Powdery mildew: how to spot, prevent and manage it

What it is: Powdery mildew is a common fungal disease that looks just like its name — a dusty white or grey coating on the leaves. It thrives in warm days and cool, humid nights, and it's one of the most common problems in an Aussie veggie patch, especially on pumpkin, zucchini and cucumber.

How to spot it:

• White or greyish powdery patches on the tops of leaves, spreading until they cover the whole leaf.
• Yellowing underneath the coating, then leaves drying out and turning brown.
• Distorted new growth and, in bad cases, fewer and smaller fruit.
• It usually starts on the older, shaded leaves low on the plant.

Plants it affects: Powdery mildew hits a wide range of crops. The usual targets are the cucurbits — pumpkin, zucchini, cucumber and melon — along with tomatoes, peas, beans and many herbs and flowers.

Growing any of these? See our guides to zucchini, cucumbers, peas, pumpkins, coriander and strawberries.

What causes it and how it spreads: The fungus loves crowded plants with poor airflow, high humidity and leaves that stay damp. Spores blow in on the wind and spread fast in the right conditions. Unlike most fungal problems it doesn't need wet leaves to take hold, which is why it can appear even in a dry spell.

How to prevent it:

• Give plants room to breathe — good spacing and airflow is the single best defence, and easy to plan in a raised bed.
• Water the soil, not the leaves, and water in the morning so any splash dries quickly.
• Grow in full sun, and choose mildew-resistant varieties where you can.
• Go easy on high-nitrogen feeding — soft, sappy growth is more prone to it.

How to manage it:

• Remove and bin affected leaves as soon as you spot them (don't compost them).
• A milk spray (one part milk to nine parts water) is a well-known garden remedy that works surprisingly well as an early treatment.
• If it keeps spreading, a copper-based fungicide or a wettable sulfur spray will bring it under control — the fallback once the gentle steps aren't enough.

For the bigger picture, see our guide to managing pests and problems safely.

Plan your patch: our free planting calendar shows what to plant now where you live. Ready to grow? Browse our raised garden beds or build your own with the garden bed builder.

Image: RoRo, CC0, via Wikimedia Commons.