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B.C. Home » Agriculture and Lands » Pest Management

Bacterial Canker of Stone Fruits

Causal agents: Pseudomonas syringae pv. syringae; P. syringae pv. morsprunorum

 

Bacterial canker has been an increasing problem in the British Columbia interior, particularly on young cherry trees. The disease is caused by the bacterium Pseudomonas syringae. Cherries are very susceptible, but Psuedomonas also infects other stone fruit, pears, apple rootstocks 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. Outbreaks are often associated with prolonged periods of cold, wet weather with late spring frosts.

Symptoms:

Symptoms on young cherry trees are typically elongated 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 brown, circular lesions on leaves which fall out to give a "shothole" type symptom. Fruit lesions include small, brown spots which may be slightly sunken on immature fruit. Leaf and fruit symptoms are not common in the Okanagan, but may be seen in areas with higher rainfall.

Laboratory diagnosis should be done in the spring, as the bacterium is difficult to isolate once hot summer weather arrives. Bacterial canker is similar in appearance to Cytospora canker.

	Bacterial canker Note discolouration of wood and gumming Photo courtesy Agriculture & Agri-Food Canada
	Bacterial canker of cherry Photo courtesy Agriculture & Agri-Food Canada

Life Cycle:

Bacterial canker infections are thought to take place 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 that may weaken trees.
2. Don't 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 during dry weather in mid-summer is unlikely to spread the disease, but disinfecting tools after cutting infected trees is probably a good precaution. 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.
7. Before establishing new orchards, have soil tested for pathogenic nematodes. High nematode populations, particularly the 'ring nematode' are associated with increased losses due to bacterial canker. Fumigate soil before planting if nematode populations are high.

January 2007

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