Pythium Blight

The disease Phytium blight, also known as Cotton Blight, Grease Spot is caused by several different Pytium species. Cool season turfgrasses are susceptibe to Pythium. Under favourable conditions the whole turfgrass could be destroyed within a few days.


Pythium symptoms will first appear as small, irregularly shaped spots, which will join together to form large patches that will often be long streaks. Pythium often develops into these streaks because it is the direction of water movement (drainage) most often ruts from golf carts and mowing equipment. The turfgrass leaves will at first look and feel water-soaked, greasy, or slimy. Once dew or moisture dries up the blade will shrivel up and collapse, often causing a matted brown turf. The turfgrass will begin to develop patches that fade to a light brown or gray color. With high humidity in early morning or throughout the day, diseased leaves may be covered with the white, cobwebby, mold like growth of  mycelium.


Pythium may survive in the soil for extended periods of time, often coming from debris from past infected plants or spores living in the soil.Pythium spreads by the movement and growth of mycelium and spores from plant to plant. Pythium thrives in hot and humid weather typically day temperatures of 27°C to 35°C in areas that have little air movement but high moisture content. In lower temperatures of 13°C to 18°C and extended periods of wet weather Pythium is still prevalent.

Pythium will most commonly appear during the "150 rule", which is when day + night temperatures are over 66°C . Pythium is most common when dew remains on the grass blade for 14 hours or more. Turf stands that have excess nitrogen and lush growth are very vulnerable to Pythiuum and will spread rapidly due to high nitrogen levels. Alkaline soils (above a pH of 7) and calcium deficient soils also tend to favor Pythium.3 Pythium survives over winter as oospores found in the soil. The pathogen therefore is easily spread with the movement of diseased plants, soil movement, surface water, or even from shoes. Pythium also causes "Damping off", "seed decay", or "seedling blight" of turfgrasses. This is most common in Perennial ryegrass (Lolium perenne) and happens in areas that are high above the recommended seeding rates. Not only does Pythium devastate the Turf canopy but the oomycete can also attack the roots and crowns, which will reduce growth, become off-colored, and cause thinning of turf.


  • Infection and disease development is associated with daytime air temperatures above 30°C
  • night temperatures are above 20°C
  •  relative humidity for 15 or more hours above 90%.
  • high nitrogen nutrition appear to be more susceptible to the disease as are young/germinating seedlings, so care must be taken during overseeding or establishment in hot weather

The model shows a riky period on the 27th of July (see below), because at that day more than 15hours of a relative humidity of above 90% was determined and the temperature was between 20 and 35°C. The wet period afer that date does not show risk, because temperature was too low.


Kentucky bluegrass and Fescue are less susceptible to Pythium blight than Perennial ryegrass and Bentgrass. Creating an environment that includes adequate drainage, good air movement, and balanced fertility will help prevent the disease. Removing dew during hot and humid weather will also help prevent Pythium. Dew can be removed from the grass from mowing, using a backpack blower, or dragging a hose across the grass. Install an internal drainage system if you have severe drainage problems. Improve air circulation (Installing fans, removing trees or shrubs) and avoid irrigation practices that will leave moisture on the grass blades for extended periods of time. An adequate aeration program will relieve compaction and improve drainage. Aeration should annually disrupt between 15-20% of the total surface area. Be sure that you are not applying excess nitrogen to your soil. Avoid using quick release fertilizers, try using slow release ammonium sources. If your soil pH is above 6.5 then use ammonium sulfate, which will acidify your soil. Most balanced fertility programs for Kentucky Bluegrass lawns will consist of applying two to five lbs of nitrogen/1000 sq.ft. a year.