Brown patch in St Augustine lawns normally occurs in late summer or early fall when the air temperature is between 75-85 degrees and the humidity is high. It can, however, occur in the spring when conditions are right. It is a fungus disease characterized by the familiar circular patches of yellow or brown grass. These circles can be anywhere from less than a foot in diameter to ten or more feet. Brown patch affects the leaves of the grass, which may be pulled away very easily from the stem.
Brown patch will not kill St Augustine grass, but it can make a lawn very unsightly and weaken the turf enough that other weed grasses may invade and take hold. There is no “cure,” but it can be effectively controlled. The disease needs the proper temperature and moisture to grow, and it thrives on nitrogen. Watering too much and watering at the wrong time are major contributors to brown patch. Water deeply and only when the grass starts showing a bit of stress, and water in the early morning. Nighttime watering does not allow the leaves to dry and gives the fungus spores the moisture they need to grow. Ideally, limit your fertilization schedule to twice a year, in April when the lawn begins active growth and late September (remember, brown patch loves nitrogen). St Augustine does just fine with two feedings.
At the first sign of brown patch, you should apply a fungicide, following instructions on the label. Finally, apply preventive fungicides in early fall only if your lawn has been susceptible to brown spot in the past or if there is an outbreak in lawns upwind from your lawn.
Bob Ellis, Smith County Master Gardener
Texas A&M AgriLife Extension Service