Guide · Disease

Brown patch: what it is and what to do

The #1 summer disease on tall fescue — and a big one on St. Augustine.

What it looks like

• Roughly circular tan or brown patches, from a few inches to several feet across.
• On humid mornings, a darker, grayish "smoke ring" around the edge of the patch.
• On tall fescue: tan leaf lesions with a dark brown border. On St. Augustine (where it's called large patch in spring and fall): the leaves pull easily from rotted stems at the edge of the patch.

Why it happens — the 150 Rule

Brown patch is a fungus (Rhizoctonia) that takes off when nights stay warm and humid. A handy rule from the turf industry: add the day's high to the night's low. At 150°F or more, disease pressure is high. A 92° day and a 70° night = 162 — brown patch weather.

What makes it worse: evening watering (leaves stay wet all night), heavy nitrogen in summer, mowing too short, and thick, lush growth.

What to do

1. Water only in the early morning (4–9 am) so the leaf dries during the day.
2. Stop summer nitrogen. Feed cool-season lawns in fall instead.
3. Mow at the top of the range with a sharp blade, and bag clippings from diseased areas.
4. Consider a fungicide only once you've identified it — azoxystrobin and propiconazole are the standard actives, and alternating them slows resistance. On lawns with a history of it, a preventive application as the hot, humid stretch begins works better than treating after the fact.

Will it come back?

Usually, yes — the leaves die but the crowns of tall fescue often survive. As the weather cools in fall, it regrows. Thin spots that don't recover are what fall overseeding is for.

Always follow the label. The label is the law for rates, timing and safety.

The Lawn Obsessed USA app watches the 150 Rule for your ZIP code and calls out the days that cross the line.