As warm (hot!) summer weather approaches the risk of brown patch disease developing in your lawn increases. Because this disease is fast moving and potentially destructive to large areas of grass, it is important to recognize the symptoms and begin treatment immediately. Even better, learn the factors that contribute to brown patch disease developing in the first place, as proper management can reduce the chances of developing it.
Brown patch is caused by the fungus Rhizoctonia solani. Though it will grow and spread at temperatures over 65F, it becomes most active and aggressive when day temperatures are over 80F, night temperatures are over 70F, and humidity is high. In other words, pretty much all summer here in the Piedmont and Sandhills. Different grass strains have differing resistance to brown patch, with some being highly resistant and some being very susceptible.
Symptoms are tan-brown patches of dead grass that may seem to appear virtually overnight. Spots may be round or irregular in shape. Affected spots range from a few inches to several feet in diameter, and smaller patches may merge to create larger, irregular patches. Close inspection of individual grass blades at the edge of the affected areas will reveal the irregular tan-brown spots with a dark border that eventually kill the individual grass blade, then the grass plant, spreading rapidly through your lawn.
Besides high temperatures and humidity, high levels of nitrogen can encourage brown patch. Fast release fertilizers, while they may cause rapid greening, will encourage brown patch spread by producing soft, lush growth that is more easily infected by the pathogen. Nitrogen doesn't cause brown patch, but it can make a mild infection much worse. Once the infection is controlled, lawns actually recover better with proper fertilizer applications.
Mowing can also contribute to the development of brown patch disease. Too low (under 2.5” for fescue, under 1.5" for bermuda and zoysia) reduces the plant’s ability to produce food energy, weakening it and making it susceptible to infection. Let grass get too high (over 3.5” for fescue, over 2" for bermuda and zoysia) and humidity at the plant level increases, increasing the risk of infection. Mow frequently enough to remove no more than 1/3 of the length per mowing, as cutting more severely can stress the grass, increasing the risk of disease.
To minimize the risk of developing brown patch in your lawn, only fertilize at the appropriate rate for summer, if at all, and use a slow release fertilizer. Keep mowing height at around 3.5” for cool season grasses and 1.5-2" for warm season grasses, and mow frequently enough that you don’t stress the grass by removing too much at once. Aerating in spring can increase airflow around grass plants, reducing humidity and the risk of disease.
Keep in mind that if your turf is a susceptible type it may be difficult to completely prevent brown patch disease and preventive fungicide applications may be in order as soon as temperatures begin to reach the 80F mark. Fungicide applications will not reverse any damage that has already occurred, making preventive applications important if you have low tolerance for any visible damage.
If brown patch does develop it is important to treat promptly, as under the right conditions it spreads very quickly, and continues to spread as long as conditions are favorable. Treatment must be continued for as long as disease pressure remains, generally every two to three weeks depending on the product used. Look for fungicide labels specifying brown patch control that is for use on lawns. You may also prefer to use a lawn care service like Nature's Select to apply fungicides, as commercial applicators have access to controls not available to the homeowner (plus you don’t have to mix and apply or store the extra).
So, even though brown patch disease can be very damaging, knowing how to minimize the likelihood of developing the disease, early identification in case of infection, and prompt treatment or prevention can minimize the damage brown patch causes to your lawn.