Growing Green

By Robin Trott, Extension Educator

Another cool, wet spring has led to a recurring problem with maple trees. Blotchy black leaves and unusual bumps abound, and samples are flooding my desk from residents concerned that their tree may be dying. If you are noticing unusual looking leaves on your maples you might be seeing Maple Anthracnose or Maple Gall Mites.

Anthracnose is the name of several common fungal diseases that affect the young leaves of ash, maple and white oak trees. Symptoms include irregular spots and dead areas on leaves that often follow the veins of the leaves. Affected tissue can vary in color, but is often tan, brown or black. Severely affected leaves often curl and may fall off.  

Do not panic. For many trees, anthracnose is a cosmetic disease, making a tree look a little ragged, but not killing the tree. Anthracnose spores are in the soil, and infections can be reduced by removing and disposing of fallen, infected leaves in the autumn. Leaves can be buried, burned (where allowed) or composted. When composting, make sure that your compost pile reaches high temperature (approximately 140°F). 

Maple galls are caused by extremely small mites only 1/125 inch long. The adult mites spend the winter under the bark and other protective places on the trees. In the early spring the adults move to the developing leaves and begin feeding. The leaf responds to the small irritation by rapidly producing extra cells that form the abnormal growth at the feeding site. The gall encloses the mite which continues to feed and lay numerous eggs within the gall. Reproduction is prolific and as the new mites mature, they leave the gall and move to other newly emerging leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity continues until mid-summer when it starts to decline. Adult mites leave the foliage in the fall and move to the overwintering sites.

Heavy infestations of galls cause leaves to be disfigured. At the worst, leaves become curled or rolled up, and may change color and drop prematurely. These effects are not detrimental to the overall health of healthy, well-established trees. The galls are unsightly and may appear to be severe, but the effect on the tree is not significant.

If you have concerns about unhealthy plants in your garden, visit What’s Wrong with My Plant at or call your local University of Minnesota Extension Office.  Douglas County residents may contact me at 320-762-3890.

Until next time, happy gardening!