Rhizoctonia Blight (Brown Patch, Large Patch, Yellow Patch)

There are three types of Rhizoctonia blight: large patch, a blight of warm-season grasses that is caused by Rhizoctonia solani; brown patch, a blight of cool-season grasses that is also caused by R. solani; and yellow patch, a blight of annual and Kentucky bluegrass caused by R. cerealis. When weather conditions are not favorable for disease development, both species can survive as sclerotia in the thatch and soil or as mycelia in plants and debris.

Large patch

Large patch on warm-season grasses appears as light green patches in fall. Infected areas may become bright yellow and then turn brown as the grass emerges from dormancy in spring. Spring symptoms can persist in cool, wet weather for an extended duration; bermudagrass can recover quickly as the weather warms, while other grasses, such as zoysiagrass, take many weeks to recover. There is often a soft, dark brown to purplish rot of the lower portion of the leaf sheaths that can develop into a reddish-brown necrosis of the leaf sheath and stem under dry conditions. In severe cases, plants will be affected by an extensive soft rot of the stems.

Brown patch

Brown patch affects cool-season grasses during periods of hot weather. On closely mowed turf, patches of blighted turf will often have a purplish edge or smoke ring appearance in the early morning hours. Initial patches may first appear purplish-green and turn brown as the disease progresses. On taller turf plantings, patches will appear as blighted turf that turns dull tan to brown. The fungus causes dull tan lesions on leaves that may develop a reddish-brown margin. Plants killed by the fungus will often have a light brown color, and turn brittle, but will not have a wet, greasy appearance.

Yellow patch

Yellow patch occurs primarily on annual bluegrass and Kentucky bluegrass in cool, moist weather. Symptoms include the development of yellow or tan-colored patches of turf. On leaves, there is a yellow to tan chlorosis that extends down from the leaf tips. Gray-tan lesions may develop on lower leaf portions and often on Kentucky bluegrass, some leaves may become reddish or reddish purple. In periods of extended cool, wet weather, affected leaves may become necrotic, but in many cases the main symptom is only a yellow chlorosis.

Large patch: Bermudagrass, zoysiagrass, St. Augustinegrass, kikuyugrass
Brown patch: bentgrasses, fescues, ryegrasses, bluegrasses
Yellow patch: annual bluegrass, Kentucky bluegrass, bermudagrass, perennial ryegrass, zoysiagrass, bentgrasses

Generally, Rhizoctonia diseases are more severe under conditions of poor drainage, high compaction, thick thatch layers, long periods of leaf wetness, low mowing heights, excessive mechanical damage, and high nitrogen fertilization.

Large patch develops in fall and spring when warm-season grasses are going into or coming out of dormancy.

Brown patch is common when temperatures are in the range of 75° to 95°F, with the optimal conditions for leaf colonization being temperatures of 85° to 90°F with high humidity or extended leaf wetness periods.

Yellow patch usually develops when air temperatures are from 50° to 65°F and there is high humidity or extended periods of leaf wetness. In many cases, the turf will recover when temperatures go above or below this range.

Irrigation and leaf wetness management is an important part of controlling Rhizoctonia diseases. Fungicide applications can be necessary if leaf wetness and soil moisture cannot be adequately managed (for instance, as a result of rain or high humidity). Combine cultural management techniques with fungicide applications for the best management of the disease.

Cultural practices that improve water and fertility management are useful in preventing the development of Rhizoctonia blight. Reduce shading and improve soil aeration and water drainage. Irrigate in the pre-dawn or early morning hours to promote leaf drying. Irrigate only when needed to a depth of 4 to 6 inches. Remove dew from leaves by poling or light irrigation. Avoid nitrogen fertilization that results in a soft foliage growth. Maintain thatch at less than 0.5 inch.

For areas where large patch and yellow patch are chronic, fall fungicide applications may be necessary; otherwise, make fungicide applications soon after the first symptoms of disease are seen. Some fungicides that are active against R. solani may not be specifically labeled for R. cerealis.