Older Nectria cankers are easily recognized in forest stands because of their typical “target” shape. “Target” cankers have rings, each of which represent a year’s growth. Younger cankers tend to be grown over by bark and callus tissue attempting to heal the wound. Such cankers are difficult to recognize, but close examination of the affected area may reveal tiny red bodies, which are the fruiting bodies of the Nectria fungi. Mortality rarely occurs from this disease, but stems may break at canker locations during high winds.
Control of Nectria canker is the same as for Strumella canker—the removal of infected trees during thinning operations.
STRUMELLA CANKER
Strumella canker of hardwoods, caused by Strumella coryneoidia, most frequently attacks trees of the red oak group. Other hosts include species in the white oak group. Beech, basswood, blackgum, shagbark hickory and red maple are also occasionally affected. This disease is found in the East, from the southern Appalachians to northern New England.
Strumella canker on black oak.
Strumella cankers are of two types: diffuse, and the more common “target-shaped.” The first develops on smooth-barked saplings and rapidly girdles and kills the tree. Killing results because callus tissue, which tends to heal over cankers, does not have time to develop. Target-shaped cankers are more common. “Targets” are formed by the alternation of killing of bark by the fungus around the canker’s perimeter and the formation, in turn, of a callus ridge by the host tree. The fungus is active usually in the dormant season, while callusing occurs in the spring. As with most canker-causing fungi, Strumella usually enters the tree through a branch stub. Cankers are quite large and may reach several feet in circumference or length. The presence of the causal fungus is revealed by dark brown, cushion-like structures about one to three millimeters in diameter on the dead bark and surrounding tissue.
No feasible control method is available under forest conditions. However, the disease impact can be greatly reduced by removing cankered trees during thinning operations.
SPICULOSA CANKER
Spiculosa canker, caused by Poria spiculosa, is found on bottomland oaks in the South. Occasionally this disease will also degrade hickories and honeylocust.