HOW to
Identify and Minimize White Trunk Rot of Aspen
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Phellinus tremulae (=Fomes ignarius var populinus) causes a heart rot of aspen that causes more volume loss than any other disease of aspen. Severity of the disease increases with stand age. In fact, incidence of white trunk rot is a major consideration in determining aspen rotations. Although no consistent relation exists between site and decay, generally less volume is lost in vigorous stands on good sites. On the same site, resistance to decay varies among aspen clones.

In standing trees look for:
In standing trees look for: hoof-shaped conks, photo
. .
bullet point Hoof-shaped conks (fruit bodies), the most reliable sign of diseased trees. Conks are dark brown to black on the upper surface with many small cracks, and light brown on the lower, pore surface. The interior of conks is dark brown with numerous white flecks. Conks are perennial: new layers of pores form on the lower surface each year; thus the interior of old conks appears layered. Although the absence of conks does not necessarily mean the absence of decay, several small conks or a few large conks usually indicate advanced decay.
Punky knots, photo bullet point Punky knots filled with dark brown material resembling the interior of conks
bullet point Trees with large branch stubs, broken tops, cankers, fire scars, and other old stem wounds.
bullet point Phellinus is a wound parasite so such trees are likely to have extensive heart rot, especially in overmature stands.
Phellinus wound parasite-vermature stand, photo
early stage decay, photo advanced decay, photo
bullet point Yellowish-white wood surrounded by a zone of dark, discolored wood. This is characteristic of the early stage of decay.

bullet point Spongy wood, yellowish in color, surrounded by irregular black zones of discolored wood. This indicates advanced decay.
bullet point No direct control is known, but loss can be minimized by harvesting aspen stands before decay becomes advanced. In many parts of the Lake States, aspen stands begin to deteriorate rapidly when they reach 50 to 60 years of age. damaged stand, photo
bullet point Consider potential hidden decay along with visible decay when planning harvest schedules.
bullet point The volume of wood affected by hidden decay (early stage of decay or trees with no external signs of disease) can be equal to the volume of decay in trees with disease signs.
bullet point Harvest damaged stands (fire, wind, ice, etc.) early Trees in such stands are especially susceptible to white trunk rot.
bullet point Make regeneration cuts in overmature, defective aspen stands to bring these sites back to full production.
bullet point Manage aspen to achieve uniform well stocked stands in which natural pruning will reduce the number and size of potential infection sites.

Michael E. Ostry
Plant Pathologist
North Central Research Station
St. Paul, MN

James W. Walters
Plant Pathologist
NA State & Private Forest
St. Paul, MN
Copies available from:
North Central Research Station
1992 Folwell Avenue
St. Paul, MN 55108

Revised 1983

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NOTE: Some States have restrictions on the use of certain pesticides. Check your State and local regulations. Also, because registrations of pesticides are under constant review by the U.S. Environmental Protection Agency, consult your local forest pathologist, county agriculture agent, or State extension specialist to be sure the intended use is still registered. caution image
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