General Information. Bacterial canker, also known as bacterial blast and bacterial gummosis, attacks species of Prunus and Pyrus. This disease, caused by Pseudomonas syringae, is widely distributed and occurs on many kinds of plants. In addition to being particularly troublesome in the Pacific Northwest, bacterial canker has been reported in England, Continental Europe, Australia, and New Zealand. The blossom blast phase of the disease occurs on almonds, cherries, peaches, and pears. Bacterial gummosis or bacterial canker of stone fruits, caused by Pseudomonas syringae, has recently become more severe in eastern Washington. It usually occurs as enlarging lesions or cankers on trunks and scaffolds of young, vigorous trees, especially sweet cherry. Great amounts of gum are usually associated with active cankers, but gumming is not always a positive indication of this disease. Bacterial canker can be distinguished from Cytospora canker as described elsewhere in this program. A distinct margin at the top and bottom of the canker is typical of Cytospora. In contrast, the margin of a bacterial canker is irregular, with frequent streaks of discolored strands extending into the healthy tissue.

Symptoms. The bacterium attacks vigorous young cherry trees. Cankers develop on the trunk or scaffold branches and great quantities of gum may form around the enlarging cankers. The diseased bark becomes dark, looks water-soaked, and the area sinks, leaving a definite margin between diseased and healthy tissue. If cankers girdle the branches or trunk, the leaves above the canker roll and turn yellow. Growth stops and the branch or tree eventually dies. In some areas of the West, a "dead bud condition" is another phase of the same disease. This phase of the disease can be confused with nutrient deficiencies. Normally the cankers do not extend below the soil line and the tree may send up suckers with characteristic symptoms. Trees 2 to 12 years old are most susceptible. Nursery trees may develop trunk cankers that make them unfit for planting. Rain or water splash, and pruning tools spread the bacterium. Bacteria overwinter in active cankers, in infected buds, and on the surface of infected and healthy trees and weeds.

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Epidemiology and Forecasting. Rain or water splash, and pruning tools spread the bacterium. Bacteria overwinter in active cankers, in infected buds, and on the surface of infected and healthy trees and weeds. The bacterium reproduces best between 21 and 25 C. The disease seems to be more severe after cold winters and prolonged spring rains. Blossoms injured by frost are suspected infection courts

Management. Prune off cankered limbs well below visible canker. Avoid pruning in early spring and fall when bacteria are most active. Sterilize pruning tools before pruning healthy trees. Trees may be protected with copper fungicides applied to the trunks and lower branches. The area around the base of young trees should be kept free of weeds and trash. This helps keep the trunk and crown dry. Since nursery infections occur in the fall or during mild winters, spraying young cherry trees with a copper fungicide before digging will help provide control. Tree susceptibility is reduced by low available phosphate in the soil. Sometimes top-working Mazzard cherry scaffolds 8 inches from the trunk when they are at least 0.25 inches in diameter has resulted in lower canker incidence on the commercial variety. The only resistant sweet cherry variety is Corum. Since infection is believed to occur primarily through leaf and fruit stem scars, protective sprays containing copper applied when an airblast sprayer will blow all except terminal leaves from the tree will help control the disease. Similar sprays in late winter, while not as effective as leaf fall sprays, may give some degree of control. Examine trees often and, if cankers appear, scrape and disinfect as described in the fire blight section elsewhere in this program.

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Online References

Jones, A.L., Sutton, T.B., and Biggs, A.R. 1996. Bacterial Canker. West Viriginia University Kearneysville Web Site (www.caf.wvu.edu/kearneysville/disease_descriptions/bactcank.html)

Teviotdale,B.L., and Gubler, W.D. 1995.. Bacterial Canker of Peach. UC Pest Management Guidelines, University of California Statewide Integrated Pest Management Project Web Site (http://www.ipm.ucdavis.edu/PMG/r602101011.html).

Pscheidt, J.W. 1996. Bacterial Canker. An Online Guide. 1998 Plant Disease Control Handbook. Oregon State University Department of Botany and Plant Pathology