At least three aspen diseases afflicted aspen during late spring through mid-summer, turning leaves a reddish-brown. A complex of fungal leaf diseases are causing this damage including Marssonina leaf blight (Marssonina populi), aspen ink spot (Ciborinia whetzelii), and Shepard’s Crook (Venturia spp.). The widespread defoliation of aspen trees this year has been reported from Montana down to Utah. Significant variation occurs among aspen clones in their susceptibility to infection by the disease agents.
Primary infection of aspen occurs generally soon after leafs emerge in spring. Casual agents of these aspen foliar diseases reproduce by means of specialized cells (spores). These specialized cells usually originate from leaves that were infected the previous year and have fallen or are still hanging on the tree. Most aspen leaf diseases begin as dark brown or black flecks scattered over a leaf. The spots enlarge with time to form black blotches usually with yellow to tan borders. From a distance, affected trees have a bronze cast and a weak, almost see through appearance. Large trees are most affected in their lower crown while all the leaves of small trees are infested.
Because these foliar diseases rarely lead to tree mortality and management approaches are often impractical, management is not often considered in a forest setting. Homeowners can rake fallen leaves and prune out the dead tops or branches of infected trees to reduce the spread of the disease during subsequent rains later this summer and next year. Increasing space between trees may create a less favorable microclimate for spread and infection. Fungicides can be used to prevent infection, but they must be applied at bud break before infection occurs.
Gymnosporangium rust fungi attacks have been identified as the cause for foliar diseases occurring on hawthorn and other deciduous trees and shrubs belonging to the Rose family (Rocaceae) in eastern Idaho. Infections occur in spring through late summer on leaves and twigs of their host. They appear as rusty, yellow/orange spots that rupture the outer layer of plant tissue. Some infections may also cause a swelling of tissues resulting in the formation of a gall. The rust fungi produce several distinctive reproduction stages (spores); some of the spore stages infect one host while the other must parasitize a different alternate host such as juniper.
These fungi are native pathogens to the area and are not a major cause for alarm. Mortality as a result of these foliar diseases is rare. As the summer progresses the brown leaves will drop and trees may seem dead at first glance, but typically they will have enough energy reserves to break bud the following year and develop new leaves. There are several protective fungicides that can be applied for controlling infections caused by rust fungi on ornamental and orchard rosaceous hosts; they must be applied in the spring, before flowering and continuing for 4-6 weeks until the spore stage from the principal host becomes inactive.