The Deadly Disease With Many Names

Pythium blight, grease spot, spot blight, root rot, cottony blight, and snow blight are the “attractive” names given to this lawn disease. Each name describes a different phase of the disease life-cycle and none of them are a pretty sight.

Damage Symptoms: What Pythium Does

The first signs of Pythium lawn disease are usually small patches of turf that look water-soaked and dark or purplish in color. When the blades of grass in these areas are handled, they have a greasy or slimy feel. That's where the name grease spot comes from. These infected grass blades become light tan and shriveled, thus the name spot blight. When a lot of these infected areas appear, they begin to grow together and overlap. Pythium lawn disease infections usually concentrate in low, wet areas or along the natural course of drainage across the turf. When humidity remains high, the shriveled and collapsed leaves become matted and covered with a fluffy mass of white fungal threads or mycelium. This is sometimes called cottony blight. The most serious and severe damage occurs during hot, humid weather, when the disease can spread so quickly that an entire stand of turf can be destroyed in just one day. Pythium survives, as spores (fungus seeds) and as mycelium (fungus strands), in and on diseased turf tissue, as well as in organic material like thatch. Wet turf grass is always required, before Pythium can cause lawn disease. Periods of wet grass and thatch, along with high relative humidity, present especially favorable conditions for the disease to grow. This lawn disease is most dramatic when it suddenly appears during very hot and humid weather, when the nights remain very warm and muggy.

Making Your Lawn An Anti-Pythium Environment

Since Pythium is almost always associated with wet turf, identifying any poorly draining areas where water tends to pool or stand can help prevent this type of lawn disease. Correct the drainage to promote rapid drying of grass plants. Attempt to water early in the day, so that the turf can dry before evening, and water as deeply but infrequently as possible. Controlling thatch is strongly suggested, by vertical mowing or regular lawn aeration. It’s also important to avoid over fertilizing your lawn. Using a properly balanced fertilizer for your grass care helps avoid nutrient imbalances that can stimulate grass diseases. It can also help to increase air movement by selectively pruning lower tree branches to eliminate pockets of still humid air. And finally, don't mow when the grass is wet, as this tends to spread the disease. Fungicides can also be effective in suppressing Pythium, but prevention is a much better and more economical turf care solution.


  • Under ideal conditions, Pythium can destroy an entire lawn in a single day.
  • Areas where turf remains wet tend to be more affected by this lawn disease.
  • Look for Pythium when temperatures and humidity are both very high.
  • Water deeply, but not too often, and water so that grass blades dry quickly (like in the morning.)

Contact your neighborhood Spring Green with any lawn question you have. We've got the answers you need. 

Learn more about... 

Red Thread

Rust Lawn Disease

Snow Mold