St. Augustine Gray Leaf Spot
Gray leaf spot (Pyricularia grisea) is the #1 disease of St. Augustine lawns, especially in the humid Southeast. Look for diamond-shaped or oval lesions on blades with gray centers and dark brown borders. Triggered by the combination of high nitrogen fertilization, high humidity, warm nights (above 70°F), and ragged mowing cuts. Prevention is the best strategy: reduce nitrogen applications from June through August, sharpen mower blades frequently, improve air circulation by pruning nearby shrubs, and water in the morning only. If active infection occurs, apply azoxystrobin (Heritage) or thiophanate-methyl. Do NOT apply nitrogen to a lawn with active gray leaf spot — it makes the outbreak worse.
Frequently Asked Questions
What does gray leaf spot look like?
Small diamond or oval-shaped gray-brown lesions on leaf blades, often with a dark border. In severe cases, entire blades wither and the lawn looks scorched. Most visible on new growth.
How do I prevent gray leaf spot?
Reduce nitrogen applications from June through August. Mow with sharp blades (ragged cuts create infection points). Water in the morning only. Improve air circulation by pruning nearby shrubs.
How do I treat gray leaf spot?
Apply azoxystrobin (Heritage) or thiophanate-methyl fungicide at first sign of symptoms. Reduce nitrogen immediately. Water deeply but less frequently. Recovery takes 2-4 weeks with proper treatment.
Related Guides
Get a free lawn plan for your yard
Completely free. No credit card required.