The Free Press, Mankato, MN


June 19, 2013

Anthracnose affecting shade trees

MANKATO -- Our home and garden questions at the Extension Office are very commonly weather-related.

This year, trees are suffering from two consecutive dry seasons. Alternative gardening decisions are being made to work around the late spring and cold, wet soils. Weather records from last week showed that we had some precipitation every day, and that the temperatures stayed between 50 and 60 degrees.

Because this cool, wet weather continues to persist, a common leaf blight disease of shade trees known as anthracnose thrives and spreads. Anthracnose is a fungal disease that causes leaf blotches, leaf distortion, shoot blight, and leaf drop. This disease is caused by several related fungi that thrive in cool wet conditions. Ash, maple and oak trees are all commonly infected with anthracnose and symptoms of the disease have been seen on all of these trees this spring.

The disease is most common in spring when new shoot and leaf growth are combined with temperatures ranging from 50-68 degrees and spring rain. Anthracnose can also reoccur in the summer when cool, wet weather is paired with succulent leaf growth.

Despite blackening of leaves and shoots, anthracnose actually only results in a minor stress on the health of the tree. Only young, growing leaves and shoots are susceptible to infection. Mature leaves are relatively resistant to the disease. Once warm, dry weather arrives in Minnesota, leaves will mature and trees will replace lost leaves with a new flush of growth. As long as cool, wet weather persists, however, expect this fungal disease to spread throughout the trees canopy.

Fungicides are not necessary unless the tree has been completely defoliated many years in a row. Because fungicides are protectants, the timing of application is sometimes tricky, and can vary widely from year to year.

Cultural protection from anthracnose damage includes good sanitation at the end of the season. Rake up and remove all dropped leaves to eliminate the residue where the fungal spores overwinter. This time of year, prune limbs with infected leaves to remove the fungus and to increase sunlight and air circulation within the tree's canopy. When planting new trees, select species which are resistant to anthracnose.

More information can be found about this disease at the University of Minnesota Extension publication on anthracnose in shade trees. Information provided by Michelle Grabowski, U of M Extension plant pathologist.

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