Pythium blight is a destructive disease that affects turfgrass, often causing severe damage in a matter of hours. This affliction is caused by water molds belonging to the genus Pythium, which are classified as oomycetes, not true fungi. Pythium blight thrives under specific environmental pressures, quickly overwhelming stressed turf. Understanding its signs and acting fast is necessary for recovery.
Recognizing Pythium Blight
The first indication of Pythium blight is the sudden appearance of small, sunken, circular patches of grass, less than six inches in diameter. Infected grass leaves look dark, water-soaked, and have a greasy or slimy texture when rubbed. When humidity is high, a visible gray or white, cottony growth called mycelium may be seen on the affected leaves in the early morning.
As the disease progresses, these spots rapidly expand and merge, often following the path of water drainage or mowing equipment, creating streaks or irregular large patches. The grass within these areas quickly collapses, shriveling and turning light tan or reddish-brown.
Pythium blight is most common in cool-season grasses during hot, humid weather when daytime temperatures are above 85°F and nighttime temperatures remain above 65°F. Prolonged leaf wetness, often exceeding 12 to 14 hours, combined with high soil moisture from poor drainage, creates ideal conditions.
Immediate Fungicidal Control
Chemical intervention is necessary to halt Pythium blight once symptoms appear due to the disease’s rapid destruction. Treatments must specifically target oomycetes, as common turf fungicides used for diseases like dollar spot or brown patch are ineffective. The most reliable active ingredients for curative control include mefenoxam (Group 4) and propamocarb (Group 28).
These specialized fungicides are systemic, allowing them to be absorbed and translocated throughout the plant tissue for effective control. For curative application on active disease, a high water volume is necessary to ensure the product penetrates the canopy and reaches the crown and root zone. Fosetyl-al (Group 33 phosphonate) is also used, but it is more effective as a preventative or when disease pressure is low.
To prevent fungicide resistance, it is necessary to rotate between chemical classes, identified by their FRAC codes. For instance, a mefenoxam application (Group 4) should be followed by propamocarb (Group 28) or cyazofamid (Group 21). During high disease pressure, repeated applications on a shorter interval (seven to fourteen days) are required to maintain protection and stop the spread of the pathogen.
Adjusting Cultural Practices for Recovery and Prevention
While fungicides stop the active infection, long-term success against Pythium blight depends on modifying the turf’s environment. The primary goal is to minimize the duration of leaf wetness and improve the movement of water and air. This begins with altering irrigation practices by watering deeply and infrequently, and timing irrigation for early morning hours.
Watering before sunrise allows the grass blades to dry quickly with the morning sun, avoiding the prolonged wet period that promotes disease development. Improving soil drainage is equally important, often requiring core aeration to reduce compaction. Reducing accumulated thatch, which traps moisture and limits fungicide penetration, is also beneficial.
Air circulation can be improved by pruning nearby shrubs or trees that limit airflow and cast excessive shade. Avoid applying high levels of quick-release nitrogen fertilizer during hot, humid summer months, as this encourages lush growth highly susceptible to the disease. Combining these environmental adjustments with a strategic fungicide program prevents recurrence.