Lawn Care and Brown Patch
February 4, 2009
Introduction
Rhizoctonia species cause the disease of turfgrass known as Brown Patch. Kentucky bluegrass (Poa pratensis) and fine fescues (Festuca sp.) may be attacked, but Bentgrasses (Agrostis sp.), Ryegrasses (Lolium sp.), Annual bluegrass (Poa annua) and tall fescues (Festuca arundinacea) are most susceptible. Rhizoctonia solani is usually the causal agent of infections occuring in June throughout early July. Rhizoctonia zeae takes R. solani‘s place as the heat of summer arrives.
Figure 1: Leaf lesions caused by Rhizoctonia sp. (provided by Dr. Eric B. Nelson, Cornell University) 
Figure 2: Microscopic view of Rhizoctonia sp. mycelium. (provided by Dr. Eric B. Nelson, Cornell University)
Symptoms
The pattern of symptoms depends largely on cool vs. warm season grasses, mowing practices, and moisture levels. On cool season grasses the symptoms produced by a Rhizoctonia sp. infection may vary depending on the maintenance practices. Under close mowing conditions, cool season grasses that are kept very wet produce small to 50 cm blighted patches that appear quickly during optimum conditions. The patches may contain the diagnostic feature of a purplish-gray border that is referred to as a "smoke-ring". The smoke-ring is commonly missing from the symptomatic region but if present is most often observed during hot-humid weather and is most easily seen when a dew is present. Cool season grasses that are cut higher produce small light brown patches up to 15 cm in diameter that may or may not contain that diagnostic smoke-ring symptom. Cool season grasses that are cut high and kept dry may produce patches up to 30 cm in diameter. These patches may produce a symptomatic patch known as a "frogeye". Frogeye patches have apparently healthy green grass surrounded by a ring of necrotic grass that appears very flat and sunken into the ground.
Individual leaf lesions may be small, round to irregularly shaped, and tan in color with a distinctive dark brown margin (Fig. 1). As the fungus grows, the hyphae produce a characteristic branching pattern of 90° angles that are easily recognized microscopically. The width of the hypha decreases at the junction of a branch giving it a pinched appearance (Fig. 2).
Disease Cycle
Rhizoctonia spp. produce structures referred to as "bulbils" to survive unfavorable conditions such as freezing temperatures or drought. The optimum temperature range for germination of the fungus is from 21°C to 32°C (70°F to 90°F). The pathogen mainly infects leaves and sheaths of the turfgrass plant.
Brown patch disease cycle.
(image from The Compendium of Turfgrass Diseases, APS Press)
