Disease
Powdery mildew on squash, cucumbers and melons
Last reviewed September 2026 · written from Cooperative Extension and university horticulture references — see how we source data
What you’re seeing
A white to gray powdery coating on the upper surface of leaves, usually starting on older leaves near the base of the plant in mid-to-late summer, that spreads and eventually causes leaves to yellow and shrivel.
Could it be something else?
Likely causes
Ranked most common first.
- Warm days, cool humid nights. The fungus that causes powdery mildew thrives in exactly the conditions common in late summer: warm afternoons followed by cool, humid nights, without necessarily needing wet leaves.
- Crowded plants with poor airflow. Dense foliage traps humidity around leaves and slows drying, which favors fungal spore germination.
- Nitrogen-heavy growth. Overly lush, soft foliage from excess fertilizer is more susceptible than moderately-fed, sturdier growth.
How to fix it
- Remove and discard the most heavily coated leaves as soon as you spot it - this slows the spread and does not meaningfully hurt a plant that already has plenty of other foliage.
- Spray a baking-soda solution (1 tablespoon baking soda, 1/2 teaspoon liquid soap, 1 gallon water) or a 1:9 milk-to-water mix weekly on remaining foliage; both measurably slow new infection when applied early.
- Switch to a sulfur or potassium bicarbonate fungicide labeled for edible crops if the milder options are not keeping up, following label timing carefully.
- Improve airflow by thinning crowded vines and watering the soil directly rather than overhead, which keeps leaf surfaces drier without needing to fight the humidity that settles at night regardless.
- Accept some late-season powdery mildew as normal - by the time it typically appears, most of the harvest from that plant is often already in.
Common mistakes
- Waiting until the leaves are fully white. Sprays like baking soda or diluted milk only slow new infections - they work far better started at the first spots.
- Stripping off every infected leaf at once. Removing too many leaves exposes fruit to sunscald and weakens the plant. Take the worst few.
- Crowding plants. Squash planted too close stays humid under the canopy - check the plant spacing calculator before planting.
- Assuming the harvest is over. Mildly infected plants keep producing for weeks; keep picking.
When and where it shows up
Powdery mildew usually arrives in mid to late summer, after the plants have started fruiting, and spreads fastest when warm days are followed by cool, humid nights. It hits zucchini and summer squash, winter squash, pumpkins, cucumbers and melons. Unlike most fungal diseases it doesn't need wet leaves to take hold, which is why it can spread even in a dry spell.
Preventing it next time
Space plants for airflow, choose mildew-resistant squash and cucumber varieties where available, and water at the base of the plant in the morning so foliage dries quickly.
Questions gardeners ask
Can I eat squash from plants with powdery mildew?
Yes. The fungus grows on the leaves, not in the fruit. The fruit is safe, though a badly infected plant may ripen fewer or smaller fruit.
Does a milk spray really work?
Several trials on squash and cucumber have found a milk-and-water spray (about 1 part milk to 9 parts water) slows powdery mildew when applied early and weekly. It won't clear an established infection.
Should I throw infected leaves in the compost?
Lightly infected leaves are fine in a hot compost pile. Put heavily coated leaves in the trash if your pile doesn't heat up.
Is it the same mildew as on my lilacs or roses?
No. Powdery mildews are mostly host-specific, so the one on your lilac won't infect your squash and vice versa.