Identifying, preventing and controlling powdery mildew

Powdery mildew is one of the best-known fungal diseases in the vegetable garden. The white, mealy, wipeable coating on the upper side of the leaves looks as if someone had dusted the plant with flour – hence the name. It particularly affects courgette, pumpkin and cucumber, but also peas, lettuce and many ornamental plants.

Powdery mildew is also called the "fair-weather fungus": it loves warm, dry days with cool nights and humid air around the leaves. The good news is that with the right preventive measures and simple home remedies it can usually be kept well under control.

How to spot powdery mildew

At first small white to greyish-white spots appear on the upper leaf surfaces. These grow into a closed, mealy coating that can be wiped off with a finger. Later the leaves turn yellow, then brown, and dry up. Unlike downy mildew, the coating sits on top of the leaf – that is the most important distinguishing feature.

Why powdery mildew occurs

The fungus overwinters on plant debris and spreads through the air. Ideal conditions are warm days around 20–25 °C, cool nights and high humidity combined with a dry leaf surface. An infestation is encouraged by planting too densely, poor air circulation, one-sided nitrogen fertilising and drought stress at the roots.

The damage it causes

The coating covers the leaves like a veil and hinders photosynthesis. The plant is weakened, grows more slowly and produces fewer and smaller fruits. In severe cases whole leaves die off – and the harvest drops noticeably.

Preventing powdery mildew

• Robust varieties: many cucumber and courgette varieties are bred to be mildew-tolerant – it is worth checking the seed packet.
• Airy position: enough spacing so that leaves dry off quickly after rain and dew.
• Water correctly: always water at the base, never over the leaves, and in the morning rather than the evening.
• Fertilise moderately: too much nitrogen makes the tissue soft and susceptible.
• Strengthen: horsetail brew toughens the leaves preventively.
• Hygiene: remove affected leaves early and dispose of them in the household waste, not on the compost.

Controlling powdery mildew naturally

At the first signs, proven home remedies help – ideally repeated every few days:

• Milk-and-water spray: mix fresh milk with water at a ratio of 1:9 and spray onto the leaves. The micro-organisms in the milk act against the fungus.
• Baking-soda solution: 1 teaspoon of baking soda per litre of water, plus a few drops of vegetable oil and a little soft soap as a wetting agent.
• Remove affected leaves: cut off heavily covered leaves to slow the spread.
• Sulphur products: for stubborn infestations, an approved sulphur-based product.

Frequently asked questions

Can I still eat affected vegetables?
Yes. The fungus sits on the leaves; the fruits themselves remain edible – just wash them well.

What is the difference from downy mildew?
Powdery mildew sits on top of the leaf and is wipeable; downy mildew shows yellow spots on top and a grey fungal layer on the underside of the leaf.

Does baking powder work instead of baking soda?
Baking powder contains baking soda and works similarly, but is more weakly dosed – pure baking soda is more effective.

Strong, well-hardened young plants usually shrug off a light infestation. We raise our young plants to be robust – a good start to a healthy gardening year.