Pythium Diseases of Turfgrass
Pythium spp, a common soil-inhabiting water mold, causes many different diseases of turfgrass. Pythium blight, also known as cottony blight or grease spot, occurs when the pathogen attacks the turfgrass foliage. When the roots and crowns are attacked, the disease is called Pythium root rot. When newly germinated turfgrass seedlings are severely affected the disease is called Damping-off.
All turfgrass spp. are susceptible to Pythium but cool season grasses tend to be more severely affected. In New Mexico Pythium diseases can be especially severe in ryegrass used to overseed hybrid Bermuda grass.
While the symptoms are somewhat variable, the diseases are typified by an overall decline in the turf area. This decline may be gradual or rapid, depending on the environmental conditions. Affected areas may appear as irregular patches or streaks associated with water drainage or mowing patterns. Small areas of declining turf may coalesce to cover large areas (Figure 1). Individual plants have dark, water-soaked lesions. The leaves turn yellow, then tan as they die. Pythium blight may also rapidly develop into round to irregular, dark, water-soaked, greasy or slimy, sunken patches of matted grass, up to 6 to 12 inches wide (Figure 2). This symptom is generally referred to as grease spot. In the early morning when dew is present, a whitish gray to purple, cottony growth may appear on the surface of blighted grass (Figure 3). Pythium root rot occurs when the roots and crowns are attacked. Affected root systems thin and discolor (but do not turn black) as many of the feeder roots decay. The outer cylinder (cortex) of these roots exhibits a soft decay, and will slough off between fingertips when pulled. Pythium can also cause seedling damping off.
Conditions for Disease
The pathogen survives as oospores in infected plants, plant debris (thatch), and soil. The disease is spread by movement of infected plant material on equipment, people, animals, and in water. Swimming spores move short distances in water and contribute to the enlargement of diseased areas. The disease can occur anytime during the growing season, but the most serious damage generally occurs during periods of hot, wet or very humid conditions. Large areas of turf can be destroyed in 24 to 48 hours after the onset of disease-favorable weather. Waterlogged soils and a moist thatch layer, along with high relative humidity (90% or greater) and daytime temperatures in the 80's or 90's with warm nights (above 68degrees Fahrenheit F) provide ideal conditions for warm weather Pythium blight. There are species of Pythium that do well in cool weather also. Turf areas seeded or overseeded during warm, moist conditions are especially vulnerable to attack. Susceptibility increases in dense turf and in turf growing in alkaline conditions. Excessive nitrogen, excessive thatch, poor drainage, and compacted soils also contribute to disease development.
Cultural practices which help to reduce the occurrence and severity of the disease include:
Improving soil aeration (reduce thatch).
Improving water drainage.
Maintaining appropriate fertility levels.
Avoiding heavy nitrogen applications.
Utilizing proper irrigation practices.
Avoid over watering and watering at night.
Avoid light, frequent watering.
Avoid mowing or walking on wet turf.
Avoid seeding or over seeding during favorable environmental conditions.
Several fungicides are available to help control Pythium. For highly valuable turf and turf with a history of the disease, a preventive spray program using a systemic fungicide, applied prior to the onset of hot, humid weather, is recommended. Pythium has developed resistance to fungicides in some areas; therefore great care should be taken to avoid excessive use of any one fungicide chemistry.