Cool, Wet Spring: Can It Hurt Your Trees?

Our lush shade trees are one of the joys of living in eastern Iowa. Sprightly sugar maples, sturdy ash, and deep, dark Norway maples line our streets and form a cooling roof on our backyards. But if a prolonged, cool, wet spring arrives this year, (as we’re likely to have; our winters grow more mild every year) we should be on the lookout for a common enemy of our treasured trees: the fungal disease, anthracnose.

What is Anthracnose?
While anthracnose can affect a wide variety of plants, we’ll focus on its affect on trees in this article. Anthracnose, sometimes called leaf, shoot, or twig blight, is one of many fungal diseases affecting maple and ash trees, and as well as dogwoods and walnuts. North Dakota State University notes that anthracnose often occurs during prolonged cool, wet springs where it seems to be humid or moist all the time, allowing the fungi that cause it to be transmitted throughout the tree easily.

Do My Trees Have It?
Because a variety of fungi can start anthracnose, the earliest symptoms exhibited can vary. The outcome of anthracnose is often curled leaves, a portion of the leaf dying, or in some cases defoliation. Shade trees often recover from this with a second set of leaves that are not infected. In some cases, twigs or entire branches can die.

What Can I Do About It?
Anthracnose can overwinter and return if another cool, wet spring presents itself. Usually, unless affected trees defoliate year after year, anthracnose causes no permanent damage and will require no treatment. Often, a hot, dry summer will cause the anthracnose cycle to stop.

The best management of anthracnose as well as other fungal diseases is to control the culture of your trees.

1. Look for fungal and disease resistant varieties.

2. Plant trees and shrubs with their adult size in mind so they won’t crowd and discourage good airflow.

3. Prune trees in early spring to keep their crowns open to allow in light and air.

The University of California recommends that if you do have anthracnose, remove the leaves and twigs that fall to prevent cross-contamination; do not compost them. Ensure you are not over-watering and that your irrigation system is not keeping leaves constantly moist. Small trees with anthracnose may be treated with a fungicide when leaves are beginning to open in the spring, but large trees cannot be effectively treated this way.

The best way to know for sure if you have anthracnose is to contact your local extension office and bring in a sample of infected leaves. They can help diagnose the specific condition and recommend options for management.