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Bacterial canker (blossom blast)

Pseudomonas syringae pv. syringae van Hall
Pseudomonas syringae pv. morsprunorum (Wormland) Yound et al.

Leaf scars, stomata, and areas of injury are the principal sites of infection. The most conspicuous symptoms are limb and trunk cankers (A, B), blossom blast (C), "dead bud", and leaf spotting; these symptoms may or may not occur together. Cankers can girdle and kill entire limbs, reducing the tree's fruiting capacity. Infection of the trunk, particularly on young trees, often results in tree death. Crotches are particularly susceptible to infection, which often leads to extensive gumming (D). Infections of the blossoms cause blossom blast and loss of fruiting spurs (E). Infections of dormant flowering and vegetative buds result in a condition called "dead bud" in which buds fail to break dormancy in spring. On leaves, lesions are tan to brown and initially surrounded by a yellow halo. Lesions may be small or they may coalesce to form large areas of infection. They are eventually walled off and the center of the lesion drops out to give the leaf a shot-holed appearance (F). On fruit, lesions tend to be circular, brown, and sunken (G).

Distribution

Common to all fruit-growing regions in eastern North America. The disease is most common on sweet cherry and apricot.

Similar Species

Blossom blast can be confused with blossom blight, caused by Monilinia fructicola (G. Wint) Honey; fungal sporulation (although not always present or evident) helps to differentiate brown rot from bacterial canker. X-disease can cause a dieback in older cherry trees that might be confused with bacterial canker. Branch or trunk cankers may be confused with perennial canker; however, perennial cankers form alternating callus rings, whereas cankers caused by Pseudomonas do not.

Management

Although most serious on sweet cherry and apricot, the disease also affects tart cherry, peach, and plum. Orchards should not be established on poor sites such as those on acidic or sandy soils, in areas prone to flooding or drying, or in the vicinity of wild Prunus spp., which can harbor the disease. Stone fruit are most susceptible to infection in late autumn and early spring. Copper bactericides can be used to manage disease, although they are generally considered ineffective. Cankers should be pruned from trees when feasible. Infection of pruning cuts can be minimized by pruning trees during the summer rather than in spring, when the bacteria are active. Tree training methods that cause bark injury should be avoided (e.g., limb spreaders), especially on sweet cherries and apricots.

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