Downy Mildew on Lawn: Identification and Fungicide Treatment

White Downy Patches Turning Brown: Classic Downy Mildew

Morning white "downy" coating on grass blades that browns and dies matches downy mildew (Sclerophthora macrospora) perfectly. This water mold (not true fungus) thrives in cool, wet, shaded conditions.

Downy Mildew vs. Look-Alikes

Disease	Appearance	Conditions	Key Difference
Downy Mildew	White fluffy growth on leaves, yellow/brown dieback	Cool (50-65°F), wet, shade	Spores on leaf surface only
Powdery Mildew	White powdery coating	Warm, humid, shade	Wipes off easily
Pythium Blight	Greasy, water-soaked patches, white cottony mycelium	Hot, humid, wet	Rapid kill, "greasy" feel
Slime Mold	White slime → powder	Wet, organic matter	Brushes off, no leaf damage

Treatment Protocol

1. Confirm diagnosis (Extension lab or diagnostic clinic)
2. Apply labeled fungicide: Mefenoxam (Subdue Maxx®), fosetyl-Al (Aliette®), or phosphonates
3. Cultural fixes:
   • Morning-only irrigation
   • Improve air circulation (prune shade)
   • Reduce nitrogen (excess N favors disease)
   • Core aerate for drainage
4. Re-seed resistant varieties if stand lost

Realistic Expectations

Fungicides protect new growth—infected leaves won't recover. The goal is stopping spread while cultural changes alter the environment.

Prevention Calendar

• Spring/Fall: High risk periods—monitor closely
• Summer: Usually inactive in heat
• Year-round: Avoid evening watering, manage thatch

Downy mildew signals environmental mismatch. Fix the shade/water/drainage triangle, and the disease disappears.