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Plant diseases
Downy mildew
Peronospora, Plasmopara and related water moulds · Water mould
Illustration
Yellow patches on top of the leaf with grey or purple fuzz directly underneath. It thrives when leaves stay wet, so morning watering at the base and good airflow are the main defences.
How to spot it
- Pale green or yellow patches on the upper leaf, often angular and held in by the veins.
- Grey, purplish or white fuzz directly underneath the patches, on the underside only.
- Grapes: round yellow 'oil spots' on young leaves, and young bunches that brown and die.
- Patches turning brown and leaves dropping; impatiens can lose all their leaves quickly.
Easily confused with
Powdery mildew
White powder on top of the leaf or on both sides, not fuzz tucked under a yellow patch.
Where it's a problem
- Australia: Grape downy mildew is worst in higher-rainfall regions.
- USA: Cucurbit downy mildew survives winter in the warm south and blows north each year.
- United Kingdom: Impatiens downy mildew arrived in 2003; no sprays are available to gardeners.
What to do
- Remove affected leaves promptly.
- Space and prune plants for airflow.
- Water in the morning, at the base, not over the leaves.
- Clear fallen grape leaves, where it survives the winter.
- Don't replant the same kind of plant there for at least a year, and choose resistant varieties.
If you use a fungicide, choose one labelled for downy mildew and follow the label and local rules. Sprays are preventive only: once symptoms show, it's often too late.
Follow your local council's guidance for disposing of diseased plant material. Official guides usually advise against putting it in a home compost heap.
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Sources
- RHS: Downy mildews
- RHS: Impatiens downy mildew
- University of California IPM: Downy mildew
- NC State Extension: Cucurbit downy mildew
- Wine Australia: Downy mildew
- University of Maryland Extension: Downy mildew of vegetables
Facts checked Sunday 27 September 2026. General guidance only: follow your local council, agency and product label.