Wet summer brings early leaf drop to black walnut trees

The Iowa Department of Natural Resources (DNR) is seeing a slightly higher than normal amount of walnut anthracnose this summer, causing some black walnut trees to yellow and lose their leaves earlier and more extensively than usual.

Walnut anthracnose is a common fungal leaf disease. The fungus infects foliage and produces brown to black leaf spots, often followed by yellowing and premature leaf drop. Wet and humid weather favors disease development. Iowa's prolonged wet conditions this growing season have provided favorable conditions for new infections.

Anthracnose also occurs on other Iowa hardwood trees, including oak, maple, sycamore, ash, linden and hickory. The fungi that cause anthracnose are generally specific to their particular tree hosts, and symptoms and timing vary among species.

Anthracnose becomes most noticeable on black walnuts in late summer. By August, heavily affected trees may have thin crowns and lose most of their foliage. Although this can look alarming, the early leaf drop is generally more noticeable than harmful. By late summer, trees have already completed much of their annual growth and stored most of the energy they need for next year. Treatment is generally unnecessary, and healthy trees are expected to leaf out normally next spring.

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