Cause Pseudomonas syringae pv. syringae, a bacterium. The disease is common west of the Cascade Range because it is favored by cool, wet weather. Bacterial canker causes severe damage under favorable environmental conditions. Although the bacteria survive on the outside of the plant they must get inside and multiply in the space between plant cells (apoplast) to cause disease. These pathogenic bacteria inject several proteins and small-molecule toxins to get past host immune mechanisms. Once inside, the bacteria induce a watery, nutrient rich environment between the plant cells where they can multiply and continue colonization of the plant tissues. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand.
Symptoms Infected buds may fail to open in spring. Small, greasy-looking spots appear on newly opened leaves. The spots turn a dark brown and eventually may fall out, leaving a shothole appearance. In wet weather, the spots may expand and kill terminal shoots of susceptible cultivars. Shoots may appear blackened. Cankers may develop on branches. Leaves of branches may wilt during hot weather and turn brown. When infection becomes systemic, leaves of infected trees emerge smaller and yellower than leaves of healthy trees. Decline and death of the tree may follow.
Prunus sargentii 'Rancho' and P. yedoenis 'Akebono' appear to have some resistance. 'Kwanzan' cherries appear to be resistant when mature but not when young.
- If practical, destroy old seedling cherry trees growing in low areas. These trees can be a source of inoculum.
- Prune out cankered branches in summer or in dry weather late in the dormant season. Disinfect pruning tools between cuts using 10% bleach or 70% rubbing alcohol or shellac thinner.
- Replace severely infected trees with resistant cultivars.
Chemical control Apply copper products before fall rains, during leaf fall, and late dormant just before buds open. Further applications of fixed copper at reduced rates during early shoot growth in spring are recommended in British Columbia. Populations of bacteria resistant to copper products are common and may compromise control efforts.
- Arbor-OTC is registered for trunk injection, see label for details. Group 41 fungicide (antibiotic). 12-hr reentry.
- Junction at 1.5 to 3.5 lb/A. May be useful when bacteria are resistant to copper alone. Group M1 + M3 fungicide. 48-hr reentry.
Reference Pscheidt, J.W., and Byther, R.S. 2001. Prunus Diseases p 317-325 in Diseases of Woody Ornamentals and Trees in Nurseries, R. Jones., and M. Benson (ed.). St. Paul, MN: APS Press.