Tree Fruit & Grape News, July 1999

Bacterial Canker of Cherry

Gayle Jesperson, Plant Pathologist,
BCMAFF, Plant Industry Branch, Kelowna.

The spring of 1999 has been a bad one for bacterial canker, with losses of recently planted cherry trees in both the Okanagan and Creston Valleys. Weather patterns, including prolonged periods of cold, wet weather with late spring frosts have provided good conditions for an outbreak.

Bacterial canker is caused by the bacterium Psuedomonas syringae pv syringae. Cherries are very susceptible, but it also infects other stone fruit including pears, and many species of ornamental trees. Young cherry trees are more susceptible than established cherry trees, and trees under stress are much more susceptible than healthy trees with optimal growing conditions.

Symptoms:
Signs on young cherry trees are typically elongated, sunken cankers that are soft or spongy to the touch and gumming copiously. Cankers may expand rapidly in the spring causing girdling of the main trunk or branches. Bacterial canker can also kill buds and sometimes causes leaf or fruit lesions, although these are not common in B.C. orchards.

Diagnosis:
The BCMAF Plant Diagnostic Lab in Abbotsford can identify bacterial canker, and has confirmed numerous cases this spring. It is important to submit suspected bacterial canker samples early in the season. Once the hot summer weather arrives, the bacterial populations decline and it is difficult to isolate bacteria from the cankers.

Life Cycle:
Bacterial canker infections take place mainly in the fall and winter during cool, wet weather. Trees are particularly susceptible during autumn leaf fall, when fresh leaf scars may become infected. Cankers may not be obvious until the spring, when they start to expand rapidly. Frost damage in the spring may promote additional infections.

The bacteria overwinter in canker margins, in healthy buds and also systemically in the vascular system. In the spring bacteria are disseminated by rain to blossoms and young leaves. The bacteria can survive in an epiphytic phase on the surface of symptomless leaves and blossoms, and also on other plants or weeds in the orchard during the summer.

Control:

1. Minimize stress on young or recently planted cherry trees, as it is a major predisposing factor. For example, provide adequate water to prevent drought stress; avoid planting in areas with poor drainage or high frost potential; supply optimal levels of nutrients; check soil pH; and control other pest and disease problems than may weaken trees. Transplanting is also a stress that may "bring out" nursery acquired infection.
2. Do not let weeds grow up around the trees. They cause unnecessary stress by competing for moisture and nutrients, increase humidity, and may harbour populations of the bacteria that cause canker.
3. Remove severely affected trees, and prune off dead or dying branches. Pruning in mid-summer is unlikely to spread the disease, but disinfecting tools after cutting infected trees is probably a good precaution. Don't prune during cool, wet weather. Avoid pruning in early spring and fall when bacteria are most active.
4. Trees with minor gumming may recover. Cankers may heal if the tree is able to contain the canker with callus tissue. Small cankers may also be cut out with a sharp knife, being careful to disinfect cutting tools between trees.
5. Applications of copper oxychloride (fixed copper) in the fall to protect leaf scars, and in early spring before bloom can be helpful, however copper resistance is widespread in the Pacific Northwest in orchards and nurseries with a history of copper use. It is not known whether copper-resistant strains are present in B.C.
6. The F12-1 Mazzard rootstock is reported to be resistant. If propagating cherry, use only scions from virus-free, canker-free trees. Nursery stock may be infected with bacterial canker. Discuss with the nursery you are ordering from ahead of time about replacement policies for diseased trees. 

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