GrowingHQ
DiseaseLeavesStems

Powdery mildew

A white, floury coating on the upper leaf surface, most familiar on courgettes and cucumbers late in the season. Unlike most fungal diseases it does not need the leaves to be wet — it needs the roots to be dry and the air to be humid, which is why it arrives in a warm dry August.

Caused by: Several host-specific fungi, including Podosphaera xanthii on cucurbits and Erysiphe species on other crops

Described view

Blackcurrant shoot tips in one frame: leaves puckered into raised red-purple blisters on the upper surface with pale aphids sheltering in the hollows underneath, and separately a shoot whose young leaves are distorted and carry a white powdery bloom over the upper surface and the stem

What brings it on

Warm days, humid nights and plants under water stress. The fungi are host-specific: the mildew on your courgettes did not come from your peas.

source

Sourced guidancehigh confidence

  • Royal Horticultural Society: Powdery mildews
  • UC Statewide Integrated Pest Management Program — University of California Agriculture and Natural Resources: Powdery mildew on vegetables

Checked 2026-08-13

Usually seen: July, August, September

Preventing it

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

  • Keep the roots moist. Mulching and watering thoroughly at the base does more to prevent powdery mildew than anything applied to the leaves.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Powdery mildews — cultural control

    Checked 2026-08-13

  • Space plants so air moves between them, and avoid crowding against a wall or fence.
    source

    Sourced guidancehigh confidence

    • UC Statewide Integrated Pest Management Program — University of California Agriculture and Natural Resources: Powdery mildew — cultural practices

    Checked 2026-08-13

  • Choose resistant cultivars where they exist. Courgette and cucumber breeding has made real progress here.
    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Cucurbit cultivar selection

    Checked 2026-08-13

Dealing with it once it is there

  • Remove the worst-affected leaves to slow the spread and let light into the plant. On a late-season courgette this often buys several more weeks of picking.
    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Powdery mildews

    Checked 2026-08-13

  • Late infection on a crop that has already produced well is not worth treating. Powdery mildew arriving in September on a courgette that has cropped since July is the end of that plant's season, not a problem to solve.
    source

    GrowingHQ editorial positionmoderate confidence

    A judgement about when intervention is worth the effort, not a statement about the fungus. Labelled so a grower can disagree with it.

    • GrowingHQ editorial synthesis — GrowingHQ

    Checked 2026-08-13

What it affects

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.