Large patch fungus active in lawns during fall

Q. Brown circular areas are showing up in my lawn. Is this a fungus? If so, how do I control it?

A. This is probably a common lawn disease called large patch, caused by the fungus Rhizoctonia. Plant pathologists recently changed the name of this disease from brown patch to large patch because brown patch is a different stain of Rhizoctonia that affects cool-season grasses grown in more Northern States. Large patch affects our warm-season turfgrasses here in the South.

Large patch usually appears as circular or somewhat circular brown areas in lawns during the milder weather of fall and spring. In some situations, these circular areas can grow together, forming irregular dead areas with borders that resemble portions of circles or arcs.

Patches can be several feet across but usually begin as small spots only a few inches to a foot across. The fungus is most active along the border of the patch expanding into healthy surrounding grass. Along the margin, the grass blades turn yellow and then brown. Eventually, 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.

Using index finger and thumb to pull upwards on a yellow infected leaf results in the blade easily pulling free from the plant, revealing the leaf base is tan to brown in color and is rotted in appearance. This technique is used to confirm large patch as the problem.

A number of fungicides are labeled to control Rhizoctonia in home lawns. It’s critical to follow the product’s directions in application method, rate and frequency to achieve control.

In addition to chemical control, it’s important to follow good lawn management practices. Avoid fertilizing after September, particularly with a high nitrogen fertilizer. Too much nitrogen at the wrong time is somewhat like putting gas on a fire – it promotes large patch. Our lawns need a balanced fertilizer containing equivalent amounts of nitrogen and potassium, preferably in a slow-release form. Irrigate only when necessary and do so only during early morning hours (between 2 and 8 AM). Since mowers can spread this disease, mow diseased areas last.

For more information on lawn diseases, including large patch, contact the UF/IFAS Extension Office in your County or use the following link to access a UF/IFAS publication on large patch fungus. https://edis.ifas.ufl.edu/lh044

Larry Williams is the Extension horticulture agent with the Okaloosa County Cooperative Extension Service, University of Florida. Contact Larry at 689-5850 or email lwilliams@myokaloosa.com.