Controlling Powdery Mildew on Greenhouse Squash

Understanding the Disease

Powdery mildew (gray-white fuzzy patches) thrives in high humidity, poor air circulation, and moderate temperatures—common in greenhouses. It weakens plants and reduces yields.

Preventive Spray Program

• Start at first true leaves or early June.
• Spray every 10 days with organic fungicide: potassium bicarbonate (e.g., GreenCure), neem oil, or copper soap (Soap Shield).
• Rotate modes of action to prevent resistance.

Cultural Controls

• Ventilate aggressively—fans, vents, doors open daily.
• Space plants widely (3+ feet) for airflow.
• Water at soil level only—never overhead.
• Remove infected leaves immediately; destroy, don't compost.

Resistant Varieties

Choose PM-resistant cultivars: 'Dunja' zucchini, 'Tivoli' spaghetti squash, 'Success PM' yellow squash. Check Johnny's, Territorial, or High Mowing seed catalogs for resistance codes.