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Fungal disease
Powdery mildew
Podosphaera aphanis
Starts underneath the leaf where nobody looks, and ends up on the fruit where everybody does.
What you see
- 01Small white colonies on the undersides of leaves
- 02Colonies merging until they cover the lower leaf surface; leaves curl upward
- 03Mature berries going seedy in appearance, with powdery white colonies on the surface
What brings it on
Unlike most fungal diseases it does not need free water on the leaf, so it can run in dry weather — which catches growers out who assume a dry season means a clean one. Protected structures with poor air movement are particularly favourable.
Why it costs money
Affected fruit is unsaleable as fresh. Because the early stage is on the underside of the leaf, it is usually well established before it is noticed.
What to do
- Turn leaves over when you scout — the top surface tells you nothing until it is late
- Ventilate polyhouses and tunnels properly; still, humid air is what it wants
- Avoid dense canopies and excess nitrogen
Spray strategy. Scout the undersides from early in the crop and act on the first colonies rather than on curled leaves. Rotate modes of action.
Also check