Powdery Mildew

• White or gray powdery coating on the surface of leaves, stems, or fruit
• Often starts as small circular patches, then expands to cover the whole leaf
• Affected leaves may curl, yellow, or die prematurely
• Unlike most fungal diseases, powdery mildew is more common in warm, dry weather with cool nights
• Tends to appear first on squash and cucumbers, then spreads

Powdery mildew is caused by several related fungi that produce airborne spores. Unlike most pathogens, it favors warm, dry weather and can infect without free water on leaves. It spreads rapidly via wind. Each powdery mildew species is host-specific — the pathogen on squash cannot infect tomatoes. The fungus overwinters as sexual fruiting bodies (chasmothecia) on plant debris.

1. 01Remove infected leaves and dispose of them — do not compost
2. 02Apply a baking soda spray (1 tbsp baking soda + 1 tsp horticultural oil per gallon of water) on first signs; repeat weekly
3. 03Potassium bicarbonate sprays tend to be more effective than baking soda and are OMRI-listed
4. 04Neem oil applied preventively when conditions favor disease can slow spread
5. 05Plant resistant varieties — many modern squash and cucumber varieties have significant powdery mildew tolerance
6. 06Improve airflow by spacing plants generously and removing competing growth

If powdery mildew is killing plants before fruit matures despite treatment, an extension office can confirm the species and advise whether the issue is in part a watering or fertility problem making plants more susceptible.

• Powdery Mildew on Vegetables— University of Minnesota Extension
• Powdery Mildew — Cucurbits— University of New Hampshire Extension