Overview

Pythium blight first appears as small, sunken, circular patches up to 1 foot in diameter during hot, humid weather. Leaves within affected areas are matted, orange or dark gray in color, and greasy in appearance. Gray, cottony mycelium may be seen in the infected areas when the leaves are wet or humidity is high. The disease spreads rapidly along drainage patterns and can be tracked by equipment. This disease can cause widespread damage to a turf stand very quickly when conditions are favorable for development.

Pythium blight may develop when night temperatures exceed 65°F (18°C) and leaves are continually wet for 12 to 14 hours for several consecutive nights. For this reason, severe Pythium blight epidemics are commonly observed the morning after a late afternoon or early evening thunderstorm in the summer. Daytime temperatures above 85°F (29°C) also encourage Pythium blight development, possibly due to increased stress. Excessive soil moisture and succulent foliar growth also favor disease development. Perennial ryegrass and annual bluegrass are most prone to Pythium blight and can sustain significant damage in 2 to 3 days when conditions are favorable. Creeping bentgrass and tall fescue are more resistant to the disease, but can be severely affected if conditions are conducive for prolonged periods.

Cultural Control

Reduce prolonged leaf wetness by watering in the early morning hours, before sunrise. Avoid excessive rates of nitrogen to prevent lush, succulent foliar growth, which is very susceptible to Pythium blight. Cool-season turfgrasses should not be fertilized with more than 0.25 lb N per 1,000 square ft when conditions favor Pythium blight activity. Avoid establishing turf in low-lying areas that will collect water. If necessary, install subsurface drainage to prevent wet soil conditions. Relieve compaction and maintain soil drainage through hollow tine aerification to minimize thatch accumulations. Do not mow or irrigate when Pythium mycelium is present on the foliage to minimize spread of the pathogen. Collect and promptly dispose of clippings from infected areas and ensure that mowing equipment is washed before going to an uninfected area.

Chemical Control
Due to the potential for rapid development of this disease, high value areas and susceptible grasses should be protected with a preventive fungicide program. Applications should be initiated when night temperatures consistently exceed 65°F (18°C) for cool-season turf and repeated on 14 to 21-day intervals as directed on the fungicide label when conditions are favorable for Pythium blight development.

Fungicides are often available in different formulations. Most of the time they are formulated to be applied through a sprayer, however there are some granular versions available that are applied through a rotary spreader. When in doubt, you can hire a landscape professional to make these applications because they are licensed and trained on how to best apply these products.