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Symptom

White powdery coating

A dusty white film that wipes off is powdery mildew, which thrives when roots are dry and air is humid. Late summer squash, courgettes, peas and grapes get it most.

Courgette leaves covered in white powdery mildew

Rub it with a finger. Powdery mildew smears off; insects or wax do not. Check the soil too, since dry roots make mildew worse.

Likely causes

Ordered from the usual answer to the less likely one. Each links to the full problem.

  1. Cucumber leaves dusted with white powdery mildew Fungal disease Common

    Powdery mildew

    White powder that rubs off, on the upper surface first.
  2. Basil leaves yellowing and blotched with downy mildew Fungal disease Less likely

    Downy mildew

    Grey-purple fuzz on the underside, yellow patches above.
  3. White cottony mealybugs at the base of aloe leaves Pest Less likely

    Mealybugs

    Cottony insects, not a flat coating.