Gray leaf spot can be a very destructive disease of perennial ryegrass in golf course fairways and sports fields, particularly on young seedlings emerging from autumn overseeding. The disease may also occur on tall fescue, but often progresses more slowly and is less severe than in perennial ryegrass.
The first symptoms are small, water-soaked leaf lesions which quickly become oblong and gray to light brown in color, often with a purple to dark brown border. Old spots may have a yellow halo and, as the disease progresses, the leaves become completely blighted. Dying leaves often develop a characteristic twist at the leaf tip that resembles a fishhook and helps to differentiate this disease from brown patch or Pythium blight. Infected patches may remain small or may rapidly expand to light large, irregularly-shaped areas. When disease is severe, large areas or the entire planting may die in a matter of weeks, leaving behind only resistant turf species and weeds. Leptosphaerulina, a weak pathogen of senescing turfgrass tissues, may also be associated with turf dying due to gray leaf spot.
The pathogen is favored by prolonged hot (82 to 90 F) and wet conditions, a high level of nitrogen fertilizer, and slowed growth due to growth-regulating chemicals.
Restrict the use of quick-release nitrogen sources prior to and during hot, moist weather. To the extent possible, reduce turfgrass stress due to soil compaction and growth-regulating chemicals, and water the turf deeply in the morning and as infrequently as possible. If possible, delay autumn overseeding until cooler weather when disease is less favored. Avoid the use of highly susceptible varieties. Do not overseed into turf infested with active gray leaf spot until cooler temperatures are realized and/or a fungicide is applied.
Where gray leaf spot has been diagnosed in previous years and is expected to reoccur, preventive applications of fungicides should be made just prior to, and throughout, the period when conditions favor disease development. Otherwise, apply fungicides early in disease development; once the disease becomes established it is very difficult to manage using fungicides.