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Fungi cause lawn problems in fall

Posted: October 11, 2011 - 6:33pm

Last week it finally had the feel of fall. The cool temperatures made it possible for gardeners to get back to work in the yard.

While in the yard, many have discovered small circular areas in the lawn that are off- color or orange. These areas are disease problems.

Most of us are familiar with the two major diseases that attack turf grass in the fall: brown patch and take-all disease. But, there are some minor disease problems that also will attack turf grass in the fall. These diseases make the turf look bad, but seldom kill the grass. Some of these diseases are dollar spot, rust and melting-out disease.

Dollar spot is a disease that typically occurs in the spring and fall. It is caused by the fungus Sclerotinia homoeocarpa . This fungus likes temperatures between 60-85 degrees and continuous high humidity. This disease is particularly favored by warm days, cool nights and intense dews. When the disease is severe, it also infects turf growing in areas with low levels of nitrogen and in dry soils.

Dollar spot is mainly a problem on Bermuda and zoysia, but it can infect centipede.

Symptoms of dollar spot include sunken, circular patches that measure up to several inches on turfgrass. The patches turn from brown to straw color and can eventually coalesce, forming irregular areas. Leaves might display small lesions that turn from yellow-green to straw color with a reddish-brown border.

The lesions can extend the full width of the leaf. Multiple lesions may occur on a single leaf blade. In the morning when there is dew on the grass, mycelia may be present. These are the reproductive part of the fungus and will appear as spider webs on the grass. When the dew dries, the webs will disappear.

Rust is more of a problem in the fall than at other times of the year. It is caused by the fungus Puccinia sp. , and primarily attacks zoysia. This disease attacks plants that are slowing down their growth and starting to go dormant.

This disease will cause orange lesions to appear on the leaves. These spores can be wiped off and will leave an orange streak. Rust will seldom kill the grass, but it does weaken it going into the winter. There can be some thinning of the turf next spring.

The next disease problem is caused by a number of fungi. It goes by the name "melting-out." The fungi which cause the problem are Curvularia spp. , Drechslera spp. and Bipolaris spp. The disease will show up as leaf spots that are purplish-brown to black spots with tan centers. The lower leaves of the infected plants become shriveled and blighted.

Severe leaf spot can lead to melting-out when leaves and tillers are infected, causing severe thinning of the stand. This disease occurs at temperatures from 40 to 80 degrees in areas that experience more than 10 hours a day of foliage wetness for several days.

The disease is favored by high amounts of nitrogen and a low mowing height. Bermuda, zoysia are the main grasses affected by this disease, but centipede can be affected as well.

The chance of these diseases can be reduced by following proper management practices, first by controlling the thatch layer in the turf. Thatch is a layer of organic material that builds up on the soil surface, and consists of stems, stolons and other woody parts of the grass plant. It is not made up of grass leaves. Thatch less than a half-inch thick doesn't cause problems. Once thatch gets deeper than a half-inch thick, it should be removed. Thatch is the home for fungi. This is where they live and work.

Thatch will inhibit pesticides, fertilizers and water from reaching the soil surface. Thatch can be removed with a verticutter, aeration or by topdressing. These practices should be done in the spring when the grass is growing.

Another practice that can help with these diseases is proper fertilization. Most of these diseases are associated with low nitrogen levels. The grass doesn't need nitrogen now because it is close to going dormant. But fertilizing the grass in late August or early September will help the grass with these diseases.

Water the grass when needed. Watering less often but for a longer period of time will help the grass dry out between irrigations.

The last thing that needs to be done is to use a fungicide. Dollar spot very seldom causes any problems and the grass will recover, but rust and melting-out can thin the grass. Most of the lawn fungicides found in garden centers will control these disease problems.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at cphillipshort@comcast.net or at (706) 836-2152.

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