Importance. -- Nectria cankers, caused by N. galligena, are frequently found on some oak species. These cankers are most important in trees less than 20 years old. The canker can girdle and kill young trees or make them weak and subject to wind breakage.

Identifying the Fungus. -- The fungus can be identified by the creamy-white fruiting structures that appear on cankers soon after infection. It can also be identified by the small, red, lemon-shaped perithecia (another fruiting structure) near canker margins after 1 year.

Identifying the Injury (figure 64). -- Well-defined localized areas of bark, cambium, and underlying wood are killed by the invading fungus. A new concentric callus ridge develops each year around the expanding canker annually and bark sloughs off the older parts of the canker. After several years, the annual concentric callus ridges resemble a target.

Biology. -- The fungus overwinters as a saprophyte in cankers and produces spores for new infections during the spring. Windblown and water-splashed spores infect tree wounds and branch stubs. Secondary infections result from spores produced on new spring cankers.

Control. -- Canker occurrence may be minimized in high-value areas by avoiding pruning during wet weather, avoiding causing new wounds, pruning (removing) cankered branches, and sterilizing pruning tools before moving to an uninfected tree.