Plant problems

There is white stuff on my plant's leaves

Most likely causes

Listed in the order they are worth ruling out. It is normal for more than one of these to be happening at once, so work down the list rather than stopping at the first match.

1. 1

   Spittlebug foam, which needs nothing from you

   A blob of white froth on a stem in late spring looks alarming and is the most harmless thing on this list. Inside is one young spittlebug, and the foam is the cover it builds while it feeds.

   What to check first

   Look at where the white sits and what it is made of. A wet, bubbly mass of froth wrapped around a stem or a leaf joint, like a dab of spit, is spittlebug foam, and wiping it aside reveals the small nymph underneath.

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2. 2

   Powdery mildew, a white dust across the top of the leaf

   The most common answer to white on the leaves. It looks like flour dusted across the upper surface, often starting on the lower leaves, and it favors shade, crowding and still air rather than wet weather.

   What to check first

   Check which surface carries the white. A dry, powdery coating sitting on top of the leaf is this. Fuzz only on the underside, beneath yellow dots on the top surface, points to the last cause on this list instead.

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3. 3

   Mealybugs, if the white tufts sit in the joints and move

   Cottony white tufts tucked into the leaf joints, along the undersides and where leaf meets stem are mealybugs, small insects that cover themselves and their eggs in white wax. Published guidance calls the cottony white wax the usual first sign of their presence.

   What to check first

   Look at a tuft closely, ideally with a phone camera zoomed in. Mealybugs are individual oval insects under the fluff, and they move, very sluggishly, where a fungus or a residue stays perfectly still.

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4. 4

   Woolly aphids, alarming fluff on woody stems

   On a shrub or tree, patches of white fluff clinging to twigs and branches are usually woolly aphids, insects whose bodies are covered with a white fluffy wax that resembles wool. Extension guidance calls them more alarming than damaging.

   What to check first

   Part the fluff with a twig and look at what it is attached to. Small soft-bodied insects underneath, sitting on woody stems and branches rather than dusted across the leaf surface, separate this from a mildew.

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5. 5

   Downy mildew, fuzz under the leaf in cool damp weather

   The opposite of powdery mildew in almost every way. The growth sits on the underside of the leaf, it favors cool damp spells rather than dry ones, and water landing on the foliage, including watering over the top of the plant, is part of what sustains it.

   What to check first

   Turn a marked leaf over. Mats of fluffy growth on the underside, white to gray with sometimes a bluish or purple tint, sitting directly beneath small yellow dots or pale patches on the top surface, is this and not powdery mildew.

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