Brown patch

Rhizoctonia solani

Big circular patches of blighted grass in a humid summer, sometimes ringed with a gray smoke halo at dawn. Water in the morning and stop feeding nitrogen.

Fungus Serious Soil-borneAirborne sporesSpread in waterSpread on tools & hands Spreads to neighbors

What you'll see

Roughly circular brown patches from 30 cm to two meters across, appearing over a few days in muggy weather.
A gray-purple smoke ring around the edge of the patch in the early morning, gone by mid-morning.
Individual blades with irregular tan lesions edged in dark brown, rather than clean bleached spots.
Fine cobwebby mycelium visible in the dew at first light.
Grass pulling out easily at the crown in the worst of the patch, while the roots stay sound.

Why it happens

A soil fungus that is present in almost every lawn all the time and only becomes a disease when the conditions turn its way — lush, soft, nitrogen-fed leaf tissue kept wet through a warm night. It spreads across the turf by growing from blade to blade, and on mower wheels and shoes.

Night temperatures above 20 °C with humidity high enough to keep the grass wet for ten hours or more. Tall fescue and ryegrass suffer most; a heavy spring feed, evening watering and thick thatch make it far worse.

Treating it

Stop all nitrogen feeding until the weather breaks; feeding through an outbreak is pouring petrol on it.
Water deeply but only in the early morning, so the blades dry within a few hours.
Raise the mowing height, mow only when the grass is dry, and collect the clippings while it is active.
Improve air movement by thinning overhanging branches and shrubs at the lawn edge.
Reserve a fungicide for high-value turf; a garden lawn recovers on its own once the humidity drops, because the crowns survive.

Keeping it away

Feed lightly and in autumn rather than heavily in late spring.
Dethatch and aerate a lawn that has built up more than a centimeter of thatch.
Water in the morning, deeply and infrequently, never in the evening.
Overseed with a resistant tall fescue blend where brown patch is an annual event.
Keep the mower blades sharp; a torn blade tip is an open wound in exactly the wrong weather.

Who gets it

Lawn grasses — tall fescue, perennial ryegrass and bentgrass worst, Kentucky bluegrass less so.