Anthracnose, a fungal leaf blight of several different shade trees, is not uncommon in Minnesota. This disease causes brown spots and cupping or distortion of young leaves. The fungal pathogens that cause anthracnose thrive in cool wet weather common in early spring. In a typical year, trees recover from anthracnose as summer brings hot dry weather and leaves become naturally more disease resistant as they mature.
This year several cases of severe anthracnose have been reported in late June and July; later than usual. Maple trees with advanced cases of anthracnose have large brown blotches on fully expanded leaves. In white oaks, the typical anthracnose leaf symptoms can progress to shoot blight. In the shoot blight stage of anthracnose, several leaves turn completely brown along with the young shoot, resulting in clumps of dead leaves at the ends of branches. These leaves may remain attached for the entire growing season and into the next. As a result clumps of dead leaves seen this year may be a result of last year's infection.
The unusual timing and severity of symptoms observed from anthracnose of shade trees this year could easily be confused with damage from other pests, pathogens or abiotic causes. The cause of the problem must be properly identified before an effective management decision can be made. For assistance in diagnosing plant problems, samples can be sent to the University of Minnesota Plant Diagnostic Clinic.
Anthracnose is a minor stress on the tree and will not significantly affect the health of the tree unless the majority of the canopy is affected several years in a row. There is no treatment effective against anthracnose that can be applied at this time of year.