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Bacterial Spot

Bacterial spot (AKA "bacteriosis" and "bacterial shot hole" and "XAP") is a disease caused by the bacterium Xanthomonas arboricola  pv. pruni (formerly Xanthomonas campestris pv. pruni). It affects peaches and nectarines as well as plums, prunes, and apricots. It infects wounds and leaf scars during leaf drop, and it overwinters in these or in protected parts of woody material. The bacteria multiply during warm weather (above 65°F) and ooze out during spring rains. Wet conditions are especially favorable to infection of fruit.

The first symptoms of bacterial spot are small, water-soaked, greyish areas on the undersides of leaves. The spots develop an angular appearance, and the color deepens to dark brown or purple. If the centers of the holes drop out, leaves will have a distinct "shot hole" appearance. These leaves will ultimately turn yellow and drop. Although other diseases will cause shot hole, bacterial spot is distinguished by the angular form of the lesions and by symptoms that appear on fruit, which are small, round, dark spots on the surface. These spots may become pitted or crack, allowing entry to fungal infections. When the bacteria infect twigs, dark, water-soaked cankers may appear, which may become cracked and provide the pathogen with a safe home for the winter.

Bacterial spot is very difficult to control, especially if weather is favorable to its spread. Trees that are young or otherwise weak are especially vulnerable. Similarly, a mature, vigorous, healthy tree is less likely to suffer from this disease. The best defense is to plant resistant varieties. Many of the PF peaches are resistant, as are several of the "har-series" apricots. It should also be noted that a cultivar developed in a warm, dry climate becomes more susceptible if planted in a cool, wet climate, so it is worth choosing stone fruit varieties that are well suited to your specific conditions.

For more information, see Stone Fruit IPM for Beginners from Michigan State University.

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