Nectria canker (Nectria galligena) is one of the most common diseases of hardwoods in North America. It affects more than 60 species of trees and shrubs. In northern hardwood stands, the species most commonly affected are white and yellow birches, red and sugar maples, oaks and aspen.

One- or two-year-old cankers appear as dark, flattened or depressed areas on smooth-barked stems or branches. Young cankers may be difficult to detect. Nectria canker differs from Eutypella canker in that the dead bark over it soon falls off to expose concentric ridges of callus tissue. Because these ridges look somewhat like a target, Nectria cankers sometimes are called “target cankers”(Fig. 84). The concentric ridges are formed by the alternating annual extension of the fungus into surrounding healthy bark during the dormant season, and the subsequent production of callus around the newly invaded area by the host tree during the next growing season. Because the canker enlarges slowly, the tree stem is rarely girdled and killed.

Fruiting bodies of the causal fungus generally appear in the fall at the margins of the cankers. They are bright red to reddish-orange, spherical, and 0.02 inch (.05 mm) or less in diameter. Spores that spread the disease are produced in the fruiting bodies. When these occur in large clusters they appear as red spots when viewed with the naked eye. A hand lens is necessary for good observation. Nectria does not decay wood, and the wood behind a canker usually is sound unless it also is invade by decay fungi.

Although Nectria cankers are more important on birch than on maple, they markedly lower the longevity of the tree by increasing susceptibility to decay. Infection occurs mainly through small wounds on young trees. Removing affected trees during thinning operations will greatly reduce the importance of the disease. Stand on poor sites often are the most severely cankered.