Iowa’s wet and humid summer is causing some black walnut trees to turn yellow and lose their leaves earlier than usual.
The Iowa Department of Natural Resources says it is seeing slightly more walnut anthracnose than normal this year. The common fungal disease causes brown or black spots on leaves, followed by yellowing and premature leaf drop.
Prolonged wet conditions have created an ideal environment for new infections. Symptoms are typically most noticeable in late summer, and heavily affected walnut trees may have thin crowns or lose most of their foliage by August.
While the appearance can be concerning, the DNR says the early leaf loss is generally more noticeable than harmful. By late summer, trees have already completed much of their annual growth and stored most of the energy needed for the following year.
Treatment is usually unnecessary, and otherwise healthy black walnut trees are expected to produce leaves normally next spring.
Anthracnose can also affect other Iowa hardwoods, including oak, maple, sycamore, ash, linden and hickory. However, the fungi responsible are generally specific to each type of tree.