Oak leaves dropping (but not from oak wilt)

By Linda Williams, forest health specialist, Woodruff. Linda.Williams@wisconsin.gov; 920-360-0665

Heavily-spotted oak leaves began to drop in early August. Spots appeared on both the front and back of leaves. Photo: Linda Williams

Heavily-spotted oak leaves began to drop in early August. Spots appeared on both the front and back of leaves. Photo: Linda Williams

Last year, Forest Health News published an article about oaks prematurely dropping leaves although they were not infected by the fungal pathogen Ceratocystis fagacearum, the cause of oak wilt disease. Oak trees infected with oak wilt disease in springtime rapidly wilt and drop green leaves in July or August. However, oak wilt disease is not the only reason oak trees prematurely drop leaves.

In 2017, the suspect was a fungal leaf disease called Cylindrosporium leaf spot, which typically causes dark brown and light tan circular spots on infected leaves. By the end of last year, neither the DNR’s forest health nor the University of Wisconsin’s Plant Disease Diagnostic Clinic lab cultured the fungus from suspicious leaves. The issue was tabled until more samples could be obtained this year.

The first leaves with suggestive spots started coming down in early August. Fallen oak leaves were heavily spotted, while leaves displaying only a few spots remained on branches. Although there may appear to be an alarming number of leaves on the ground, trees infested with Cylindrosporium do not suddenly drop all of their leaves and do not rapidly die as they do with oak wilt disease. Symptoms of Cylindrosporium leaf spot disease were observed on oak trees in Langlade, Lincoln, Oneida, Richland, Sawyer, Washburn, and Vilas counties. Leaf samples were collected and submitted to the forest health lab; perhaps testing this year will confirm the disease’s presence. Watch for more updates in the Forest Health News as the year goes on.

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