Hosts:  Aspen, Douglas-fir, willow, and cottonwood

Symptoms/signs:  Powdery yellow to yellow-orange spores produced on both sides of leaves or needles.

Biology:  The life cycle of many rust fungi are very complex. In the early spring, infected dead aspen leaves on the ground release basidiospores that can only infect Douglas-fir. A different spore produced on Douglas-fir is wind disseminated and infects aspen leaves in the summer. Yet another spore, the golden-yellow uridiniospore, appears on both sides of infected aspen leaves within 2 weeks. This is called the repeating stage because these spores germinate to reinfect aspen leaves throughout the growing season. An overwintering spore is produced on the underside of infected leaves in late fall.

Effects:  During wet summers this fungus can cause the leaves of highly susceptible clones to shrivel and drop prematurely, reducing growth.

Similar Insects and Diseases:  Willows are host of several species of Melampsora, each which has a different alternate host. These species cannot be differentiated morphologically on the willow.

References:  38, 92