If you have not seen this disease yet in 1998, you are probably in one of the dry areas of the state. Bacterial leaf spot is caused by a bacterium (Xanthomonas) that thrives in rainy June and July weather. It infects peach, nectarine, almond, apricot, plum, prune, and cherry. Illinois may not have high numbers of these species, but there are many ornamental equivalents (for example, flowering cherry) that also host this disease. Bacterial leaf spot is also known as bacterial shot-hole.
Numerous small spots (pinprick to 1/5 inch) form in the leaves. This stage often goes unnoticed. These spots are circular and watersoaked, but soon enlarge to become angular and deep purple to rusty brown or black. The centers of the spots often dry and tear away, so you may see this only as a shot-hole appearance or even a wind-tattered effect. Infected leaves turn yellow and drop early. Symptoms resemble nitrogen deficiency, but fruit specialist Dr. Steve Ries points out that whereas nitrogen deficiency usually causes the holes to be concentrated near the midvein on the leaf, bacterial blight causes scattered spotting. The bacterial spot bacterium also attacks twigs and fruits, reducing fruit quality and yield or reducing aesthetic appeal, in the case of ornamental species.
Some peach cultivars have resistance to this disease. Most apricot varieties are susceptible, as are many nectarine varieties. Some resistant cultivars of peach are listed in Report on Plant Diseases No. 810, Bacterial Spot of Stone Fruits.
Bacterial leaf spot can be managed by using balanced fertility practices and pruning the trees to improve air circulation. This makes conditions less conducive to disease development.