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Fungal disease causes those ubiquitous brown patches

Around this time of the year, many homeowners face a baffling mystery - brown circular patches that inexplicably appear on their lawns.

At the extension office, we get a lot of calls from concerned residents wondering, "What are those brown spots on my lawn?"

The cause is often a fungal disease called brown patch - or large patch on warm summer turf - and it can quickly destroy the beauty of a lawn.

The symptoms of brown patch are thinned patches of light brown grass, roughly circular in shape. These areas range in diameter from a few inches to several feet. The grass in the center of the patch will recover, resulting in a doughnut-shaped pattern.

Upon closer inspection, the cool-season grass blades show small, irregular, tan leaf spots with dark brown borders. Warm-season grasses with this disease seldom have leaf spots but will have rotted leaf sheaths near the soil surface. In the early morning when the weather is hot and humid, a smoky gray can be observed around the areas that are brown and diseased.

All types of turf grasses are affected by this disease. It's one of the most common diseases on turf grasses, especially on tall fescue.

Brown patch prefers high humidity and warm temperatures - over 80 F during the day and over 60 F at night. In lawns where the foliage stays wet for several days in a row, brown patch can be more of a problem. Cutting the grass at a lower height than recommended amount also makes it more susceptible to the disease.

The best way to control brown patch is to use cultural lawn-care practices, which are much easier and less costly than using chemical fungicides. Here are a few ideas:

•Avoid applying fertilizers high in nitrogen to cool-season grasses in late spring and summer, and to warm-season grasses mid to late fall. The fungus prefers grass with lush, new growth that results from too much nitrogen.

•Have the soil tested to determine fertility levels, and add lime if the pH is less than 6.5.

•Irrigate infrequently, and let the water penetrate deeply - 4 to 6 inches. Do the watering early in the morning so the grass will be drier during the evening hours.

•Keep lawns mowed at the recommended height and prevent thatch buildup. Make sure the mower blade is sharp before use.

•Good drainage also reduces the susceptibility of the grass to brown patch.

Using fungicides to control brown patch is difficult, but regular applications can vastly improve appearance. Start applying the fungicides when symptoms are first observed, and continue to apply at the recommended label intervals until the disease is controlled. Usually, only two or three applications are necessary to control the disease.

For chemical control of brown patch on either cool- or warm-season grasses, begin fungicide applications when nighttime low temperatures reach 70 F once every 14 days.

Fungicides such as captan, maneb, mancozeb and myclobutanil are recommended for controlling brown patch. Find out more about these chemicals and their usage by visiting the 2007 online Georgia Pest Management handbook, www.ent.uga.edu/pmh. For more information and consultation on brown patch or any other topic of concern, contact the Gwinnett County Extension Office.

Timothy Daly is an agricultural and natural resource agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or timothy.daly@gwinnettcounty.com.