Raychis

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What is the white stuff on my plant?

White on a plant covers two mildews, three insects, a mineral crust and a mould living on the compost, and each wants a different answer. Rub a patch between finger and thumb before you do anything else, because what happens next narrows it to one or two.

Rub it, then look at your finger

White dust that comes off as powder and leaves faintly damaged leaf behind is a mildew. White fluff that squashes and leaves an orange or pink smear is an insect. A hard white crust that stays put belongs to the pot or the water rather than the plant. And white specks that fly up in a cloud when you touch the plant are whitefly.

Work down the list

  1. Powdery mildew

    What you would see
    A white or grey dusty bloom on the upper surface of leaves and on the stems, spreading in warm dry weather, worst on plants short of water at the root.
    The check
    It rubs off, and the leaf underneath is discoloured rather than clean. Courgettes, roses, honeysuckle and asters carry it every August.
    Powdery mildew

  2. Mealybug

    What you would see
    Tufts of white waxy fluff wedged into leaf joints, along stems and under leaves, on a houseplant, with sticky leaves below.
    The check
    Squash one. A mealybug leaves an orange or pinkish smear, and a fungal growth leaves nothing but a mark.
    Mealybug

  3. Whitefly

    What you would see
    Tiny white moth-like insects on the undersides of leaves that rise in a cloud when the plant is disturbed, with sticky honeydew below them.
    The check
    Shake the plant. Nothing else on this list flies.
    Whitefly

  4. Salts from hard water and feed

    What you would see
    A hard white or buff crust on the compost surface, around the drainage holes, and up the outside of a terracotta pot, with the plant otherwise unmarked.
    The check
    It is on the pot and the compost rather than on the leaves, and it does not spread. Flush the pot through several times with plenty of water.
    Nutrient deficiencies

  5. Downy mildew

    What you would see
    A fine off-white to grey bloom on the underside of a leaf, directly beneath an angular yellow patch on the top.
    The check
    Turn the leaf over. Growth underneath with yellowing above is downy mildew, and dust on top with a green underside is powdery mildew.
    Downy mildew

  6. Scale insects with a waxy covering

    What you would see
    White or pale scurfy bumps along stems and leaf veins, immobile, on bay, citrus, camellia and many houseplants.
    The check
    Push one with a thumbnail. A scale pops off as a shell with the insect underneath, and a mildew smears.
    Scale insects

  7. Harmless mould on the compost

    What you would see
    A white cottony or cobweb-like growth on the surface of the compost in a pot, appearing after watering and in low light.
    The check
    Look at whether it is on the plant at all. Compost moulds live on organic matter and touch nothing living. Scrape it off, water less often, and improve the airflow.

  8. Woolly aphid

    What you would see
    Fluffy white cotton wool tucked into bark cracks and around pruning wounds on apple, cotoneaster and pyracantha, from May onwards.
    The check
    Rub a patch and look for the purple-brown aphids under the wool, and for the lumpy swellings the feeding leaves in the bark.
    Aphids

Still not sure which one it is? Photograph the leaf and compare it against the full library.

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