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DiseaseLeaves

Downy mildew

Pale yellow patches on the upper leaf surface with a greyish or purplish furry growth directly beneath them, the patches often bounded by the leaf veins into angular blocks. It is routinely confused with powdery mildew and behaves in the opposite way: downy mildew needs wet conditions, powdery mildew thrives in dry ones.

Caused by: Peronosporaceae · also called false mildew

Described view

Two courgette leaves compared at the same scale: one with the loose white flour-like powdery mildew lying on the upper surface and rubbing off under a thumb, and one turned over to show downy mildew — angular yellow blocks on the upper surface bounded by the veins, with grey-purple fuzz on the underside directly beneath them

What brings it on

Cool, wet, humid weather and crowded plantings with still air. The angular patches bounded by veins are the distinguishing feature, and the mould is on the underside rather than the top.

source

Sourced guidancehigh confidence

  • Royal Horticultural Society: Downy mildew

Checked 2026-08-13

Usually seen: May, June, July, August, September

Preventing it

Ordered by how much difference each measure actually makes, not by how often it is repeated.

  • Space plants for airflow, water at the base rather than over the leaves, and avoid watering late in the day when foliage stays wet overnight.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Downy mildew — prevention

    Checked 2026-08-13

Dealing with it once it is there

  • Remove affected leaves and improve ventilation. On a badly affected annual crop the practical response is usually to harvest what is usable and clear the rest.
    source

    Sourced guidancemoderate confidence

    • Royal Horticultural Society: Downy mildew — control

    Checked 2026-08-13

Signs that lead here

Each of these lists the other things that produce the same sign, and how to tell them apart.

On sprays and treatments

GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.

Plant-specific guides

Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.