What is the white powdery coating on my plant leaves?
A white or grey film on foliage is almost always a fungus growing on the leaf surface rather than dust or spray residue. Which fungus it is depends on which side of the leaf it sits on, and that one observation settles most cases without a lens.
The one observation that decides it
Turn the leaf over. A powdery bloom on the UPPER surface that rubs off with a finger is powdery mildew, and it thrives in warm dry air. Fuzzy growth on the UNDERSIDE, with yellow patches showing through on top, is downy mildew or leaf mould, and both need prolonged leaf wetness. Dense cottony white mould on stems, later holding hard black pellets, is white mould.
Start from the plant in front of you
9 things it could be
Ordered by how much they can cost you — if the first entry fits what you can see, rule it out before considering the milder ones.
Dark, water-soaked greasy patches on leaves that rapidly turn brown; white fuzzy sporulation on leaf undersides in humid mornings; firm brown lesions on stems and fruit. Whole plants can collapse within days.
Conditions: 10–25°C · humidity ≥90% · wet leaves / rainFirst move: Remove affected material nowAngular yellow patches on the upper leaf bounded by veins, with grey-purple fuzzy growth on the underside. Leaves brown and die from the inside of the plant outward.
Conditions: 10–23°C · humidity ≥85% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Cucumbers, Zucchini, Lettuce, Onions, Beetroot, Spinach and 8 more
Fuzzy grey-brown mould on flowers, fruit and stems; soft watery rot; tan spots with concentric zoning on leaves. Spores puff out in clouds when disturbed.
Conditions: 5–22°C · humidity ≥90% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Grapes, Strawberries, Raspberries, Blackberries, Redcurrants, Gooseberries and 10 more
Sudden wilting and yellowing; a white fan of fungal threads at the soil line studded with tan, mustard-seed-sized resting bodies; rapid plant collapse.
Conditions: 25–35°C · humidity ≥80% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysWater-soaked stem and pod rot covered in dense cottony white mould, later embedded with hard black pellets (sclerotia); sudden wilting of branches above the rot.
Conditions: 10–24°C · humidity ≥90% · wet leaves / rainFirst move: Remove affected material nowSeen on: Lettuce, Beans, Sunflowers
Pale green to yellow blotches on the upper leaf surface with olive-green to brown velvety mould on the underside; leaves curl, brown and drop, starting low in the canopy.
Conditions: 18–27°C · humidity ≥85% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Tomatoes
White cottony masses in leaf axils, on stems and roots; yellowing and stunted growth; sticky honeydew and black sooty mould; ants attending the colonies.
Conditions: 20–32°CFirst move: Treat, then re-check in 5–7 daysWhite to grey powdery coating on the upper surface of leaves and stems, later yellowing, curling and premature leaf drop. Fruit may be stunted or sun-scorched as the canopy thins.
Conditions: 15–28°C · humidity ≥60%First move: Treat, then re-check in 5–7 daysSeen on: Cucumbers, Zucchini, Peas, Pumpkins, Apple trees, Grapes and 15 more
Round patches 5–30 cm across of yellowed, then greyish-brown, matted grass. In damp mornings the edge of a patch carries white or pale pink cobwebby mycelium. Patches merge into large dead areas in a bad winter.
Conditions: -2–8°C · humidity ≥90% · wet leaves / rainFirst move: Treat, then re-check in 5–7 daysSeen on: Lawn