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.
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.
from the West Virginia University Extension
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.
Small, round olive-brown to black spots form on the fruit.
They are usually sunken and frequently surrounded by a water-soaked
On peaches, spots usually form on the side exposed to the sun.
Spots may slowly enlarge and merge to cover large irregular areas
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.
plums, symptoms are different than on peaches. Large, sunken, black
spots form on some varieties; on others, small pit-like lesions
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.
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.
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.
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.
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.
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.
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.
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.
from Michael A. Ellis, Ohio State University Extension, 1999