Hypoxylon canker, caused by the fungus Hypoxylon mammatum, is a disease that affects aspen, poplar, and willow. The fungus causes cankers on branches or stems, which usually appear sunken and/or discolored. Hypoxylon canker is most commonly found on trembling aspen (Populus tremuloides), some hybrid poplars, and some introduced species. Big tooth aspen (Populus grandidentata) is occassionally infected, while balsam poplar (Populus balsamifera) is rarely infected. Native willows appear to be resistant to the fungus, but some introduced species are susceptible.

The means by which this disease is spread is unknown, but it is believed that the fungus enters young stems and branches through wounds. It then grows beneath the bark, causing a tannish discoloration. Two years after infection, the bark turns dark, blisters, and cracks. Under the bark, small gray pillars are produced. Three or more years after infection, somewhat larger gray to black, hard, raised structures are formed on the older portion of the canker. Spores produced on these structures are dispersed to wounds where new infections begin.

Young canker

Conidal stage       (2 years after infection)

Final stage      (3 or more years after infection)

The fungus continues to colonize the tree and usually within 3 to 7 years the tree is girdled and dies. Dead trees often break at the original point of infection.

Wounds created by insects such as Saperda sp. And Cicada sp. have been shown to be an entry point for fungal spores. Species of Saperda, Obera, and Cicada create wounds during oviposition (egg laying) which are occasionally infected by Hypoxylon. In years when insect numbers are high, disease incidence has also been shown to increase. Woodpeckers moving from tree to tree are another possible vector of the disease.

There is no chemical control for Hypoxylon canker. Looking for dead or dying branches and removing them during dry periods, before the canker reaches the main trunk, can save landscape trees.

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Revised 1/99