Hypoxylon canker of aspen
From Bugwoodwiki
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USDA Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State Priv. For., For. Insect and Disease Management., Broomall, PA. p. 123, illus.
Hypoxylon canker of aspen, caused by the fungus Hypoxylon mammatum, is one of the most serious diseases of aspen in the United States. The disease abounds east of the Rocky Mountains wherever aspen occurs. Of the native poplars, quaking aspen is very susceptible, bigtooth is moderately susceptible, and balsam poplar is rarely infected. There are marked differences in clone susceptibility.
A young canker begins on the bark as a slightly sunken, yellowish-orange area with irregular margins. As the infection spreads, the outer bark blisters and drops off to expose the pillars formed by the fungus. Three years after infection, small patches of hard, gray, raised fruiting structures develop where the fungal pillars were found previously. Old cankers, which may be several feet long, are rough and blackened in the center. The older cankers have margins that are characteristic of young cankers. Cutting into the diseased bark of the young cankers or near the margin of older cankers reveals a white mycelial fan.
The mode of spread is unknown. Attempts to infect trees artificially with sexual spores (acospores) have been unsuccessful. The wounds caused by poplar borers (for example Saperda inornata) and other insects provide entry points for the fungus.
 Photo by USDA Forest Service - Rocky Mountain Region Archive, USDA Forest Service, Bugwood.org |  Photo by Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org |
