Extension Communications Extension 4-H Youth Building Ames, Iowa 50011-3630 (515) 294-9915

7/5/04

Contacts:
Christine Engelbrecht, Plant Pathology, (515) 294-0581, cengel@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Garden Column for the week of July 9, 2004

Spring Rains Bring Lawn Diseases

By Christine Engelbrecht
Plant Pathologist
Iowa State University Extension

Rainy spring weather brings out ducks, puddle-jumping children and fungi. The recent wet weather has caused a proliferation of fungal diseases on home lawns. Some of the most common diseases found on lawns recently are dollar spot, ascochyta leaf blight and melting out. The fungi that cause these diseases do not produce large showy mushrooms or toadstools. Instead, they simply kill the grass, leaving behind unsightly straw-colored patches that frustrate homeowners.

Dollar spot gets its name from its appearance on golf greens, where it shows up as small, dead lesions the size and shape of a silver dollar. On home lawns, though, dollar spot causes much larger dead patches in the lawn. The round spots can be up to six inches in diameter. Individual dead spots may coalesce into large, irregular shaped patches. The individual blades of grass typically each have a dead, straw-colored area that spans the entire width of the leaf, bordered by reddish brown bands. Dollar spot occurs on all turfgrass species and is most common from late spring to late autumn.

Ascochyta leaf blight is another fungal disease that causes patches of dead grass. It is named for the fungus that causes these dead patches. Unlike dollar spot, ascochyta leaf blight usually causes much larger, more uniform areas of dead grass. The individual grass blades die from the tips rather than from lesions in the centers. Often, the dead leaf tip has a pinched appearance. Ascochyta leaf blight tends to show up from late winter through summer.

A third fungal disease of turfgrass is called leaf spot, or melting out. This is the most common disease on lawns in this region. Straw-colored, oval spots with dark brown margins appear on individual grass blades and can coalesce to cover the majority of the blade. The overall effect is that the lawn looks thinned ("melted out") with sickly, diffuse straw-colored patches. Melting out is especially severe on Kentucky bluegrass, although it can also affect other grass varieties.

All three of these diseases are caused by fungi that favor high humidity and moist conditions. The recent rainy weather has promoted outbreaks of these diseases on many local lawns by providing an excess of moisture that allows fungi to run rampant.

Although dollar spot, ascochyta leaf blight, and melting out are caused by several different species of fungi, the management of all three diseases is similar. Management strategies include tactics that enhance the overall health and vigor of the lawn.

* Fertilize adequately because nitrogen deficiency makes grass more susceptible to disease. However, avoid over-fertilizing in the spring, as too much lush foliar growth can also promote disease.

* Avoid letting the grass be wet for long periods. Water deeply, as infrequently as possible. Water in the morning rather than the evening to allow leaf blades time to dry off before dark.

* Mow the grass regularly at two inches in the spring and fall and two and a half to three inches in the summer. Overly frequent mowing favors infection by injuring the grass blades, opening wounds that may be infected by the disease-causing fungi.

* Keep mower blades sharp.

* Avoid mowing when grass is wet to keep from moving the fungus over the lawn.

* Core aerate the lawn to reduce thatch buildup (the layer of organic matter between the grass blades and roots).

* Several fungicides are available that are effective against these and other fungal grass diseases. Typically these should be applied before symptoms appear.

Following these basic guidelines can help to ensure that your lawn is as healthy as possible, no matter how much rain may fall.

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ml: isugarden

Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 184K and the bottom picture's fullsize photo is 148K. Caption: A lawn infected with ascochyta leaf blight. Photo courtesy of American Phytopathological Society Press.
Caption: Close up of dollar spot lesions on grass. Photo courtesy of American Phytopathological Society Press.

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