(excerpt) Trembling aspen, Populus tremuloides Michaux, and bigtooth aspen, P. grandidentata Michaux, are hosts of numerous species of injurious insects and microorganisms (Harrison 1959). Only a few of those organisms, however, are directly responsible for mortality of healthy trees. The fungus Hypoxylon pruinatum (Klotzsche) Cke. is most important in that respect, killing 1-2%o f the standing volume annually in the Lake States (Anderson 1964). It invades and spreads in cambial tissue, killing it and eventually the branch or stem by girdling. Initially, a canker appears as a sunken, yellowish-orange area in the bark (Anderson 1956). In a later stage the outer bark raises in blister-like patches and sloughs off exposing blackened, crumbling cortex.
Nord, John C. and Knight, Fred B.
"The Importance of Saperda Inornata and Oberea Schaumii (Coleoptera: Cerambycidae) Galleries as Infection Courts of Hypoxylon Pruinatum in Trembling Aspen, Populus Tremuloides,"
The Great Lakes Entomologist: Vol. 5
, Article 7.
Available at: http://scholar.valpo.edu/tgle/vol5/iss3/7