Pythium blight on lawns

Of all the diseases that affect cool-season lawns, Pythium blight is the one that can destroy a large section of turf overnight. Literally overnight: conditions that favor it are extremely specific, but when they occur, the disease spreads at a pace that is alarming compared to the gradual decline.

Of all the diseases that affect cool-season lawns, Pythium blight is the one that can destroy a large section of turf overnight. Literally overnight: conditions that favor it are extremely specific, but when they occur, the disease spreads at a pace that is alarming compared to the gradual decline of most turfgrass diseases.

Pathogen overview

Per Penn State Center for Turfgrass Science, Pythium blight is caused primarily by Pythium aphanidermatum, with some contribution from Pythium ultimum and other Pythium species. These are water molds (oomycetes) rather than true fungi — a distinction that matters for fungicide selection.

The disease primarily affects:

• Perennial ryegrass — most susceptible
• Annual bluegrass (Poa annua)
• Tall fescue and Kentucky bluegrass — less susceptible but affected
• Newly seeded turf of any cool-season species — extremely vulnerable