Powdery Mildew on Squash, Cucumbers and Peas

5 min read · Reviewed August 15, 2026 · Garden Planner editorial team (pending independent horticultural review)

Identify

White or grey powdery patches on the upper leaf surface, spreading to cover leaves that yellow and dry. Unlike most fungal diseases it does not need wet leaves; warm days, cool nights and shade favour it. Downy mildew, a different disease, shows yellow angular patches on top with grey fuzz beneath, and needs wet leaves.

Prevention

• Full spacing (zucchini 90 cm, cucumbers trellised) for airflow and sun.
• Resistant cultivars (labelled PM or PMR); this is the most effective measure.
• Steady water at the root; drought-stressed plants succumb faster.
• Avoid excess nitrogen.
• Remove crop debris in autumn.

Management

Remove the worst leaves early and bin them. Sprays of potassium bicarbonate or horticultural oil suppress spread if applied at the first spots, weekly; milk sprays have some support in trials but are inconsistent. None cures existing patches. On late-summer squash it is usually reasonable to let it run while harvesting what remains.

Companion notes

No plant has been shown to reduce powdery mildew on a neighbour. Crowding is the main cause the planner can address: it reserves each cucurbit’s full footprint and flags vines sprawling over other crops.