"Bacterial spot" or "shot hole" is a commonly encountered disease, especially on older peach trees. This disease may also be found on susceptible varieties of apricot and plum but is more frequently seen on peach and nectarine. Bacterial spot is caused by the bacterium Xanthomonas campestris pv. pruni. The disease occurs most often on Long Island, but has been occasionally reported in the Hudson valley and Western New York.
The disease can affect foliage, tender twigs, and fruit. Home orchardists may recognize the disease by the characteristic shot hole leaf symptoms. At first, the leaf spots are small, light green or whitish in color, and distinct from surrounding tissue. The spots are generally concentrated near the tips of the foliage. Severely infected leaves may turn yellow and fall to the ground. On sensitive varieties, a few lesions will lead to severe defoliation. Heavy defoliation early in the summer can reduce the size of the fruit and weaken the tree.
Leafspots similar to bacterial spot can be caused by a variety of other factors, including X-disease, water stress, nitrogen deficiency, and spray injury. Bacterial spot can usually be distinguished from the others by the angular nature of the lesions and by the fruit symptoms.
Fruit symptoms initially appear as small circular brown spots on the surface of the fruit. Later pitting and cracking may occur around the spots. Although this symptom damages the appearance of the fruit, it does not destroy the fruit's edibility. However, the resultant cracking may provide suitable sights for ingress of decay fungi. Late season infections are superficial and give the fruit a mottled appearance.
Infections of twigs produce cankers from spring to fall. At first the cankers are purplish water-soaked spots, but later become more or less circular to elliptical in shape and sunken. Cankers allow the bacterium to overwinter. On plum and apricot, bacteria may survive in cankers for more than one season.
The conditions which favor disease development include warm, moderate temperatures, frequent light rains, heavy dew, and considerable winds. If all, or most, of these conditions prevail, severe infection can be expected. When weather favors it,infection by the bacteria may occur anytime from shuck split until post-harvest. Hard, driving rains are more important in starting new infections, thus the disease can be proportionally more severe on one side of the tree than the other.
Some peach varieties are more susceptible than others are but virtually all commercially grown peach varieties can be infected. The most susceptible varieties include Blake, Jerseyland, Suncrest, Suncling, Sunhigh, Redcrest, Autumnglo, and Ran Cocas. Varieties with the highest resistance should be grown. These include Biscoe, Candor, Cresthaven, Dixired, Earliglo, Emery, Encore, Harbelle, Harbinger, Harbite, Harkin, Jefferson, Jerseyglo, Loring, Madison, NJ 240, NJ 248, Norman, Ranger, Redhaven, Redkist, redskin, Sentinel, and Sunhaven. Most apricot varieties are susceptible (Goldcot, SH-50, & SH-7). Many nectarine varieties are also susceptible. The disease is usually more of a problem in warmer, downstate locations and/or in wet years.
Vigorously growing peach trees are less susceptible to infection than weak ones. Good tree vigor should be maintained by proper pruning, judicious application of fertilizer, and watering when necessary. Excess nitrogen may aggravate the disease.
Planting susceptible trees in close proximity to one another can contribute to the buildup of the disease. A post-harvest fixed copper application may help in preventing some cankers from forming. The disease usually is not devastating in the home orchard.
Prepared by: Karen L. Snover, The Plant Disease Diagnostic Clinic, Cornell University, Ithaca, NY; last updated, 8/99.
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