Symptoms

The symptoms of bacterial spot are quite different from other diseases of stone fruits. They may be confused with nitrogen deficiency and spray injury. The disease first appears as small, water-soaked, grayish areas on the undersides of leaves. Later the spots become angular and purple, black, or brown in color. The mature spots remain angular and are most numerous at the tip ends and along the midribs of leaves. The infected areas may drop out, giving the infected leaves a shot-holed, tattered appearance. On plum, the shot-hole effect is more pronounced than on other stone fruits. Infected leaves eventually turn yellow and drop. Severe defoliation often results in reduced fruit size, increased sunburn, and fruit cracking. As a result, tree vigor and winter hardiness are also reduced. Other leafspot diseases and spots due to spray injury tend to be much more circular in outline. Often, these are not confined by veins in the leaf as is bacterial spot. Leaf spots due to nitrogen deficiency are normally red in color.

Fruit infected early in the season develop unsightly blemishes and may exhibit gumming. Since the infected areas cannot expand with increased fruit size, the spots crack. Pits or cracks on the fruit surface extend into the flesh and create large, brown to black depressed areas on the fruit surface. Lesions that develop during the preharvest period are usually superficial and give the fruit a mottled appearance. On plum, the fruit symptoms are likely to be quite different in that large, black, sunken areas are most common. On a few varieties, small pit-like spots occur.

There are two distinct twig cankers on peach and nectarine. Lesions that develop on green shoots and twigs in the summer are called summer cankers. These are small to large purple-black lesions, slightly sunken to deeply cracked, and circular to elliptical in shape. Lesions that develop after bud break are called spring cankers. They develop on the previous season's growth beginning from about the time of bud swell through the bloom period. They may appear as small, somewhat blister-like, darkened areas often around or near a bud. Later, the epidermis ruptures and the bacteria become exposed. Spring cankers also are seen as a tip dieback of the twig. Summer cankers are usually located between nodes and spring cankers are located at nodes.

The cankers on plums and apricots eventually appear quite different. On susceptible varieties, the bacteria may survive for 2 or 3 years, slowly enlarging and deepening the cankered area. The results are deep-seated cankers deforming the small branches so they have a knotty appearance. Some of these branches may be killed or they may break from the weight of the fruit. Bacterial spot may affect sweet and tart cherry leaves. While rarely happening, the leaf symptoms are like those on peach.