Pythium blight

Pythium aphanidermatum

also called Cottony blight, Grease spot

Turf that turns dark and greasy overnight in muggy weather, collapsing in streaks that follow the water. The fastest-moving lawn disease there is.

Water mold Serious Spread in waterSoil-borneSpread on tools & hands Spreads to neighbors

What you'll see

Small dark, water-soaked, greasy-looking patches appearing overnight in hot humid weather.
Blades matted and slimy underfoot, sticking together as if walked on when wet.
White cottony mycelium over the patch at dawn, gone within an hour of the sun reaching it.
Patches running together into long streaks that follow the drainage — down a slope, along a path edge, where water stands.
Grass in the patch dead and brown by the afternoon, with the damage done in a single night.

Why it happens

A water mold living in the soil and thatch that produces swimming spores. Free water is the entire requirement: it moves in surface water, on mower wheels and on shoes, which is why the pattern follows drainage lines and mowing passes rather than making circles. It kills fast because it attacks the crowns directly.

Night temperatures above 20 °C combined with 14 hours or more of leaf wetness — the muggy nights of high summer. Overwatered, over-fertilized, poorly drained turf on a still evening is the exact recipe.

Treating it

Stop watering completely until the weather breaks and the surface dries out.
Do not mow or walk across an affected area while it is wet; you will draw the streaks yourself.
Stop all nitrogen feeding; lush growth is what it feeds on.
Improve the drainage and the airflow at the wet end of the lawn where it always starts.
Reseed the dead patches once the weather has cooled, as pythium kills the crowns outright and the grass will not come back on its own.

Keeping it away

Water deeply and early in the morning, never in the evening in summer.
Avoid heavy nitrogen in hot weather, and lime an acid soil rather than letting it stay sour.
Fix the drainage in the low corner and reduce thatch.
Improve air movement by thinning shrubs and low branches at the lawn edge.
Do not seed too thickly; overcrowded seedling turf is where pythium damping-off does its worst.

Who gets it

All lawn grasses, and seedling turf especially; the same organism causes damping-off in seed trays.