This year’s record-wet spring brought many blessings to Northern Colorado, including a bountiful hay crop and reduced wildland fire danger. For our aspens and cottonwoods, however, the cool wet spring was perfect for the spread of leaf blights, also referred to as “leaf spot disease.”

These leaf blights are caused by fungi, usually in the genus Marssonina, though sometimes it occurs in conjunction with another leaf blight, Septoria. The spores of these fungi are present in the previous year’s leaves on the ground, and they proliferate and infect the new growth in wet conditions.

Small, brown spots can appear on the infected leaves in the spring after the trees have leafed out. With Marssonina, these brown spots often have a yellowish margin. The symptoms become increasingly noticeable in July and August.

The leaves, still green, may dry up and drop off the tree. The tree may grow new leaves if this happens early enough in the season. As the summer goes on, the symptoms move up the crown of the tree, often giving it a thin appearance and the leaves of a brown or bronze color.

The good news is, despite the apparently drastic impacts, leaf blights like Marssonina in aspen and cottonwoods generally do not kill the tree. A healthy tree will leaf out normally the following year, provided there is not a recurrence of the wet conditions that precipitated the outbreak.

There is no real practical treatment for leaf blight on a forest-wide scale. It is present in the environment and will appear when the conditions are favorable. Some things can be done when there are only a few trees in a yard or landscape situation.

Raking up and disposing of the infected leaves can be the most effective action to help prevent future infections. Providing adequate space between landscape trees may also help by providing for air flow and drying.

The other good news is that this year’s leaf blight is not going to ruin our fall foliage, because only some stands of trees are visibly impacted, and these are in the midst of many that are not. Why are some stands impacted when their neighbors are not?

Our local aspen is quaking aspen (Populous tremuloides). It tends to grow in clones, a group of trees sharing the same root system and, thus, the same genetic makeup. Clones usually range in size from one to 20 acres, though there is an aspen clone in Utah called “Pando” that is 106 acres in size and is thought to be the heaviest organism on earth.

At an estimated age of 80,000 years, it may also be the oldest. Fortunately, many of our aspen clones are resistant to Marssonina and other leaf blights, and these stands show little or no impact from this year’s leaf blight.

Questions about this or other problems with trees may be directed to the Routt County Extension office, at 970-879-0825, or the Colorado State Forest Service at 970-879-0475.

John Twitchell is district forester for the Colorado State Forest Service.