June 5, 2003

 

Ash Anthracnose Causing Problems

Some ash trees that are losing leaves may be suffering from ash anthracnose, according to Bob Stack, North Dakota State University plant pathologist and professor. "The cool, showery weather of the past few weeks favors infection by the anthracnose fungus," according to Stack.

"The cool temperatures also slow the development of the ash leaves, giving the fungus a longer time to infect the tree."

Once leaves start to fall, it is too late for any fungicide sprays to be effective because infection has already taken place.

Stack advises those with problems not to panic. "While unsightly and worrisome, a single defoliation by anthracnose will not permanently damage the tree. When warmer weather comes, the tree will make new leaves to replace those lost."

For the current season, rake and destroy the leaves to reduce the disease carryover. The leaves can be burned, buried or composted. Prune-out cankered branches and remove twigs to promote air movement within the crown. Fertilize the tree next spring to promote vigorous growth.

Ash leaves are "compound leaves, composed of mostly seven to nine leaves arranged along a stalk or "rachis." Leaves that fall appear to have their normal healthy green color with no browning or yellowing. Close examination may reveal a few tiny discolored spots of brown or purple. These spots are especially noticeable on the rachis.

"If you see repeated anthracnose damage in successive years, a preventive fungicide treatment may help," Stack says. "A treatment needs to be carefully timed to the bud break of the tree to be effective. A fungicide containing the active ingredient chlorothalonil can be used or contact your county agent or the NDSU Plant Diagnostic Lab for other possible recommendations."

Ash anthracnose has been found in Midwestern states for more than 100 years, occuring in periodic boom or bust cycles. It may be severe for several years followed by years when it is rare.

During the mid 1980s anthracnose was common in N.D., especially in the Red River Valley. During that time several studies were conducted by NDSU to better understand the disease and how it develops.

More information on ash anthracnose along with other tree diseases is available on the Web at http://www.ext.nodak.edu/extpubs/plantsci/hortcrop/pp697-2.htm

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Source: Bob Stack, (701) 231-7077, robert.stack@ndsu.nodak.edu
Editor: Rich Mattern, (701) 231-6136, richard.mattern@ndsu.nodak.edu