Issue 5, May 26, 2015

Rhizoctonia on Turfgrass

Brown Patch, a common disease of turf, is caused by the fungus Rhizoctonia solani. It is a warm-season disease which affects the leaf tissue of infected plants. However, there is a much milder cool-weather Rhizoctonia disease called Yellow Patch which bookends summer Brown Patch in the spring and fall. With the cloudy, wet spring in the Midwest, a few samples have shown up at the Plant Clinic.

Yellow Patch is a cool-weather disease; it primarily occurs from late March through mid-May and sometimes reappears from late September through early November. The disease is favored by periods of prolonged cool, wet weather. Yellow patch is caused by Rhizoctonia cerealis. This pathogenprefers cloudy, wet weather on shaded lawns while R. solani thrives in the muggy nights and dry days of summer. R. cerealis is the much less destructive of the two pathogens, and turf will generally grow out of the symptoms after a few days of warm, dry weather.

Rhizoctonia mycelium in turfgrass roots.

Areas affected by Yellow Patch are irregular to circular in shape, and can be as large as 3' in diameter. Foliage will first turn yellow, followed by tan, and finishing as a bronze color. The edges of the patches may become brown and look similar to dormant grass if the weather becomes too cold. Yellow Patch is most commonly observed on lush grass with a high nitrogen content. Excessive thatch (over 0.75 inch thick) is also associated with the development of this disease.

Because turf will generally grow out of Yellow Patch when the weather changes, Yellow Patch is mostly considered to be a cosmetic disease. Chemical control is rarely warranted. Instead, maintaining adequate nutrient levels while reducing nitrogen fertilization and improving drainage are recommended to manage Yellow Patch. If over 0.75 inches of thatch has accumulated, dethatching may help, but a fertilization program that avoids the accumulation of thatch is better. In problem areas where Yellow Patch has been a recurring issue, two or more preventative applications of a fungicide labeled to control Yellow Patch in late fall have been shown to suppress the development of the disease in spring, and to encourage the recovery of turf in which symptoms develop. (Sean Mullahy and Diane Plewa)

Diane Plewa
Sean Mullahy

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