Foliar Diseases
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Melampsora Rust
Melampsora spp.

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

Figure 177. Figure 177. Rust pustules of Melampsora spp. on aspen leaves.

Symptoms/signs: Several Melampsora species occur in the region. Melampsora medusae causes conifer-aspen leaf rust; M. occidentalis causes conifer-cottonwood rust; and M. epitea, which causes willow rust. 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: The orange spores are distinct to Melampsora rust on the listed hosts.

References:  39, 93