Mystery Solved: Aspen Leaf Spot & Mosaic Virus

“What are these spots on my aspen leaves?” 

We received numerous calls this summer about aspen trees that looked sickly and were thought to be dying. Most concerning was that they were also prematurely dropping leaves in the middle of summer.

This valley-wide incident was a result of the abnormally frequent rainstorms our location received this past spring. These rain events caused leaf spot disease and mosaic virus to flourish in aspen trees. More crowded and shaded landscapes were more affected as decreased airflow and shade prevents leaves from drying after rain and increases conditions favorable to disease development.

When a tree becomes infected, its natural defense is to shed the heavily infested leaves, hence the summer leaf drop. Fortunately, new buds are then formed, and new leaves emerge either that year or the following depending on how late in the season the leaves are shed from the tree.

It's important to understand that this is a mostly cosmetic issue and rarely causes tree mortality on its own. Successful treatments must be preventative and are not effective after symptoms are observed. For these reasons, control treatments are not typically needed or recommended. Have no fear, your trees will be okay!