 1954

N.C. 1954

N.Y. 1954

Conn. 1956

Conn. 1956

Conn. 1956

Mo. 1952

Del. 1952

111. 1947

111. 1947

Denmark— 1972

N.Y. 1964

Del. 1952

N.Y. 1963

N.Y. 1965

N.Y. 1968

N.Y. 1963

N.Y. 1965

Colo. 1947-48

Del. 1952-53

111. 1954

Iowa 1953

N.Y. 1954

N.Y. 1962

Ohio 1953

Wash. 1952

Colo. 1945-48

Oreg. 1954

Colo. 1948

725

17U

10Uh

U82

2U

2W, 2U1

10U

75U

177

378

378

177

177

378

650

650

650

673, 67J^

W3

753a

753a

38J^

1111

W3

1111

10U

10Jf3

1111

10U3, 10U

9U

W2, W3

75U

863a

598

482

1032

oI

td>o>>OOm>

Cl o

cl m>

Ui

9U

52U, 526

9U

See footnotes at end of table.

O

CO
TABLE 8,—Summary of chemical compounds used to control fire blight on apple and pear—Continued

o

Chemical compound Tree Concentration^ Number of Disease Disease Research

applications severity^ control^ location Year Reference

MISCELLANEOUS

ABG-1000 24 Pear 20 ppm

Bronopol SP. 25 ^jo !_ 4%

Captan ^^ do 2 lb/100

2 lb/100

Dichlone ^^ Apple .5 lb/100

Dimethyl sulfoxide (DMSO) Pear 2%

Emmi 26 do 600 ppm

Expt. Fungicide 5379 do 1 lb/100

HPMTS 27 do .3%

Lime Apple 4 lb/100

MBR-10995 28 Pear 150-300 ppm

150-600 ppm

OM-1562 29 do 2 lb/100

Phenacridane chloride ^^ Apple 100-400 ppm

Phosphamidon ^i (Dimecron) Pear 1 qt/100

Puratized Agricultural Spray ^2 do 1 pt/100

TD-225 33 do 800 ppm

UC-17525 34 do 900 ppm

UC-19297;35 UC-20712 «e Apple 1,000 ppm

Uni G 554 Pear .4%

Vanicide 51 ^^ do 2 lb/100

Vanicide Z-65 ^s do 2 lb/100

Zinc sulfate Apple 2 lb/100

Pear 2 lb/100

4 L-M* 2

2 M 3

1 L-M 2-3

1 M* 1

4 S 1

1 M 1

1 S 3

3 L 2

2 M 3

6 L-M 0

4 L-M* 2-3

6-8 L-M 2-3

1 L* 4

7 M 2-3

1 L 0

1-2 L 0

1 L 1

1 L 4

2 M 2-3

2 M 3-4

3 L 0

2 M 2

1-2 L 1

1-2 L 0

Colo. 1973

Denmark — 1972

N.Y. 1961

N.Y. 1962

m. 1954

N.Y. 1965

Mich. 1962

Wash. 1952

Denmark — 1972

Wis. 1936

Colo. 1973

Calif. 1973

Mich. 1962

111. 1961

Mich. 1962

Colo. 1948

Mich. 1962

Mich. 1962

N.Y. 1964

Denmark — 1972

Wash. 1952

N.Y. 1954

Colo. 1948

Colo. 1948

597

380

U82

75U

lOU

316, 537

884c

384

51U

597

775

537

136

316, 537

9U

537

316, 537

480, 1111

384

378

944

944

1 Concentration: 1 oz=28 g, 1 lb=454 g, 1 pt=473 ml, 1 qt=946 ml, 1 gal-3.8 1; /A=per acre (2.5 acres-1 ha); /100=per 100 gal of water; bordeaux mixture (2-6-100)

indicates 2 lb of copper sulfate plus 6 lb of hydrated lime per 100 eral of water; all other dash siens indicate a ranee.

2L=light, M=medium, S=severe; asterisk (*) indicates artificial inoculation.

30=none, l=poor (1-35 percent), 2=moderate (36-75 percent), 3=good (76-89 percent), 4=excellent (90-100 percent).

''Copper 8-quinonnolate.

^Pounds of copper sulfate plus pounds of hydrated lime in water.

^Copper salts of fatty and rosin acids.

"^Copper oxychloride sulfate.

^Unidentified copper compound.

^Copper oléate "Cu-dol," 2.6 percent copper.

^^Copper oxides.

i^Tetracopper calcium oxychloride, 55 percent (metalHc copper, 45 percent).

i2Hydrous aluminum silicate.

i3N-[(Trichloromethyl)thio]-4-cyclohexene-l,2-dicarboximide.

>

O

O

a

a

>

td

O

O

a

CO

O

>

s

Q

a
^^2,3-Dichloro-l,4-naphthoquinone.

i^2-Heptadecyl-2-imidazoline acetate.

^ ^Manganous ethylenebis(dithiocarbamate).

i'^Zinc ethylenebis(dithiocarbamate).

i^Mixture of ethylenebis(dithiocarbamato)zinc and dithiobis(thiocarbonyl)iminoethylene)bis(dithiocarbamato)zinc.

i96-(Acetylmino)-4-methy1-1,2-dithiolo [4,3-b] pyrrol-5(4/7)-one.

20Ferric dimethyldithiocarbamate.

2iZinc ion and manganous ethylenebis(dithiocarbamate) complex. ^

22Disodium ethylenebis(dithiocarbamate). ^

23Zinc dimethyldithiocarbamate. t^

^^Unidentified experimental compound (Abbott). \jj

252-Bromo-2-nitro-l,3-propanediol. ^

26erw¿o-l,4,5,6,7,7-Hexachloro-A^-(ethylmercuri)bicyclo[2.2.1]hept-5-ene-2,3-dicarboximide. O

2'^2-Hydroxypropyl methanethiosulfonate. ^

2^New organic compound fi:*om 3M Co. I

^^Copper salt of 2-pyridinethiol 1-oxide. ^

^^9-[4-(Hexyloxy)phenyll-10-methylacridinium chloride. ^

3i2-Chloro-2(diethylcarbamoyl)-l-methylvinyl dimethyl phosphate. ^

^^Phenylmercuric triethanolammonium lactate. Q

^^Pyrimidine compound. M

^%¿/)/¿a-(Bromoacetyl)vaHnamide. ^

^W-Hydroxy-5-nitro-2-furancarboximidamide. h>

^%/p/¿a-(2-Bromoacetoxy)acetanilide.

3'^Sodium dimethyldithiocarbamate (27.6 percent) plus sodium salt of 2-benzothiazolethiol (2.4 percent). Ö

^^Zinc salt of 2-mercaptobenzothiazole (65 percent). ^

>

m

O

O

>

O

O

a

m

hj

>

C/2

O
106 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

657, 773-775) have been fairly successful in controlling blight in pear trees with copper hydroxide

(Kocide) and other forms of copper, especially in

orchards where the E. amylovora population has

become resistant to streptomycin. In general, the

copper compounds appear to give better blight control when disease severity is low to moderate, and

they produce more phytotoxicity when applied during damp or wet periods.

Copper-lime dust has also been used on the west

coast (35, 36, 39, 84,3). Under light disease conditions

these dusts have given moderate to satisfactory control, but again fruit russeting limits their use. There

is some evidence, however, that fruit russet may be

associated with unfavorable weather conditions

(370, 387).

Antibiotics

The most important development relative to fíre

blight control was the discovery and application of

antibiotics in the early 1950's. It was natural for this

development to take place, especially after the successful use of antibiotics in human diseases difficult

to control.

The earliest studies on controlling fire blight with

antibiotics were in 1951 by Murneek (673, 674) of

Missouri. Thiolutin and streptomycin were applied

once when 90 percent ofthe Jonathan apple blossoms

were open. An average of 48 infections per tree

developed with the thiolutin treatment and 94 with

streptomycin compared with 224 in the untreated

control. In the next 3 or 4 years, several other workers (32, 240, 333, 403, 1032) conducted additional

field studies that proved the efficacy ofstreptomycin

in controlling fire blight in both apple and pear orchards. In general, streptomycin gave better control

of this disease than did copper and normally caused

no fruit russet. The lack of injury probably appealed

to many growers, even though the antibiotic program cost considerably more than the alternate copper schedule.

Streptomycin came into general commercial use

during the late 1950's and early 1960's. Both streptomycin nitrate (177, 525, 650, 725) and streptomycin sulfate (35, 39, 52, 59, 82,177, 333, 336, 344, S78,

380, 387, 403, 503, 524, 526, 527, 537, 596, 597, 650,

682, 754, 755, 844,1031,1032,1043,1044,1061,1115)

were evaluated on apples and pears and both resulted in commercial control of fire blight. The sulfate form of streptomycin was generally used,

whereasthe nitrate form was only used occasionally.

In the early antibiotic research a widely used formulation contained a combination of 15 percent

streptomycin sulfate and 1.5 percent oxytetracycline (Terramycin) (34, 35, 240, 241, 333, 334, S37,

405, 498, 524, 526, 650, 922). Laboratory studies

showed £'. amylovora developed resistance far more

slowly with the combination than with streptomycin

alone (273). This combination was used commercially

for many years until the oxytetracycline was removed during the 1960's, apparently because the

combination showed no advantage when used in the

field.

One wonders whether streptomycin-resistant

strains of E. amylovora would have occurred in

California (657, 658, 949), Washington (182a), and

Oregon (189) in 1971-72 if growers had been still

using the combination of streptomycin and oxytetracyline. However, streptomycin-resistant E.

amylovora has been found in areas where little or no

streptomycin was used previously as well as in orchards where it was applied many times for several

years. Conceivably one might substitute oxytetracycline for streptomycin in orchards where only

resistant E. amylovora occurs.

On the other hand, data ^^ from Beltsville tests

indicate that such a substitution should not be made

in orchards where only streptomycin-susceptible ¿Z.

amylovora exists. These tests demonstrated that

oxytetracycline was much inferior to streptomycin

under these conditions. Surveys in 1972-74 revealed only two streptomycin-resistant isolates—

one from a Golden Delicious apple tree regularly

sprayed with streptomycin and one from an unsprayed tree. Beer and Norelli (81b) were unable to

find any streptomycin-resistant isolates of E.

amylovora in 19 orchards surveyed in western New

York.

In one orchard in Washington, Covey (182a) made

detailed isolations of resistant and susceptible E.

amylovora. In 1972, all isolates were found to be

streptomycin resistant, whereas in 1973, only half of

the isolates were resistant. Based on streptomycinresistance surveys, these mixed populations may

make spray recommendations more comphcated in

the future.

During the early research, streptomycin was restricted to bloom application for fear that toxic residues might remain on the fruit if it was applied

during the postbloom period. Because the bloom

Unpub. data, Fruit Lab., U.S. Dept. Agr., Beltsville, Md.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 107

period on the west coast, especially for pears, extends over a long time, it was possible legally to

make many applications. However, elsewhere in the

country where the bloom period usually is short,

only one to three applications were made. Obviously

during long bloom periods young fruit might be

present on trees still in bloom. However, in pome

fruit-growing areas east of the Rocky Mountains

much blight infection develops during the postbloom

period. In fact, studies at Beltsville demonstrated

that at least 50 percent of the blight infection on

pears and apples occurs during this period (503).

Streptomycin sprays extended into this period

adequately controlled postbloom infection (503^ 856,

857), The few blighted shoots found at this time in

the streptomycin-sprayed trees could easily be removed by pruning. However, pruning of unsprayed

trees took considerable time because of their many

infections (i98, 503).

At the same time, studies in California (657) and

Beltsville ^'^ demonstrated negligible streptomycin

residue on pear fruit when sprays were discontinued

30 days before harvest. Similar studies on apples at

Beltsville showed negligible residue if the sprays

were stopped 50 days before harvest (50i, 505). In

New Zealand, little or no residue was detected on

apples if the last spray was applied 5 weeks before

harvest (252). Unfortunately this schedule was not

followed in the Maryland studies, hence the 50-day

limitation. Because of these residue studies a tolerance of 0.25 ppm of streptomycin residue was permitted by the U.S. Food and Drug Administration in

July 1968 with a 30-day-to-harvest limitation on

pears. Shortly thereafter a similar tolerance was set

for apples with a 50-day-to-harvest limitation.

In general, the data in table 8 show that streptomycin applied to apples (39,177,333,336, ^03, Í80,

505, 527, 682, 754, 759, 856, 857, 1031-1033) and

pears (35, 39, 59, 378, 380, i82, 503, 52i, 526, 537,

596, 598, 658, 682, 775, 10Í3) gave moderate to excellent control of fire blight. Sometimes satisfactory

commercial control was demonstrated even under

severe disease conditions. Streptomycin appeared

to be superior to any other treatment in most tests

reported and showed little or no phytotoxicity. It

usually caused no fruit russeting, but occasionally

slight chlorotic spotting of foliage was evident. This

soon disappeared when spray applications were discontinued.

Most streptomycin applications are made during

the daylight hours. However, Powell (759) of Illinois

first demonstrated that streptomycin is more effective if applied at night. Using spray intervals of4-13

days (shorter following rainy periods), nighttime

applications gave excellent control of fire blight on

Jonathan apples compared with moderate control for

the daytime schedule. Many growers have not yet

accepted nighttime spraying, apparently because it

deviates from their usual pattern of activity. However, if farmers once shifted to such a program,

especially when better control can be obtained, they

might be more willing to accept nighttime spraying.

Better control reported by Powell (759, 761) and

Goodman (338) appears to be related to both temperature and relative humidity. Less streptomycin

is required for a given degree of control at 18° C (65°

F) or higher than at 10° (50°) or lower. Winter and

Young (103Jf) and Petersen (736) also reported better control with streptomycin when applied during

higher temperatures. There is also greater streptomycin absorption by leaf tissue at 95-100 than at

35-40 percent relative humidity. These interactions

explain the increased control obtained with nighttime applications and the recommended 100 ppm of

streptomycin during the bloom period when temperatures are cool.

Comparisons between concentrate and dilute

sprays for fire blight control have been tested since

1972. In California, Beutel et al. (82) found that

concentrate and dilute sprays of 50 and 200 gallons

per acre (75 and 300 1/ha), respectively, of streptomycin on pears gave equally satisfactory control of

bhght. The more streptomycin they used, the better

the control. In 1976, Beer (80a) showed similar results on apples in New York with streptomycin

applied in 60 versus 360 gallons per acre (90 and 540

1/ha). However, significantly poorer control was obtained when streptomycin was applied in 25 gallons

per acre (371/ha). In streptomycin tests on apples in

Missouri comparing dilute and low volume sprays at

400 and 120 gallons per acre (600 and 180 1/ha), respectively, Shaffer and Goodman (858) demonstrated that about twice as much antibiotic was absorbed by fruit buds with the low volume sprays.

These results suggested that one could reduce the

antibiotic 50 percent and obtain the same degree of

control as with dilute sprays. However, studies ^'^ at

Beltsville under severe fire blight conditions showed

better control when streptomycin was kept near the

level recommended on the label for dilute sprays.

Although streptomycin sprays were used more

extensively than dusts, California investigators
108 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

(S2-3Í, 36, U, ^^) formulated, developed, and made

limited use ofthe latter. Dusts used at 50 pounds per

acre (9 kg/ha) gave satisfactory fire blight protection, but when the dosage was reduced to 30 pounds

(5.5 kg/ha), the amount of fire blight increased

sharply {3J^). In the same tests, however, streptomycin sprays gave better control than the dust

formulations. Copper A formulated with streptomycin in dusts gave satisfactory fire blight protection in

some tests without evidence of fruit russet (36).

Early studies showed that bentonite as a dilutant or

extender for streptomycin was unsatisfactory because it failed to release the antibiotic (33). However,

substituting pyrophyllite for the bentonite worked

well (32, 33, 36). Some streptomycin dusts are still

used under certain conditions in some fruit areas.

Streptomycin has also been used experimentally

as an afterharvest fruit dip to eliminate surfaceborne E. amylovora. These dip treatments were

developed by Dueck (238) of Canada as a precaution

against possible spread of the pathogen on fruit surfaces to fire blight-free areas. Of those compounds

tested, streptomycin sulfate at 250 ppm and acetic

acid at LO M were most effective. Several combinations of these compounds killed all bacterial cells on

the surface. When apples were immersed in a water

suspension of bacterial ooze (8.2 x 10^ cells per milliliter), a 10-minute dip in 1.0 M acetic acid was

completely effective. In other dip tests with Bartlett

pear fruit, Wilson (1028) demonstrated enhanced

ripening but no control of E. amylovora with an

isopropanol treatment. In Washington, Wright

(10Jf2) reported definite infection of healthy fruit

through contact with diseased fruit but failed to do

so when bacterial inoculum was added to a hot water

dip (32° C, 90° F).

Most other antibiotics evaluated for control of fire

blight in the field were less effective than streptomycin (^7, 39, U3, WS, 75If, 82Í). However, Billing

et al. (87) in laboratory studies found various cultures OÎE. amylovora sensitive to chloramphenicol,

streptomycin, and Terramycin but resistant to

penicillin. In vitro tests conducted by Morgan and

Goodman (663) and Morgan (1119) showed Aureomycin and neomycin equal to streptomycin, but

polymixin, streptothricin, and viomycin were

slightly better, whereas Chloromycetin was definitely less effective by the agar diffusion technique.

Laboratory studies by Martinec and Kocur (630)

showed also that 49 strains oîE. amylovora were

sensitive to tetracycline, erythromycin, and neomycin but resistant to tyrothricin, nystatin, and bacitracin and variable with chlortetracycline. Colasito

et al. (180) also noted that circulin in vitro was more

effective than streptomycin. Both laboratory and

field tests by Klos (537a) indicated that LMA-B-100

(spectinomycin) significantly controlled fire blight.

However, in later studies by investigators in Michigan as well as several other States, it was found to be

too phytotoxic for practical use.

Several attempts to improve the effectiveness of

streptomycin by adding adjuvants to the antibiotic

spray have had limited success (10, 33U, 3^6, 365,

399, W2, 595, 598, 725). Fungicides and insecticides

combined with streptomycin showed variable results (17J^, 85i, lOUy 1111 y lUO). Cing-Mars and

Crete (17JÍ) reported some success when using

glyodin and captan in combination with streptomycin on apples, but dichlone had no effect. Likewise,

studies by Shaffer and Goodman (857) suggested

that glyodin and sulfur aid control when added to

streptomycin sprays on apples. They also found that

sulfur, captan, Polybor, zineb, dodine, and endrin

inhibited absorption of streptomycin. Carbaryl,

DDT, lead arsenate, and parathion increased absorption of streptomycin, whereas dieldrin and

phenyl mercuric acetate did not alter uptake of the

antibiotic by apple leaves (851^). In greenhouse

trials, Hamilton and Szkolnik (380) found that captan

reduced the effectiveness of streptomycin when exposed to simulated rain.

In laboratory experiments. Zehr (lOJ^S) found that

maneb, Polyram, and glyodin supplemented the activity of streptomycin when they were combined in

vitro, but dodine and captan had little or no effect.

Combining fungicides in hope that they would enhance the bacterial action of streptomycin was not

very successful with Polyram, maneb, and dimethyl

sulfoxide in greenhouse and field tests (lOJ^i, lliO).

Other investigators indicated variable results with

certain miscellaneous compounds (80a, 379, 496,

568, 922). Additional laboratory and greenhouse

studies indicated that adding glycerin to streptomycin sprays tended to increase the effectiveness ofthe

antibiotic (365, i82, 725). However, field studies

failed to demonstrate any significant increase with

this combination compared with streptomycin alone.

The addition of superior '70' oil to streptomycin lessened fire blight control in pear in New York State

(Í82). In vitro studies by Keitt et al. (517) showed

high toxicity of mercury compounds of E. amylovora, but sulfur was ineffective.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 109

Carbamates

The carbamate compounds were used commercially for several years on vegetables and fruit to

control various fungus diseases before anyone field

tested them for fire blight on pome fruit. In Delaware, Heuberger and Poulos ií03) and Heuberger et

al. {W2) conducted much of the early field testing of

carbamates to control vegetable fungus diseases and

were the firstto evaluate these compounds on apples

for control of fire blight. About the same time,

Sprague ^^ in Washington also tested them on pears.

These investigators found that zineb gave moderate

control of fire blight. However, blight severity in

these studies was light to moderate, which probably

accounted for the favorable results.

In other tests, usually under more severe fire

blight conditions, carbamates failed to give satisfactory control of fire blight ^80, J^82, 52JÍ, 525, 598,

75Jf, 10^3, ion). On some occasions, however, control on apples or pears with carbamates appeared to

be acceptable (380,38Í, 9Uy lOU)- In general, under

severe blight conditions the carbamates appear to be

far less effective than the antibiotic streptomycin.

Miscellaneous Compounds

In the early 1930's, Thomas (OiO) determined in

the laboratory by phenol coefficients the bactericidal

efficiency of 14 disinfectants with respect to certain

plant pathogens. Only mercuric chloride, ethyl mercury chloride, and all mercurial derivatives exhibited considerable activity against i/. amylovora.

In Denmark, Johansen ÍÍ78) reported in 1969 that he

had evaluated several commercial fungicides against

E. amylovora in the laboratory. Maneb, mancozeb

mercury, and copper were bactericidal, whereas

thiram was bacteriostatic. In vitro studies by Jones

(1111) also indicated that nabam, maneb, and UC19297 showed some promise for controlling fire

blight.

Many miscellaneous compounds have been field

tested for fire blight control, as shown in table 8.

Most of them gave little or no control of the disease,

and only a few appeared promising (136, 316, 378,

380, 403, 482, 5U, 537, 597, 754, 775, 944, 1044,

1111). However, one new organic compound

(MBR-10995) showed promise as a potential commercial bactéricide for controlling fire blight (597,

775). In hmited tests in several geographic areas,

control with MBR-10995 appeared to be at least

equal to control with streptomycin. At Beltsville, ^^

600 ppm of MBR-10995 were required to give control

on apples or pears equal to control with 100 ppm of

streptomycin. These results indicate that this compound should be considered in future evaluation

trials.

An interesting approach for control of twig blight

resulted from preliminary studies by Klos (537a) and

Klos and Ritchie (538), who demonstrated that retarding tree growth with a chemical such as Alar

significantly reduced the amount of blight in pear

twigs during the summer.

In Belgium, Veldeman (976h) suggested indirect

control of fire blight in hawthorn through chemical

destruction of the flowers. He obtained good results

in reducing blossoms and fruit set, with moderate to

severe leaf damage, by spraying hawthorns with

aqueous solutions of sodium hydroxide.

Most chemical and biological control treatments

are usually applied as sprays or dusts. Recently a

trunk injection technique has come into vogue.

Holes are drilled into the tree trunk and concentrate

or dilute materials are siphoned by either gravity

feed or under pressure into the tree. Apple trees

injected with thymol showed some resistance to the

progress of E. amylovora (841). Tests ^^ at

Beltsville indicated that MBR-10995 injected into

the trunks of fire blight-infected pear trees prolonged their life. Although these results were preliminary, they introduced a new approach to controlling diseases in trees.

Apparently additional screening of chemical compounds, including growth regulators, in laboratory,

greenhouse, and field tests, as well as improved

application techniques, should be conducted by industry. State, and Federal investigators to find a

more effective, less expensive, but longer lasting

control for fire blight.

Monitoring

Like fungicides, bactéricides are frequently

applied before any part of the plant becomes visibly

infected. For fire blight, such a protective schedule

sometimes includes from three to eight applications

depending on the part ofthe country where the trees

are grown. In California, many growers apply from

10 to as many as 18 applications (59, 657, 844)- Ifthe

pathogen can be reliably monitored, the number of

1^ Pers. commun., Tree Fruit Res. Cent., Wenatchee, Wash. Unpub. data. Fruit Lab., U.S. Dept. Agr., Beltsville, Md.
no AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

applications can be reduced. In general, protective

applications are restricted, because streptomycin is

ineffective if applied after infection occurs {1^05, 503,

650), With such a program, there is always the possibility that serious fire blight would not have developed anyway, and thus much time, effort, and

money would have been wasted.

Miller et al. (655) and Thomson et al. {950, 951) in

California have conducted research on a system of

monitoring pear flowers for the presence of E.

amylovora. Their studies showed that, although

over 50 percent of the healthy flowers in some orchards were infested with about 10^ cells of E.

amylovora per flower (fig. 17, C-E), subsequent

disease incidence was only one to three infections

per tree. Furthermore, these studies showed that

the susceptibility of pear flowers to disease varied

from year to year, during the season, and among

flowers of a cluster. Because so many apparently

healthy flowers and other plant parts harbor potentially dangerous E. amylovora (fig. 17), some way

must be found to kill or inactivate them so that fire

blight will not flair up when conditions are optimum

for disease development.

Sutton and Jones (915a) in Michigan used a similar

isolation method to detect i?. amylovora in infected

and apparently healthy apple tissues. E. amylovora

was not detected in samples from 12 apple orchards

before infections were prevalent in the orchards nor

in the severely blighted orchards until numerous

additional infections were apparent. More recently

Beer and Opgenorth (8Id) in New York isolated E.

amylovora from the surface of some nonoozing fire

blight cankers in one pear and four apple orchards,

sometimes before blight symptoms appeared. However, the pathogen was not usually recovered from

canker surfaces before bloom and not from blossoms

in the early stages of bloom. These investigators

pointed out that E. amylovora was not detected in

blossoms in sufficient time to prevent infection by

the immediate application of bactéricides.

When a monitoring system is perfected for both

pear- and apple-growing areas, we may expect more

effective spacing of sprays, which will result in considerable savings for the grower. In California,

Thomson ^^ estimated a savings of one-half to

three-quarter milUon dollars during the 1975 growing season after growers used fewer spray appUcations based on the monitoring system.

Pers. commun., Dept. Plant Path., Univ. Calif., Berkeley.

Insect Control

Aphids, flies, ants, tarnish plant bugs, and

leafhoppers have been implicated in the spread of

fire blight (iO, U5, 7U, 726, 901, 902, 905, 906),

Flies and ants are attracted to bacterial ooze, where

their bodies may become contaminated. They then

carry the fire blight bacteria to uninfected parts of

the plant, where new infections may start if conditions are optimum. These insects should be kept

under control with insecticide applications.

Bees rarely if ever feed on bacterial ooze and are

not the principal agents involved in producing the

first spring blight as once thought (chap. 8). However, bees after visiting blighted blossoms can

spread fire blight to uninfected blossoms (857, 7UU,

750, 806-808, 933), Therefore keeping beehives out

of an orchard during the pollination period is unnecessary, but blight should be controlled during

bloom.

During the remainder of the growing season certain piercing and sucking insects often inhabit some

legumes and other cover crops in or near the orchard. When such crops are harvested or plowed

under, these insects may migrate to the fruit trees.

Therefore insecticides are recommended in some

areas as additives to regular fire blight spray control

programs (188a).

Biological Control

Some investigators have attempted to control fire

blight with biological means. Thomas and Ark (98U)

were among the first to test several isolates, mostly

bacteria, in petri dish cultures for antagonism towardE'. amylovora. Some ofthese isolates inhibited

growth ofthis organism. Parker (722) also conducted

studies during 1928-31, whereby he introduced into

the blossoms the organisms that were antagonistic

to the fire blight organism. The antagonists were

mixed with E. amylovora and introduced into the

blossoms together or sometimes separately. These

studies indicated a tendency for some of the antagonists to reduce the percentage of fire blight infection. Ark and Hunt (88) continued to search during 1936-41 for other antagonists and found two

isolates from soil, which in vitro tests were active

against E. amylovora, as well as certain other

phytopathogenic micro-organisms. In the early

1950^s, Stessel et al. (897) screened about 70,000

colonies of soil micro-organisms in petri dish studies

for activity against various plant pathogens. Three
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 111

of these organisms exhibited considerable activity

against i". amylovora.

In the mid-1960's, Goodman (SJ^O) reported in vitro

and in vivo interactions between components of

mixed bacterial cultures isolated from apple buds.

Single-cell isolates were derived from a bud isolate

35A—a virulent white (35A-W) organism sensitive

to five E. amylovora bacteriophages and an avirulent yellow (35A-Y) form sensitive to two phages.

When mixed together, the yellow organism produced sufficient acid to inhibit growth ofthe virulent

white organism and thus prevent it from causing

infection. Also, about this time Klos {537a) showed

that spraying Bartlett pear blossoms with an unidentified yellow organism 24 hours before inoculation significantly reduced blight infection.

Higgle and Klos (790, 791) and Klos and Ritchie

(538), working with isolates ofE. herbicola from fire

blight cankers, also demonstrated buildup of acid

inhibitory to E. amylovora. On the other hand,

Chatterjee et al. (16i) suggested thatE. herbicola, a

yellow organism often associated with Jî. amylovora

in fire blight-diseased tissues, possesses a betaglucosidase, which breaks down arbutin to hydroquinone and D-glucose. Hydroquinone accumulated in cultures of E. herbicola grown in arbutin

broth to ca. 1,000/ig per milliliter in 24 hours but to a

much less extent in E. amylovora cultures. They

suggested that the accumulation of high levels of

hydroquinone by E. herbicola might contribute to

the fire blight resistance of the plant by inhibiting

the metabolic activity ofE. amylovora and reduce

its effectiveness as a phytopathogen.

In tests in Great Britain with pear slices and hawthorn shoots, one isolate ofE. herbicola often prevented infection when inoculated before E. amylovora, but there was little or no protection when

they were applied together (25J^).

In Missouri, Goodman (3J^1) later showed that

Pseudomonas tabaci, as well as a yellow

Erwinia-like isolate (35A) and avirulent isolates

of E. amylovora, protected against infection from

a virulent strain of E. amylovora. However,

Xanthomonas pruni was unable to induce the

protective effect. In California, Reil et al. (775)

reported in 1974 that an application of bacteria

antagonistic to E. amylovora gave some control

of fire blight but not as effectively as a chemical

treatment.

Although these biological control studies indicate

some potentially useful control of the fire blight disease through antagonistic organisms, none of these

control measures are sufficiently effective to replace

chemical treatments. If some way can be found to

enhance the activity of these organisms, they may

be useful in the control program.
CHAPTER 13

NATURE OF RESISTANCE

Since the middle of the 19th century, differences

in degree of fire blight resistance have been known

to exist between cultivars of pear and apple. More

recently such differentiation has been shown between species ofPyrus and Malus as well as among

cultivars, species, and clonal selections of quince,

hawthorn, pyracantha, and other genera of the family Rosaceae. However, blight resistance in plant

material as discussed here is not necessarily innate.

Depth of blight penetration in plants is strongly

affected by (1) the age, vigor, and nutrition of the

host; (2) environmental factors, particularly temperature and humidity; (3) soil types, moisture content, and cultural practices; and (4) a combination of

one or all of these factors with the time of bloom.

Therefore the most realistic measure of the degree

of blight resistance for any cultivar, seedling, or

clonal selection can best be determined when the

plant material is grown and tested under optimum

conditions for blight development.

Plant material may be evaluated in many different

ways and with various techniques for fire blight

resistance. An attempt to grow uniform apple and

pear seedlings for experimental use was reported at

the Phytobacteriology Workshop in Missouri {Siia),

Because of the great diversity of techniques and

results obtained by fire blight investigators, the

procedures should be standardized in order to obtain

uniform results.

To express the degree of fire blight resistance in

pear and apple, many investigators have developed

various blight rating systems. Following inoculation

of succulent shoots of young pear material,

Thompson et al. (9^8) estabhshed four blight-resistance classes based on the extent of infections. Lamb

{550) measured the length of shoot killed and used

values in excess of 100 percent to indicate infection in

older wood. For large orchard trees, Mowry {667)

developed a blight rating index using a factor obtained by multiplying the number of infected twigs

times 5 and the age of infected wood times 20.

To score large numbers of clonal and seedHng

trees in the pear breeding program at Beltsville, a

simple, efficient, and dependable rating system was

112

developed to estimate the severity of damage {1071).

This system is based on (1) the number of branches

infected, (2) the age ofthe wood into which the blight

organism has penetrated, and (3) the overall percentage of tree blighted. The scale is a descending

rating from 10 to 1 and is a visual estimate of tree

damage. A score of 10 indicates a tree with complete

absence of blight symptoms, whereas a score of 1

means a tree is entirely dead from fire blight. Figure

25 is a schematic diagram of a tree showing various

degrees of blight and the location of blight for each

percent score. This scoring system can also be used

to determine the degree of blight resistance in apples.

.^

^

^

<t

yf.<^

89-99%

FIGURE 25.—Schematic diagram of USDA

system used to evaluate degree of blight

and pear trees.

fire blight scoring

resistance in apple
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 113

From these 10 blight scores, 4 classes of blight

resistance are arranged as follows:

Resistance class Score Percent blighted

Highly resistant 10-8 0-6

Moderately resistant 7-6 7-25

Susceptible 5 26-50

Very susceptible 4-1 51-100

To simplify ratings of large numbers of cultivars,

score 5 is combined with scores 4-1 into one susceptible class.

The tables in this chapter contain the names of

most Pyrus species, as well as those of the best

known pear and apple cultivars. The cultivars are

divided into those named prior to 1920 and those

since then (118,119), The listing of degrees of blight

resistance is based mainly on records of natural

blight as given in the literature, although some

ratings were assigned arbitrarily. Also, different

sports of a cultivar may have lower or higher blight

ratingsthan the original cultivar. For specific details

on any one species, cultivar, or clonal selection, refer

to the pertinent reference cited in the table.

Pear

The genus Pyrus has been classified under the

subfamily Pomoideae and contains approximately 20

true species, all indigenous to Europe, Asia, and

Africa {771, 1015). Challice {1092) and Challice and

Westwood {155) made a detailed taxonomic study of

the genus. Based on a total of 51 chemical and botanical characters, they classified the species into the

following main groups: (1) East Asian pea pears, (2)

larger fruited East Asian pears, (3) North African

pears, and (4) European and West Asian pears.

In table 9 are listed the 21 true and 13 miscellaneous species, the latter classified as natural or arboretum hybrids. The following descriptions and

records of blight resistance are for the five most

economically important Pyrus species.

Pyrus communis

Practically all our cultivated varieties of pears

have been derived from P. communis L., the native

pear of Europe. It is also known as the common,

domestic, or cultivated pear. The finest cultivars of

this species are superior in quality to those of any

other species. Although these cultivars vary considerably in their degree of blight resistance, none of

the high-quality European pears are known to be

sufficiently resistant to thrive in regions where fire

blight is serious.

The losses by fire blight sustained by cultivars of

P. communis have undoubtedly been greater than

those by any other disease. In addition, fire blight

has unquestionably been responsible for greater losses in American pears than all other pear diseases

combined. Following 2 years of epiphytotic blight

conditions at Beltsville, nearly 90 percent of the

trees in a worldwide collection of more than 500 pear

cultivars were destroyed {707), Most ofthe cultivars

in the resistant blight classes were of oriental origin.

Such cultivars as Kieffer, LeConte, and Garber,

hybrids of P. comm^unis cultivars with oriental

pears, were long believed to be ratherresistant {528,

716,1011), but even these have often been blighted

severely {707, 908, 909, 1037, 1070).

As with most other Pyrus species, many clonal

selections and cultivars of P. communis are very

susceptible to fire blight, whereas others are highly

resistant. Cultivars such as Conference, DeVoe,

Forelle, and Laxton's Superb are very susceptible,

Whereas Orient, Richard Peters, and Waite are

highly resistant. In between these two extreme

classes are two general groups of pears—cultivars

that are rather consistently resistant to moderately

resistant, such as Ayres, Mac, and Moonglow, and

cultivars that show variable resistance, indicating

that some trees are highly resistant and others moderately resistant to very susceptible {1070). Cultivars in this last group include Comice, Dawn,

Duchess, Kieffer, and Maxine (Starking Delicious).

Many other pear cultivars, such as Bartlett, Beurre

Bosc, Beurre d'Anjou, and Clapp Favorite, are generally susceptible to fire blight. However, they are

usually not as susceptible as those in the very susceptible group.

In table 10 are listed 400 cultivars of P. communis. Some of them, such as Kieffer, Old Home,

and Carrick, may contain a fair degree of hybridization with other species. These cultivars bearthe best

known names used during the 1900's for which blight

records are given in the literature. Of the 287 cultivars named prior to 1920, about 40 percent are

classified as susceptible and 40 percent are only

moderately resistant. They contain almost all the

most familiar pear names. Eleven percent of the

cultivars are resistant to highly resistant, including

Beurre Fouqueray, Old Home, Orel, Professeur Molón, and Sudduth, and about 9 percent are variably resistant, such as Douglas, Duchess d'Angouleme, Garber, LeConte, Lincoln, and Winter

Nelis.
TABLE 9.—Average degree offire blight resistance in Pyrus species based on literature cited

Scientific name

Common name or

type of hybrid

Native

location

Degree of

resistance ^

Shoot Trunk

Type of

infection ^

Reference

TRUE, NATIVE PYRUS SPECIES

amygdalifonnis Vill. (syn. parviflora Desf.)

betulaefolia Bunge

calleryana Decne.-

Almond pear Asia (west), Europe (south)-

Birchleaf pear China (north, central)

Callery pear China (south, central)-

caiicasia Fed. (syn. pyraster)

communis L. Domestic pear —

cordata Desv.

dimorphophylla Makino -

elaeagrifolia Pall.

faurüi Schneid.

gharbiana Trab.

hondoensis Nak and Kik.

koehnei Schneid.

longipes Coss. and Dur. ~

mamorensis Trab.

nivalis Jacq.

Japanese pear

Evergreen pear--

Snow (perry) pear

U.S.S.R. (Caucasus), Europe, Asia (west)

Europe, Asia (southwest)

France, SpainJapan -

Asia (west), Europe (southeast)-

Korea

Morocco

Japan

Taiwan (south)

Africa (north)

Morocco

hia Bush.-Ham. ex D. Don. Pashi pearpyrifolia (Burm.) f. Nak. (syn. serótina) Sand pearEurope (south)India, Pakistan

China, Korea, Japan -

regeln Rehd. {heterophylla Reg. and Schm.)

salicifolia Pall.

syriaca Boiss.

ussuriensis Maxim, (syn. sinensis)

Willowleaf pear—

Afghanistan, TurkestanIran

Asia (west)

Ussurian pear China (north), Manchuria, Korea, Siberia

M S A UO, 780

O S — N 381, 953, 960, 1072

R — 599 H-1 O

R-S S A UO, 780 a

— M A 780

R — N 231, JÍ79, 599, 667, 779, 953, 959 a

S — N 22, 667

>

S-R — A, N UO, 576, 1069, 1072

155

S-R S A, N UO, 780, 948, 1037, 1072

S-R — A 565 Ö

td — S A 155, 780 o

M-S s A

155

UO, 780, 1072

o

R-M M A UO, 780 h-1

o

R-M S A, N 780, 1069, 1072 a

S R-S A UO, 780 m

S S A, N 780, 1072

— s A 780 a

S s A, N 780, 1072 ;i s M-S A UO, 780

R-S

R

M-S A, N

N

565, 780, 948, 1069, 1072

479, 953, 959, 1037

o

S — > N 599

___ S A 780 ^

M-S S A, N UO, 780, 1072 O

S — A UO a

R R A 780, 948 5

R ___ N 22, 140, 381, 479, 550, 556,

667, 779, 1035, 1072

M — N 953

S — N 599

S-R — A, N 565, 1069

MISCELLANEOUS PYRUS SPECIES

AND HYBRIDS

balansae Decne. Asia (west)- M S 780
bretschneideri Rehd.-

canescens Spach. -

cotinifolia Hort. —

fascicularis Hort. -

glabra Boiss.-

Natural hybrid — China (north) -

malifolia Spach.

michauxii Bosc. ex Poir.-

ovoidea Rehd.

pérsica Pers.

phaeocarpa Rehd.-

serrulata Rehd.—-

sinaica Thouin. —-

dU"

Arboretum

hybrid.

do--

Apple-leafed pear

Natural hybrid —

Arboretum

hybrid.

Natural hybrid -

Asia (west)-

China (north) -

R

R-S

S

M

S

R

China (north) —

-do China (central) -

..do

M-S

S

S

S

M

S

M-S

S

R

S

S

M

A 780

A 550

A, N 780, 1072

A 780

A, N

A

A

A, N

A, N

A

A

A, N

A, N

780, 1072

780

780

780, 1072

381, 780

780

780

780, 1072

1072

iR=resistant or 0-6 percent, M-moderately resistant or 7-25 percent, and S=susceptible or 26-100 percent of tree blighted.

2A=artificial inoculation, N=natural infection.

2

td

o

I

>

td

>

Q

>

>

ui

O

o

m

>

Cl

o

>

en
116 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS ORIGINATED BEFORE 1920

Abbe Fetel

Alamo

Amelia Baltet

Ames

Amoot

Andres Desportes

Ansault

Baldwin

Bariker

Baronne Leroy

Barrillet Deschamp

Barseck

Bartlett (Williams;

Williams' Bon Chretien).

305

389

317, Jf39, 716 610, 706, 707, 909

707

389

305

Bayerische Winterbirne -

Belle de Beugny

Belle de Ferone

Belle Lucrative

Belle Poitevine

Belmont

Bergamote dAutomne—

Bergamote d'Eté

Bergamote Sargeret

Besi d'Hery

707, 1067

395

389

707

Beurre Arenberg-—

Beurre Auguste

Beurre Baltet Père -

Beurre Bosc 779

389

389

305

Beurre Chaudy

Beurre Clairgeau

Beurre dAlexander Lucas -

Beurre d'Amanlis

Beurre dAngleterre

Beurre d'Anjou

S89

20

389

S89

389

172, 305, 731 160, 27J^, 389, 395,

h39, 528, 556,

706, 726, 769,

1011, 1108

707

1037

305, 707

395

550, 667

1067

389, 707

9JÍ8

21, lOJ^, 160, 27U,

317, 389, 395,

WO, Jf86, 528,

550, 587, 635,

667, 706, 707,

726, 769, 872,

909, 923, 9J^8,

1011, 1012, 1037,

1066, 1070, 1136

389

389

389

1037

1067

706, 707

707

Jf39

20, 10J^, 27U, 317,

389, 395, U39,

528, 587, 660,

667, 706, 707,

726, 769, 1011,

1070, 1108

305, 389, h86, 550,

707

lOJf, 707

707

298, 317, 550, 587,

667, 707, 908,

1037

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 117

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Beurre de Bollwiller

Beurre de Jonghe

Beurre de Nantes

Beurre de Saint NicolasBeurre Diel

Beurre Dumont

Beurre Flon

Beurre Fouqueray

Beurre Giffard

Beurre Gris

Beurre Gris d'Hiver

Beurre Hardy

389

707

707

389

389

305, 389

706

1067

389, 1067

305, 389

305, (2)

389

389

389

389

389

389

305

Beurre Millet

Beurre Phillipe Delfosse —

Beurre Superfm 20, 305, 395

Bezi de la Motte 556

Blanquet Précoce

Blanquet Superfin

Bloodgood 395

Bon Chretien Bonnamour— 389

Bon Chretien de Vermont- 389

Bonn d'Ezee 389

Bordeaux 305

Buffum 395 20, 389

Burkett 389, 776, 780

Butirra Capiaumont 707, 1067

Calebasse Madam Charles

Fürst.

Champion

Charles Cognée

Charles Escaig

Chamin

Chasseurs

China

1037

706

389

707, 1067

Citron de CarmesClapp Favorite—

Clarksville 305

Colonel Marchand 389

Colonel Wilder 305, 389

Columbia 305, 389

Comte de Lambertye 389

See footnotes at end of table.

707

707

395, 707, 909, 1037

1^39, 667, 707

68, 707

305, 389, 707

305

389, 395, 667, 706,

707, 1070

707

550

Jf39

707

706, 707

389

20, 21, 75, 160, 172,

27U, 305, 317,

889, 395, JfOO,

^39, J!f86, 528,

550, 587, 667,

706, 707, 726,

769, 923, 1011,

1067, 1070
118 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Comte de Paris 389

Comte Lelieur 389

Conference 192,389, (2)

Conkleton —

Conner's Japan 305

Conseiller a la Cour 389

Dana Hovey 439

Dearborn 395

Délices dAvril 389

Délices d'Hardenpont 389

dAnger.

Directeur Hardy 389

Dixie 389

Docteur Desportes 389

Docteur Jules Guyot 192, 389

Dorset 305

Douglas 22, 389, 395, 550, 317, 776, 1067

707, 90Ji,

Doyenne Boussock 395

Doyenne dAlencon 305

Doyenne d'Eté 305

Doyenne de la Grifferaye — 389

Doyenne de Saumur 389

Doyenne du Comice 706, 1070 75, 389, 528, 769,

1011

Doyenne Georges Boucher -

Doyenne Goubalt

Doyenne Gris 389, 707, 1067

Doyenne Madam 389

Levavasseur.

Duchesse 20, 21, 528, 769,

1011

Duchesse dAngouleme 172, 707, 1067, 389, 706

1070

Duchesse dAngouleme 118, 707 389

Bronzçe.

Duchesse de Berry 389

d'Eté.

Duchesse de Bordeaux 389, 706

Duchesse de Brissac 389

Duchesse de Mouchy

Duchesse d'Orléans

Early Green Sugar 389

Early Harvest 389

Early Sugar- 305

See footnotes at end of table.

lOJ^, 667, 706, 707

68, 305, 389, 395,

667, 706, 707

305

389

298, 528, 667

20, 389, 707

707

707

305, 395, 587, 660,

667, 707, 1067

Jf39, 707

389, 707

160, 305, 486, 587

667

439, 707

707

707

389

707

707
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 119

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Easter Beurre 305

Elizabeth 389

Emile d'Heyst 305

Estella 389, 605 317, 780

Eureka 389 550

Eva Baltet 389

Fame 305, 660

Farmingdale 2U, 550, 626 389, ^35, 780

Favorite

Figue d'Alencon 389, 707, 1067

Flemish Beauty 159, 305, 556

Flemish Pear 556

Florida Sandpear 776

Fluke 776

Fondante de Moulins- 389

Lille.

Fondante de Noel

Fondante des Bois 389

Fondante de Thiriott

Forelle

Fox 305, 389

Fred Beandry 305

Frederick Clapp — 305

Garber 228, 528, 716, 1011 21, 305, 587,

923

General Galliene 389

German Sugar 317, 780

Glou Morceau

Good Christian 389

Grégoire Bordillon 389

Greisa No. 1 389

Grosse Louise 389

Grune Jagdbime

Hawaii

Henri Desportes 389

Hessle 707 1067

Hofrath's Birne 389

Hood ^39

Hourdequin 389

769,

20, 389, 395

20, 395, 667, 707

298

667

667

667

22

20, 16C), 298, 317,

395, ^39, U86,

528, 550, 587,

610, 611, 667,

706, 707, 769,

908, 909, 923,

1011 , 1108

21, 621

707

19Jf, 707

305, 389, 707, 1067,

1070

389

WO, 667, 707, 80Jf,

908, 909, 1037

298

305, Jf39, 707

707

9, 707

See footnotes at end of table.
120 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

HowellCULTIVARS ORIGINATED BEFORE 1920—continued

20, 160, 556, 706

IdahoJapan Golden RussetJargonelle

Jeanne d'Arc

Josephine de MalinesKieffer

Koonce -

Krull—-

Krylov--

Kuroi—-

305

305, 317, 395, J^3

528, 550, 611,

703, 706, 716,

1011, 1108

381, 707, 1067

381, 707, 1067

305, 389

68, 75, 706

20, 21, 160, 317,

389, 587, 726,

769, 922, 932,

9J^8,1012,1067,

1070, 1136

21

776

707, 1067

1037

Lady Clapp

Lawrence

Lawson 20, 305, 395

Laxton's Wonderful

LeBrun

LeConte 528, 703, 1011

305, 389

389

389, 587, 769

389

21, 317, 389, 587,

923

LeLectier

Lemon 2U, 389, 780

Leon Leclerq

Lieutenant Poidevin

Lincoln 67, 305, 395, Jf3

582, 706, 80J^

Longue Verte

Longworth 389, J^35, 707, 780 667, 1067

Louise Bonne de Jersey 75, 305, 395, (2)

(Bonne Louise

d' Avranche).

Louis Pasteur 389

Louis Vilmorin 389

Lucretia

Lucy Duke 776 395

Lyerlie 305

Madame Andre Leroy

Madame Caroline d'AirólesMadame Ernest Baltet

Madame Favre

Madame Hutin

Madame Lye Baltet —

Madame Treyve

Madeline

Magnolia

389

389

706

389

389

389

298,305,317,389,

611, 707, 769,

908, 909, 1011

20, 172, 395, WO,

528, 587

1037

707, (2)

707

20, 707

230, 27Jlf, WO, 707,

80Jf, 908, 909,

1037

667, 908, 909,1037

389

20, 550, 667, 909

667

J^39, 707

389

20, 21, 305, 395,

JfOO, 909, 1037

707

707

707

550, 667, 707, 909,

1037

389

20, 635, 707

550, 707

707

305

389, 550, 707

707

Jf39, 707

707

305, 395

305

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 121

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

^ ,,. Resistant . ^ , Susceptible Cultivar ,,- „- resistant /r ^x (10-8) ^^_g^ (5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Margaret 895

Marie Louise 389, 395 68, 707, (2)

Marguerite Marillat 389 20, 707

Messire Jean 389

Miller 550

Monchallard 389, 707

New Zealand Winter 706, 707

Bartlett.

No Blight 93

Notaire Lepin 389, 707

Old Home 67, 96, 213, 2U, 317, A35, 550, 706, 667

871, 889, Jf89, 707, 1067

626, 779, 780,

9^8, 1136

Olivier de Serres 707

Orner Fache 389

Onondaga 805 895

Orel 889, 776

Osband 20

Ozark 305

Para de Rosetta 1037

Passe Colman 895 707

Passe Crassane 889, 707

Petit Blanquet 889 707

Phillipe Chaveau ^39

Pierre Corneille 707

Pineapple 889,792-79^,908, 230

909, 1037

Pitmaston 889

Poete Beranger 389

Pound 805 707

Précoce de Trévoux 889 75, 685, 707

Prenices de Maria 389

Lesueur.

President Drouard 395, 707

Prince Marianne 104^

Professeur Grosdemage 389 707

Professeur Molón 707, 1067

Ramsey 909

Reeder 805, 895 389

Reliance 389, 707

Riehls Best 395 305

Robert de Neufville 489

Roi Charles de 660 805, 889 707

Wurtemberg (RCW).

Rosee de Juillet 889 707

Rosseny 805

Rousselet de Reims 389

Royale Vendee 889

Rutter 395 20, 805

See footnotes at end of table.
122 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Saint Andre—

Saint Ghislain -

Saint Giles

389

Seckel 172, 305, 395, 611,

706, 1108

Selenga

Sénateur BellSerrurier

Shaparh 1037

Sheldon 706

Sir Harry Veitch

Sladky 707, 1067

Smyth

Snyder

Sodak 609

Southern Queen 909

Souvenir d'Emile Coue -

Souvenir du Congress -

Spalding

Stark Tyson

Stout

96

Summer Doyenne -

Superb

Surprise- 2U, 389, 776, 780

Suzette de Bavey -

Taronta

Tedrow Beauty -

Thompson

Thomley

Tolstoy -

706

63

Triomphe de VienneTriumph

Tyson 395, 706

Urbaniste

Uvedale St. Germain

Variolosa 2U, 626, 780

Vermont Beauty 305

Verte Longue d'Automne—

21, 68, 160, 27Jf,

317, 389, 528,

550, 556, 587,

726, 769, 923,

9^8, 1011

389

389

305, 550

389

305, 389

389

389

Stuttgarter Geisshirtel

Succès de la Meilleraye

Sucre Vert 389

Sudduth 67, 96, 395, 707,

1067

Sugar Top 707 1067

395

389

389

305

20, 305, 389, 528,

587, 707, 1037

21, 9A8, 1037

389

389, 707

^39, 707

298,^00,667,707,

908, 909, 1037

20,389,J^39,528,

587, 667, 707,

909

Jf39, 707

^39, 707

305, 307

909

707

707

707

20

707

389, 707

1037

707

707

707

389, 707

667, 908, 909

395

389

389, 660

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 123

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

^ , . Resistant . , / Susceptible Cultivar .-^ Q. resistant ._ .

(lU—ö) ._ ^. vo—i;

CULTIVARS ORIGINATED BEFORE 1920—continued

Vicar of Winkfield 305 20, 389, 395

Vice President Aubert 707 1067

Walker 317

Wallis Kieffer 550

Warner 556 389

White Doyenne 305, 389 20, 395

White Star 389

Wilder 395, 716 909, 1037

Wilder Early 395 389, 707 20, 1067

Winter Bartlett 389 660

Winter Cole 706 707, 1067, (2)

Winter Nelis 305, 395, 599 68, 389, 528, 587, 172, 298, 660, 667,

703, 706, 769, 707, (2)

1011

Worden Seckel 706 305, 389, 395 667, 707

Yermak 383

Zoete Brederode 707

Zuckerbime 389

CULTIVARS INTRODUCED SINCE 1920

Aurora 706 118, 1009, 1070

Ayres 118, 233, 667, 706 389, 610, 707,1067

Bantam 12, 118, J^39

Beierschmitt 68, 389, U39, 706

Bristol Cross 192

California

Campas 118, ^39, 706

Carrick 232, 317, U39, 9^8 118, 861

Cayuga J^39, 706 118

Caywood

Chapin

Christmas HolidayClyde

Colette 706

Covert

667

667, 707,9^8,1070

706, 707

119, 372

667, 707

667, 707, 909

707

707

707

707

707

m. 667, 707

Dabney 706, 707, 716, 1067 118, 233, 667

Dawn 706, 707 117, 118, 1067, 872, 9^8, 969

1070

DeVoe 118 ^39,667, 706, 707,

955, 1067, 1070

Dropmore (^)

Dymond— 118

Early Faulkner 118

Early Seckel 706 707

Eldorado 328, 1070

Enie 93

Ewart —- 118, 389, ^39, U2, 667, 707

550, 660, 706

Finland 118, 381

See footnotes at end of table.
124 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS INTRODUCED SINCE 1920—continued

Finsib

Funks ColoradoGilmore Pride —

Golden Spice

Gorham

118

Grand Champion

Hansen Seedless 382

Harbin 381

Harper

118

389, 706

389

118

Henderson SpecialHighland

Honeysweet

Hoskins

317

118

Illinois Bartlett -

John

^7ia

118, 233, 716

667 118

Johnson 118

Late Faulkner 118

Laxton's Early Market

Laxton's Foremost

Laxton's Progress

706

Laxton's Record

Laxton's SatisfactionLaxton's Superb

Laxton's VictorLee

LeRoi (Red d'Anjou)-

Luscious

Mac

McllhennyMagness—

J^36

67, 68, 117, 118,

317, J^7Jf, 706,

707, 861, 872,

1065,1067,1070

118

(«)

119

118,

118

27U,

1070

9U8, 1066

Manning Miller -

Marks

Max-Red Bartlett —

Mendel 93

Menie 93

Mericourt 118

118

119

706

528, 909

Miney--

Ming —

Moe

Mooers -

93, 118, U2

118, 381

93, 118, 550, 707

117, 118, 233, 707,

1067

68, 27Jf, 383, 667,

707, 726, 909,

1070

667

118, 119, 553

.^39, 707

118, A39

118, ^39, U2, 706,

707

707

118, Jlf39

75, 118, 192, 19^,

^39, 571, 707,

796

118, ^39, 707

707, 1070

667, 1060, 1063,

1067

m, 667, 707

587

707, 1067

1067

667

667

667

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 125

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS INTRODUCED SINCE 1920—Continued

Moonglow 118, 317, U71, 667, 27J^, 706, 707

872, 969, 1066,

1070

Morgan 118, 317, 707, 1067

Nikto 118

Nixon 118

Nye Russet Bartlett

Okolo

Olia

707, 1067

Orcross 118

Oregon 18 118

Orient 118, 227, 230, ^39,

610, 706, 707,

716, 827, 871

Ovid

Packham's Triumph

ParkerPatten -

Peter- ed)

Phelps —

PhilessonPhilip -—

Pontotoc 119, 716

Pulteney

Reed 550

Regal Red Cornice

Richard Peters 96, 118, 389, 696,

706, 707, 1067

Rogue Red

Russet Bartlett

Saponsky

Shea

Simon

Sirrine

97, 381

Southworth -

Spartlett—

Stanley

Star

Starking Delicious

(Maxine).

Starkrimson

Stewart Bartlett118, 317, 389, Jf36,

^39, Jt73, 706,

709, 861, 9J^8

Sungari 609

Sure Crop 118, 706

Tait Dropmore

Tait 1 J^39

317

706

93

118

389

118

(^)

707, 1067

389

118

(^)

820

118,

706

118

118,

(')

118

118

119,

706

589

U6

118, 389

707

298, 667

667, 707

68, 118, 389, 707,

1037, (2)

118, WO, 528, 587,

667, 707

UOO, 528, 587, 667

1108

667

J^39, 707, 1108

667

707

U39, 706 707

551

707

118, 707

550, 667, 707, 1070

118, 886

707

118

667, 707

97, 115, 185, 707,

953, 1070

667

667, 707

550

See footnotes at end of table.
126 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 10.—Ratings ofpear cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

383

CULTIVARS INTRODUCED SINCE 1920—continued

Tanya 118

Texking 118

Thomas

Tioma

Vistica Nectar 118

Waite (Canner) 118, 317, ^39, 550,

610, 660, 706,

707, 709, 716,

1067

Willard 706, 707

707

707

667

^Blight susceptibility classifications are primarily as originally published; some ratings

were assigned arbitrarily. Resistant is 0-6 percent, moderately resistant is 7-25 percent,

and susceptible is 26-100 percent of tree blighted.

^Pers. commun.. Dye, D. W., Dept. Sei. and Indus. Res., Auckland, New Zealand.

^Pers. commun., Gumming, W. A., Canada Dept. Agr., Morden, Manitoba.

*Pers. commun., Stushnoff, C, Univ. Minn., St. Paul.

*Pers. commun.. Nelson, S. H., Univ. Saskatchewan, Saskatoon.

^Pers. commun., Lombard, P. B., South Oreg. Agr. Expt. Sta., Medford.

In the last part ofthe table are Hsted 113 cultivars

named and released since 1920 (118^ 119), Of these,

about one-third are reportedly predominantly

resistant, one-third susceptible, and of the remaining one-third, one-half are moderately resistant and

the other halfvariably resistant and susceptible. For

22 percent of the pears introduced since 1920, there

was no mention of a blight rating.

Early pubhshed records of blight resistance in

certain pear cultivars were later followed by contradictory reports. Dawn was at first reported as

fairly susceptible to fire blight (9^8, 969), but in later

tests it proved to be more resistant {JÍ7JÍ, 1067). On

the other hand, Stewart Bartlett was recommended

as a replacement for Bartlett based on considerable

blight tolerance {886), but laterit was shown to be as

susceptible as Bartlett {115, 185, 1070).

Besides the four classes of cultivars—highly resistant, moderately resistant, variably resistant,

and susceptible—there appears to be a fifth class,

namely cultivars that are resistant to blossom and

shoot blight but are very susceptible to trunk blight

(pi. 4, C and D). The most recent example of this

type of blight was observed in 8- to 10-year old

Magness trees at Beltsville (1060,1067,1070) and in

young trees in isolated orchards in north-central

Arkansas (1006). In all cases, the fire blight

pathogen appeared to have entered the trees

either through natural wounds in the trunk (1060,

1068) or through severe hail damage in older

branches of the tree (1066). At Beltsville, trunk

blight has also been observed in several Giant

Seckel trees (female parent of Magness) as well

as in 12 clonal accessions of different Pyrus

species.

In general, trunk blight has been fairly uncommon, since previously it had only been recorded

by Reimer (780). Following extensive artificial

inoculations, he reported this phenomenon only in

the cultivars Douglas, Orel, and Surprise and in

several Pyrus species (table 9) and oriental

cultivars (table 11).

Pyrus caUeryana

The callery pear (P. caUeryana Decne.) is native

to South and Central China, especially in the

Yangtze Kiang River Valley (780). The trees are

medium to large, very vigorous, and usually bloom

early. This species can be readily distinguished from

all other species, especially P. communis, by its
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 127

TABLE 11.—Ratings of oriental pear cultivarsforfire blight resistance

based on literature cited ^

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

Ba Li Hsiang 389, 550, 707, 780,

859, 1067

Champa Li

Chieh Li 605, 707, 780, 1067

Chien Pa Li 389, 780

Chin Li

Chiu Tze

Chojuro

E Li

Fo Chin Hsi

Guar Li

Hau Kai

Hsiang Sui Li 389, 605, 707, 780,

1067

Huang Hsiang Sui 389, 780

Hung Guar Li 389, 780

Hung Li 389, 707, 1067

Ishi Wase

Kikusiu

Kuan Hung

Lo Suan Li 780

Ma Li

Man Yuan Hsuang 780

Ma Ti Huang 780

Meigetsu

Mein Suan Li 780

Mhikie

Mieajo

Mikado

Mi Li 389, 780

Nai Tze Hsiang

Nan Li

Nyisiki

Okusankichi

Pai Li 389, 1^35, 667

Pan Chien Sui 389, 780

Ping Ding

Ping Li 389

Seigyoku

Suan Li 707, 780

Ta Mo Pan

Tang Li 707, 780, 1067

Ta Suan Li 707

Ta Tau Huang 707, 780

Tzu Ma Li 389

Tzu Su Li 1037

Ya Kuang Li 389

Yakumo

Yarr Li

Yui Li

780

780

780

780

389

780

780

780

1067

707, 780

780

1067

780

780

707

707

780

707

780

707

707, 780

707

707

780

707

707

667, 707

707

707

707

780

707

707

707

707

707

707

1067

707

707

780

1037

^Blight susceptibiKty classifications are primarily as originally pubhshed; some ratings

were assigned arbitrarily. Resistant is 0-6 percent, moderately resistant is 7-25 percent,

and susceptible is 26-100 percent of tree blighted.
128 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

medium to large, ovate, green leaves with a glossy

surface and marginal teeth along the edge.

Nearly all selections, including the Bradford cultivar, are highly resistant to fire blight, Fabraea (F.

maculata Atk.) leaf spot, and insect pests. However, other P. calleryana accessions are reportedly

susceptible to fire blight (IW, 780, 1072). When

more than 400 open-pollinated Bradford seedlings

were rated at Beltsville, the distribution was as

follows: Highly resistant (10-8), 42 percent; moderately resistant (7-6), 19 percent; and susceptible

(5-1), 39 percent. Bradford appeared to be rather

variable in transmitting its high degree ofresistance

to its progenies (1072). Reimer (780) reported that

only 11 percent of the trees inoculated in the trunk

showed severe infection.

Pyrus ussuriensis

The ussurian pear (P. ussuriensis Maxim.) is native to extreme northern China, Manchuria, and

eastern Siberia and is the hardiest of all pears. The

trees are moderately vigorous, and the wild forms

and some cultivated varieties bloom earher than any

other species. The flowers are larger than those of

any other pear and vary from pink to white (780).

In P. ussuriensis two general groups are recognized: Group I, representing the true native species,

and group II, which includes the domestic hybrids

between this and other species.^^ In the species

planted at Beltsville, clones of the wild type (group

I) were the most resistant of all the species tested

and were assigned a mean blight score of 9.4 (highly

resistant) (1072). However, 11 clones of the hybrid

type (group II) showed considerable susceptibility

(mean score of 4.3). Of the 36 accessions with P.

ussuriensis parentage, such as Illinois 76 and Pai Li,

64 percent of the clones were rated moderate to

highly resistant.

In the collections of oriental cultivars and clonal

selections at Beltsville (table 11), those representing

group I (Ba Li Hsiang, Hsiang Sui Li, and Suan Li)

all showed high resistance, whereasthose in group II

(Pai Li and Ya Kuang Li) succumbed to blight (707).

This reemphasizes the beliefthat cultivars closest to

the wild type are more resistant to fire blight than

hybrids with one susceptible parent. Of the 48 cultivars listed in table 11, approximately one-third

were assigned to each of the 3 blight classes.

21KIKUCHI, A. SPECIATION AND TAXONOMY OF CHINESE PEARS.

[English transi, fromJapanese.] [Unpublished. Copy on file Dept.

Hort., Oreg. State Univ., Corvallis.]

Reimer (780) reported that some cultivars of P.

ussuriensis (Guar Li, Chin San, Hu Pi Hsiang, and

Ta Tze Hsiang) produced a high percentage of

blight-resistant seedlings, whereas seedlings of

other cultivars (An Li, Chieh Li, Chien Pa Li, and

Hau Kai) proved too susceptible to be of any value.

He emphasized, however, that this may have been

due in part to the effect of the unknown pollen parent, particularly so with Chieh Li and Chien Pa Li,

which are highly resistant cultivars. From his trunk

inoculation studies, Reimer (780) found that seedlings ofthe cultivated types ofP. ussuriensis ranked

second in resistance after those of P. calleryana.

The proportion of infected trees varied from 20 percent in Ba Li Hsiang to 77 percent in Chieh Li. The

extremely high degree of resistance in the former

was confirmed at Beltsville (707, 1067).

Pyrus pyrifolia

The sand pear (P. pyrifolia (Burm.) f. Nak.) is

synonymous with P. serótina. It is native to Central

and East China and Japan. Cultivars of this species

are also grown in China and Japan. The tree is very

vigorous and productive but does not bloom as early

as P. ussuriensis. Leaves are very large, with a

long, tapering point and coarsely serrated margins.

The fruit is usually round or slightly flattened. Table

11 includes several cultivars with P. pyrifolia parentage.

Inoculation tests by Reimer (780) and observations on natural blight in Beltsville (1072) have

shown that this is another extremely variable

species, ranging from highly resistant to very susceptible. Many cultivars with P. pyrifolia parentage, such as Campas, Hawaii, and Twentieth Century, have died from blight at Beltsville (707). Ofthe

49 clones with this species parentage (Okusankichi

and Meigetsu), only 28 percent were in the resistant

classes (10-8, 7-6) (1072). Reimer (780) reported

that P. pyrifolia cultivars ranked third in resistance

regarding trunk blight.

Pyrus betulaefolia

The birchleaf pear (P. betulaefolia Bunge) is indigenous to Central and North China. The tree is

moderately vigorous with horizontal or drooping

branches. The young shoots and leaves are very

distinct from those of all other species; usually they

are covered with a light grayish pubescence. Blossoms are very small and are produced in great abundance. The fruit is borne in clusters of from 5 to 10

and is smaller than that of any other species.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 129

Severe fire blight in P. betulaefolia has been reported (UO, 605, 780, 960,1072), However, Reimer

{780) found, among seedlings of this usually susceptible species, 18 seedlings that were highly resistant

to fire blight. As reported for hybrids ofthe previous

four species, those of P. betulaefolia varied from

highly resistant to extremely blight susceptible.

In general, our observations at Beltsville agree

with those of Reimer {780) that the five most important Pyrus species, ranked in descending order as to

their degree of blight resistance, are ussuriensis,

calleryana, betulaefolia, pyrifolia, and communis.

In each species, however, the range of resistance

makes impossible the assignment of a certain degree

of resistance to a given species.

Apple

Rehder {771) Usted 24 species of apples or crab

apples indigenous to China, Japan, Siberia, and the

United States. The cultivated forms have variously

been designated Malus pumila Mill., M. sylvestris

Mill., andM. domestica Borkh. {958), Cummins and

Aldwinckle {203) reported that several clonal selections of M. X atrosanguinea, M. fusca, M.

prunifolia (Willd.) Borkh. xanthocarpa, and M.

surgenti Rehd. are highly resistant to artificial shoot

tip inoculation. There are numerous other species,

which include the many crab apple cultivars.

The flowering crab apples are excellent ornamental trees, valued for their flowers, foliage, and fruit.

However, many of the crab apple cultivars are very

susceptible to four common diseases, including fire

blight. The indigenous crab apples in the United

States were presumably one ofthe native hosts from

which the blight pathogen spread to the cultivated

apples and pears introduced from Europe. Therefore this host has been exposed to blight infection

more than any other genus in the family Rosaceae.

In Colorado, Crandall {190) observed in alternating

rows of Martha and Whitney crab apple cultivars

that the Whitney trees were severely attacked, but

not a single Martha tree was blighted.

Studies on the nature of resistance in crab apple

have been conducted in many parts of the country

{63,381, 870), The most extensive were by den Boer

and Collins {98) in Iowa, Nichols {685-688) in

Pennsylvania, Kozel and Dugan {5^3) in Ohio, and

Jefferson {Í76) at the U.S. National Arboretum. One

of the largest collections with about 1,100 trees in

300 cultivars of flowering crab apples in America is

presently maintained by the Des Moines Water

Works in Iowa {98),

The following crab apple cultivars and selections

have consistently been found resistant to fire blight

and four otherserious crab apple diseases {J^76, 687):

Adams ^

Ames White

Baskatong

Beverly ^

Burton

Centennial ^

Gibbs Golden Gage

Golden Gem

Golden Gem (BD 1155-58)

Golden Gem (PLT 788-58)

Henry Kohankie

Honeywood 14

Malus hybrid (scab immune clone - GR700-58)

Minnesota 1492

Morden 19-27

Mount Arbor Special

Professor Sprenger

Robinson ^

1 Commercially available.

These four diseases are apple scab {Venturia inaequalis (Cke.) Wint.), powdery mildew {Podosphaera leucotricha (Ell. and Ev.) (Salm.)), cedar

apple rust {Gymnosporangium juniperivirginianae Schw.), and frog eye leaf spot {Pysalospora obtusa (Schw.) Cke.). In addition to these

multiresistant trees, several other crab apple cultivars are resistant to fire blight alone {^76, 687),

Descriptions of crab apple origin, as well as flower

and fruit characteristics, have been published {i76,

687, 688),

As with pears, there are marked differences

among species and cultivars of apples in their response to attack by fire blight. In table 12 are hsted

390 cultivars of apples, including the best known

names used during the 1900's for which bUght records are given in the literature. Of the 193 cultivars

introduced before 1920, about 28 percent are classified as resistant, 27 percent moderately resistant,

28 percent susceptible, and 17 percent variably resistant and susceptible.

In the last part ofthe table are listed 197 cultivars

named and released since 1920 {118,119,1007,1009),

They comprise only 32 percent of the 620 cultivars

listed {118,119), Ofthe 197 cultivars for which blight

ratingswere reported, 41 percent are listed asresistant, 26 percent moderately resistant, 17 percent susceptible, and 16percentvariablyresistantandsusceptible.
130 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 12.—Ratings ofapple cultivarsforfire blightresistance based on

literature cited ^

^ ,,. Resistant . , r Susceptible Cultivar .,„ „, resistant /r^x

(lO-ö) .„ a\ (o-l) (7-b)

CULTIVARS ORIGINATED BEFORE 1920

Adenburg 301

Akin 17 22, 622

Alexander 17, 769 698 22,160,298,317,

396, 611, 622,

666, 667, 725,

919, 920, 1011

Amur 698

Ananas Berzenicki 104'

Anaros 698

Anis 698

Antonovka 698, 946 665

Arkansas (Black Twig) 17,22,539,610, 850 301,666,667

622, 827

Arobka 396

Athabaska 698

Autumn Strawberry 22, 622, 859

Babbit 396, 850

Bailey Sweet - 396

Baldwin 17, 22, 396, 622, 274, 317, 400, 528, 160, 298, 587

769, 1011 667, 714, 850, 919,

920

Ballarat 17 6

Baltimore 850

Baxter 22, 622

Beauty of Boskoop 75

Bellflower 850

Ben Davis 17, 22, 63, 114, 21, 317, 400, 404, 298,301,587, 713

611, 622, 769, 528, 714, 850,

1011 919, 920, 923

Benoni 17 21, 22, 622, 923

Bentley Sweet 850

Bietigheimer 22, 622

Bismarck 22, 539, 622 396

Black Ben Davis 22, 539, 946 622, 666, 667 114

Black Gilliflower- 17,22, 622 396

Blushed Calville 698

Bonum 693

Canada Red 850

Champion 22, 622

Charlancoff- 114 698

Charles 698

Charles Ross 573

Chenango 17,21, 2&

Collins 923

Columbia 539, 665, 698

Constantine 396

Cox Orange Pippin 17, 396, 573 946 666, 667

Cranberry Pippin 17,75,396

Crawley Beauty 573 17

Crusoe 698

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 131

TABLE 12.—Ratings ofapple cultivarsforfire blightresistance based on

literature cited ^—Continued

^ ,^. Resistant . , / Susceptible

C"'*'^^"- (10-8) '•^'^^i'"' (5-1)

CULTIVARS ORIGINATED BEFORE 1920—Continued

Delaware Red 920

Delicious (Red Delicious) — 6, 17, 22, 105, lU, 63, 271^, 301, 317, 298, 587, 920

396, U5, 513, 528, 666, 667,

622, 693, 769, 919, (2)

827, 850, 919,

1011

Dr. Mathews 22, 160, 622, 666,

667

Domine 22, 622

Duchess of Oldenburg 17, 21, 22, 63, 301, lU, 317, 528, 665, 160,587,666,

(Oldenburg). 396, 539, 587, 769, 1011 667, 850

611, 622, 698,

827, 919, 920,

923

Dudley 17, 22, lU, 622, 611

651

Early Cooper 301

Early Harvest 22, 611, 622 17, WU, 693, 9^6 301,666,667,850,

923

Early Ripe 22, 622 667

Egremont Russet 17,573

Elsa 698

English Codling- P^

Esopus Spitzenburg 919 17, 22,396, ^3^,

(Spitzenburg). 528, 622, 769,

804, 850, 920,

1011

Fallawater 17 622

Fall Pippin 17 1011 22, 396, 528, 769,

827

Fameuse (Snow) 17, 9^6 22, 63, 622, 919, lU,160,27^,611,

1011 651, 666, 667,

920

Five Crown 827 179

Florence 665 698 827

Gano 22, 63, 611, 622 ^00, 850, 920, 923 301

Giant Geniton 22

Gideon 17, 396

Gilbert Winesap 827

Golden Delicious 17,22,75,223, 301, 317, m, 528, 587

668, 693, 9U5, 666, 667, 859, (2)

9U6

Golden Pippin 396

Golden Russet 400 17

Golden Summer WU

Golden Sweet 22, 404, 622, 946 667

Golden Winesap 17, 22, 622

Granny Smith (*)

Gravenstein 17, 22, 396, 622, 528, 919, 946, (*) 587, 611

821

See footnotes at end of table.
132 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 12.—Ratings ofapple cultivarsforfire blight resistance based on

literature cited ^—Continued

^ ,^. Resistant . ^ , Susceptible Cultivar .,^ „. resistant /r ix (10-8) ^^_gj (5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Grimes (Grimes Golden, 17, 22, 301, 396, 21, 63, lU, kOU, 27U, 587, 610, 667,

Grindstone). 622, 769, 9i6, 528, 769, 850, 713, 806

1011 919, 920, 923

Hawley 17 396

Hibernal 731 63, 317, 665, 919 298, 698

Hibkee 698

Hoover 17, 693

Horse ^0^ 946

Hubbardston 17, 396 22, 622, 859

Huntsman 17,22,622 859 396

Hyslop 17 22, 622 611, 698

Ingram 22,622 770,859 611,923

James Grieve 17 104-

Janet

Jefferis 946

Jonathan 22, 622

17, 22, 622

17, 63, 528, 769, 21, 75, 114, 274,

770, 919, 920, 301, 317, 396,

1011, 1047 400, 404, 587,

611, 666, 667,

804, 827, 923,

698

946

Keetosh

Kildare 827

King 946 528, 611, 769

King David 17, 22, 622, 827, 400, 850

946

Kinnard 622

Lady 17, 850, 946 396

Lady Sweet 22, 622 396

Lawver 22, 622, 919 396

Limbertwig 693

Liveland Raspberry 17, 22, 622

Lowell 22, 622

Lowry 17

Mclntosh 17, 22, 105, 223,

622, 651, 769,

946, 1011

MacMahon 22, 946

Maiden Blush 946

Mahnda 17

Mann 17

Markham 827

Melon 22 622

Minkler 22, 622, i^)

Missouri Pippin 859, 923

Morden Russet 698

Moscow pear 665, 698 —-

662, 946

63, 114, 274, 317, 160,396,425,587,

400, 528, 859, 610, 611, 850

919, 920

63, 919 114, 404, 611, 651,

920

17, 404, 1047 22, 396, 622, 667,

713, 804, 850,

923

859

850

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 133

TABLE 12.—Ratings ofapple cultivarsforfire blight resistance based on

literature cited ^—Continued

„ ,^. Resistant . ^ ^ Susceptible Cultivar ,.„ „. resistant /r ^x (lO-ö) ,_ „, (5-1) (7-b)

CULTIVARS ORIGINATED BEFORE 1920—Continued

Mother 17, 22, 622 396

Nero 622 22

Northern Spy 17, 22, 611, 622, 9J^, 27Jk, 317, WO, 298, 396, 587, 699,

769, 1011 528, 919, i^) 850, 9U6

North Western 22

Northwestern Greening-— 11J^, WO, 622, 651, 17, 63, 27h, 528, 666, 667

850, 98U 587, 919, 920

Ontario 17, 611

Opalescent 17, 527 666, 667

Ortley 17, 22, 622 396

Osman 698

Paragon 22, 622, 827, 9W 17 66, 667

Patten 22, 622, 665

Patten Greening 698

Pedro 698

Pennock 765, 1011 528, 587

Petrel 665, 698

Pewaukee 665, 769 396

Pioneer 665 698

Piotosh 17 698

Polgo 827

Prince 698

Rails 22, 622, 9W 22, 396

Ramsdale 22

Ramsdell Sweet 622

Redant 698 665

Red Astrachan 17, 22, 396, 622 667

Red Canada 17, 22, 539, 622 396

Red June (Wilson Red 22, 622 WJf, 693 22,27J^, 622,9W

June).

Red Melba 827, 9W 9W

Rhode Island Greening 528, 665, 726, 1011 17, 22, 9U, 160,

27J^, 396, U25,

587, 622, 667,

769, 827, 850

Romanite Little Red 22, 622

Roman Stem 17, 22, 622

Rome (Rome Beauty) 693, 9W 528, 769, 919, 920, 17, 22, 27Jf, 301,

1011 317, 396, 587,

622, 662, 666,

667, 850, 923

Rostherns 698

Roxbury Russet 17, 396 22, 622, 9W

St. Clair 945 667

St. Lawrence 17 396

Salome — 17, 22, 622 850 114

Scarlet Pimpernel 573

Schoharie 17 622

Scugog 698

Senator 22,622

See footnotes at end of table.
134 AGRICULTURE HANDBOOK 51O, U.S. DEPT. OF AGRICULTURE

TABLE 12.—Ratings ofapple cultivarsforfire blightresistance based on

literature cited ^—Continued

^ ,^. Resistant . ^ / Susceptible Cultivar ,,_ _- resistant ,_ ,. (10-8) ^^_g^ (5-1)

CULTIVARS ORIGINATED BEFORE 1920—continued

Shannon Pippin ^0^

Shiawassee 17 22, 622

Smith's Cider 71S, 850

Smokehouse 9J!^6 17, 528, 666, 667, 396,587, 713, 827

769, 1011

Sonora 827

Spilaw 827

Stark 17, 22, 622, 769, 587, 850

1011

Starr 827 528 94, 587, 666, 667,

1011

Stayman (S. Winesap) 17, 22, 301, 622, 27^, 317, 528, 539, 75,587, 666, 667

769, 827, 923, 610, 920

9^6, 1011

Sturmer 6

Sulton 396, 693

Summer Rambo (Rambo) 17, 622, 9^5 528, 9^6 587, 769, 850,

1011

Sutton (Sutton Beauty) 17 22,396,622,850

Swaar 396

Sweet Bough 396 22, 622

Sweet Russet -- 698

Tolman Sweet 396 22, 622 17, 21, lU, 160,

27h y 611, 666,

667

Tompkins King- 396 17, 919, 920, 9^6 22, 622, 726,850

Twenty Ounce 17, 726, 769, 1011 22, 9^, 396, 622,

666, 667, 946

Vandevere 22, 622 17

Wagener 17, 22, 396, 573, 666, 667, 919, 920, 400,528,587, 726,

622 946 769, 827

Walkers Beauty 622

Wapella 17, 698

Washington Strawberry 396

Watkers Beauty 22 622

Waukon - 698

Wealthy 17, 22, 396, 622 21, 63, 143, 528, 114,223,274,298,

665-667, 698, 301, 317, 400,

920, 1011 587, 611, 651,

769, 827, 923

Weissemer Dessert 22

White Ohio Pippin 827

White Pippin 17,850 22,622

Williams Red (Williams) — 17, 22, 528, 622, 587 666, 667

769, 945, 946,

1011

Willow (Willow Twig) - 21, 22, 274, 298,

317, 396, 666,

667, 923

Windsor 22, 622

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 135

TABLE 12.—Ratings ofapple cultivarsforfire blightresistance based on

literature cited ^—Continued

^ ,^. Resistant . , , Susceptible Cultivar .,„ „, resistant /r i\

(lö-^> (7-6) ^^-^^

CULTIVARS ORIGINATED BEFORE 1920—continued

Winesap 17,21, 22, 301, 317, 666, 850 667

622, 693, 827,

H6

Winter Banana (Banana)— 827 17, 22, 622, 666, 298, 317, 920

667, 919

Winter Pearmain 22, 622, 667 396

Wolf River 17, 22, 622 63, ^00, 656, 919 920, 9^6

Yellow Bellflower 17, 22, 622, 91^6 667

Yellow June WJf

Yellow Newtown (Newtown 17, 9^6 22, 396, 622, 667,

Pippin, Albermarle Pippin 850

Green Newtown).

Yellow Transparent 17, 396 22, 622, 666, 667, 21, 63, 9A, 11^, 160,

(Transparent). 770, 919, 920, 27ky 301, 317,

York ImperialAbbott

Acheson1011 UOO, WJf, 513,

528, 587, 610,

611, 651, 665,

769, 80^, 850,

859, 9Jtß

21, 528, 769, 770, 17, 22, 301, 513,

850, 1011, 10k7 587, 610, 622,

662, 666, 667,

713, 859, 923

3 SINCE 1920

(^)

U3, 665

Adanac ('*)

Advance 118

Almey 17, (3), («)

Alton 17

Anderson (*)

Anoka 827, 946 666, 667

Atlas 17 143, 665, («)

Bancroft 17, («)

Barrie- (^)

Barry 17, 667, 946

Battleford 698

Beacon (Fenton, Miller 118, 121 17, 143, 665, 946 317, 666, 667, 827,

Red). 984

Bismar- 609

Blair- 17, («)

Blaxtayman 827

Blaze 17, 945, 94ß 118, 223, 666, 667

Breakey 698

Brightness ('*)

Burgundy 17 1010

Caravel 17, 946, (^), («)

Carlton 17

See footnotes at end of table.
136 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 12.—Ratings ofapple cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS INTRODUCED SINCE 1920—continued

^7, (5)

17

Carroll

Chieftain

Chipman

Christmas Red ('*)

Close 17, 9J^6, 95Jp

Collet (5)

Conard 17, (^)

Cortland 22, 769, 1011

Crandall 91,5

Crimson Spy (^)

Dawn

Delcon 17, 223, C^)

Dr. Bill 665

Dowingland

Dunning

Early Mclntosh 17, 22

Early Vance Spur

Edgar 17, {^)

Edith Smith

Empire 17, 699, 91,6

Empire Red

Exeter

Faurot 17

Fireside 17, 665

Folwell

Fyan 17

Gala

Garland 17, (^)

Geneva Mclntosh 17

Geneva Ontario 17

George

Gertie (^)

Glennal 665

Glenton (s)

Goalie

Godfrey (^)

Goldo 17

Goodland (^)

Greendale 943

Greene Spy 17

Grove 118, 9^6

Haralson -

Harvester -

Hawkeye GreeningHeaver #5

Holiday

Honeygold

17, 143, 223, 539,

665, 698, 946

(^)

(^)

17, 946

118, 216, 274

75, 666, 667

17, 75, 143, 274,

425, 528,

665-667

17, 223

274

667, 946

119

1008, (3)

143, 665

118, {')

118, 274

(^)

118

665

17

17, n

274, 317

143, 665

17, 440

(^)

667, 946

400, 587, 945

667

667, 946

118

17

(^)

666, 667, 827

666, 667, 94-6, 984

400

(')

143, 665

17, 667

118

667, 946

667

(«)

118, (9)

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 137

TABLE 12.—Ratings ofapple cultivarsforfire blight resistance based on

literature cited ^—Continued

^ ,,. Resistant . , , Susceptible Cultivar .,„ „- resistant /r -.x (10-8) ^^_gj (5-1)

CULTIVARS INTRODUCED SINCE 1920—continued

Hume 17, {^)

Idajon 667 118

Idared 118, 223 17, 27h, 298, 667,

9U6, (3)

Jerseymac 17

Jerseyred 118, 539, 9J^6 666, 667

Joan 17, 118, U3, 665, 27U

9U6

Jonadel 17, 223, 539, 559, 27J^, 666, 667 118

9U6

Jonagold 17, {^)

Jonagram (^) 9J^6 17, 223

Jonamac 17

Jono 17, 119

Joyce 17, 539, 698

July Delicious 9Jt5

Julyred 667, 9Jt5 17, 9^6

Kendall 17, 666, 667 118, 9^6, 1007

Kidd's Orange (^)

Kingscourt (^)

Lakeland 17, 118 223,666,667,9^6

Laking (^)

Lambton (^)

Lawfam (^)

Lina 609

Linda 827 17

Lobo 17, 22, 9U6 lJf3, 665

Lodi 22, 9Jtö U3, 665 17, 75, 227, 27U,

298, 317, 610,

666, 667, 9Jf6

McLean (4) —

Macoun 17, 22 666, 667, 9J^6 (3)

Maga 609

Manan (^)

Manbee llf3, 665, {^) —

Mandan ^ 17, 118 —

Manitoba 1J^3, 665, 698 --

Manitoba Spy 698

Manred (^) -—

Mantet 17 666, 667, 698, 9J^6

Mantón 698

Martin 118

Medina 22, 91^6 17, 666, 667

Melba 17, 22, 223, 98U U3,665, 667 27U, 298

Melrose 17, 9J^6 («)

Merton Russet (^)

Metzgar 118

Milton 665, 667, 9J^6 17

Minjon 17, 9^6 1^3, 665 223, 27J^, 667

MoUie's Delicious 9J^5, 9U6 17 610, 925

See footnotes at end of table.
138 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

TABLE 12.—Ratings ofapple cultivarsforfire blight resistance based on

literature cited ^—Continued

Cultivar Resistant

(10-8)

Moderately

resistant

(7-6)

Susceptible

(5-1)

CULTIVARS INTRODUCED SINCE 1920—continued

Monroe 17

Morden RussetMoris

Mortof

Mount

(«)

Mutsu (Crispin)-

Newfane

Niagara

Norspy

Ogden75, 9Jt5

17

Oriole 17

Orleans 17, 22, 223, 9J^6

Ostem 698

Oxbo 9J^6

Park (4)

Patricia 17, {^)

Patterson

Paulared 17

Piotet

Prima 17, 215

Priscilla 119, 1025

Prolific

Puritan 17, 9^6

Quebec Belle (^«)

Quinte 17, 9^6, (^)

Ranger 17, i^)

Raritan

Red Atlas («)

Red Baron 17

Redford 17

Redhook 9^6

Red Prince 17, 668

Red Sauce

Redsnow (^)

Red Standard 17

Redwell 17

Redwin Spy (^)

Regent 17

Reid -

Rescue

Reward

Richared Delicious 17

RoanokeRosemanRuby9^6

9Jt6

Rutherford

Sandow

Secor

Seeando Red Rome 262 -

Semlar

17

17

609

667, 9^6, (3)

105,

223,

9J,6,

667,

(^)

94^6

667

U3, 665, 9^6

609

223,

(^)

(^)

665

17

(^)

667

17

17

17

118

17, (^)

665

(^)

105

667

U3, 665, 698

17, 223, 27J^, 667

17

17, (3)

9^6

271 667

298

(«)

945

1^3, 665

667

17, 827

27U

118

698

1A3, 665

m

118

528, 558, 667, 827

827

See footnotes at end of table.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 139

TABLE 12.—Ratings ofapple cultivarsforfire blight resistance based on

literature cited ^—Continued

„ ,,. Resistant . ^ ^ Susceptible Cultivar ,,„ „. resistant ,r ^^ (lö-ö) (5-1) (7-6)

CULTIVARS INTRODUCED SINCE 1920—continued

Sharon 17, 667 223, 558, 665 27J^

Shenandoah 17, 9U6

Sir Prize 1025a

South Dakota Golden 609

Spangelo 665, {^)

Spartan 17, 75, 223, 9Ji.6 27U 666, 667

Spencer 17 667

Spigold 9J^6 17

Spijon 17

Starking Delicious 827

Stevenson 665, i^)

Summerred 17 9^6

Superior 118

Sweet DeUcious 17, 9U6

Sweet Mclntosh 17

Tolmo 609

Toshkee 17, {^)

Trenton («)

Turley 22, 223, 622, 827, 17, 27Jp

9Jf6

Tydeman's Early Red 17, 118

Ultra Red DeUcious 119

Unity C*)

Victory 17,118,665-667

Volga 609

Wakaga 609

Watts 698

Wayne 17 699, 9^6, (^)

Webster 17

Wedge 17, WO

Wellington 17 610

Winter Queen ("*)

Wright 17, C) 118

Yellow Sweet 118

^Blight susceptibility classifications are primarily as originally published; some ratings

were assigned arbitrarily. Resistant is 0-6 percent, moderately resistant is 7-25 percent,

and susceptible is 26-100 percent of tree blighted.

2Pers. commun., Dye, D. W., Dept. Sei. and Indus. Res., Auckland, New Zealand.

^Pers. commun., Cummins, J. N., N.Y. State Agr. Expt. Sta., Geneva.

^Pers. commun., Nelson, S. H., Univ. Saskatchewan, Saskatoon.

^Pers. commun.. Gumming, W. A., Canada Dept. Agr., Morden, Manitoba.

^Pers. commun., Hutchinson, A., Hort. Res. Inst., Vineland Sta., Ontario.

"^Pers. commun., Hansen, K. W., Mo. State Fruit Expt. Sta., Mountain Grove.

*Pers. commun.. Ferrée, D. C, Ohio Agr. Res. and Devlpmt. Cent., Wooster.

^Pers. commun., Stushnoff, C, Univ. Minn., St. Paul.

i^Pers. commun., Coulombe, L. J., Res. Sta., St. Jean, Quebec.
140 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Aldwinckle and Preczewski (16a) in 1976 reported

results of artificial shoot inoculations on 92 apple

cultivars in the field and 79 in the greenhouse. They

observed a strong correlation in the amount of blight

between the two locations. In general, their blight

ratings confirm the data in table 12.

Since 1946 the U.S. Plant Introduction Station at

Glenn Dale, Md., has introduced and evaluated hundreds of apple cultivars. In addition to those in table

12, Ackerman U, 5) published ratings of natural fire

blight for a 3-year period of 63 early-ripening and 58

late-blossoming apple cultivars, most of which

were introduced from Europe. The majority of the

cultivars showed less than 5 percent of the tree

blighted, indicating a high degree of resistance.

Studies by Ahn (1079) indicated that the

mechanism responsible for fire blight resistance in

apple shoots is closely related to catechol oxidase

activity in the host tissues. Extracts from resistant

cultivars Starking Delicious and Haralson showed

higher total catechol oxidase activity than susceptible cultivars Beacon and Wealthy. However, total

phenol and flavonol content did not correlate with

resistance to E. amylovora, but leucoanthocyanin

content appeared to be higher in resistant than in

susceptible cultivars.

Rootstoeks

Pear and apple cultivars in the United States are

usually propagated on seedling or clonal rootstocks.

For many years both cultivars were propagated on

the so-called French seedlings imported from

France prior to the 1930's (953). Since that time

seeds of commercial cultivars from domestic sources

have replaced imported seed. Also, many cultivars,

especially apples, are grown on clonal rootstocks.

Pear

For several centuries, quince has been used as a

pear stock, mainly because of its dwarfing effect. A

series of clonal stocks, designated as Quince A, B, C,

and Provence, have been developed from it. Unfortunately these stocks are winter tender, incompatible with most pear cultivars, and usually susceptible

to fire blight and Fabraea leafspot (953), In addition,

Adams quince was reported in 1972 to have been

developed in Belgium (656a). It appears to enhance

precocity and increase yields in the cultivar Doyenne

du Comice, but we know of no available information

regarding its susceptibility to fire blight.

About 1915, Reimer (780, 782) began the search

for pear rootstocks and interstocks resistant to fire

blight. Following extensive trips to China he imported and tested seed from several native oriental

Pyrus species. He found P. ussuriensis and P. calleryana fairly resistant to blight, whereas P.

pyrifolia and P. betulaefolia were susceptible (table

9). In addition, rootstocks of P. pyrifolia and P.

ussuriensis were susceptible to pear decline and

produced serious black end on fruit of the scion cultivar (590, 953). In New York, Kieffer performed

more successfully on several oriental pear species,

whereas Bartlett, Seckel, and Anjou were most successful on French roots (959).

Blight-resistant seedling stocks were also developed by Reimer (780, 782) by crossing Old Home

and other resistant cultivars with Farmingdale.

Wiien he compared Bartlett, Bosc, and Howell on

French seedling roots and Old Home-Farmingdale

roots, 60 percent of those on French seedling were

blighted following artificial inoculation of the susceptible scion cultivars, whereas only a few of the

latter stocks were blighted. These resistant stocks

are being used considerably as body stocks, interstocks, or rootstocks (^7i, 1014). In a 1975 report,

Lombard and Westwood (590) identified pear

rootstocks that are resistantto the woolly pear aphid

(Eriosoma pyricola Baker and Davidson) and various diseases, including fire blight and decline. These

rootstocks are adapted to a wide variety of soil conditions and sites and produce trees larger or smaller

than standard.

In extensive rootstock trials in California, Day

(213, 21Í) found that Old Home interstock on quince

roots planted with the graft union 25-30 cm below

ground produced Old Home roots above the quince

and thus furnished blight resistance below ground as

well as in the trunk and scaffold branches. However,

Higdon (Jf06) reported unidentified graft union failure in 15-year-old trees with Oregon-18 trunks on

Oregon-18 x Farmingdale seedling roots and on 6-

year-old Forelle trees topworked onto P-87 trunk

stocks.

In addition to these pear stocks, stocks of several

other genera ofPomoideae are graft compatible with

Pyrus. English hawthorn (Crataegus oxyacantha

L.) has been reported to have dwarfing effects and to

be particularly tolerant of cold, wet soils (958). Olden

(708a) reported that Amelanchier canadensis (L.)

Medic, promoted earlier fruiting than quince, although anchorage was inferior, and that Sorbus

aucuparia L. was also compatible enough with pear.

However, these species are susceptible to fire blight
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 141

and have never been extensively used in pear pro- tion, after several severe blight years at Beltsville,

duction. Degrees of dwarfing have been recognized we observed very severe fire blight in York Imperial

for several years in seedling selections of the USDA on M 26 rootstock in contrast to light infection when

pear breeding program at Beltsville. US 309 and US on M 111 or seedling rootstock (513). This high inci342, both seedlings from Michigan 437 x RCW, dence of fire blight on M 26 has been reported by

propagated on Bartlett seedling rootstock, show msiny others (105, 200, 22i,25i, 798, 799). However,

definite dwarfing and a high degree of fire blight M 26 to date has had no effect on the susceptibility of

resistance. Additional research is needed to deter- Golden Delicious in large orchard plantings in

mine the effect of the dwarfing character of these Pennsylvania. Similar results were reported by Ahn

selections on commercial scion cultivars. (1079) using Mclntosh on M 9, M 26, and M 111.

The increased sensitivity of the scion cultivar to

^^ ^ blight infection on different clonal rootstocks is well

The history of apple rootstocks dates back much estabUshed. This has been attributed to an earlier

farther than that of pear. The Greeks and Romans flower production induced by the rootstock, which

were familiar with dwarf, self-rooting forms of ap- provides more infection sites for the blight pathoples (958). Following selection of several types of gen. Thus the rootstock also may have altered cerrootstocks in western Europe, the well-known Mai- tain physiological processes in the tree, producing

ling rootstocks were classified in Great Britain dur- more flowers and also rendering the scion cultivar

ing 1912-25. The Malling-Merton rootstocks with niore blight susceptible. Therefore the two factors

Northern Spy as one parent were developed in appear to be concomitant rather than causal.

1925-50. In an extensive rootstock breeding project in New

We have summarized the fire blight susceptibility York, Cummins and Aldwinckle (15, 16, 200, 201)

data for the most common Mailing and Malling-Mer- screened large numbers ofseedlings from controlled

ton rootstocks evaluated by other investigators (15, crosses during their first year of growth for suscep16, 105, 200, 20J,, 223, 22J,, 668, 728, 799, 91,5). The tibility to collar rot, the woolly apple aphid

mean blight ratings for natural infection plus artifi- (Eriosoma lanigerum (Hausmann)), and fire blight,

cial inoculation of ungrafted rootstocks were placed They found Malus prunifolia (Willd.) Borkh.

in four general categories (table 13). Thus, of the 28 xanihocarpa, M. x sublobata (Zab.) Rehd., M. x

clonal rootstocks, 9 were in the light susceptibility robusta (Carr.) Rehd., and a high proportion of

class, 12 in the moderate, 5 in the severe, and 2 in the seedlings in progenies from crosses with them to be

very severe class (513). blight resistant. In 1974 they (202) reported an inIn assessing the degree of blight resistance of the teresting correlation between different degrees of

various rootstocks, we should distinguish between suckering of apple rootstocks and incidence of fire

the direct damage offire blight to the rootstock itself blight. In 2- to 7-year-old orchards, tree mortality

and the effect of the rootstock on increasing suscep- was highest on M 9, M 26, and Alnarp 2 rootstocks.

tibility in the grafted scion cultivar. Cultivars such In Arkansas, Rom (798) observed a relationship beas Jonathan, Mutsu, Golden Delicious, Raritan, and tween burrknots on 4-year-old Jonathan trees on

Lodi varied in their degree of blight susceptibility different rootstocks and fire blight lesions on the

depending on the rootstock (105, 668, 9Í5). In addi- stocks.

TABLE 13. — Classification of most comraon Mallirig and MallingMerton apple dwarfing rootstocks forfire blight susceptibility ^

Susceptibility Clonal rootstock No.

class Mailing Malling-Merton

Light 2, 7, 15 102, 103, lOJ^, 105, 110, 111

Moderate i, -4, <?, 10, 12, 13, 25, 27 106, 107, 109, 112

Severe 6, 9, 16 113, lU

Very severe 3, 26

^Data compiled from Anthony and Clarke {26), Cummins and Aldwinckle {200), Doll {223),

Parker et al. {728), and Rom and Slack {799).
142 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Artificial Inoculation

Techniques

Many techniques have been tested and employed

for artificial inoculation of plant material to determine the degree of fire blight resistance in pears,

apples, pyracantha, cotoneaster, and other rosaceous hosts. The oldest method, which is still used,

includes variations of needle inoculation of the succulent shoot tip. In 1925, Reimer (780) placed a drop

of bacterial ooze or pure culture ofE. amylovora on

the shoot tip or other part ofthe tree to be inoculated

and passed a needle several times through this drop

into the tissue. In 1934, Shaw (859) extensively

studied the variations in fire blight resistance among

31 rosaceous species of several genera, including

Pyrus. In addition to Reimer's needle-puncture

method, he used a hypodermic needle to inject about

0.1 ml of concentrated bacterial suspension into the

shoot tip. Bruner (1087) reported in 1953 ofdipping a

dissecting needle into a bacterial suspension and

then puncturing the leaf axils or stem internode of

the plants. These three methods of inoculation have

since been used by many scientists studying fire

blight (81, 97, 139, UÍ, 550, 561^, 565, 9^8, 1090,

1091, 1101, 1108, 1136).

At Beltsville we have experimented with several

techniques oftissue injury in combination with spray

inoculation (501, 105^, 1062). Sandblasted and

spray-inoculated pear trees in the greenhouse on

raised beds (fig. 26, A) showed that petioles were

most commonly infected (fig. 6, B and C). Occasionally stems became infected through wounds caused

by large sand particles. Vascular tissues in the

leaves supported the blight pathogen better than

interveinal tissue. Sandblast injury of petioles and

midribs often resulted in systemic infection. Crosse

et. al. (197) and Crosse and Shaffer (198) reported

similar findings with Jonathan apple.

In Canada, Layne (563, 56Jf) reported that the

needle inoculation method was more effective than

spray inoculation on 7- to 10-month-old pear seedHngs. Following inoculation he placed the plants in a

humidity chamber maintained at 90-100 percent relative humidity for 3 days. This method, though slow

and laborious, resulted in 90-100 percent infection

compared with only 60 percent infection without a

humidity chamber.

Severalinvestigators have successfully inoculated

pear blossoms by spraying them with a bacterial

suspension (^i 7, 651, 7JÍ2,1057). In Maryland, Dunegan et al. (2Í2, 2U3) reported on field inoculation of

9-year-old pear seedlings during bloom. They

sprayed the trees with an aqueous suspension oíE.

amylovora and an abrasive applied with an orchard

sprayer. Blight developed in 52 percent of the trees

the first year, and with additional sprays it developed in 57 percent ofthe trees the following year.

In New York, Beer (80) obtained as much as 40

percent infected blossom clusters when 6-year-old

Idared apple trees were sprayed in full bloom with a

backpack-type gasoline-powered mist blower. He

has also reported techniques for field evaluation of

spray materials to control fire blight of apple and

pear blossoms (80c).

Carpenter and Shay (11+7) and Carpenter (1090,

1091 ) were the first to attemptthe development of a

mass inoculation technique to screen seedlings in

their pear breeding program. When leaves and succulent shoots were sprayed with a bacterial suspension at 15 pounds per square inch (1 kg/cm^), only 10

percent of the plants became infected. When 2-

year-old nursery trees without succulent shoots

were sprayed at 50 pounds per square inch (3.5

kg/cm^) with Carborundum added to the inoculum,

39 percent of the trees were blighted. They concluded that there was no visible resistance in the

seedlings prior to the 8-leaf stage but that intermediate types of resistance existed at the 18- to

24-leaf stage.

Maximum movement of the blight organism in

plant tissue is usually obtained when inoculated

plants are kept under optimum environmental conditions for blight development. Small lots of plants

are usually placed for 3 to 14 days in growth chambers or small humidity chambers maintained at

26° C (79° F) and 80-100 percent relative humidity

(565, 105Í, 1058, 1068). During 1966-69, several

types of humidity chambers were tested at

Beltsville, mainly as enclosures over raised beds of

seedlings in the field (fig. 26, A). Maintaining optimum temperatures was difficult, but fair success

was obtained in May and June when a layer ofburlap

shading was used above the plastic chamber.

As a result of these techniques, we decided to

handle large populations of seedlings at Beltsville

with a specially prepared rapid inoculation device

(fig. 27) under a large plastic canopy over the

greenhouse bench (fig. 26, B). The inoculation apparatus consisted of a set of large aluminum forceps

containing a florist pin holder on one side and a

sponge on the opposite side (1059,1068, 1075,1077).
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 143

FIGURE 26.—Methods used in inoculating and planting pear seedlings at Beltsville, Md.: A, Small plastic enclosure with supplemental

burlap shading for experimental control of temperature and humidity over raised beds of inoculated seedlings; B, environmental

control under plastic structure in greenhouse for optimum blight development in inoculated pearseedlings; C, 1-year-old seedhngs

in double rows in field; D, blight in double rows of 6- to 8-year-old seedlings.

The sponge was attached by a hose to a bottle of

inoculum and clamped to a backpack made of

aluminum tubing. With this inoculator, two technicians could inoculate rapidly and uniformly about

4,500 pear seedlings per day with approximately 4

gallons (15 1) of inoculum.

Following inoculation, the tentlike structure was

closed and two humidifiers were activated to provide

a relative humidity of 85-100 percent within the

enclosure. With the regular greenhouse heating system and the aid of two large window fans and a wet

pack on the opposite side of the greenhouse, the

temperature during April and May was maintained

between 21° and 27° C (70°-80.5° F). The use ofthis

inoculation device resulted in a high percentage of

blighted seedlings (107^, 1077). Although such

screening has provided desirable selection pressure

in determining blight resistance, it may eUminate a

higher percentage of trees than necessary.

Wrather (J.139) reported in 1973 of successfully
144 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

PN-6391

FIGURE 27.—Apparatus for rapid inoculation of seedlings with Erwinia amylovora at Beltsville, Md.: A, Backpack equipment,

consisting of aluminum tubing (t ) and rubber hose (rh ), showing plastic inoculum bottle (6 ) attached to inoculator (insert) by plastic

hose iph ); flow of inoculum may be regulated by valve (v); B, closeup of inoculator, consisting of aluminum forceps with hinge and

spring attachments; florist pin holder (p) and sponge (s) are in opposite wells.

inoculating etiolated pear and apple seedlings by

puncturing the stem and placing a drop of inoculum

with a known number of cells per milliliter in the

opening. He found the seedlings as resistant to fire

blight as mature plants following exposure to light

for 14 days. In Michigan, Ritchie and Klos {795)

needle-inoculated similar young seedlings to study

E. amylovora isolates for pathogenicity. Ahn (1079)

studied the wilting response of several apple cultivars, mountainash (Sorbus), and hawthorn

(Crataegus) species to phenolic compounds. Differences in wilting time between susceptible and resistant apple cultivars were significant when shoot tips

were inserted in 4-methylcatechol 0-0 diacetic acid.

From East Germany, Schaefer et al. (S^Oa) reported more successful blight development in young

apple and pear trees in the greenhouse with a "needle prick" (Nadelstich) than a "squeeze-bruise"

(Quetsch) inoculation method.

Inoculum

In all these inoculation techniques not much is

known about the actual concentration ofinoculum or

number of bacterial cells necessary to produce infection and symptom expression. In 1937, Hildebrand

Ul8) conducted one of the early inoculation studies

with small numbers of E. amylovora cells. When

single cells were introduced into nectaries of excised

apple flowers, 60 percent became infected; with 10 or

more cells per nectary, all flowers became infected.

However, similar tests with intact flowers on pear

trees in the greenhouse failed to produce infection.

In 1976, Aldwinckle and Beer (Ua) reported that

they obtained blossom infection in Golden Delicious

apples using as few as 10 cells per blossom.

Initiation of shoot blight appears to require more

cells per inoculation for infection to develop. Powell

(758) demonstrated a direct relationship between

inoculum concentration per milliliter and incubation
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 145

period in Jonathan apple. With 200 cells, no infection

resulted; with 2 x 10^ cells, 40 percent of the shoots

were blighted after incubation for 7-10 days,

whereas with 2 x 10'^ cells, 100 percent of the shoots

were infected after 4-5 days. Lelliott (575), on the

other hand, in field studies showed a mean EDso

(dose at which 50 percent of plants respond) of 2 x

10^ cells per milliliter for leafinfection and 6.6 x 10^

cells for stem infection. Wounded seedling leaves

required 20 times fewer cells for infection than did

unwounded leaves.

Preliminary studies ^^ at Beltsville indicated that

fewer cells were required to produce disease symptoms in apples than in pears. Furthermore, fewer

cells of a streptomycin-sensitive strain of E.

amylovora appeared to be required to produce

symptoms than of a streptomycin-resistant strain. If

repeated studies show this to be true, one might

expect the streptomycin-sensitive strain to eventually overcome the resistant strain.

Sources of bacterial inoculum have varied among

investigators from a bacterial suspension produced

by extraction from freshly blighted tissue in vacuo

ií75, 9Í8) to a cell suspension prepared from a virulent E. amylovora culture grown on solid or liquid

media in vitro {565,105Í, 1068), as well as a mixture

of several cultures (^^7, 765). In our research^^ at

Beltsville we have usually applied concentrations of

5 X 10'^ cells per milliliter from a single isolate and

results have generally been comparable with the

previously mentioned types of inocula.

Disease Evaluation

The degree of blight resistance in young seedlings

has usually been expressed by measuring the total

length of the seedling at time of inoculation and the

visually blighted part when blight ceased to move

any farther. From this, the percent blighted tissue

could thus be calculated. Seedlings with a trace to 25

percent ofthe shoot blighted are usually classified as

highly resistant (scores 10-8), from 26 to 50 percent

as moderately resistant (scores 7-6), and more than

50 percent as susceptible to very susceptible (scores

5-1). In a slight variation of this system, Thompson

et al. {U75, HS) and Thompson {1136) estabhshed

four resistance groups based on the average length

of blighted tissue per tree as follows: 0-1.9, 2-9.9,

10-19.9, and 20 or more inches (0-4.9, 5-24.9, 25-

49.9, and 50 or more cm).

Unpub. data, Fruit Lab., U.S. Dept. Agr., Beltsville, Md.

In our studies at Beltsville a general correlation

was observed in degree of shoot blight resistance

between young budded pear trees artificially inoculated in the greenhouse and mature trees ofthe same

cultivars naturally blighted in the field {1057). Similar results were reported from New York on degree

of blight between young apple cultivar trees in the

greenhouse and nursery {16a).

Breeding Programs

Breeding programs to improve pear and apple

treesfor blight resistance were started in the United

States about the mid-19th century. At this time,

blight was reported in various cultivars and the first

plant exploration trips were made to Europe, Asia,

and the Orientto collect resistant germ plasm. Table

14 summarizes the most significant pear and apple

breeding programs and selection-evaluation sites in

North America since about 1870-80. Detailed summaries of pear and apple breeding programs in

North America prior to World War II have been

published {11, 623, 62Í).

Pear

Nearly all pear cultivars introduced from Europe

proved susceptible to fire blight. In the dry, irrigated valleys of the Pacific Coast States, where

nights are relatively cool, the disease is less serious

than in the Eastern and Central United States. By

1840 the sand pear (P. pyrifolia) was growing in

America and became widely distributed in the eastern part ofthe country. Soon after, hybrids between

the sand pear and cultivars of the common pear (P.

communis) began to appear. Garber, Kieffer,

LeConte, Pineapple, and more recently Ayres,

Mooers, and Orient are the best known of these

gritty, coarse-fleshed fruit hybrids. Because oftheir

culinary quality, however, they were widely grown

in home orchards and small commercial plantings.

All pear breeding programs in North America had

one main objective, i.e., to obtain high-quality dessert pears with a high degree of fire blight resistance. Of the inactive pear breeding programs not in

existence today, the one by Patten in Iowa in 1867 is

probably the oldest. His work was mainly to develop

pears sufficiently hardy to thrive in the upper Mississippi Valley. He grew mainly open-pollinated

seedlings ofP. ussuriensis, which have proved more

hardy than pears from any other source {730, 731).

In 1922 the cultivar Patten (Orel 15 x Beurre d' Anjou) was introduced by the Iowa Agricultural Exper-
TABLE 14.—Active and inactive pear and apple breeding programs and selection-evaluation sites in North America

Location Period

(approximate) Plant Research activity ^ Breeder or evaluator Reference

hi^

Oi

UNITED STATES

California:

Palo Alto 1920-35

Davis 1928-

Georgia:

Experiment 1912-36

Blairsville, Byron 1959-

Idaho: Moscow 1909-40

Illinois:

Urbana 1908-22

1922-

1942-68

Carbondale 1951-

Indiana: Lafayette 1945-

1947-

lowa:

Charles City 1868-1932

Ames 1880-1960

Kentucky: Lexington 1950-55

Maryland:

College Park 1906-35

Beltsville 1890-1932

1932-60

1960-

Michigan:

South Haven 1916-35

East Lansing 1945-

Minnesota: St. Paul 1890-

Mississippi: State College 1925-

Missouri:

Columbia 1905-60

Mountain Grove 1901-

New Jersey: New Brunswick 1948-

New York:

Ithaca -r 1888-92

Geneva 1892-

Pear - BP

Pear, apple BP, ES

Apple, pear ES

do BP, ES

Apple BP

do--

Apple, pearBP

BP, ES

Pear BP, ES

Apple, pear ES

Apple BP, ES

Pear BP, ES

Apple, pearApple

Pear

BP, ES

BP

ES

Apple, pear BP

Pear BP

do BP, ES

do BP, ES

do BP, ES

Apple, pear ES

do BP, ES

Pear ES

Apple, pear ES

do BP, ES

do BP, ES

-do- - BP, ES

Pear, apple BP, ES

Wight, W. F.2 62J^

Chandler, W. H., Tufts, W. P., Griggs, W. H. 372

Parish, J. J., Stuckey, H. P. 908, 909, 1037

Hardigree, J. B.,2 Thompson, J. M.^ 386, 945, 946

Vincent, C. C, Longley, L. E. 981

Crandall, C. S. 191

Anderson, H. W., Dayton, D. F., McMunn, R. L. 21, 22, 215, 622

Hough, L. F., Barrett, H. C. 67, 68, 435

Mowry, J. B. 666-668

Shay, J. R., Williams, E. B. 121, 215, 861, 1025, 1136

Carpenter, T. R., Janick, J. 147, 473, 474-475, 948,

1090, 1091

Patten, C. G. 730, 731

Budd, J. L., Beach, S. A., Lantz, H. L. 556-559, 624

Olney, A. J. 709

Auchter, E. C.,^ Close, E. C. 54

Waite, M. B.,2 Shear, E.^ 624

Magness, J. R.,2 Moon, H. H.^ 624, 660

Brooks, H. J.,2 van der Zwet, T.^ 116, 117, 707, 1054, 1065,

1068, 1070-1073

Wight, W. F.,2 Johnston, S. 479, 624

Tukey, H. B., Carlson, R. F. 146, 958, 960

Wilcox, A. N., Alderman, W. H., Stushnoff, C. 9, 12, 13, 912

Overcash, J. P. 716, 871

Whitten, J. C, Chandler, W. H., Murneek, A. E. 624

Faurot, F. W., Shepard, P. H., Hansen, K. W. 624

Hough, L. F., Bailey, C. H. 57, 58, 436

Bailey, L. H., Cox, H. R., Hsiong, S. L. 186, 443, ^^^ 1108

Howell, G., Beach, S. A., Oberle, G., Lamb, R. C. 200, 386a, 549-552, 1009,

1087

>

S

Q

a

a

>

a

td

o

o

a

Ö

o

>

o

s

Q

a

a
North Carolina: Raleigh 1955-

North Dakota: Mandan 1915-66

Ohio: Wooster 1915-66

1966-

Oregon:

Talent 1911-50

Medford 1950-

Corvallis 1961-

Pennsylvania: State College 1929-39

South Dakota: Brookings 1895-1945

Tennessee:

Knoxville 1921-30

Merricourt 1931-60

Virginia: Blacksburg 1910-

Apple BP, ES

Pear - ES

Apple, pear BP, ES

do ES, BP

Pear BP, ES

do ES, BP

do ES

do ES

do ES

Apple, pear BP, ES

Pear, apple ES

Pear BP, ES

Apple ES, BP

Klein, L., Cummins, J. N., Way, R. D., Aldwinckle, H., lU, 17, 200-20J,,

CANADA

British Columbia: Summerland— 1930- do BP, ES

Manitoba: Morden 1916-70 do BP

Nova Scotia: Kentville 1950- Apple, pear BP, ES

Ontario:

Ottawa 1899-1968 Apple BP

Vineland 1913- Apple, pear BP, ES

Harrow 1962- Pear BP, ES

Quebec: Saint Jean 1968- Apple BP

Saskatchewan: Saskatoon 1925- do BP

^BP=breeding program; ES=evaluation site.

^Employed by USDA.

Hamilton, J.

Clayton, C. N.

Pfaender, M.,^ Baird, W. P.,2 Oitto, W. A.'

Keil, J. B., Howlett, F. S.

Oitto, W. A.,2 Blake, R. C.^

Reimer, F. C.

Lombard, P. B.

Westwood, M. N.

Nixon, E. L.

Hansen, N. E., McCrory, S. A.

McClintock, J. A.

Drain, B. D.

Drinkard, A. W., Horman, F. W., Moore, R. C.

Oberle, G. D.

Lapins, K. 0.

Kerr, L., Ure, R., Quamme, H. A.

Bishop, C. J., Crowe, A. D.

Hunter, A. W. S., White, F. H., Macoun, W. T.,

Spangelo, L. P. S.

Dickson, G. H., Anderson, E., Tehrani, G.,

Hutchinson, A.

Layne, R. E. C, Quamme, H. A.

Rousselle, G., Lereau, M.

Patterson, H., Nelson, S. H.

1006-1010

62U

118

1,39--U2

705, 706, 1070

776, 777, 779, 780, 782

uo. 589, 590

uo. lOU, 1015

696

381, 609

599--601

227, 229--233

662, 700, 701

623, 62U

623, 62J,

198a-198c

93, 207, 459, 623, 62J,

219

563-566, 76J,a, 765

623, 62J,

w

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148 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

iment Station. Between 1918 and 1928, crosses and

backcrosses between Patten's seedlings and P.

communis cultivars were made, but much of this

work was lost when the station was closed in 1932.

A similar program of breeding hardy, blightresistant pears was conducted by Hansen {381) at

the South Dakota Agricultural Experiment Station

in Brookings. Several cultivars were named, such as

Finland (Finland Early Yellow x OP), Ming (P.

ovoideae x Louise Bonne de Jersey), Hansen Seedless (P. sinensis x Marguerite Marillat), Tanya

(Ideal X P. ussuriensis), and Valya (Lincoln x Russian sand pear).

During 1925-60, a major pear breeding and selection-evaluation program was carried out in Tennessee. Started under McClintock {599, 600), it was

continued and expanded by Drain {227). About 1933,

McClintock discovered a resistant pear seedling and

later named it Late Faulkner. Since 1925 the work

consisted mainly of crossing resistant species, such

as P. pyrifolia, P. calleryana, and P. ussuriensis,

with the more resistant cultivars of P. communis.

By 1943, of 1,586 seedlings containing one-fourth P.

pyrifolia, more than 50 percent developed no fire

blight; in 710 seedlings with one-eighth P. pyrifolia,

70 percent did not develop blight {229). Several pear

cultivars have been introduced from this program

{118, 230, 231), such as Ayres (Garber x Beurre

d'Anjou), Dabney (Seckel x Garber), Carrick, Hoskins, Mericourt (all three from Seckel x Late Faulkner), Mooers (Duchess d'Angouleme x Late Faulkner), Morgan (Bartlett x Late Faulkner), and

Orient (P. comm^unis x P. sp. China).

Between 1942 and 1968, another large pear

breeding program was conducted at the University

of Illinois at Urbana. Pear species and cultivars had

been tested since 1919 {21, 22). In addition, some

very resistant pear trees (Old Home, Farmingdale,

and Longworth) were discovered in and near Illinois. Hough {J^35) used primarily cultivars and

selections of Chinese species, such as Pai Li and P.

ussuriensis 76, and crossed them with several P.

communis cultivars. Later Barrett {67, 68) continued this program and broadened the base of

blight-resistant material, using Lincoln, Old Home,

Sudduth, and several resistant Illinois selections. In

1968, this work was discontinued. Many selections

from Hough's program were moved to Rutgers University in New Jersey and have been used extensively there by him as well as in the USDA program

at Beltsville. Many selections from Barrett's program are currently under observation at the N.Y.

Agricultural Experiment Station at Geneva.

Other past pear breeding programs, smaller but of

importance in their time, were active in California

(Palo Alto), Georgia (Experiment), Kentucky

(Lexington), Maryland (College Park), Minnesota

(St. Paul), New York (Ithaca), North Dakota (Mandan), and Pennsylvania (State College). From the

Pennsylvania program originated the highly resistant Richard Peters cultivar (Kieffer x OP) {696).

Of the active pear breeding programs, the most

productive was at the N.Y. Agricultural Experiment Station at Geneva. This program has consisted

mainly in hybridizing P. communis cultivars to produce high-quality fruit {5^9, 550). Between 1920 and

1945, the following 11 cultivars were introduced:

Cayuga, Caywood, Chapin, Clyde, and Early Seckel

(all open-pollinated seedlings of Seckel); Covert,

Ovid, and Willard (all Bartlett x Dorset seedlings);

Gorham (Bartlett x Josephine de Malines); and

Phelps and Pulteney (Winter Nelis x Russet

Bartlett seedlings). Most recent introductions were

Aurora (Marguerite Marillat x Bartlett) in 1964,

Sirrine (seedling) in 1970, and Highland (Bartlett x

Comice) in 1974 {551, 553). It should be emphasized,

however, that all these introductions from New

York are blight susceptible and must be protected in

areas where fire blight is a problem.

The pear breeding program in New Jersey was

undertaken by Bailey and Hough {57, 58) in 1948. It

provided some very interesting and important

blight-resistant selections, many with P. pyrifolia

and P. ussuriensis parentage. An apparently

blight-resistant pear seedling (NJ 1) was used extensively in this breeding program. The tree and

foliage are intermediate betweenP. pyrifolia and P.

communis cultivars. This selection has mediumsized, very short pyriform, green-skinned fruit,

which is not buttery but contains a few stone cells. In

1968, Hough and Bailey {^36) introduced three

blight-resistant pear cultivars for the fresh market— Star and Lee (both from a cross of

Beierschmitt x NJ 1) and Mac (Gorham x NJ 1). At

Beltsville the cultivars Star and Lee were found to

be susceptible, whereas Mac was moderately resistant {707, 1070).

The oldest continuous pear breeding program is

that of the U.S. Department of Agriculture at

Beltsville. In 1908, Waite (fig. 3) started hybridizing

pears at Arlington Farms in Virginia {990). He intercrossed the moderately resistant Kieffer, Seek-
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 149

el, and Anjou cultivars with the susceptible Bartlett.

Many thousands of seedlings were inoculated with the blight organism, but few resistant

seedlings were recovered. One selection of unknown

parentage was introduced in 1938 as the Waite cultivar (fig. 28, A), but it was never widely planted.

In 1938 the breeding program was moved from

Arlington to Beltsville, where it was under the direction of Magness (62^, 660). Several European

cultivars with the most blight resistance, such as Roi

Charles de Wurtemberg (RCW), were intercrossed

with promising selections from preceding programs.

In 1960 three new pear cultivars were introduced

(fig. 28, B-D). Magness (Seckel seedling SPI 49490

X Comice) is a male sterile, high-quality dessert

pear, resistant to twig blight but susceptible to

trunk blight (1060,1063). It was later found susceptible to blossom blast caused by a Pseudomonas

species (506). Moonglow (US-Mich. 437 x RCW) is a

medium-quality dessert and processing pear, with

moderate resistance to fire blight. Dawn (US-Mich.

437 X Comice) is a high-quality dessert pear, which

at first was thought to be susceptible to fire bhght

but later proved fairly resistant (1070). Figure 29, A

and By shows that numerous Magness, Moonglow,

and Dawn trees were ahve following severe bhght

epiphytotics. The missing Magness trees died from

trunk blight. The many empty spaces due to loss of

trees from fire blight indicate the relative resistance

of these three cultivars to this disease.

In 1960, Brooks et al. (117) continued the pear

breeding program when five objectives were initiated: (1) Extensive collection of species and cultivars from around the world for evaluation of blight

resistance; (2) evaluation and use of several oriental

Pyrus species, hybrids, and cultivars as parents; (3)

planting of large numbers of seedlings annually for

evaluation on a 10-year rotation schedule; (4) mass

inoculation of seedlings in the greenhouse for evaluation of blight resistance; and (5) establishment of

genetic studies on blightinheritance, tree juvenility,

and fruit characteristics. Since 1966 a cooperative

progi'am has been maintained with the Ohio Agricultural Research and Development Center at Wooster

(117, 706).

The extensive planting of many pear cultivars collected worldwide to evaluate their blight resistance

resulted in the loss of about 90 percent of the

trees (fig. 29, C) and about 50 percent of the trees in

a planting of commercial cultivars (707, 1070). The

best known resistant and susceptible cultivars are

listed in table 10. Degree of resistance also was

examined in 107 selections in 17 Pyrus species (table

9), 85 selections from controlled interspecific crosses, and a large number of species hybrids (1069,

1072). Degree of blight resistance varied among

species, clones, and selections within species and

species hybrids. In interspecific crosses, resistance

was not consistently transmitted either by crossing

a highly resistant with a very susceptible species or

by crossing two highly resistant ones. The highest

overall resistance appeared to result from crossing

two moderately resistant parents (1072).

Combinations of low and intermediate levels of

fire blight resistance within P. communis have resulted in numerous resistant selections with fair to

good fruit quality (1057a). Subsequent combinations

of these and other additive sources of resistance

have resulted in such resistant cultivars as Magness

and Moonglow.

Nearly 32,000 pear seedlings at Beltsville and

10,000 at Wooster, Ohio, from more than 600 progenies derived from controlled pollinations have been

planted as of 1975 and are being evaluated for blight

resistance, fruit quality, tree vigor, and time and

amount of flowering (1073). At Beltsville the seedlings have been spaced 4 feet in the row and in

3-foot-wide double rows, which are 16 feet apart (1.2

X 1.0 X 5.0 m) (fig. 26, C andZ); fig. 29, D). Thus, we

were able to plant 1,400 trees per acre (3,500/ha),

and the trees will have attained sufficient growth so

that all required data can be collected within 8-10

years from the planting date. In addition, 3,000

seedlings screened for blight resistance in the

greenhouse are being evaluated for natural blight in

the field to determine the relationship between

natural and artificially induced blight. All data are

computerized so that genetic studies can be made for

these characteristics.

One of the first significant contributions from this

computerized breeding program is the determination of a dominant gene. Se, causing sensitivity in P.

communis to fire blight (9^7, 1076). A total of 13

Cultivars and 4 selections are Sese and 3 selections

are sese. At this time we question previous reports

of the apparent existence of dominant genes EWi

and EW2, carrying resistance in P. pyrifolia and P.

ussuriensis, respectively (19, 229, 9^8, 1136).

In the mid-1970's, Bell (108i) and Bell et al. (81e)

reported on a possible correlation between pear fruit

quality and fire blight resistance, based on data collected in the USDA pear breeding program. They
150 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

FIGURE 28.—Pear cultivara released by the U.S. Department of Agriculture: A, Waite; B, Magness

PN-«392

C, Moonglow; D, Dawn.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 151

PN-6393

FIGURE 29. —Aerial view of USDA pear plantings at Beltsville, Md.: A, Randomized and replicated block ofthree pear cultivars: Large

trees, Magness; medium-sized trees, Moonglow; smallest trees, Bartlett; S, single-row planting ofmajor pear cultivars: Magness,

row 1 (bottom); Bartlett, rows 2 and 3; Moonglow, row 4; and Dawn, row 5; C, planting of2 trees each of500 pear cultivars collected

worldwide; seedling progenies in upper half of photograph; D, several blocks of pear seedlings planted in double rows, as shown in

figure 26, C and D. Note: With few exceptions all missing trees in these plantings were destroyed by fire blight.

concluded that blight resistance and fruit quality of

the progenies studied were not genetically linked or

physiologically associated and that simultaneous

selection for resistance and high fruit quality appeared feasible. Possibly the breeder should concentrate on germplasm of high fruit quality available

within P. communis and discard susceptible seedlings U7ib). Bell et al. i8lf) also studied regressing

progeny means on midparental phenotypes and obtained heritability estimates for fire blight resistance in pear. They determined that approximately

half of the variability in resistance in pear is additive, but also they found evidence for nonadditive

genetic effects compatible with the qualitative gene

for sensitivity reported by Thompson et al. (9^7).

Between 1916 and 1919, the USDA pear program

also maintained a cooperative breeding project with

the Michigan Agricultural Experiment Station at

South Haven. Parent trees in this program had

either moderate blight resistance or desirable fruit

quality. Of all progenies, seedlings from Barseck

(Bartlett x Seckel) x Bartlett had the best fruit

quality in combination with moderate resistance to

fire blight. One selection from this parentage, USMich. 437, has since been used extensively as a parent at Beltsville. Also, Johnston (in Brooks and

Olmo, 118) in 1935 introduced the cultivar Campas

(parentage unknown) for canning purposes.

Another of the large pear breeding programs in

North America is that ofthe Canada Department of
152 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Agriculture (CDA) at Harrow, Ontario. This program was started in 1962 by Layne (561, 56i) in

order to produce high-quality, blight-resistant pears

for southwestern Ontario. The program consists of

crossbreeding most of the known sources of blight

resistance with the best commercial cultivars of high

fruit quality (566), All seedlings from controlled pollinations are artificially inoculated and the resulting

seedlings grouped into four blight-resistance classes

according to the amount of blight.

Studies at Harrow on about 7,500 seedlings in 48

progenies have shown that both quantitative and

qualitative inheritance offire blight may be involved

(565). The breeding value of the most efficient P.

communis sources (Magness, Maxine, and USMich. 437) appeared to be superior to the P. ussuriensis and P. pyrifolia sources, principally because of the larger, higher quality fruit associated

with P. communis.

The first phase of a cooperative study between

CDA and USDA has been completed. Its purpose

was to determine the possible correlation of fire

blight resistance of young pear seedlings inoculated

in the greenhouse with mature seedling trees naturally infected in the field (765, 1057, 107i). PreHminary results indicated a general correlation, and

early screening appears useful in breeding fire

blight-resistant pears by eliminating the most susceptible seedlings.

In 1947-48, the CDA Research Station at Ottawa

introduced a series of four blight-resistant pear cultivars—Enie, Menie, Miney, and Moe—to be

grown in home gardens in eastern Ontario and

Quebec (93, Í59). Menie was selected from a cross of

Kurskaya x Flemish Beauty, whereas the other

three cultivars originated from the cross Zuckerbime X Clapp Favorite. This station was closed in

1968.

Lombard and Westwood (590) and Westwood et

al. (1015) have maintained a large collection ofPyrus

species, hybrids, and cultivars at Corvallis and

Medford, Oreg. This material has been evaluated for

resistance to fire blight and for use as rootstocks

against Phytophthora root rot, blast, pear decline,

and root aphids (UO),

Other active pear breeding programs and selection-evaluating sites are located in California

(Davis), Georgia (Byron), Illinois (Carbondale), Indiana (West Lafayette), Michigan (East Lansing),

Mississippi (State College), Missouri (Mountain

Grove), and North Carolina (Raleigh), the provincial

station at Vineland, Ontario, and the station of the

CDA at Kentville, Nova Scotia.

Recent pear cultivar introductions are Rogue Red

(Seckel x Farmingdale seedling) from Oregon in

1969, Spartlett (Bartlett seedling) from Michigan in

1973, California (Max-Red Bartlett x Comice) from

California in 1974, and Honeysweet (Seckel x US

220) from Indiana in 1976. The first three cultivars

are only moderately resistant to fire blight (1J^6,372,

589), whereas Honeysweet appears to be rather resistant (Jf7Jfa).

Soon after the appearance of fire blight in Europe

in 1957, a pear breeding program was undertaken in

England. By 1966, several inoculation techniques

were evaluated at the John Innes Institute to determine seedling resistance to E. am^ylovora (631).

This program is being continued at East Mailing.

Alston (19) reported in 1972 that among such other

breeding criteria as late maturing, long storage, and

dwarftree habit, the main goal againstfire blight isto

avoid parent trees that produce secondary blooms.

Apple

Objectives in apple breeding have varied in different sections of North America. Because fire blight

has usually been less severe on apples than on pears,

the main objective has not been to combine blight

resistance with superior fruit quality, although some

breeders have been advocating this since 1930.

Throughout most of the central United States increased winter hardiness is highly desirable as in the

northern Great Plains, New England, and Canada.

An extensive review of apple breeding prior to 1938

was published by Magness (62i).

In breeding for disease resistance, scab probably

has been considered more important than fire blight.

However, fire blight is a serious disease on

Jonathan, Wealthy, York Imperial, and several

other cultivars. On the other hand, Delicious, Ben

Davis, and the Winesap family of cultivars are fairly

resistant.

One of the first apple breeding programs was

begun in 1868 by Patten in Charles City, Iowa. He

introduced several cultivars, and some selections

from his work were later used extensively by the

Iowa Agricultural Experiment Station at Ames by

Budd et al. (558, 559) until about 1960. In 1909

another extensive apple breeding program was

begun by Vincent and Longley (981) in Idaho. Results from nearly 12,000 seedlings indicated that the
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 153

cultivars Jonathan, Wagener, and Esopus Spitzenburg provided the highest dessert quality when used

as parents, whereas seedlings derived from Ben

Davis had the best keeping quality. About the same

time, Crandall (191) started a breeding program at

Urbana, 111., which was later continued by Dayton et

al. (215) and Mowry (666, 667).

At the Ohio Agricultural Research and Development Center in Wooster, apple breeding was begun

by Keil in 1915 and continued in 1929 by Howlett.

Other apple breeding programs, nearly all of which

were terminated by World War II, were in Georgia

(Experiment), Maryland (College Park), Missouri

(Columbia), New York (Ithaca), North Dakota

(Mandan), and South Dakota (Brookings). The program at Mandan was directed by the U.S. Department of Agriculture. Between 1952 and 1965, the

following apple cultivars were introduced at Mandan: Custer, Dakota, Garrison, Heart River, Killand. Mandan, Peace Garden, Prairie Gold,

Stephens, and Thorberg {118). For additional apple

cultivars introduced from these programs, see the

extensive review by Magness {62Jt) and the fruit

variety register by Brooks and Olmo {118). Those

reported cultivars with ratings for fire bhght resistance are listed in table 12.

Of the active apple breeding programs, the oldest

is that of the N.Y. State Agricultural Experiment

Station at Geneva started by Beach. The main objectives initially were to produce high-quality, productive, late-keeping cultivars that would be fully hardy

under New York State conditions. Many well-known

cultivars were used to obtain thousands ofseedlings

for evaluation of these characters. During 1914-68,

52 apple cultivars were introduced {1007). The best

known of these were Cortland (Ben Davis x Mclntosh), Early Mclntosh (Yellow Transparent x

Mclntosh), Empire, originating from an open-pollinated seedling in an orchard containing only Mclntosh and Delicious trees {1008), Lodi (Montgomery

X Yellow Transparent), Macoun (Mclntosh x Jersey Black), Milton (Yellow Transparent x Mclntosh), Monroe (Jonathan x Rome Beauty), Niagara

(Carlton x Mclntosh), and Wayne (Northwestern

Greening x Red Spy). Recent apple introductions

from Geneva include Jonamac (Mclntosh x

Jonathan) in 1972 and Burgundy from a cross of

Monroe x (Macoun x Antonovka) in 1974 {1008,

1010). Ofthese cultivars, Lodi and Wayne appear to

be susceptible, whereas the remainder are moderately resistant to fire blight (table 12).

In 1975, Gardner {1101) reported that he had identified near immunity to fire blight in several clonal

selections of Malus species, following numerous

shoot tip inoculations in the greenhouse and field

nursery. From crosses between these resistant

clones and the susceptible M 9, M 16, and M 27

rootstock clones, about one-eighth ofthe seedlings in

each progeny exhibited the resistance level of the

resistant parent. In progenies from crosses between

the susceptible Mailing parents, 90-100 percent of

the seedlings were killed by fire blight, and of the

survivors more than 50 percent of the shoot lengths

were blighted. Gardner concluded that resistance

appeared to be oligogenic, with control by dominant

additive genes carried in the heterozygous condition.

A comprehensive apple breeding program is being

carried out in Indiana. It was undertaken in 1945 by

Shay et al. {861 ) and continued by Dayton et al. {215)

and Williams et al. {1025). It is a highly cooperative

endeavor among the agricultural experiment stations in Indiana, Illinois, and New Jersey. Names of

new cultivars carry the prefix PRI, an acronym

formed by the three cooperating institutions—Purdue, Rutgers, and Illinois. Four new cultivars have

been introduced from this program: Prima (14-510 x

N.J. 12324a) in 1970, Priscilla (Starking Delicious x

610-2) in 1972, and Sir Prize (Golden Delicious (4x)

X 14-152) in 1975. These three cultivars are not only

highly resistant to scab but also to fire blight in the

field in Indiana {215, 1025, 1025a). In addition,

Priam (Golden Dehcious x 26829-2-2) x Jonathan)

was introduced in France in 1974. This cultivar is

only slightly susceptible to mildew; its resistance to

fire blight was not mentioned {215a).

In 1959 the USDA undertook a minor apple

breeding program at Blairsville, Ga., to serve the

Southeastern United States {886). The principal

objectives were fire blight resistance and superior

horticultural characteristics, including late blooming, early maturity, and fruit color. Following blight

epidemics in 1963, 1967, 1970, and 1971, Thompson

{H6) reported fire blight ratings on approximately

300 fruiting apple cultivars and selections on various

rootstocks and 100 nonbearing cultivars. In addition, he published data on tree precocity. Also, about

12,000 seedlings from controlled pollinations are

being evaluated. The headquarters for this breeding

program has been moved to Byron, Ga.

Other locations for current apple breeding programs and selection-evaluation sites are Illinois
154 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

(Carbondale), Michigan (East Lansing), Minnesota

(St. Paul), Missouri (Mountain Grove), Virginia

(Blacksburg), and Vineland, Ontario.

Pyracantha

At the U.S. National Arboretum in Washington,

D.C., Egolf (260a) has maintained a large

pyracantha breeding program since 1959. Cold hardiness and disease resistance have been emphasized.

Since 1960, 80,000 seedlings have been produced,

from which 72 selections have been test planted for

evaluation. Mass inoculations of the plants have

been performed with the rapid inoculator (fíg. 27) to

screen them for fire blight resistance (262). The cultivar Shawnee, considered a spontaneous hybrid of

Pyracantha koidzumii (Hyata) Rehd. and P. crenato-serrata (Hance) Rehd., was introduced in 1966

and the cultivar Mojava (P. koidzumii x P. coccinea

Roem. cv. Wyatt) in 1970 (259, 260). Both are resistant to fire blight and scab (Fusicladium pyrinum

(Lib.) Fckl. var. pyracanthae Thuem.).

Hawthorn and Cotoneaster

A breeding and selection-evaluation program has

been undertaken in Denmark since 1970 to find

blight resistance in hawthorn (Crataegus) (882).

Large numbers of plants and clonal selections of

several species from various countries in Western

Europe are being evaluated for fire blight resistance

in the field (Jt.89). Blight-resistant cultivars of hawthorn are of utmost importance because they are

extensively used as windbreaks for fruit orchards

and generally as ornamental shrubs.

At the U.S. National Arboretum, Egolf (^^i) has

inoculated large numbers of cotoneasters to select

possible resistant germplasm. In West Germany,

Zeller and Meyer (10Jf6h, 10^6c) are evaluating

numerous hawthorns and cotoneasters against

natural blight infection. A cooperative endeavor is

underway among fire blight investigators from

Denmark, West Germany, and the Netherlands to

hybridize, select, and evaluate these host plants for

resistance to fire bhght and for important horticultural characters.

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1974. FIREBLIGHT — LET'S KEEP IT OUT OF AUSTRALIA. Tasmanian Jour. Agr. 45: 175-178.

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1962. ANTAGONISTIC EFFECT OF A BACILLUS SUBTILIS

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1972. TOXIC EFFECT OF AUTOCLAVED MEDIA CONTAINING

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1962. EVALUATION OF EARLY RIPENING APPLE INTRODUCTIONS. U.S. Plant Introd. Sta., Glenn Dale,

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1929. THE FIREBLIGHT MENACE. SEVERE EXPERIENCE

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1976. LEAKAGE OF ELECTROLYTES AND PHENOLS FROM

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— and GOODMAN, R. N.

1972. POLYPHENOL OXIDASE AND PEROXIDASE ACTIVITY

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1973. MONO- AND DI-PHENOL OXIDASE ACTIVITY AND

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1956. MOVEMENT OF CERTAIN ANTIBIOTICS IN CUTTINGS

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1940. A NEW APPLE AND PEAR. Minn. Hort. 68: 45.

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1946. NEW MINNESOTA FRUITS: REDWELL APPLE,

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FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 155

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1974. FIELD SUSCEPTIBILITY OF 46 APPLE CULTIVARS TO

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1976. NUTRIENT STATUS OF APPLE BLOSSOMS AND THEIR

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1973. ARTIFICIAL INOCULATION OF APPLE ROOTSTOCK

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1943. FIRE BLIGHT EPIDEMIC IN 1942. Ohio State Hort.

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1972. PEAR BREEDING AT EAST MALLING. Internatl.

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1920. DISEASES OF ILLINOIS FRUITS; FIRE BLIGHT OF APPLE; PEAR BLIGHT OR FIRE BLIGHT. 111. AGR.

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156 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

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1961. A NEW PEAR INDUSTRY FOR NEW JERSEY. N.J.

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1971. FIRE BLIGHT OF POME FRUITS: THE GENESIS OF THE

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1917. SUSCEPTIBILITY OF APPLE VARIETIES TO BLIGHT IN

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1961. BREEDING NEW PEAR VARIETIES RESISTANT TO FIRE

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1971. DESSERT PEARS IN MIDWESTERN GARDENS. Fruit

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194 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

(1104) GROVES, A. B.

1928. A COMPREHENSIVE BIBLIOGRAPHICAL RESUME OF

THE DISEASE KNOWN AS FIRE BLIGHT OF APPLES AND

PEARS. Univ. Ark. M.S. thesis, 105 pp.

(1105) HALL, C. J. J. VAN.

1902. BIJDRAGEN TOT DE KENNIS DER BACTERIEELE

PLANTENZIEKTEN. Univ. Amsterdam doct.

diss., 198 pp.

(1106) HENDRY, A. T.

1966. STUDIES OF ERWINIA AMYLOVORA AND ASSOCIATED

ORGANISMS. Univ. Guelph M.S. thesis, 96 pp.,

illus.

(1107) HILDEBRAND, D. C.

1962. RELATION OF THE ARBUTIN-HYDROQUINONE COMPLEX IN PEAR TO THE DEVELOPMENT OF FIRE

BLIGHT. Univ. Cahf. Ph. D. diss., 112 pp., illus.

(1107a) HOCKENHULL, J.

1975. FIRE BLIGHT SURVIVAL IN HOST TISSUES, OVERWINTERING AND SOURCE OF PRIMARY INOCULUM OF THE

PATHOGEN, ERWINIA AMYLOVORA. Roy. Vet. and

Agr. Univ., Copenhagen, doct. diss., 84 pp.

(1108) HsiONG, S. L.

1938. STUDIES OF SOME CROSSES OF KIEFFER PEAR WITH

SPECIAL REFERENCE TO THE INHERITANCE OF DESSERT AND KEEPING QUALITIES, DISEASE RESISTANCE AND OTHER PLANT CHARACTERS. Cornell

Univ. Ph. D. diss., 97 pp., illus.

(1109) HUANG, P. Y.

1969. MORPHOLOGY AND ULTRA STRUCTURE OF WILD AND

FILAMENTOUS TYPES OF ERWINIA AMYLOVORA. Univ. Mo. M.S. thesis, 70 pp., illus.

(1110)

1974. ULTRASTRUCTURAL MODIFICATION BY AND

PATHOGENICITY OF ERWINIA AMYLOVORA IN APPLE

TISSUES. Univ. Mo. Ph. D. diss., 134 pp., illus.

(1111) JONES, A. L.

1963. USE OF ORGANIC PESTICIDE SPRAYS AND OF INSECT

CONTROL TO PREVENT FIRE BLIGHT SHOOT INFECTIONS. Cornell Univ. M.S. thesis, 55 pp., illus.

(1112) LAKE, C. F.

1973. SUMMER PRUNING OF APPLES IN MINNESOTA. Univ. Minn. M.S. thesis, 53 pp., illus.

(1113) LEWIS, L. N.

1961. STUDIES ON THE NUTRITION OF BARTLETT PEAR

TREES (PYRUS COMMUNIS L.) AND FIRE BLIGHT (ERWINIA AMYLOVORA BURR.). Mich. State Univ.

Ph. D. diss., 85 pp.

(1114) LEWIS, S. M.

1964. MODE OF PENETRATION AND MOVEMENT OF A STREPTOMYCIN RESISTANT STRAIN OF ERWINIA AMYLOVORA IN APPLE LEAF AND STEM TISSUE. Univ. Mo.

Ph. D. diss., 81 pp., illus.

(1115) LUEPSCHEN, N. S.

1960. FIRE BLIGHT CONTROL WITH STREPTOMYCIN AS INFLUENCED BY TEMPERATURE AND OTHER ENVIRONMENTAL FACTORS, AND BY ADJUVANTS ADDED

TO SPRAYS. Cornell Univ. Ph. D. diss., 91 pp.

(1116) MCCORMICK, A. C.

1917. HISTOLOGICAL STUDY OF PEAR BARK IN RELATION TO

BLIGHT RESISTANCE. Oreg. State Univ. M.S.

thesis, 30 pp., illus.

(1117) McCowN, M.

1928. BORDEAUX MIXTURE IN THE CONTROL OF FIRE

BLIGHT OF APPLE. Purdue Univ. M.S. thesis, 32

pp., illus.

(1118) MCINTYRE, J. L.

1974. PROTECTION OF PEAR AGAINST FIRE BLIGHT BY BACTERIA, BACTERIAL SONICATES AND BACTERIAL

DEOXYRiBONUCLEic ACID. Purdue Univ. Ph. D.

diss., 148 pp., illus.

(1119) MORGAN, B. S.

1955. LABORATORY STUDIES ON THE USE OF ANTIBIOTICS

IN PLANT DISEASE CONTROL. Univ. Mo. M.S.

thesis, 75 pp., illus.

(1120) MORRILL, G. D.

1969. OVERWINTERING OF THE FIRE BLIGHT PATHOGEN

INSIDE LIVING HOST TISSUE IN CACHE VALLEY,

UTAH. Utah State Univ. M.S. thesis, 40 pp., illus.

(1121) NEAL, J. W., JR.

1964. INVESTIGATION ON APHIDS IN ASSOCIATION WITH

FIRE BLIGHT. Univ. Mo. M. S. thesis, 92 pp., illus.

(1122) PARKER, K. G.

1934. FIRE BLIGHT: OVERWINTERING, DISSEMINATION AND

CONTROL OF THE PATHOGEN. Cornell Univ. Ph. D.

diss., 42 pp.

(1123) PlERSTORFF, A. L.

1929. STUDIES ON THE FIRE BLIGHT ORGANISM, BACILLUS

AMYLOVORUS(BURR.)TREV. Cornell Univ. Ph. D.

diss., 108 pp.

(1124) PLURAD, S. B.

1967. APPLE APHIDS AND THE ETIOLOGY OF FIRE BLIGHT

DISEASE. Univ. Mo. Ph. D. diss., 69 pp., illus.

(1125) REINHARDT, J. F.

1954. THE RELATIONSHIP OF COPPER IN SUCCULENT

SHOOTS OF YOUNG JONATHAN TREES TO FIRE BLIGHT

INFECTION. Univ. 111. M.S. thesis, 22 pp.

(1126)

1957. THE EFFECT OF SUB-FREEZING TEMPERATURES ON

THE VIABILITY AND PATHOGENICITY OF THE FIRE

BLIGHT PATHOGEN, ERWINIA AMYLOVORA (BURR.)

WINSL. ETAL. Univ. 111. Ph. D. diss., 104 pp.,

illus.

(1127) REINKING, 0. A.

1915. THE OVER-WINTERING OF BACILLUS AMYLOVORUS. Univ. Wis. M.S. thesis, 61 pp.

(1128) RiGGLE, J. H.

1970. STUDIES ON ERWINIA HERBICOLA, THE YELLOW

BACTERIUM ASSOCIATED WITH ERWINIA AMYLOVORA AND ITS RELATIONSHIP TO FIRE BLIGHT OF

PEAR. Mich. State Univ. M.S. thesis, 61 pp., illus.

(1128a) SCHRODER, C. M.

1977. UNTERSUCHUNGEN ZUR SYMPTOMATOLOGIE, DIAGNOSE UND EPIDEMIOLOGIE DES FEUERBRANDES

VERURSACHT DURCH ERWINIA AMYLOVORA (BURRILL) WINSLOW ET AL. Christian-Albrechts Univ.

Ph. D. diss., 180 pp., illus.

(1129) SHAFFER, W. H., JR.

1961. SOME FACTORS AFFECTING THE VIRULENCE OF ERWINIA AMYLOVORA. Univ. Mo. M.S. thesis, 80

pp., illus.
FIRE BLIGHT - A BACTERIAL DISEASE OF ROSACEOUS PLANTS 195

(1130) SHAW, L.

1929. STUDIES ON FIRE BLIGHT: I. THE IDENTITY OF A YELLOW SCHIZOMYCETE FOUND ASSOCIATED WITH

BACILLUS AMYLOVORUS; II. THE REACTION OF B.

AMYLOVORUS TO THE PRESENCE OF THIS YELLOW

ORGANISM. Univ. Ark. M.S. thesis, 57pp., illus.

(1131)

1932. STUDIES ON RESISTANCE OF APPLE AND OTHER

ROSACEOUS PLANTS TO FIRE BLIGHT. Univ. Wis.

Ph. D. diss., 30 pp., illus.

(1132) STULIN, T. M.

1977. PROPERTIES AND MODE OF ACTION OF AMYLOVORIN,

A WILT-INDUCING POLYSACCHARIDE FROM FIRE

BLIGHT EXÚDATE. Cornell Univ. M.S. thesis, 67

pp., illus.

(1133) SMITH, J. H.

1965. TAXONOMIC CHARACTERISTICS OF A YELLOW BACTERIUM ISOLATED FROM THE TISSUE OF JONATHAN

APPLE TREES. Univ. 111. M. S. thesis, 25 pp., illus.

(1134)

1967. ETIOLOGICAL STUDIES OF ERWINIA AMYLOVORA

(BURR.) WINSL. ET AL. AND ELECTROPHORETIC COMPARISONS OF IT WITH OTHER BACTERIA. Univ. 111.

Ph. D. diss., 68 pp., illus.

(1134a) STAHL, F. J.

1976. TRANSMISSION OF ERWINIA AMYLOVORA TO PEAR

FRUIT BY LYGUS SPECIES. Colo. State Univ. M. S.

thesis, 42 pp., illus.

(1135) STEWART, V. B.

1913. THE FIRE BLIGHT DISEASE IN NURSERY

STOCK. Cornell Univ. Ph. D. diss., 56 pp., illus.

(1136) THOMPSON, S. S.

1961. METHODS OF EVALUATION OF RESISTANCE TO FIRE

BLIGHT OF APPLE AND PEAR. Purdue Univ. M.S.

thesis, 53 pp., illus.

(1137) THOMSON, S. V.

1969. THE OVER-WINTERING OF THE FIRE BLIGHT BACTERIA OUTSIDE OF LIVING TISSUE IN UTAH. Utah

State Univ. M.S. thesis, 35 pp., illus.

(1138) TULLíS, E. C.

1929. STUDIES ON THE OVER-WINTERING AND MODES OF

INFECTION OF THE FIRE BLIGHT ORGANISM. MICH.

State Univ. Ph. D. diss., 32 pp.

(1139) WRATHER, J. A.

1973. INHIBITION OF FIRE BLIGHT IN POME FRUITS BY

NON-PATHOGENIC BACTERIA AND EFFECT OF LIGHT

ON SUSCEPTIBILITY OF ETIOLATED SEEDLINGS. Purdue Univ. M.S. thesis, 64 pp., illus.

(1140) ZEHR, E. I.

1965. DIMETHYL SULFOXIDE AND CERTAIN FUNGICIDES

AS SUPPLEMENTS TO STREPTOMYCIN FOR FIRE

BLIGHT CONTROL. Cornell Univ. M.S. thesis, 49

pp.

SURVEY OF FIRE BLIGHT RESEARCH IN THE UNITED STATES AND CANADA

In 1974, an ad hoc committee^^ of the Apple and

Pear Disease Workers surveyed the fire bhght research in the United States and Canada. The purpose of this survey was to determine the following

facts about this research: (1) Studies in progress,

(2) nature of the research, (3) personnel involved,

(4) supporting agencies, and (5) location. The questionnaire was returned by 71 professional workers

from 26 States of the United States and 2 Provinces

of Canada.

Table 15 summarizes the pertinent data. The

committee found that 47 professional investigators

devoted part or full time to fire blight research in the

United States and Canada. About 9.6 scientific

man-years (SMY) were spent on fire blight research

in the United States and 2 SMY in Canada during

1974. Included with scientists in the United States

were seven professional investigators (1.3 SMY)

employed by commercial companies, who evaluated

new chemicals for fire blight control. For each SMY

in the United States, there were 2.7 support person2^This committee included H. L. Keil (chairman), H. S.

Aldwinckle, S. V. Beer, R. P. Covey, A. L. Jones, E. J. Klos,

W. R. Landis, W. J. Möller, J. D. Moore, and T. van der Zwet.

nel compared with about 2.2 in Canada. Among the

42 U.S. investigators, 27 were considered to be

working in pathology, 10 in horticulture, 4 in

bacteriology-physiology, and 1 in biochemistry.

Breeding of pears, apples, or both for fire blight

resistance and evaluating progenies from these

programs predominated the research activity of

many professionals. Although one cannot determine

from the questionnaires exactly how much time an

investigator devoted to breeding, we know from

personal contact that about 3.5 SMY were spent on

research associated with breeding. Georgia

(USDA), Indiana, Maryland (USDA), New Jersey,

Ohio (cooperative with USDA), and Canada (Harrow, Ontario) had active breeding programs. Minor

programs were conducted in California, Michigan,

Minnesota, New York, and North Carolina, where

1-10 percent of a professional investigator's time

was spent principally on variety evaluation.

The largest breeding program (2.0 SMY), which

was supported by the U.S. Department of Agriculture, was devoted to breeding pears resistant to fire

blight. This research was conducted at Beltsville,

Md., and Wooster, Ohio. In addition, 0.25 SMY was

spent on fire blight resistance in apples (USDA-
TABLE 15.—Data on fire blight research in the United States and Canada, 197i ^

Location Investigator Discipline

Full-time Support

effort personnel

Number Percent

Source of

support

Objectives

^

Oi

UNITED STATES

California:

Berkeley - Schroth, M. N. — Pathology -

Möller, W. J. doZoUer, B. G. doDavis- Ryugo, H. Physiology —

Starr, M. P. BacteriologyRichmond - Landis, W. R.2— PathologyColorado: Grand Junction Luepschen, N. S. -doDelaware: Wilmington Davidson, S. do —

Georgia: Byron Thompson, J. M.- HorticultureIllinois: Urbana Ries, S. M. BacteriologyIndiana: Lafayette - Kuc, J.^ BiochemistryWilliams, E. B.— Pathology

Janick, J. HorticultureMaryland: Beltsville vanderZwet, T.~ Pathology —

30

30

40

10

25

20

25

25

10

10

10

15

100

5.U

— > State, Fed. Ext., Hatch, Indus. Study ecology, biology, and variation of Er-

— / winia amylovora; chemical and biological control; monitor bacterial population on pear

trees.

.05 State Evaluating hybrid resistance in pears.

,5 do Study ecological genetics of£'?"M;mm, particularly virulence in E. amylovora.

1.0 Food and Mach. Corp. Field evaluation of chemicals for blight control.

1.0 State, Hatch, Indus. Evaluating new chemicals for blight control

and field inoculation techniques.

.5 Dupont Evaluating new chemicals for blight control.

Fed. (USDA) Breeding and evaluating apple selections for

blight resistance.

.1 Hatch Evaluating new chemicals for bhght control

and effect of mulching on blight control.

sol ' V USDA coop, agreement Study biochemical factors in resistance; fac-

/ tors contributing to symptom development; in

vivo and in vitro studies ofsubstances synthesized by infected host; inducement of resistance factors by avirulent strains ofE. amylovora.

.3 State, Hatch Breeding pears for blight resistance.

1.5 Fed. (USDA) Breeding pears for bhght resistance; genetic

studies of blight inheritance; improvement of

inoculation techniques; study relation of artificially inoculated seedlings with naturally infected trees; determining degree of resistance

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Keil, H. L.- -doMichigan: East Lansing Klos, E. J. -

-doCarlson, R. F.— HorticultureMinnesota: St. PaulJones, A. L.—

Sutton, T. B.2 -

O'Malley, J.

Pathology -

do-

-doStushnoff, C. HorticultureMissouri: Columbia - Goodman, R. N. - Pathology —

Huang, H. S.^ -— BacteriologyNew Jersey: New Brunswick Hough, L. F. HorticultureBailey, C. H. do—-

New York:

Geneva

See footnotes at end of table

50

20

5

15

100

10

Aldwinckle, H. S. Pathology —

Szkolnik, M. do

Gilpatrick, J. do —

Cummins, J. N. - HorticultureLamb, R. C. do

Way, R. D. do--

20

10

10

4

.5 Fed. (USDA) -

.7 State, Indus.-

State-

::} Fed. Ext.

1.5 3MCo.-

.5 State—

251A 1.2 1.2 State, Nati. Sei. Found.

100 I Hatch, Indus.

10 .05 State, Hatch10 - do2.5 -do-

--do-

--do-

--do-

-do-

--doin various pear tissues; study pathogenicity of

single and mixtures ofE. amylovora isolates;

developing dwarf-resistant pear varieties.

- Evaluating new chemicals for blight control;

determining £'7-M;mm cell numbers to produce

blight symptoms; determining if tissue developed after infection acquires any resistance; determining role of bacterial strands in

dissemination; effect of tree injections on

eradication and control; determining susceptibility of apple dwarfing rootstocks.

Evaluating new chemicals for blight control;

study ways to increase effectiveness of compounds; determining role of yellow Erwinia;

altering tree physiology by fertilizer and

biological compounds to reduce susceptibility;

monitoring£'. amylovora populations; surveying for resistant individuals; study role of insect vectors.

Breeding and selecting pear varieties resistant to blight.

Developing better timing of fire blight treatments; monitoring£". amylovora populations.

Evaluating new chemicals for bhght control.

Breeding and evaluating apple selections for

blight resistance.

Study host specificity, biochemical properties,

and mode of action of E. amylovora toxin;

developing bioassay with toxin for evaluating

blight resistance of pear and apple seedlings.

> Breeding pears for blight resistance.

ÍEvaluating new chemicals for blight control;

developing inoculation techniques to evaluate

resistance ^f apple cultivars and seedHngs in

breeding program.

-- Breeding and evaluating apple rootstocks for

blight resistance.

- Breeding and evaluating pear seedHngs for

blight resistance.

- Breeding and evaluating apple selections for

blight resistance.

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TABLE 15,—Data on fire blight research in the United States and Canada, 197Jp i—Continued

Location

Full-time Support

Investigator Discipline effort personnel

Number Percent

UNITED STATES—continued

Ithaca Beer, S.V. PathologyMiddleportNorth Carolina:

Goldsboro —

Raleigh -

Ohio: WoosterOregon: Medford

Virginia:

Blacksburg -

Winchester -

Washington:

WenatcheeHarnish, W.-

Perry, R. S.-

-do-

-doBrooks, D. H. — do—--

Clayton, C. N.—- do—--

Blake, R. C. HorticultureCameron, H. R.-- Pathology —

Drake, C. R. -- -doHickey, K. D.^ — doCovey, R. P. doSource of

support

Objectives

50

1

1

5

1

100

5

40

L9 State, Hatch

2.0)

2.0

— Evaluating control practices; study

epidemiological, physiological, and biological

factors affecting bhght infection and resistance.

Food and Mach. Corp. Evaluating new chemicals for bhght control.

.1 Imper. Chem.

.01 State

- Fed. (USDA)

State, Hatch -

State, Indus.-

.1 State, Indus.-

State--

Yakima - Biehn, W.^- -do- .7 Ciba-Geigy Corp.-

Field testing of chemicals for bhght control.

Evaluating pear seedlings in greenhouse and

field for blight resistance.

Breeding pears for bhght resistance.

Evaluating susceptibility of potential

rootstocks.

Determining whether pear industry can be estabhshed in Virginia.

Evaluating new chemicals for bhght control.

Evaluating new chemicals for blight control;

study effect of environmental factors and

bacterial population on blight development;

surveying extent of streptomycin resistance.

Evaluating new chemicals for bhght control.

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CANADA

British Columbia:

Summerland.

Lopatecki,L.C.2- —-do- lo Cañad. Dept. Agr. Study characteristics of E. amylovora bacteriophage and its role in bhght etiology; in

vitro and in vivo studies ofE. amylovora and

related nonpathogenic bacteria.
Mclntosh, D. L.- ._do 10

Ontario:

Guelph -

Harrow -

Gibbins, L. N.—- Bacteriologyphysiology.

Quamme, H. A.-- HorticultureBonn, W. G. Pathology —

30

.75 Ganad, and Brit. Columbia

Dept. Agr.

2.0 Nati. Res. Council

of Canada.

100

50 ':"} Ganad. Dept. Agr.

Part- and full-time professional fire blight investigators inU.S.

Canada —

Support personnel for fire blight research inU. S.

Canada

42.0 (SMY9.6)

5.0 (SMY2.0)

25.9

4.3

(MY)

(MY)

lA November 1977 survey indicated only minor changes in personnel and percentage of full-time effort.

^No longer at this location.

Monitoring populations of £'. amylovora in irrigated and nonirrigated pear and apple trees;

study ehmination ofE. amylovora from fruit

being exported.

Study oxidative metabolism ofE. amylovora

through aerobiosis and anaerobiosis transition; bacteriocinogeny, lysogeny, and nature

of temperature-induced variation in E. herbicola.'

Breeding and evaluating blight-resistant varieties of pear and dwarfing rootstocks; study

inheritance of resistance; developing satisfactory control measures by studying epidemiology and environmental factors; monitoring £".

amylovora in orchards.

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200 AGRICULTURE HANDBOOK 510, U.S. DEPT. OF AGRICULTURE

Georgia). New Jersey devoted 0.2 SMY and Indiana

0.15 SMY to their pear breeding programs. The

Canadian Department of Agriculture supported 1

SMY to breeding pears resistantto fire blight. Also,

a small amount of time was used in evaluating

rootstocks for fire blight resistance.

New chemicals for fire blight control were tested

in California, Colorado, Illinois, Maryland (USDA),

Michigan, New York, Virginia, and Washington. An

estimated 2-3 SMY were devoted to this work by

the academic institutions. In addition, at least seven

commercial companies were developing new chemicals for fire blight control. This commercial effort

amounted to about 1.4 SMY, which seemed hardly

enough when E. amylovora strains resistant to

streptomycin have been found in several areas ofthe

United States. Resistance ofE. amylovora to streptomycin was studied in California, Maryland

(USDA), and Washington.

Studies on the etiology, biology, and ecology oîE.

amylovora were conducted in California, Colorado,

Illinois, Maryland (USDA), Michigan, Missouri,

New York, and in British Columbia, Canada. In

addition, epidemiology was investigated in Indiana,

Maryland (USDA), Michigan, New York,

Washington, and Ontario.

Techniques for monitoring bacterial populations

were being developed in California, Michigan,

Washington, and Canada. As soon as they are perfected, we anticipate much more activity in this

phase of fire blight research. In California, researchers were spending a small amount of time

studying the ecological genetics of the genus Erwinia, particularly the virulence of£'. amylovora to

its host. In Canada, some time was devoted to

bacteriology-physiology studies.

In the United States, fire blight research was

supported by funds of Federal and State governments and by industry. Without industry, this left

about 8.3 SMY dependent on government for support. Federal money for fire blight research was

appropriated through the Agricultural Research

Service, Extension Service, Hatch funds, and the

National Science Foundation. About 4-5 of the 8.3

SMY devoted to fire blight research were supported

by Federal sources and the remainder by State

funds. The Canadian Government completely supported the 2 SMY.

Ifwe consider that fire blight usually causes more

damage to pears and apples in the United States

than in Canada and that the United States produces

16 and 7 times more pears and apples, respectively,

then the U.S. Government appears to support proportionately far less fire blight research than the

Government of Canada. Since much ofthe fire blight

research in the United States is fragmented, better

coordination of this research is needed so that a

concerted effort can be made to solve the many

problems associated with the disease.

T^U.S. GOVERNMENT PRINTING OFFICE: 1979—245-584