-- 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.
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.
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.
-- 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.
-- 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