Control of peach bacterial spot in southern orchards

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UNITED STATES DEPARTMENT OF AGRICULTURE

BULLETIN No. 543

Contribution from the Bureau of Plant Industry

a WM. A. TAYLOR, Chief

>
SH Pu.

Washington, D. C.

CONTROL OF PEACH BACTERIAL SPOT IN
SOUTHERN ORCHARDS.

By Joun W. Roperts, Pathologist, Fruit-Disease Investigations.

CONTENTS.
Page. Page
ENETOGTCHION Ceres erento oo Uke eee {>} Varietal susceptibility. . 2-2-2 hac. cee a ease 3
Description of the disease............-------- 20| ‘Control experiments. : 2... 2.222 g5 2220 2k i
Cause Of the disease... 52. - 4. - 2-2 -. 5... 2255 3 | Summary and conclusions................... ti
INTRODUCTION.

In most of the peach-growing sections of the eastern half of the
United States the disease commonly called bacterial spot, or bacteri-
osis, is becoming increasingly important, especially in the more
southerly sections. It is impossible to give any estimate of the
damage which it causes annually, (1) because it is mainly a disease
of the foliage and (2) because of its great seasonal variation. It has
been reported as occurring in practically the whole of the eastern
United States, including Massachusetts, Connecticut, New Jersey,
Delaware, Maryland, Pennsylvania, Michigan, Illinois, Indiana, Ohio,
Kentucky, Tennessee, Virginia, North Carolina, South Carolina,
Georgia, Alabama, Arkansas, Missouri, Nebraska, and Texas. As
a rule, it is most serious in the more southerly peach-growing sec-
tions, but during recent years many specimens from farther north
were sent to the Bureau of Plant Industry for identification and
for suggestions as to control measures. During the years 1914 and
1915 the writer found the disease to be serious in the Ozark region of
Arkansas, being surpassed in destructiveness only by scab and brown-
rot.

Note.—This bulletin is of interest to peach growers in the eastern half of the United States and particu-
larly those of the more southerly parts of this section. It is also of interest to plant pathologists,

84987°—Bull. 543——17

2 BULLETIN 543, U. S. DEPARTMENT OF AGRICULTURE.

DESCRIPTION OF THE DISEASE.

Leaves, fruit, and twigs are affected, but the injury to the leaves
usually is the most serious phase of the disease.

The first indication of injury to the foliage is the appearance of
small, nearly transparent, water-soaked areas. These are par-
ticularly noticeable when a leaf is held between the observer’s eyes
and the sun. Later, the spots enlarge, turn darker, and at length
become dry and brittle. Then, as a final stage, they crack away
from the living tissues and often fall out entirely, giving the leaf
the so-called shot-hole appearance (Pl. I, figs. 3 and 4). Sometimes

a number of spots, especially those near the margin of the leaf, will

coalesce, giving the leaf a burned or blighted appearance. Later,
these dead areas, composed of several spots which have run together,
may break away and fall to the ground, giving the leaf a peculiar,
ragged appearance. Infections may be so numerous as to injure
every leaf on the tree, they may be localized so as to affect seriously
only the foliage of certain limbs, or the infection may be mild all
over the tree, so mild at times as not to be especially important.

Though the disease in its later stages on the leaves is difficult to
distinguish from spray injury, damage by shot-hole fungi, and some
other diseases, when it appears on the fruit, especially after it has
passed the first stage of its development, it is not easily so confused.
Minute spots, scarcely darker than the skin of the young fruit, denote
the first appearance of the disease. These spots soon become some-
what enlarged and gradually become darker in color. Later, as the
fruit enlarges, small cracks appear in the diseased areas (PI. I, figs. 8
and 9). Beginning with this stage, the disease is most characteristic
and is not easily confused with any other type of injury. Later, with
further growth of the fruit, the cracks are extended, and finally several
may run together, making long irregular fissures (PI. I, figs. 1 and 2).
The flesh of the fruit is protected by the formation of a corky layer
in these cracks, but nevertheless the fruit itself presents a ragged and
irregularly cracked surface which, except in mild cases, renders it
unfit for market.

A grayish water-soaked spot is the first indication of the disease
on the twigs of the current season. This soon becomes dark and later
is sunken. If there are a number of infections close together they
may coalesce, forming a rather large canker, which may persist, with
rather abundant flow of gum. Plate I, figures 5, 6, and 7, shows
advanced stages of the disease on twigs.

Usually in the Ozarks the first infections on leaves, twigs, and fruit
occur in May, or sometimes even earlier. The disease does not
become conspicuous, however, until much later; in the Ozarks
about the middle of June in the case of leaves and twigs and about

wing

; 3 and 4, leaves sho
; 8 and 9, fruits showing

gs

in

bacterial cankers on twi
figures 1 and 2.

PEACH BACTERIAL SPOT.
he disease on fruits that are nearly ripe
hown

5, 6, and 7,

earlier stages of the disease than ares

; .
?

1sease

FRUIT, LEAVES, AND TWIGS OF ELBERTA PEACHES, SHOWING VARIOUS STAGES OF
late stages of the d

Bul. 543, U. S. Dept. of Agriculture.
land 2, Advanced stage of t.

CONTROL OF PEACH BACTERIAL SPOT. 3

July 15 in the case of the fruit. At these times the leaves are begin-

ning to become ragged and shot holed and the affected fruits are |

showing conspicuous cracks.

As far as the peach is concerned, the direct killing of twigs and
branches is rare, and this phase of the disease is not in itself to be
considered as very serious. The twig lesions are of importance,
however, in that it is in these that the causal organism passes the
winter. Orchards in which direct damage to the fruit causes much
loss are also rather rare. It is the injury to the leaves, sometimes
amounting to complete defoliation and often to 25 to 50 per cent
defoliation, and the consequent damage to tree and fruit through the
curtailment of the supply of plant food, which make this an important
disease.

CAUSE OF THE DISEASE.

The disease is of bacterial origin and is caused by Bacterium pruni
Erw. F. Smith. |

Smith? in 1908 first successfully inoculated the foliage of the
peach with the organism which he had obtained in pure cultures
from the plum. |

Rorer * in 1909 reported successful inoculations on peach leaves
with pure cultures of the organism from the same source. He also

- obtained the organism from twigs and described the disease on the

fruit.

Many successful cross inoculations were reported upon by Rolfs *
in 1915, confirming the work of Smith and Rorer and establishing
beyond doubt that the disease on foliage, fruit, and twigs is caused
by the same organism.

Bactervum prum is described as a short rod, occurring singly, in
pairs, or sometimes in chains, motile by polar flagella, from 0.001
mm. to 0.0018 mm. long and about half as wide. Warm, moist
weather conditions, with the trees in a weakened state, due to previous
injury by freezing, lack of pruning, lack of fertilization, etc., are most
favorable for infection and subsequent growth of the causal organism.

VARIETAL SUSCEPTIBILITY.

Practically all varieties are attacked, at least to some extent, by
this disease. The Elberta, the leading commercial peach, is very
susceptible. It is very difficult to estimate the relative suscepti-

1 Smith, Erwin F. Observations on a hitherto unreported bacterial disease, the cause of which enters
the plant through ordinary stomata. Jn Science, n.s., v. 17, no. 429, p. 456-457. 1903.

2Smith, Erwin F. Occurrence of Bacterium pruni in peach foliage. In Science, n. s., v. 30, no. 763,
p. 224. 1909. Abstract of paper read at the meeting of the Society of American Bacteriologists, December,
1908.

3 Rorer, J. B. A bacterial disease of the peach. In Mycologia, v.1, no. 1, p. 23-27. 1909.

4Rolis, F.M. A bacterial disease of stone fruits. N. Y. (Cornell) Agr. Exp. Sta. Mem. 8, p. 381-436,
fig. 59-70. 1915. Literature cited, p. 435-436.

4 BULLETIN 543, U. S. DEPARTMENT OF AGRICULTURE.

bility of different varieties, but the Bilyeu, Elberta, Carman, Cham-
pion, Oldmixon, Sneed, and Waddell appear to be more susceptible
than such varieties as the Hiley, Belle, Fox, Edgemont, Rivers, Early
Crawford, and Salway. Rolfs' gives an extensive list of varieties,-
with estimates as to relative susceptibility and resistance. All
varieties were found by him to be susceptible to some extent.

CONTROL EXPERIMENTS.

It was not until 1913 that the problem of control came directly
before the Bureau of Plant Industry. During that year one section
of an orchard in the Ozark region of Arkansas in which spraying
experiments for the control of brown-rot and scab were being con-
ducted suffered severely from an attack of this disease. Twigs, leaves,
and fruit were badly infected, and no control by the spraying was
apparent. It may be said both from the experiments of this bureau
and from those of Rolfst that the sulphur mixtures (not lime-sulphur
solution), which, are the only known fungicides that can be used with
safety on the peach during the growing season, will not control this
disease. In Georgia in 1909 the writer noticed that trees which were
in good growing condition, that is, which had been well pruned,
fertilized, and cultivated, were practically free from the disease,
whereas those which had been allowed to go without pruning, fer- -
tilization, and cultivation were to a considerable extent damaged
by the disease. Similar observations were made in Georgia in 1912,
but the disease did not then appear to be of sufficient importance
to demand experiments in control.

When, therefore, in 1913 it seemed desirable to work out proper
means of control, the first things that suggested themselves were nat-
urally pruning, fertilization, and cultivation. The writer’s observa-
tions were further supported by a statement in the report of the
Director of the Missouri Agricultural Experiment Station for 1912?
to the effect that in the case of winter-injured trees the plats fertilized
with nitrogen were nearly free from the bacterial disease, while in
adjacent unfertilized plats the damage was very great.

During the seasons of 1914 and 1915 the same orchard in which
the spraying experiments had been conducted in 1913 was used
for experimental work in the control of bacterial spot. This orchard
consisted of a block of Elberta trees which was bounded on one side
by aroad, on the second side by a cultivated field, on the third side by
a meadow, and on the fourth side by an apple orchard. The disease
had been particularly severe in the corner adjacent to the meadow
and the apple orchard. Accordingly, the experiments were con-

1 Op. cit.
2 Mumford, F. B. Fertilizing peach trees. Jn Mo. Agr. Exp. Sta. Bul. 111 (Rpt. [1911]12), p. 247-248,
1913.

CONTROL OF PEACH BACTERIAL SPOT. | 5

ducted in that section of the orchard. The trees themselves had
been well pruned and were of good average size. The soil was, |
in texture, good peach soil, but inclined to be somewhat lacking
in natural fertility.

In 1914 treatment was as follows: Plat 1, consisting of 18 trees,
received 2 pounds of nitrate of soda per tree; plat 2, consisting of 6
trees, received 1 pound of nitrate of soda per tree; plat 3, consisting of
12 trees, received 3 pounds of nitrate of soda per tree; plat 4, consist-
ing of 18 trees, received 3 pounds of bone meal per tree; plat 5, con-
sisting of 12 trees, received 6 pounds of bone meal per tree; and plat
6, consisting of 12 trees, received 4 pounds of acid phosphate per tree.
The fertilizer was applied on May 4 in arimg or band about the trees,
directly below the ends of the branches. The application of fertilizer
was immediately followed by cultivation.

On June 8 the disease was evident, though not as yet serious, in
all the plats, including both treated and untreated trees. The
amount of infection in all plats appeared to be about equal. Each
plat had some trees which were next to the meadow, the grass of
which extended nearly to the trunks of the trees. In all cases these
trees, deprived by the meadow of their proper share of food materials -
and moisture, were the worst affected. By July 2 plats 1, 2, and 3
(the nitrate plats) showed few leaf infections except in the case of
the trees close to the meadow. Even these, however, were much
freer from the disease than were the untreated trees. The leaves of
the unfertilized trees showed considerable injury, about 20 per cent
of them being affected. Plats 4, 5, and 6 (the bone-meal and acid-
phosphate plats) appeared to be somewhat better than the untreated
portion of the orchard, but the difference was not nearly so striking
as the difference between the nitrate-of-soda plats and the untreated
trees.

By the end of the season the untreated trees had lost fully one-
third of their leaves. Of the remaining leaves, about 30 per cent
showed some injury from the disease. On the nitrate-of-soda plats
there was little defoliation from the disease, and few leaves showed any
infection. Most of the injury was on the trees next to the meadow.
The leaves of the trees in these plats were large and possessed the
dark-green color characteristic of healthy peach leaves, which is the
usual effect of fertilizing peach trees with nitrate of soda. Plats 4, 5,
and 6 (the bone-meal and acid-phosphate plats) were in somewhat
better condition than the untreated plats, but not especially so. It
will be noted that, while at the beginning of the season injury from
the disease was slight and about the same in all the plats, it later n-
creased and became commercially important only on those plats
which had not received the nitrate-of-soda treatment.

6 BULLETIN 543, U. S. DEPARTMENT OF AGRICULTURE.

During 1914 there was no fruit on any of the trees in this orchard,
the fruit buds in the peach orchards of northwestern Arkansas having
been killed by low temperatures in January.

The experiments of 1915 were conducted in the same orchard as
were those of 1914. Plat 1, consisting of 34 trees, was treated with
nitrate of soda at the rate of 2 pounds per tree; plat 2, consisting of
24 trees, was treated with nitrate of soda and dried blood at the rate
of 14 pounds of the former and 1 pound of the latter per tree. Six
trees between these two plats were left untreated and were con-
sidered as checks, as was also the remainder of the orchard.

Owing to the press of other work, the fertilizers were not applied
until May 22. The orchard had been cultivated a week before and
was again cultivated on June 1. The trees had bloomed on April 25
and had set a good crop.

On June 10 there was very little of the disease on the nitrate plats,
but considerable was beginning to show on the untreated plats.
The large dark-green leaves of the nitrate plats were in sharp con-
trast to the relatively pale leaves of the unfertilized plats. By June
30 the contrast was much greater, as the disease had given a ragged
-appearance to the foliage of the untreated plats, while the nitrate
plats remained nearly free from infection.

On August 5 the fruit was just beginning to ripen. On certain
limbs of the untreated trees there was considerable injury to the
fruit, though not enough to be of importance commercially. The
fruit of one limb, for instance, would be badly affected, while that of
the remainder of the tree would be free from injury. There was
almost no injury to the fruit of the nitrate plats.

By the end of the season the difference in the appearance of the
foliage of the nitrate plats and the untreated ones was as great as
during the previous season. The difference between this damage
and the total damage caused by the bacterrum was about the same
on both the fertilized and unfertilized trees as during the previous
season. There was no perceptible difference between plats 1 and 2
in so far as the amount of infection was concerned. Eight of the
fertilized trees which bordered on the meadow were considerably
more affected than the others in the same plats, but were infected
much less than the unfertilized trees.

During both years the nitrate plats included the trees which during
1913 suffered the heaviest infection. Sixteen of the trees treated
with nitrate of soda in 1914 were included in the nitrate plats in 1915.
These, however, showed no superiority, in so far as resistance to the
disease was concerned, over those which had not received fertilizer
during the previous year. In the experimental plats of both 1914
and 1915 the sole difference in the treatment of the plats, including
the check plats, was one of fertilization. All had been given the

CONTROL OF PEACH BACTERIAL SPOT. f

same amount of pruning and cultivation and received throughout the
two years exactly the same treatment except as to the fertilization
outlined in these experiments. It is evident, therefore, that the
striking differences noted were due to the nitrate of soda alone. The
larger amount of infection on the trees of all plats growing next to
the meadow, upon which the grass was encroaching, indicates that
cultivation was generally beneficial to all the plats. Rolfs,! in Mis-
souri, obtained good results in the control of this disease by means of
cultivation, pruning, and the use of nitrogenous fertilizers. In those
regions of the South in which the disease has been serious, nitrogen
appears to be the most deficient of the important plant-food
materials.? .

These experiments indicate that, at least in the South, peach
orchards which are kept in good growing condition are not liable to
suffer from the disease to any serious extent. Proper pruning, cul-
tivation, and fertilization so increase the resistance of the trees that
the causal organism is unable to attack them successfully.

Pruning, besides benefiting the tree in general, may also remove
many of the twig cankers in which the bacterium passes the winter,
thus eliminating many of the sources of infection.

SUMMARY AND CONCLUSIONS.

(1) The peach bacterial spot, also known as bacteriosis, caused
by Bactervum pruni, occurs in practically all peach-growing regions
of the eastern half of the United States. It is most serious in the
more southerly parts of this region. Bacteriwm pruni also causes a
disease of the plum, affecting especially the Japanese varieties.

(2) Twigs, fruit, and leaves are affected, but the most serious
injury is to the leaves.

(3) Experiments carried on by the writer and others indicate that
the disease may be kept in check in southern peach orchards by
proper pruning, cultivation, and especially fertilization. Nitrate of
soda was by far the most efficient fertilizer used. Trees in which a
high state of vigor and health is maintained are commercially
resistant to the disease.

1 Op. cit.

2 For a discussion of peach tillage and fertilization, see Gould, H. P., Growing peaches: Sites, propagation,
planting, tillage, and maintenance of soil fertility, U.S. Dept. Agr., Farmers’ Bul. 631, 24 p., 7 fig. 1915.

WASHINGTON : GOVERNMENT PRINTING OFFIC® : 1917

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