Gray Leaf Spot Information
Causal Agent(s)
- Pyricularia grisea
- Magnaporthe oryzae
Hosts
- Perennial ryegrass
- Tall fescue
- Centipedegrass
- St. Augustinegrass
- Bahiagrass
- Bermudagrass
Symptoms & Signs
Leaf lesions from the gray leaf spot pathogen are oblong with tan centers and dark-purple to brown margins. Lesions coalesce as disease progresses girdling the leaf, causing it to twist and curl the leaf tip downward which results in the characteristic “shepherd’s crook” symptom. On high humidity mornings, fuzzy, gray mycelium may be visible on infected tissue. Stand symptoms are diffuse in warm season grasses, while in cool season grasses stand symptoms are patches under a foot in diameter and are orange to yellow in color. If perennial ryegrass or tall fescue are left untreated, severe outbreaks of gray leaf spot can lead to canopy collapse with matted and greasy looking leaves.
Images of gray leaf spot symptoms:
When Does it Occur?
Gray leaf spot is most severe on newly planted turfgrass. The risk of excessive turfgrass loss tends to decrease as stands mature. Gray leaf spot can occur when nighttime temperatures are above 70 °F and daytime temperatures are 80-90 °F, but is most prevalent during rainy summer months. The pathogen requires around 14 hours of continuous leaf wetness to initiate infection. Succulent leaf tissue from heavy summer N fertilization is highly susceptible to infection.
Cultural Management
Where possible, plant resistant cultivars of perennial ryegrass, tall fescue, or St. Augustinegrass. Reduce leaf wetness periods by scheduling irrigation to occur when the foliage is already wet, increase airflow to promote drying, and remove dew on high-value areas. Collect clippings to reduce spreading disease. Do not apply late-summer N to cool-season turfgrasses.
Chemical Management
Early curative fungicide applications can be made when symptoms are first observed in warm-season grasses and tall fescue, which are less susceptible to rapid disease development. Perennial ryegrass is extremely susceptible to rapid development of severe damage and as such preventative applications are recommended. Preventative applications should begin in June. Fungicide resistance to QoI fungicides is documented in gray leaf spot. Due to the pathogen’s rate of reproduction, development of additional resistance is a major concern. Tank-mix fungicide chemistries, especially when using QoIs, and rotate chemistries after each application to reduce the risk of resistance development.
Selected fungicides for gray leaf spot management:
Selected fungicides for gray leaf spot management
1 Trade products containing the same active ingredient(s) labelled for use on turfgrasses may be used according to label directions. Mention of specific products does not constitute endorsement.
2 Always check product label for current use rates, application directions, and rate restrictions.
3 Relative efficacy. + = Marginally effective; ++ = Moderately effective; +++ = Effective; ++++ = Very Effective; ? = Limited information at this time.
4 Fungicide Resistance Action Committee code(s). Products sharing a code contain active ingredients with the same mode of action. Always use resistance management practices.
