Pythium Blight


Downy mildew

Pythium aphanidermatum

Pythium first appears as circular reddish brown spots in the turf, ranging in diameter from 1 to 6 in. In the morning dew, infected leaf blades appear water soaked and dark and may feel slimy. When spots are wet with dew, purplish gray or white cottony fungal mycelia can be seen on the outer margins of the spots. Infected grass plants collapse quickly, and when conditions are conducive, spots may coalesce and large areas of turf can be lost in a short period of time (overnight).

Pythium blight is a good saprophyte, and survives in the thatch and soil as a water mold until proper conditions occur for it to become pathogenic. As a warm-weather disease of cool season grasses, the disease is most destructive when temperatures are between 85° and 95° F (29.4° - 35° C). When evening temperatures average 68° F or higher, outbreaks will typically first appear in low areas, or poorly drained areas where soil moisture is maintained. Humid periods further favor disease development.

  • Crops Affected: turf


    The location of this disease is home lawns and golf courses. General symptoms are a circle, streak, spot, wilted, and irregular coloring. Foliar symptoms are browning, turning orange, and water-soaked. This can occur during the months of June, July, and August. Hosts of the disease are Kentucky Bluegrass, Perennial Ryegrass, Tall Fescue, Fine Fescue, Creeping Bentgrass, Annual Bluegrass, and Annual Ryegrass.


    Good soil drainage is essential for effective management of Pythium blight. Heavy watering can increase the severity of Pythium blight, therefore, light watering, or syringing during midday will help to manage Pythium outbreaks during hot, humid weather, and nighttime watering should be avoided altogether. Improving air circulation can also help to manage the disease. High nitrogen levels which cause lush growth are thought to make Pythium blight outbreaks worse, so nitrogen should be kept at low to moderate levels during hot, humid weather. Of all the turfgrasses, the only resistance to Pythium blight appears to be in the improved bermudagrass cultivars.

    Due to the aggressive nature of the pathogen, and the potential for severe turfgrass loss, fungicides are essential tools for Pythium control. Contact fungicides such as chloroneb and ethazole, which normally have 5-to 7-day residuals can be effective if applied at appropriate times. Systemic fungicides like propamocarb, mefenoxam, and fosetyl Al, normally have 10-to 14-day residuals when disease pressure is heavy. Fungicides should be applied on a preventative bases in areas prone to disease development. Disease predictor models can be very helpful in determining likelihood of Pythium development.

    One commonly used prediction model requires maximum daily temperatures greater than 86° F (30° C) and a minimum temperature of 68° F (20° C), with at least nine hours of relative humidity above 90 percent.