Best Management Practices (BMP) for Gray Leaf Spot in St. Augustine Grass
Disease Overview
Gray Leaf Spot (Pyricularia grisea, also known as Magnaporthe oryzae) causes oval or round gray lesions with brown or purple margins, often starting on leaf blades and expanding rapidly during warm, wet conditions. It thrives in hot, humid weather, especially when nitrogen and moisture are high.
Recommended Fungicides and Application Details
| Product | Active Ingredient | Rate per 1,000 ft² | Application Interval | Notes |
|---|---|---|---|---|
| Propiconazole 14.3% | Propiconazole | 1 - 2 oz | Every 14–21 days | Do not apply under high heat stress (>85°F); apply early morning or late evening. |
| Heritage G | Azoxystrobin | 2 - 4 lbs | Every 14–28 days | Granular option; best watered in after application. |
| Azoxy 2SC | Azoxystrobin | 0.38 - 0.77 oz | Every 14–21 days | Liquid; apply with 1–2 gal water per 1,000 ft². Rotate with other fungicide classes. |
Spray Instructions
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Water Volume: Apply fungicides in 1 - 4 gallons of water per 1,000 ft² to ensure even coverage. Use higher volumes in dense or thick lawns.
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Surfactant Use:
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DO NOT use a surfactant with propiconazole unless specifically allowed on the label it can increase burn risk.
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DO use a non-ionic surfactant with azoxystrobin if targeting foliage; not always needed if watered in.
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Best Time to Apply:
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Early morning (dew present) or late evening to avoid evaporation and heat stress.
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Avoid midday applications during temperatures above 85°F.
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Resistance Management
Rotate fungicide classes (FRAC codes) every application. Gray Leaf Spot quickly becomes resistant to strobilurin fungicides (like azoxystrobin). Use tank mixes or rotate to DMI fungicides (like propiconazole) to reduce resistance pressure.
Cultural Practices for Control
Irrigation
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Water deeply but infrequently, about 1" per week, including rainfall.
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Water early morning only, allowing grass to dry before nightfall.
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Avoid frequent, shallow watering, which keeps leaf surfaces moist and encourages spore germination.
Mowing
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Mow when grass is dry to prevent spreading spores.
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Bag clippings during outbreaks to reduce fungal spread.
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Clean mower decks and blades with a 10% bleach solution (1 part bleach to 9 parts water) after each mow in infected areas.
Fertilization
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Avoid heavy nitrogen feeding during summer stress.
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Use slow-release, balanced fertilizer.
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Test soil pH; aim for 6.0 - 6.5 for optimal St. Augustine performance.
Final Notes
Gray Leaf Spot can be managed with a combination of good cultural practices and targeted fungicide use. Conditions that encourage the disease, like poor drainage, excessive watering, and over-fertilizing, must be corrected alongside treatments. Monitor disease progression and adjust practices seasonally.
Don’t just follow advice, read labels, study product modes of action, and watch for environmental conditions that favor disease.
BES-TEX Supply
4512 Adobe Dr.
San Angelo, TX 76903
325.653.1300
www.BES-TEX.com