Varieties within Prunus species vary widely in their susceptibility to this disease. Other names for the disease are bacteriosis, shot hole, and black spot. The causal bacteria, Xanthomonas campestris pv. pruni, can attack fruit, leaves, and twigs. Fruit loss on some varieties can be very high. Early and severe defoliation can affect fruit size and the winter hardiness of buds and wood.
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.
The bacteria overwinter in the twigs, buds, and symptomless plant tissue. In the spring, the bacteria are spread by rain to leaves, shoots, and fruit. Spring infections can occur anytime after the leaves begin to unfold. Temperatures above 65°F and warm rains are needed for the bacteria to multiply, become exposed, and be disseminated. After these first infections, which are rarely noticed but do initiate the disease each year, the severity of the secondary infections depends entirely on the weather. A moderately warm season--with light, frequent rains accompanied by heavy winds--favors severe outbreaks of bacterial spot. Any recent injury to the leaves or fruit, such as wind-blown soil particles and hail, may result in severe outbreaks.
Secondary spread of the bacteria can occur from oozing summer cankers and leaf and fruit lesions during warm, wet weather. The systemic movement of the bacteria from leaves and shoots contributes to the formation of cankers. These cankers can be spread by budding to healthy nursery trees.
Maximum use of resistant varieties is the most effective control measure. Growing numbers of good peaches are highly tolerant of bacterial spot. Resistance in plums, nectarines, and apricots is not as common. Nurserymen are well aware of the degree of susceptibility of the varieties they sell and they can provide good information for specific areas. Since trees in poor vigor are more susceptible, orchard management programs should be designed to maintain good vigor. Major outbreaks of bacterial canker in young orchards are often attributed to poor cultural practices.
No completely successful spray programs exist for control of bacterial spot. Chemical sprays can help reduce the amount of fruit and leaf infection but must be applied before symptoms occur. In seasons when disease incidence is light, special programs do help. In those when infections are numerous, spray programs can reduce the number of infections, but not enough to prevent defoliation and fruit infection. Chemical applications suppress the development of disease but do not eliminate it. Because chemical control is uncertain, the use of resistant varieties appears to be the best control strategy.