Many conditions can cause patches of brown, dead grass on your lawn, but only one gets the official name brown patch. Brown patch disease is a condition caused by a single species of fungus, Rhizoctonia, that often occurs in mid- to late-summer when the weather is hot and humid.
The Rhizoctonia Fungus
Rhizoctonia can affect all cool-season lawn grasses, but it is especially harmful to ryegrass and tall fescue. Kentucky bluegrass and fine fescues can occasionally be affected, but the damage is usually minimal in these species. Brown patch can also affect a variety of warm-season grasses, including St. Augustine grass and zoysiagrass.
Brown patch is a foliar disease, meaning that it harms the blades of grass but not the crown of the plant or the root system. Grass plants affected by brown patch may recover on their own, without chemical intervention.
Brown patch appears as irregular circular patches in the lawn that are brownish yellow in color and range from 6 inches to several feet in diameter. The affected leaves usually remain upright, and close inspection shows lesions on the leaves that are tan in color and irregular in shape with a dark brown border. White, cottony, mycelium can be found on dew-covered turf in the early part of the morning.
Brown patch is notable for a distinctive "smoke ring" border that is sometimes visible on the outer edge, giving the patch a "frog-eye" appearance. The ring itself is typically 1/4 to 1/2 inch wide and is most visible in the morning. Sometimes grass within the ring is entirely killed, creating a sunken look to the patch, but more often the grass inside the patch simply becomes thinner than the surrounding lawn.
The Rhizoctonia fungus may be present for some time in the soil before it manifests as brown patch disease. The fungus overwinters in the lawn grass or soil beneath in the form of fungal bodies known as sclerotia, and it can survive for years until conditions are right. Rhizoctonia is most likely to cause brown patch during the periods of high temperature and high humidity in mid- to late-summer when night-time temperatures remain above 68 F and daytime temperatures are routinely in the 80s or above. Periods of rainy weather when the air is saturated with moisture also promote the appearance of the disease.
In addition to heat and humidity, the main factors inviting brown patch are excessive nitrogen and irrigation, which means that very lush and green lawns can be susceptible to brown patch. Other causal factors include lack of air movement, poor soil drainage, excessive thatch, and compacted soils, which means that poorly maintained lawns can also be prone to brown patch disease.
Control and Prevention
Some of the conditions that are conducive to brown patch are beyond the homeowner's control. Air temperatures and humidity levels may cause a lawn to develop brown patch no matter what a homeowner does. But you can minimize the chances for a brown patch with a good lawn care regimen.
- Do not over-fertilize, and avoid feeding your lawn during hot and humid weather.
- Use proper watering practices. If you water, do it early in the day so the grass can dry out fully before nightfall. If dew is collecting on your lawn each morning, your lawn probably does not need to be irrigated at all. Standing dew can be removed by dragging a water hose over the lawn, which will lower the humidity that causes brown patch disease.
- Improve air circulation. The best way to do this is to aerate and dethatch the lawn annually. Improved air circulation will reduce the humidity that causes brown patch.
- Reseed with resistant grass cultivars. A variety of grasses are available with moderate resistance to the Rhizoctonia fungus. If a brown patch is an annual occurrence, consider top-seeding your lawn with resistant grasses.
- Apply fungicide where appropriate. Lawn care experts say that fungicide treatment is appropriate only for high-value ryegrass and bentgrass turf blends; most lawns typically recover without chemical treatments. The first spray of fungicide should be applied immediately after the symptoms first appear, especially if hot and humid weather is expected. Although fungicides can be purchased by homeowners, it's best to deal with a lawn care company staffed by professionals who are trained to diagnose and treat lawn diseases.