Large patch, formerly called brown patch, is a common lawn disease during the cooler temperatures of fall and spring.
Large patch usually appears as circular or somewhat circular brown areas in lawns during the milder weather of fall. In some situations these circular areas can grow together, forming irregular dead areas with borders that resemble portions of circles or arcs. This disease is caused by the fungus Rhizoctonia solani. All of our lawn grasses can be affected by this pathogen.
Even though patches can be several feet across, this disease usually begins as a small patch only a few inches across to a foot across. The spot usually expands in a circular pattern. The fungus is most active along the leading edge or margin of the patch expanding into the healthy surrounding grass. As the patch expands, it may take on a “doughnut” pattern with the grass recovering in the center of the circle. In shady, moist areas a circular pattern may not occur.
Rhizoctonia is most active at 73º F to 90º F. This is why it is common in fall. Adequate moisture is required for infection to occur. Moisture can be in the form of rain, high humidity or excessive irrigation. Infection can be severe where the leaf canopy is wet continuously for forty-eight hours and when the temperature is below 80º F.
This fungus functions at ground level, infecting the base of the grass blades closest to the soil. The fungus disrupts movement of water and nutrients to the upper parts of infected leaf blades, eventually killing the leaves. The infected yellow leaves pull loose from the plant very easily, revealing that the base of the leaf is tan to brown in color and is rotted in appearance. This is a technique used to confirm large patch as the problem.
Controlling large patch involves following good cultural practices in managing your lawn and possibly the use of fungicides.
Avoid excess nitrogen during potential disease development periods. Irrigate only when necessary and do so only in the early morning hours (between 2 and 8 AM) when dew is already present. Since mowers can spread this disease, mow diseased areas last and wash turf clippings off the mower before proceeding to the next site.
More information on large patch, including fungicide recommendations, is available at https://edis.ifas.ufl.edu/lh044 or from the UF/IFAS Extension Office in your County.
Larry Williams, UF/IFAS Extension Agent, Okaloosa County, November 5, 2015