White Powder on Plant Leaves — Powdery Mildew and How to Get Rid of It

Photo: Dollymoon, CC BY-SA 3.0, via Wikimedia Commons

If it looks like someone dusted your plant with flour, it is powdery mildew — a fungus that grows on the leaf surface rather than inside it. That surface habit is good news: it is one of the few plant diseases you can still turn around once you can see it clearly.

How to be sure it is mildew

Rub a patch gently with your finger. Powdery mildew smears and partly wipes away; the leaf underneath is green but dull. Three things it is commonly confused with:

• Downy mildew — grows on the underside and looks greyish-purple, with yellow blotches on top. It needs a different treatment, so check both sides before you spray.
• Hard water residue — sits in droplet-shaped rings, does not spread outward over days, and appears on leaves that get splashed rather than on new growth.
• Spider mite stippling — tiny pale dots rather than a continuous film, and usually with fine webbing at the leaf joints.

What causes it

Unlike most fungal problems, powdery mildew does not need wet leaves. It thrives in warm days, cool nights, crowded plants and still air — which is exactly a windowsill in late summer, or a greenhouse in autumn. High humidity around the plant with dry leaf surfaces is its ideal.

Treatment, in order

1. Remove the worst leaves. Anything more than half covered will not recover; cutting it removes spore factories. Bin them, do not compost.
2. Improve airflow. Space the plants out, thin dense growth, open a window. This single step does more than any spray.
3. Spray weekly. Potassium bicarbonate (1 teaspoon per litre plus a drop of soap) is the most reliable home option; sulphur- or oil-based fungicides also work. Treat the whole plant, both leaf sides, in the evening — never in direct sun.
4. Repeat for three weeks. One application kills what is visible, not what has already germinated.

Stopping it coming back

Water the soil, not the foliage. Do not crowd pots together. Feed less nitrogen late in the season — soft new growth is the tissue mildew colonises first. And if one particular plant gets it every single year, replace it with a resistant variety rather than fighting the same battle annually.

Not sure that's what you have?

Symptoms overlap, and the wrong treatment wastes weeks. Photograph the affected leaf with Sherleaf and you get the diagnosis, a severity rating and a treatment plan in about ten seconds.