Description of the Disease
The two types of Rhizoctonia blight that we see in the central Arkansas area are large patch, a blight of warm season grasses that is caused by Rhizoctonia solani and brown patch, a blight of cool season grasses that is also caused by R. solani; When weather conditions are not favorable for disease development, both can survive in the thatch and soil or as mycelia in plants and debris. 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 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.
Large patch: Bermudagrass, zoysiagrass, St. Augustinegrass, kikuyugrass
Brown patch: bentgrasses, fescues, ryegrasses, bluegrasses.
Conditions favoring disease
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.li, ½ to 951i, ½F, with the optimal conditions for leaf colonization being temperatures of 851i, ½ to 901"i,½F with high humidity or extended leaf wetness periods.
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 are chronic, fall and spring fungicide applications may be necessary; otherwise, make fungicide applications soon after the first symptoms of disease are seen. Fungicide treatments are to be applied in 28 day intervals.