Identifying common aspen ailments

It’s that time in spring when trees all over Colorado (depending on the altitude) are sprouting new leaves! While it’s exciting to see, this new growth is particularly vulnerable to foliage diseases that threaten the health of the tree. Even Colorado’s most recognizable and resilient deciduous trees, quaking aspen, can be affected.

Photo from Colorado State University.

Aspen are found across Colorado’s high-altitude mountains and play an important role in the state’s ecology and history. As a relatively fast-growing species, aspen often pop up in areas where evergreen trees have burned or fallen. They produce more forage than evergreens, which means that aspen can become a keystone species that supports animal populations. Historically, the Nuche and other indigenous people used aspen bark as a tool and also as medicine because the bark contains salicylates (chemical compounds similar to aspirin). In modern times, aspen wood is used to make a range of products and is an especially popular material for furniture.

Both wild and cultivated aspen are prone to foliage diseases that develop in wet, cool weather – exactly like springtime in the Colorado mountains! Foliage diseases threaten the health of aspen trees by causing early defoliation (leaf loss). If the tree loses its leaves early in the growing season, it will have a chance to regrow new ones; if the leaf loss happens later in the summer, the tree will be vulnerable to frost damage and other diseases or insects.

Diseases affecting aspen are primarily caused by a few types of fungi. The Aspen and poplar leaf spots fact sheet published by the Colorado State University Extension explains how to identify common diseases:

• Marssonina leaf spot: The fungus Marssonina causes the most common foliage disease in aspen and cottonwoods. It presents on the leaves as dark brown flecks that can grow and fuse together into large black patches. Marssonina can survive all winter on fallen leaves, allowing spores to infect new leaf growth when warmer, wet weather arrives in the spring.
• Septoria leaf spot: Similar to Marssonina, Septoria presents as brown spots on leaves, although the spots are lighter in color, have black outlines, and can produce small black fruiting bodies. Septoria spreads to new leaves in the same way as the Marssonina fungus.
• Ink spot of aspen: This disease is named for the raised black spots that appear on aspen leaves during the later stages of the fungus Ciborinia’s infection. These black masses are called “sclerota” and can fall out of the tree, leaving holes in infected leaves.
• Leaf and shoot blight: This disease is caused by the fungus Venturia and mostly affects young aspen. Symptoms of the disease are seen in the spring, when aspen shoots develop brown or black areas that spread down the new growth. The young shoots then turn black and curl into a “shepherd’s crook.”
• Leaf rusts: This disease is caused by the fungus Melampsora and is the least harmful to the health of the tree because it typically appears later in the summer or fall, so it does not cause aspen leaves to drop early. Leaf rusts are named for the yellow-orange pustules that develop on the underside of the leaves.

To help diagnose fungi and other issues affecting trees, use the Diagnosing tree disorders guide. For a quick visual reference, the Colorado State Forest Service created a poster to help identify what’s ailing your aspen.

CSU Extension experts also explain how to prevent aspen diseases and insect infestations. The best way is to plant tree varieties that are known to be resistant to common fungi. Another method is to make sure to remove infected leaves and shoots in the fall so that the fungi can’t spread to vulnerable new growth in the spring. To maintain healthy trees, make sure to water early in the morning so that leaves have time to dry out. Persistently wet leaves create perfect growing conditions for harmful fungi!