Symptoms: In perennial trees and shrubs, cankers are sunken, dead areas on twigs, branches or trunks surrounded by living tissue. The sunken appearance is due to the death of underlying tissue. Cankers usually have distinct margins between dead and living tissues. They may grow slowly, increasing in size year after year. Once the canker grows completely around a branch, girdling the branch, the parts above the diseased area usually die back. Cankers are most common on trees and shrubs under stress.
Cause: Cankers may be caused by fungi or bacteria. Bacterial cankers generally begin as infections of leaves or flowers. Then the bacteria rapidly progress down the petiole and twig, killing the shoot down to a major branch. Bacterial cankers then form on the large branch at the junction with the dead shoot. Growth and advancement of the canker in the larger branch generally is very slow. Improper pruning can greatly increase the number of cankers on susceptible shrubs and trees. Fungal cankers usually begin as infections at twig and branch stubs left from breakage or as pruning wounds. The fungus moves into the larger branch or trunk at the junction with the stub. As the canker slowly grows year after year, callus tissue is killed back, causing concentric rings of callus to surround the canker and giving it a characteristic "target" appearance. Large cankers on trunks weaken them structurally. Strong winds are likely to topple a cankered tree by breaking it over at the canker site.
Control: Branches with cankers should be pruned out when cankers are still small.