Hypoxylon Canker - Field Guide to Insects and Diseases of AZ and NM Forests

Symptoms/Signs: Young infections occur as slightly sunken, irregular, yellowish-orange areas around wounds or branch stubs. The underside of diseased bark appears laminated or mottled black and yellowish white. White mycelial fans form under the bark and are most evident at the top and bottom edges of the canker. Gray fungal pillars (asexual fruiting bodies) form beneath loose, blistered bark. Later, patches of gray-black, crust-like mounds form on the wood. These mounds contain perithecia, sexual fruiting bodies. The papery outer bark sloughs from older infections, exposing a blackened, crumbly inner bark. The cortex in the central portion of older cankers cracks in a checkerboard fashion, sloughing off in small patches.

Biology: In the humid Lake States area, the fungus invades the sapwood and trees often die before they are completely girdled because sapwood decay beneath a trunk canker predisposes the tree to wind breakage. In the arid Southwest, however, breakage is not common. Cankers on large trees in the Southwest may attain ages of 20 to 50 years before tree death. A live 1 m d.b.h. aspen tree in Arizona was observed with a Hypoxylon canker extending from the ground to a height of about 12 m, only half-girdling the tree. There appears to be a genetic predisposition to infection, because some clones are more susceptible to infection than others within the same geographic area.

Effects: Hypoxylon canker disease is the most important canker disease of aspen in the Lake States region, but of minor importance in the Southwest. Hypoxylon canker annually kills an estimated 1 to 2 percent of the standing aspen volume in the Lake States area. While the disease causes serious mortality in localized areas in the Southwest, mortality rates have not been determined.

Similar Insects and Diseases: It is the checkerboard pattern of older infections that distinguishes this canker disease from others that occur on aspen.

References: 38, 44, 92