Ministry of Agriculture
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. There also appear to be varietal differences in susceptibility, with 'Sweetheart' being particularly hard-hit. Outbreaks are often associated with prolonged periods of cold, wet weather with late spring frosts.
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 on cherry.
Note discolouration of wood and gumming.
Photo courtesy Agriculture & Agri-Food Canada
|Bacterial canker of cherry
Photo courtesy Agriculture & Agri-Food Canada
Pseudomonas overwinters in canker margins, in healthy buds and also systemically in the vascular system. In the spring, bacteria are disseminated by rain to buds, blossoms, young leaves and developing fruit. 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.
Infection is known to occur in leaf scars in the autumn, and in all types of natural and man-made injuries to the tree, including pruning wounds, heading cuts, scoring cuts, frost injury, and winter injury. Most infections are thought to take place in the spring and fall during cool, wet weather. However infection can occur during the summer if temperatures are not too extreme and weather is wet.
Research in Oregon has shown that infection of heading cuts resulted in the highest tree mortality, followed by infection of leaf scars and summer pruning. In comparison, freezing injury, low temperature injury and dormant pruning resulted in the lowest canker incidence and severity.
- Remove severely affected trees, and prune off dead or dying branches.
- Avoid summer pruning, scoring or making heading cuts during wet weather, or if rain is forecast within the next few days. Avoid pruning in spring and fall when bacterial populations are highest. Don't run overhead irrigation immediately after any orchard operation that wounds trees.
- The risk of spreading bacterial canker on pruning tools appears to be quite low, based on research in Oregon. Researchers were unable to transmit the disease in summer or winter by cutting through active cankers.
- Paint trunks with white latex paint to reduce south-west injury.
- 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 bacterium that cause canker.
- If possible, obtain nursery stock from a low-rainfall production area. If propagating cherry, use only scions from virus-free, canker-free trees.
- There are differences in varietal susceptibility of cherry. Oregon trials have shown that 'Sweetheart' and 'Bing' are susceptible, while 'Regina' and 'Rainier' are more resistant. There are also differences in susceptibility of rootstocks. Colt and F12-1 Mazzard are reported to be resistant, while Gisela 6 is reported to be very susceptible.
- Minimize stress on young or recently planted cherry trees, as stress 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.
- 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.
- There are currently no highly effective chemical treatments available in Canada for prevention or control of bacterial canker. Copper oxychloride (fixed copper) may be used in the fall to protect leaf scars, and in early spring before bloom, however the effectiveness of this treatment is questionable due to resistance issues. Copper resistance in populations of Pseudomonas syringae is widespread in the Pacific Northwest in orchards and nurseries with a history of copper use. It is not known to what extent copper-resistant strains are present in B.C. Copper sprays are no longer recommended in some areas with resistance, as their use can actually increase damage due to bacterial canker.
- Bacterial canker of sweet cherry in Oregon - Oregon State University
- Evaluation of Pruning Techniques and Bactericides to Manage Bacterial Canker of Sweet Cherry - NEW YORK FRUIT QUARTERLY . VOLUME 18 . NUMBER 1 . SPRING 2010
- Cherry - Bacterial Canker - An Online Guide to Plant Disease Control, Oregon State University
- Spotts, R.A., K.M. Wallis, M. Serdani and A.N. Azarenko. 2010. Bacterial Canker of Sweet Cherry in Oregon - Infection of Horticultural and Natural Wounds, and Resistance of Cultivar and Rootstock Combinations. Plant Disease 94:345-350
Updated April, 2012