Brown Patch Fungus: How to Identify and Treat It in Your Region

There's a particular sinking feeling when you walk out in the morning and find a circular patch of dead grass that wasn't there yesterday. If it's summer, if you have a warm-season grass or a transitional-zone lawn, and if the nights have been warm and humid, you almost certainly have brown patch — Rhizoctonia solani, one of the most common fungal lawn diseases in the United States.

The good news is that brown patch responds well to treatment if you catch it early. The frustrating news is that most homeowners treat the symptom and not the cause, which means it comes back every year like clockwork.

How to identify it

Brown patch creates roughly circular patches of tan or straw-colored grass ranging from a few inches to several feet in diameter. The patches often have a darker, water-soaked "smoke ring" around the outer edge — especially visible in the morning before the dew burns off. This ring is the actively expanding fungal front. According to NC State TurfFiles, this smoke ring is caused by grayish-white mycelium at the lesion margin and is the most reliable visual diagnostic feature.

On individual grass blades, you'll see lesions with tan centers and dark brown or reddish-brown borders. If you look closely at the base of the plants in an affected area in the morning, you may see a grayish-white cottony mycelium in the dew.

On cool-season grasses (fescue, bluegrass, ryegrass), brown patch affects only the leaf blades — the crown and roots stay alive. This means the grass will recover on its own once conditions change, though it looks terrible in the meantime. On warm-season grasses (St. Augustine, Bermuda, Zoysia), a disease called Large Patch — caused by the same fungus — can kill the crown, leading to permanent bare spots if left untreated.

What triggers it

Brown patch needs three things to activate: warm nights (above 70°F), high humidity or wet foliage, and a lawn that's been over-fertilized with nitrogen. Get all three and the fungus spreads rapidly — sometimes expanding inches per day during peak conditions. As University of Minnesota Extension notes, the disease is most severe when night temperatures stay above 70°F and relative humidity exceeds 95% for extended periods.

The number one human contribution is evening watering. When you water at night, foliage stays wet for 8-12 hours. The fungus needs leaf wetness to spread, and evening irrigation hands it exactly what it needs. This single change — shifting irrigation to early morning — prevents more brown patch than any fungicide.

The second most common trigger is applying a nitrogen-heavy fertilizer in summer. Lush, soft growth created by late-season nitrogen fertilization is highly susceptible. This is why the standard advice is to fertilize cool-season grasses in fall, not summer. Research published in Agronomy Journal confirmed that elevated nitrogen inputs significantly increase Rhizoctonia solani severity on turfgrass, particularly when applied during warm, humid periods.

Regional timing

Brown patch pressure varies significantly by location. In the Southeast and Gulf Coast (Houston, Atlanta, Orlando, Miami), it can be active from June through September — essentially the entire summer. In the Mid-Atlantic and transition zone (Washington DC, Charlotte, Nashville), the peak window is July and August. In the Northeast (New York, Boston, Philadelphia), it typically peaks in July during heat waves.

In the Pacific Northwest, brown patch is relatively uncommon because cool maritime temperatures rarely sustain the warm nights the fungus requires.

Treatment

First, stop making it worse. Switch all irrigation to early morning (before 8am). Skip your next scheduled nitrogen application. If you've been mowing low, raise the deck to 3.5-4 inches to reduce stress on the plant.

For active infections, a curative fungicide application is warranted. Look for products containing azoxystrobin, propiconazole, or trifloxystrobin — these are the most effective active ingredients for Rhizoctonia. Apply in the early morning, not midday.

Will the dead patches come back?

For cool-season grasses, yes — the grass in affected areas is usually still alive (crowns intact) and will recover on its own as temperatures cool in fall. You may want to overseed thin areas in September. For warm-season grasses where the crown has been killed, you'll need to either wait for the surrounding grass to fill in (Bermuda and Zoysia spread aggressively) or plug the affected areas.

Brown patch will return every year if the underlying conditions — evening watering, excess nitrogen, poor drainage — aren't addressed. The fungicide suppresses the current infection; it doesn't fix the cause.