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Bacterial Canker of Stone Fruit
Also SeeFruit Tree Diseases and Pests: An Introduction
List of Common Fruit Tree Diseases and Pests
Bacterial canker (AKA gummosis and sour sap) is caused by two closely related strains of the bacterium Pseudomonas syringae, which is also the pathogen that causes pseudomonas or "blossom blast" on pear trees. This disease primarily affects sweet and tart cherry trees, but it is also found on plums and other stone-fruit trees. The disease is most active in cool, wet weather: In fall, it enters leaf scars as leaves drop, and it overwinters in buds. In spring, infected buds will either be dead, or they will continue to grow and spread the disease. Bacteria also gain entry to branches via pruning cuts or other injuries in spring.
The symptoms of bacterial canker will depend on what part of the tree is infected. Bacteria can invade leaves during rainfall, and this will appear as dark, angular spots. The center of the spots may drop out to create a "shothole" appearance. Shoots and buds often die. The most noticeable symptoms, however, are cankers, darkened areas at the base of buds or on branches. During the active phase of the disease, cankers will often ooze prolific amounts of sap. Like any canker, these will deprive the plant of nutrition and have the potential to girdle and kill a branch completely.
Bacterial canker is a disease that most often infects weakened or injured trees. The best prevention is to remove otherwise sick trees and to carefully time pruning. Cherries should be pruned postharvest, when no rainfall is expected, and a stub about six inches long should be left when pruning out infected area. This provides a "dead end" for the disease. Clean your blades with alcohol between cuts whenever pruning a diseased tree. Some varieties of cherry are especially susceptible to canker, and these should be avoided if pressure is high: Royal Ann, Bing, Lambert, Sweetheart, and Van.
For more information, see the Michigan State University publication, Stonefruit IPM for Beginners.
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