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University
of Rhode Island GreenShare Factsheets
Peach
Diseases: Bacterial Spot
Xanthomonas pruni
Bacterial spot affects peaches, nectarines, apricots,
plums, prunes and cherries. The disease is widespread throughout
all fruit growing states east of the Rocky Mountains. Bacterial
spot can affect leaves, twigs and fruit. Severe infection results
in reduced fruit quality and yield. Fruit infection is most
serious on late-maturing varieties. If proper environmental
conditions occur, up to 50 percent or more of the fruit of
susceptible varieties may have to be discarded. Defoliation
by bacterial spot may weaken the tree, predisposing it to winter
injury and attack by other pathogens. The disease is usually
more severe where soils are light and low in fertility. Vigorous
trees are usually less susceptible to the disease than devitalized,
neglected trees.
Leaves:
Small spots 1 to 5 mm (l/25 to l/5 inch) in diameter form
in the leaves. Spots are irregular to angular and have a deep
purple to rusty-brown or black color. In time, the centers dry
and tear away, leaving ragged "shot-holes." When several spots
merge, the leaf may appear scorched, blighted or ragged. Badly
infected leaves may turn yellow and drop early. Early defoliation
is most common on trees deficient in nitrogen or where the disease
is further complicated by pesticide injury.
Nitrogen deficiency can cause leaf symptoms that are very
similar to those of bacterial spot. Care must be taken to avoid
an incorrect diagnosis. In both cases, the leaves of affected
trees turn yellow and drop prematurely. With bacterial spot,
symptoms are most likely to appear at the tip of the leaf,
but with nitrogen deficiency, symptoms are usually most evident
along the midrib. Leaf tissues surrounding shot-holes caused
by a nitrogen deficiency are more likely to have a reddish
color than when bacterial spot is involved.
Fruit:
Small, round olive-brown to black spots form on the fruit. They
are usually sunken and frequently surrounded by a water-soaked
margin. On peaches, spots usually form on the side exposed
to the sun. Spots may slowly enlarge and merge to cover large
irregular areas on the fruit. On some varieties the spots
may exude a yellowish gum after rainy periods. Skin cracking
and pitting may occur near the spots during fruit enlargement.
Fruit infected at an early stage of development are usually
the most malformed.
On plums, symptoms are different than on peaches. Large,
sunken, black spots form on some varieties; on others, small
pit-like lesions are common.
Twigs:
On peaches, two distinct types of cankers damage twigs. "Spring cankers" develop
on young twigs produced the previous summer. Spring cankers first appear as
water-soaked, slightly darkened blisters about the time the first leaves appear.
If these cankers encircle the twig, it will die. As the season progresses,
the tissues over the blister-like lesions rupture and bacteria are released.
These bacteria can be spread by windblown or splashing rain and can result
in new infections. In time, spring cankers heal and become inactive.
"Summer cankers" develop on green
twigs of the current season's growth. They usually occur
later in the summer after
leaf spots are evident. At first, they are water-soaked, dark
purplish spots. In time, they enlarge, turn brown to purple-black,
become slightly sunken and round to elliptical with water-soaked
margins.
On certain plum and apricot varieties, twig cankers may
continue to develop in two- and three-year old twigs. If the
cankers are deep-seated, they can deform or kill the twigs.
The bacterium Xanthomonas pruni overwinters in
twigs that are infected late in the season about the time leaves
are shed. The following spring, when environmental conditions
are favorable, bacteria ooze out onto the surface of these
twigs. The bacteria are then spread by windblown or splashing
rain and can result in new infections throughout the growing
season. The bacteria come in contact with healthy leaves, fruit
and current-year twigs and enter the tissues through stomata
or lenticels when surface moisture is present. Once inside
healthy tissues, the bacteria multiply and disease develops.
Warm temperatures (70-85 degrees F, 2l-29 degrees C) with
light rains, heavy dews or fogs and windy weather are most
conducive for disease development and spread. The disease makes
little progress when weather is hot and dry.
1. When planning an orchard, avoid low-lying or shaded
sites with poor air circulation and soil drainage. Any practice
that promotes faster drying of fruit and foliage will help
reduce the risk of infection. Destroy nearby wild or neglected
stone fruits (Prunus spp.). Buy and plant only vigorous,
disease-free fruit trees from a reputable nursery.
2. Prune trees annually to allow for better air circulation
and to maintain tree vigor. If possible, prune during dry weather
in the latter half of the dormant season.
3. Select peach varieties with resistance to bacteria
spot. The following varieties are somewhat resistant: Belle
of Georgia, Biscoe, Candor, Comanche, Garnet Beauty, Harbrite,
Harken, Late Sunhaven, Loring, Madison, Norman, Pekin, Raritan
Rose, Redhaven, Redskin and Sunhaven. These varieties are very
susceptible: Babygold S, Blake, Elberta, Halehaven, Jersey
Queen, Jerseyland, July Elberta, J.H. Hale, Kalhaven, Rio-Oso-Gem,
Suncling, Suncrest and Sunhigh.
4.Fertilize where needed to maintain vigorous but not
excessive shoot growth.
5.Spray applications. At present, no spray program is
completely effective for controlling bacterial spot. In the
home orchard, spraying for bacterial spot is not considered
practical.
Adapted from Michael A.
Ellis, Ohio State University Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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