Turfgrass Diseases: Brown Patch (Causal Fungus: Rhizoctonia solani)

The most susceptible turfgrass species include perennial ryegrass, tall fescue, and bentgrasses. On rare occasions, brown patch becomes a problem on Kentucky bluegrasses in mid to late summer during extended periods of high temperature and humidity.

Symptoms and Signs

On high-cut turf, brown patch symptoms usually appear as light brown circular patches ranging from a few inches up to several feet in diameter. In the early morning on dew-covered turf, white mycelium of the causal fungus can often be seen on and between grass leaves in the patch. Sometimes, all the grass within patches appears dead, but in less severe cases, turf in patches is thinned, with some foliage showing only minor symptoms. Occasionally, no circular patterns are observed and the disease appears as a diffuse blighting of turf foliage.

On tall fescue, symptoms of brown patch can be observed on individual leaves, appearing as irregular tan or light brown lesions surrounded by dark brown borders.

A distinguishing feature of brown patch on low-cut bentgrasses and annual bluegrass golf course putting greens, tees, and fairways is the presence of dark purplish rings around the periphery of circular patches. These are called smoke rings and range in width from ¼ to ½ inch. Smoke rings are more pronounced in the early morning hours, usually fading by midday.

The causal fungus of brown patch, R. solani, does not produce spores on diseased plants but does form robust hyphal strands with right angle branching and internal cross walls, which are only visible with the aid of a microscope.

Disease Cycle

The causal fungus of brown patch overwinters in the form of resting bodies called sclerotia, either within infected grass tissue or in the soil. Sclerotia are capable of surviving in soil for years in the absence of a susceptible grass host. When conditions are conducive for disease development, sclerotia germinate and produce mycelium which infects leaf and stem tissues and initiates disease activities. Mycelium spreads from plant to plant infecting new tissues and forming patches of diseased turf when surface moisture, humidity, and temperatures are high.

Disease Development

Brown patch is a midsummer disease, often occurring when night temperatures are above 68°F and daytime temperatures average 80°F or above. Rainy weather and a saturated atmosphere (100% relative humidity) greatly speed disease development. Disease severity is greater on lush, succulent, irrigated turfgrass maintained with high amounts of nitrogen fertilizer than on non-irrigated turf maintained with moderate amounts of nitrogen. Turf stands less than one year old are more likely to sustain severe brown patch damage when compared with older turf. Perennial ryegrass, creeping and colonial bentgrasses, and tall fescue are particularly susceptible to brown patch disease, whereas Kentucky bluegrass and the fine fescues are less likely to develop symptoms. Mature stands of turf affected by mild cases of brown patch often recover when cool temperatures, dry conditions, and foliar growth resume in late summer and fall.

Cultural Control

Apply only moderate amounts of nitrogen fertilizer on turf with a known history of brown patch during summer. Irrigating turf at night during hot and humid weather may exacerbate brown patch symptoms and create the need for fungicide applications to control the disease. Ideally, watering should be done in early morning or at other times of the day in time for the grass to dry before nightfall. For high-value turf, use blends of brown-patch tolerant cultivars of tall fescue, perennial ryegrass, and creeping bentgrass. When planting new lawns in areas prone to brown patch, incorporate some Kentucky bluegrass and/or fine fescue cultivars into the seed mixture.

Chemical Control

Fungicide treatment is sometimes needed on high-value perennial ryegrass or bentgrass turf stands. In areas where brown patch causes severe thinning on sports turf and golf course putting greens, tees, and fairways, preventative fungicide applications may be justified. Fungicides can also be used on a curative basis, with the first application being made immediately after the onset of symptoms, especially if prolonged hot, humid weather is expected.

Some penetrant fungicides labeled for control of brown patch disease.

Some multisite contact fungicides labeled for control of brown patch disease.

Some combination product fungicides labeled for control of brown patch disease.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on the mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide’s ability to penetrate plant surfaces or remain on plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. 2020. Chemical control of turfgrass diseases 2020. University of Kentucky, Rutgers University, and University of Wisconsin.

Latin, R. 2011. A practical guide to turfgrass fungicides. American Phytopathological Society Press, St. Paul, MN.