Diseases of the Quaking Aspen Tree

The quaking aspen remains an important species in forest regeneration because it survives wildfires and colonizes burned sites, but it is no longer recommended for landscape use. The tree may not live past 20 years in most urban settings despite proper care. Quaking aspen possess a soft bark that animals and insects wound easily. The wounds create entry points for disease that further reduce the tree's lifespan. However, pruning infected, dying, and dead branches helps discourage the spread of most diseases of the quaking aspen tree.

Several canker diseases affect the quaking aspen tree, including black canker, Cryptosphaeria canker, Cytospora canker, Hypoxylon canker and sooty-bark canker. Sooty-bark and Cryptosphaeria canker are the most prevalent and lethal. Sooty-bark strikes trees of all ages and sizes but prefers mature trees. The fungus invades cambium tissue and causes sunken area in the bark. After two to three years, the outer bark peels away to reveal dead inner bark that crumbles like soot. Cryptosphaeria canker, also called snake canker, kills saplings and the branches of mature trees. The long, narrow cankers form a snake-like pattern that spirals around the tree. As the disease progresses, the bark becomes increasingly discolored and starts to decay. Both canker diseases predispose the quaking aspen to wind breakage.

Several fungi cause leaf spots, but Marssonina populi is the most prevalent in quaking aspen. Infection occurs in spring when fungal spores migrate by wind or rainwater to new growth. The infection produces brown spots with light centers and yellow margins. The spots enlarge and coalesce, then turn into black necrotic blotches. Fungal fruiting bodies develop at the site of infection and produce white threads of spores that further spread the infection. Severe cases of Marssonina infection result in leaf drop and repeated infections may kill the tree.

The fungus Venturia macularis causes shoot blight infection during the early wet weather of spring. Shoot blight presents angular, black spots on the leaves. The spots enlarge and cover the whole leaf. Eventually, the infected leaves elongate, wilt and die. Infections at the top a quaking aspen tree may cause new shoots to bend over and form a "shepherd's crook" before dying. Young tree stands are particularly susceptible. Shoot blight abates when the weather turns hot and sunny, but repeated infections reduce the tree's overall growth.

Many types of Melampsora fungi cause leaf rust in the aspen, but the fungus Melampsora medusae tends to attack the quaking aspen. Infection occurs in midsummer when bright yellow-orange pustules and angular, yellow specks appear on the leaves. Angular, black specks, similar to those caused by Venturia macularis, develop in infected tissue in the fall. Leaf rust is not detrimental to the quaking aspen, but severe infections cause premature defoliation and allow other diseases to infect the tree.