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Disease causing ash leaves to fall

Little green leaves from ash trees, like these found along the curb in northwest Willmar near the Kandiyohi County Fairgrounds, are a common sight throughout town because of a weather-related disease. (Tribune photo by Ron Adams)

WILLMAR -- A weather-related disease is causing ash trees to lose their leaves in Willmar and in other cities in the region.

Anthracnose, a common type of fungal spot on ash trees, is caused by the fungus Discula and survives winter on infected plant tissue. During the spring and summer when the weather is cool and wet, spores are produced on infected tissue.

These spores are spread by wind and splashing rain to buds, shoots and expanding leaves where new infections occur, according to the Minnesota Extension Service.

Leaf spots are irregular in shape and usually appear brownish in color. Infections early in the season may result in large dead areas, blotching, or distortion of leaves. Severe leaf infection can cause extensive defoliation.

Anthracnose is most severe on the lower and inner portions of the tree where humidity and moisture levels are higher. This helps distinguish it from wilt diseases and root problems where symptoms develop first in the top of the tree.

Ron Gilbertson, city of Willmar public works superintendent, said the problem surfaced the first of the week.

"We started to see it this past weekend,'' he said Thursday. "People saw leaves falling. We received several calls Monday. This happened a couple of years ago, too.''

Gilbertson said there is nothing that people can do about it, other than rake up the leaves. He said the trees will come back, but will probably not be as full.

Most of Willmar's boulevard trees are ash. An inventory of the city's 16,000 boulevard trees showed 10,000 were ash. Gilbertson said the problem will affect most of the city's ash trees.

The Extension Service says anthracnose does not cause permanent damage to established trees. However, trees stressed by root restrictions, drought, heavy insect infestation or other problems are much less tolerant of anthracnose and may show decreased vigor after only a single season of severe anthracnose defoliation.

Cultural control measures should include proper watering, mulching and sanitation procedures, the Extension Service says. Raking leaves in the fall and pruning out dead or dying branches help reduce the number of new infections the following year. Fertilizing stressed trees in the spring may also help boost tree vigor, the Extension Service says.

David Little
David Little covers the Willmar City Council, Willmar Municipal Utilities and other city news.
(320) 235-1150