Pythium blight of turf
Pythium blight, a fungal disease caused by several species of Pythium, occurs during periods of hot, humid weather. Pythium blight is most severe and causes the most damage when relative humidity is above 90% for more than 14 hours, daytime temperatures are between 85° F and 95° F, and the nighttime temperatures remain above 68° F.
Early in the morning, grass infected with Pythium will appear water-soaked (dark green). Infected grass may also feel greasy or oily. In hot weather, infected turf wilts rapidly, browns, dries, and dies. These dead patches of grass, circular to irregular in shape, may rapidly enlarge up to six inches in diameter. Infected patches may also be matted and covered with gray, fluffy fungal growth when humidity levels are high. Pythium is spread by moving water and mechanical equipment. Damage is worse in low-lying areas and often follows the flow of water, the path of the lawnmower, or foot traffic across the lawn. Finally, the fungus survives winter in the soil or in infected plant material.
Like most plant diseases, Pythium blight is easier to prevent than to cure. Maintain a healthy lawn by properly watering, fertilizing, and mowing. Frequent overwatering during hot weather can stimulate this disease. Water early in the day so that the grass blades dry quickly. Avoid excessive nitrogen fertilizer, since lush grass is particularly susceptible to Pythium. Manage the thatch layer and dethatch if there is more than one-half inch of thatch. Frequent watering required to establish grass seed contributes to the development of Pythium, so avoid seeding lawns in mid-summer.
Once a lawn is infected by Pythium, the first objective is to stop the spread of the fungus. Reduce the amount of water applied to the infected areas and keep people and equipment off the grass when wet. Grass killed by Pythium will not recover. The lawn should be reseeded or resodded once the weather has cooled. Even in cool weather, the frequent watering needed to establish new grass can reactivate Pythium. When seeding, use a disease resistant seed mixture. Mature grass plants in sod are less vulnerable to Pythium than germinating seeds. No fungicide treatments for Pythium blight are available to homeowners. Contact a professional lawn care service for chemical treatment.
Smiley, R. W., Dernoeden, P. H., and Clarke, B. B. 1992. Compendium of Turfgrass Diseases. 2nd ed. American Phytopathological Society. St. Paul, MN. pp. 43-46.
Scotts Guide to the Identification of Turfgrass Diseases and Insects. 1987. The O. M. Scott and Sons Co. pp. 46-47.
Revised by Chad Behrendt and Crystal Floyd 2000