What are the white marks on my plant?

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 often get it in late summer.

   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. Fungal growth does not.

   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. If tiny white insects fly up from the leaves, it is whitefly.

   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 insect pops off as a shell with the insect underneath. 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

   Check whether it is on the plant or only on the compost. Compost mould lives on dead organic matter. 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