Fig. 1. Canker on maple tree
Photo: Chad Behrendt
Nectria canker most commonly occurs on maple and honey locust, but can also infect apple, aspen, basswood, birch, elm, oak, walnut, and other hardwood trees. The fungus Nectria galligena causes target-like cankers on many hardwoods (Fig. 1), while N. cinnabarina causes similar cankers on honey locust (Fig. 2).
Fig. 2. Nectria canker on honey locust.
Photo: Robert Blanchette
In spring and early summer, pink or cream colored, cushion-like reproductive structures (sporodochia) form on the surface of tissue infected the previous year. Other reproductive structures, perithecia, are formed in late summer to early fall (Fig. 3). These structures are initially red colored, later turning brown or black. Spores are dispersed from these structures by wind or splashing water to wounds and natural openings on nearby trees. Wounds caused by improper pruning, sunscald, frost cracks, storm damage, or other types of mechanical damage serve as entry points. Infection can occur during wet periods throughout the growing season.
Fig. 3. Reproductive structures of canker
Photo: U of MN Plant Disease Clinic
Nectria canker initially appears as a slightly sunken, elongated lesion. The surface of the outer bark is often discolored and may be open or covered with bark. Attempts by the tree to contain the infection result in the formation of a callus ridge during the growing season. If the tree is not successful, the fungus will reinfect healthy wood beyond the callus ridge the following year. As a result, perennial cankers develop a target-like appearance, due to the alternation of fungal growth and the production of callus tissue by the tree (Fig. 1). Eventually, branch dieback or death of the tree may occur if branches or the trunk are girdled by the fungus. Cankered trees are vulnerable to windthrow, commonly breaking at the canker site.
Nectria canker is most severe on stressed trees. To keep trees growing vigorously, choose proper planting sites, apply mulch around base of trees, water during dry periods, and properly fertilize and prune. Cankered branches may be pruned during dry periods. Use a 10% bleach solution (one part bleach to nine parts water) to disinfect pruning tools between each cut.