January 2018

SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.

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www.spor 32 // January 2018 Gray leaf spot on turfgrass // By JIM KERNS, PHD G ray leaf spot (GLS) is caused by the fungus Magnaporthe oryzae and is a destructive disease of St. Augustinegrass, perennial ryegrass, kikuyugrass, and tall fescue. The disease was problematic in the late 1990s and early 2000s, especially in the northern transition zone and Mid-Atlantic US. The disease seems to be increasing in incidence in recent years, especially on newly planted tall fescue and perennial ryegrass swards. Research demonstrated that seedlings are most susceptible to gray leaf spot 4 to 5 weeks after emergence, thus the disease has been a challenge for those that overseed with perennial ryegrass or in tall fescue sod production. The disease is most severe when temperatures are between 82 and 90 degrees F, so the disease could potentially develop in any area of the US if conditions are conducive. Typically, gray leaf spot is prevalent south of Interstate 80 (Figure 1). Conditions favoring disease development are warm, humid weather. Specifi cally only 9 hours of continual leaf wetness when air temperatures are between 82 and 90 degrees. When temperatures are lower (68 to 75 F) 21 to 36 hours of leaf wetness is required for infection. Basically fungal infection is strictly tied to humidity especially in the eastern US. For example, in North Carolina the disease is fi rst observed in July on tall fescue and the disease may continue even into October if conditions remain favorable for infection. Gray leaf spot subsides after a heavy frost. Symptoms on warm-season hosts such as St. Augustinegrass and kikuyugrass initially develop as small brown spots on leaves and stems. The spots can enlarge rapidly into round or oblong spots and lesions (Figure 2). Spots can extend across the entire leaf and leaves with numerous spots often die. The spots are tan to gray and have a purple or brown margin. As the disease progresses, stand symptoms appear as a general thinning or scorched similar to drought stress. In perennial ryegrass and tall fescue, symptoms initially appear as small water-soaked spots that quickly turn necrotic. The spots vary in color, size and shape, but are regularly gray to light brown in the center of the spot surrounded by a purple to dark brown boarder (Figure 3, p. 35). The spots are often oblong in shape and old spots may have a yellow halo. As the disease progresses, entire leaves are blighted and may have a fi sh hook appearance. Stand symptoms typically develop as small patches and may resemble dollar spot or Pythium blight (Figure 4). Mycelium will not be present with gray leaf spot during dew formation in the morning. The small patches can rapidly expand under favorable conditions and can be confused with brown patch. Under intense disease pressure, large swards of turf maybe killed leaving behind resistant plants or weeds. Gray leaf spot stand symptoms in perennial ryegrass and tall fescue may also mimic heat or drought stress. Gray leaf spot is diffi cult to diagnose in the fi eld. If GLS is suspected consider submitting a sample to a diagnostic lab for confi rmation. How to manage Gray leaf spot can be managed using a number of factors. Many perennial ryegrass varieties have some resistance to gray leaf spot. Consult the National Turfgrass Evaluation Program website Figure 2. Plant symptoms of gray leaf spot on St. Augustinegrass. Note the grayish interior surrounded by a purple to brown boarder. Figure 1. Interstate 80 generally can be used to delineate where gray leaf spot is most severe; typically GLS develops south of I-80.

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