The quaking or trembling aspen (Populus tremuloides) is one of the most widely distributed tree species in North America. Its range extends from New England through the lakes states to the Rocky Mountains and throughout much of Canada, however they are susceptible to tree diseases, pathogens and pests across their range. Although the pathogens that cause decay contribute to discoloration, affect the strength and stability and can result in hazardous trees, aspen snags are important wildlife habitat.
White trunk rot, caused by the fungus Phellinus tremulae, is the most important and recognizable form of stem decay. The fruiting bodies of P. tremulae are the distinctive conk fungi seen throughout the forest, these are a reliable indicator that white trunk rot is present. Conks typically occur at scars or branch stubs, where internal decay is growing out. White trunk rot occurs throughout the aspen’s range but varies in severity. These variations can be due to age, size, site quality and genotypic differences. Decay can extend 2 to 3 meters in any direction from the site of the conk, and incidence of mortality increases with additional conks. Wood affected by P. tremulae becomes spongy and discolored; early stages are a creamy white with a distinct black outline, and later wood turns yellowish and punky with a series of concentric, black zonation lines. This disease is considered an obligate stem decay fungus, causing destruction only to living trees.
Root and butt rot diseases are also caused by fungi, these enter the tree through wounds around the base and roots or through connected roots. Gymnopilus junonius and Pholiota aurivella are common culprits of infection, they cause a stringy, yellow-brown butt rot that may extend up the trunk for a few feet. Species in the Armillaria genus are common and important in the eastern part of the aspen’s range, these diseases cause white-yellow, stringy butt rot that can result in failure of immature trees and loss of volume.
Although these diseases are determinedly detrimental to aspen trees, the habitat created from decayed trees or snags is invaluable for wildlife. Aspens with stem decay are highly desirable for bird nesting, and will more often than not be selected by primary-cavity nesting birds. Secondary-cavity nesting birds, those that utilize cavities previously excavated by other species, will also benefit from habitat created by stem decay. In coniferous dominated forest systems the majority of cavity nests are excavated in aspen trees and in many cases those trees were found to be afflicted P. tremulae. Decay in live trees provides easy excavation due to softened wood combined with the protection for the nest by wood that is still sound.
Aspens have relatively thin bark which accounts for their susceptibility to permission of pathogenic organisms into the wood. There really aren’t ways to prevent these organisms from penetrating through the bark, or to reduce injury thereby. In forest stands, removing infected and weakened trees should somewhat protect healthy trees from infection, but this method is not 100% effective either. If you suspect aspens on your property or in your community are suffering from one of the above-mentioned diseases or another pest contact your arborist to develop a treatment plan and visit http://www.savatree.com/tree-disease-treatment.html.
James J. Worrall and Mary Lou Fairweather; “Disease and Discoloration of Aspen”, Forest Insect & Disease Leaflet 149, USDA and Forest Service
Revised May 2009