Importance. -- Hypoxylon cankers affect most oak species in North America. Both H. atropunctatum and H. mediterranium have been reported in oaks. They affect mainly trees that have been stressed by wilt, drought, construction damage, or other injuries. Limbs and boles weakened by Hypoxylon spp. can be a safety hazard in high use areas.

Identifying the Fungus (figure 66). -- Hypoxylon spp. can be identified by the light- to dark-colored crusty fungus tissue (stromata) over the cankered area. Numerous small black fruiting structures are embedded in the stromata.

Identifying the Injury. -- Bark sloughing and decay are associated with Hypoxylon cankers. Affected trees are subject to wind breakage.

Biology. -- The fungus infects stressed trees through wounds and either produces a canker or quickly kills the tree by colonizing the sapwood. Fruiting structures develop on the cankers and ascospores are discharged into the air and spread to new infection sites. Hypoxylon cankers are generally secondary to some other disease or stressing condition in trees.

Control. -- Remove cankered limbs before they fall on someone, and to reduce the amount of inoculurn for new infections. Maintain tree vigor and avoid wounding and stubbing of branches to minimize conditions favoring cankers.