Rhizoctonia solani (Brown Patch of Turfgrasses)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Agaricomycotina
Class	Agaricomycetes
Order	Cantharellales
Family	Ceratobasidiaceae
Genus	Rhizoctonia

Scientific Name

Rhizoctonia solani

Scientific Name Synonyms

Moniliopsis aderholdii

Corticium solani

Rhizoctonia napaeae

Thanatephorus cucumeris

Common Name

Rhizoctonia damping-off, blight and rot

Author: Lee Miller, University of Missouri

Reviewed by:Name, Organization

Pathogen

Rhizoctonia solani Kühn is an agaricomycete in the order Cantharellales, and is the asexual state ofThanatephorus cucumeris. Pathogen strains are separated into anastomosis groups based on hyphal fusion when paired. Isolates associated with brown patch of cool season turfgrasses are most commonly in the groups AG-1 1A and AG2-2 IIIB. A separate anastomosis group of R. solani, AG 2-2 IV LP, is associated with large patch of warm season turfgrasses. The mycelium of R. solani is tan to brown and relatively large (4-15 m) in diameter. The mycelium is also characterized by right-angled branching at a constricted branch point, and a dolipore septum. Clamp connections are absent. Sclerotia are never differentiated into rind and medulla. The color of the sclerotia is dark brown to black, 1-10 mm in diameter, and are spherical to irregular in shape. R. solani causes a number of important diseases in other crops including rice, potatoes, and cereals. The pathogen does not produce conidia and exists as mycelia sterilia.

Symptoms and Signs

Brown patch symptoms on longer cut turfgrass are circular or irregular-shaped solid patches of tan to bleached turf that vary in size. Some initial purplish green blighting is possible in the more susceptible regions, but will quickly fade to light brown. Occasionally, leaves on outer margins are blighted while those in the patch center are recovering, which may cause a ring-shaped or “frog-eye” patch. On individual grass blades, tan to bleached leaf lesions with dark brown margins can be observed on the leading edges of the patch. These lesions are initially small and irregular shaped, increasing in size as the disease persists. On close mown turfgrass, such as bentgrass putting greens, the stand symptom is most often a dark brown-colored patch of declining plants. In warm and humid weather, especially on close-cut turf, the margin of the patch may develop a “smoke ring” that is greyish brown to dark purple. A smoke ring indicates the mycelium is actively infecting and spreading to expand the patch margin. The smoke ring is most often observed in early morning dew.

Ecology and Spread

R. solani survives in plant debris or thatch as sclerotia or mycelium. Sclerotia begin to germinate when soil temperatures reach 60°F. Mycelia spread in a circular pattern, but do not parasitize the foliage until high humidity and high temperatures begin to stress the plant. Initially, R. solani infects the leaves when the average air temperature is approximately 73°F. In lower temperatures, the pathogen invades through stomates and mechanical damage (i.e. mowing). When air temperatures reach 80-85°F, infection occurs through direct penetration of intact epidermal cells.

Geographic Distribution

R. solani is widespread globally and occurs in all areas where turf is grown.

Management

On higher cut turfgrasses, cultural practices aimed at reducing leaf wetness duration and limiting lush leaf growth during conducive conditions may suppress brown patch enough to reduce or eliminate the need for chemical control. Chemical, as well as cultural control methods, are often necessary on lower cut, high amenity turfgrasses such as golf putting greens.

Cultural Control of Brown Patch

A minimum of 10 hours of leaf wetness or >95% relative humidity must occur to allow for brown patch infection. Brown patch severity can therefore be reduced through increasing drainage, or removing dew and guttation fluid through early morning mowing, irrigation, pole-whipping, or dragging. Evening irrigation may start the leaf wetness period and increase brown patch severity. Increasing air movement and turf vigor by removing or pruning trees or installing fans on golf putting greens can also reduce severity. High nitrogen levels, especially before or during hot and humid weather, increases brown patch severity. Nitrogen fertilizer should be modestly applied on susceptible higher cut turfgrasses in spring, and more heavily applied in fall after the summer stress period. On lower cut turfgrasses, nitrogen should be applied in small quantities as needed (spoonfeeding) during the summer months to prevent lush growth.

Chemical Control of Brown Patch

If chemical means are needed, preventive fungicide programs are best, and should be applied when night-time temperatures are approximately 70F and high relative humidity. Active ingredients within a number of fungicide chemistry classes are effective for brown patch control, including the carboximides, benzimidazoles, carbamates, dicarboximides, DMIs, nitriles, and QoIs.

Host Resistance

R. solani attacks all known turfgrasses, with some species more susceptible than others. Species that are most susceptible are tall fescue, perennial ryegrass, creeping bentgrass, St. Augustinegrass, and annual bluegrass. No cultivars are known to be completely resistant to brown patch infection, although differences in symptom severity have been observed.

Diagnostic procedures

Peel blighted leaf tissue back to expose the clear tissue at the base of the leaf sheath. Examine under a compound microscope for large, often dark brown, septate mycelium with right-angled branching. Samples can also be incubated in a humidity chamber for 24 hours and examined for mycelium occurrence. If attempting to isolate R. solani from leaf tissue, surface sterilize leaf tissue with 70% ethanol and not 10% bleach as Rhizoctonia can be sensitive. .

Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30818 as part of "Diagnostic Image Series Development for Supporting IPM in the North Central Region" (USDA-NIFA-RIPM-003349)