Struggling With Gray Leaf Spot in Tall Fescue?

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Over the past few weeks, we have consulted with multiple clients who thought either their lawn was destroyed by Pythium blight or that fungicide applications were failing at suppressing brown patch. Turns out, in most cases, they were dealing with gray leaf spot.

Brown patch lesions vs. gray leaf spot

Brown patch lesions vs. gray leaf spot on tall fescue leaf blades.

Gray leaf spot is a devastating disease of cool-season turfgrasses such as perennial ryegrass and tall fescue. The disease was of primary concern for those growing perennial ryegrasses but incidence and severity on tall fescue has been increasing. It is not a new problem on tall fescue, yet for many years it was most problematic for sod producers that were managing juvenile stands of tall fescue. Over the past four years we have observed more gray leaf spot in our fungicide trials at the Lake Wheeler Turfgrass Research and Education Center and in-home lawn submissions to the Turfgrass Diagnostic Lab.

Gray leaf spot is most severe as leaf wetness exceeds 12 hours and nighttime temperatures are above 72oF. Leaf spot can be evident in June but stand symptoms usually do not develop until July and August. Unfortunately, the disease can persist throughout September if conditions remain conducive for disease development. Gray leaf spot can affect young seedlings as well and research has shown that young plants are more susceptible than older plants. Protecting new establishments, renovations and fall seedings is important.

Gray leaf spot

Typical gray leaf spot symptom on tall fescue leaf blade.

Symptoms include small oval leaf spot that are gray in center surrounded by a darker border. In order to see the characteristic leaf spot numerous leaves may need to be examined. Stand symptoms are large, irregular areas that initially can appear grayish but quickly turn brown as the affected plants die. If spots are observed during the summer months that look like the image provided in this post, it is likely that it is gray leaf spot.

Management of gray leaf spot can be challenging. Although QoI fungicides provide excellent brown patch suppression, our research shows that only 50% control should be expected with QoI fungicides alone. Pre-mix products that incorporate a QoI and DMI or a QoI with thiophanate-methyl are necessary to suppress gray leaf spot. Pathogens that incite brown patch are considered low risk for developing fungicide resistance. Therefore, we do not suggest rotating chemistries for brown patch management, yet the gray leaf spot pathogen is prone to resistance development. We suggest rotating a tank mixture of a QoI and DMI with thiophanate methyl and a QoI to protect against resistance development.

A slight decrease in mowing height may also help reduce the severity of gray leaf spot. Current recommended mowing heights for tall fescue range between 3.5 to 4 inches, so targeting 3.5 inches can reduce severity of the disease. Avoid applying more than ¼ lb N/1,000ft2 as it could make the disease more severe. Finally, avoid irrigating late afternoon or evenings as increasing leaf wetness will increase the severity of gray leaf spot.