Field notes
What causes brown rings in St. Augustine and zoysia lawns in spring and fall?
Usually large patch, often called brown patch: a fungus that shows up as circular brown areas with yellow-orange edges when temperatures drop below about 70 degrees. Overwatering makes it worse.
Large patch is the most common disease of St. Augustine and zoysia lawns in Texas, according to Texas A&M AgriLife. It is caused by a soil fungus and is active when temperatures fall below about 70 degrees, so it shows up in fall and again in spring. Look for circular patches, a few feet to many feet across, with a yellow-orange border where the grass is actively dying.
Wet soil drives it. The best prevention is watering less often and only in the early morning, so the grass dries during the day, and fixing heads that soak one spot. Avoid heavy nitrogen fertilizer in fall. If it keeps coming back, a licensed applicator can apply a fungicide before the season starts.
Bermuda lawns rarely get large patch, which is one reason bermuda is the standard for full-sun lawns here.