In the Great Lakes region hypoxylon canker is the main cause of premature death to quaking aspen. It is one of the most destructive diseases to aspens in forest settings and can also be detrimental in residential landscapes. It is common for trees infected with hypoxylon canker to break in the wind.
Pathogen and susceptible plants
Entoleuca mammata (Syn. Hypoxylon mammatum) is the fungus that causes hypoxylon canker on aspen, poplar and willow (Table 1). The fungi also resides on but does not cause disease on several other shade trees including maple (Acer), oak (Quercus), elm (Ulmus), birch (Betula), alder (Alnus), Mt. ash (Sorbus), hornbeam or blue beech (Carpinus), apple (Malus), and pear (Pyrus).
Table 1. Trees affected by the disease in Minnesota.
|Most susceptible||Occasionally affected||Rarely affected|
|Quaking aspen (Populus tremuloides) and hybrids||Bigtooth aspen (P. grandidentata)||Balsam poplar (P. balsamifera)|
European aspen (P. tremula) and hybrids
|White poplar (P. alba) and hybrids||Eastern Cottonwood (P. deltoides)|
|Violet willow (Salix daphnoides)|
|Black poplar hybrids (P. nigra)|
M. Grabowski, UMN Extension
Figure 3. Fungal stromata pushing through bark on a canker older than 3yrs
- Leaves on infected branches are initially small then turn from yellow to brown. Dead leaves often remain attached.
- Cankers often form at branch unions, stubs, wounds or galls.
- Cankers are slightly sunken, round to oblong, with irregular wavy margins.
- Young cankers have smooth yellow-orange to orangish-brown bark compared to healthy, young green grey bark.
- Old cankers have yellow-orange margins but develop a blotchy salt and pepper appearance in the center as bark blisters and peels away in small patches.
- Small gray pegs of fungal mycelia can be seen under blistered bark in cankers that are 2 yrs or older.
- Small gray to black cushion-like fungal spore producing structures form in clusters on the exposed wood of cankers that are 3 yrs or older.
- Wood beneath old cankers is often decayed and occupied by wood-boring insects.
The Hypoxylon canker fungus survives from year to year in cankers on infected trees. Gray to black cushion-like spore producing structures, that are 2-5 mm across, form crust-like patches on cankers that are 3 years or older. Spores produced on these structures are ejected through the air after rainy weather. If spores land on a susceptible host, infection will occur if air remains damp for up to 48 hours with temperatures above 60°F. Healthy bark is resistant to infection, but the fungus can infect through young dying twigs, branch unions with cracks or wounds made by insects. The fungus grows first through the wood, cambium and then into the bark, killing cells as it spreads into healthy tissue. The fungus continues to colonize the tree and usually within 3 to 8 years the tree is girdled and dies. Infected trees often break at the original point of infection due to wood decay under the canker.
- There are no fungicides that prevent or cure Hypoxylon canker.
- Avoid planting species that can become infected (see Table 1) where existing Hypoxylon infections are found on neighboring trees.
- Prune to remove dead or dying branches during dry periods, before the canker reaches the main trunk.
- Remove structurally weak trees that have cankers along the main trunk in order to avoid damage to people and property from tree breakage.