Powdery Mildew on Plants: How to Identify, Treat and Prevent It
Powdery mildew looks like someone dusted flour across your leaves. Catching it in the first week keeps it from jumping to every plant in the room.
How to identify powdery mildew
Powdery mildew is a fungal disease caused by several species of fungi in the order Erysiphales. On houseplants and garden plants it shows up as:
- White or pale-grey powdery patches on leaf surfaces — first in small circular spots, then spreading
- Leaves that curl, yellow, and drop as the infection progresses
- Stunted new growth, especially on susceptible plants like cucumber, rose, zinnia, begonia, and African violet
- Worst on plants with poor airflow, in humid rooms, or in crowded plantings
Unlike downy mildew, powdery mildew sits on the leaf surface and can often be wiped off gently with a finger — a quick identification test.
| What you see | Likely cause |
|---|---|
| White powder that wipes off, on the top of leaves | Powdery mildew |
| Fuzzy grey growth on the underside, yellow patches on top | Downy mildew |
| White cottony tufts in leaf joints that move when poked | Mealybugs |
| Hard white speckles that do not wipe off | Mineral deposits from hard water |
If the white marks do not match, see the full guide to white spots on plant leaves.
Treatment: the 4-step plan
- Isolate the plant from others immediately. Spores travel on air currents.
- Remove the worst-affected leaves with clean scissors and throw them in the trash — not compost.
- Spray with a simple home mix: 1 teaspoon baking soda + a few drops of liquid dish soap in 1 litre (1 quart) of water. Coat both sides of the leaves.
- Repeat weekly for three weeks, and improve airflow — a small fan on low does more than any spray.
Commercial alternatives
For stubborn infections, neem oil (5 ml per litre of water) or potassium bicarbonate sprays work well. Skip sulfur sprays on cucurbits (squash, cucumber, melon) — they cause leaf burn.
How to prevent powdery mildew coming back
- Keep humidity below 60% where possible — powdery mildew loves humid, still air
- Space plants so leaves don’t touch
- Water at the soil, not the leaves
- Increase airflow with an open window or a small fan
- Choose resistant cultivars if you grow roses, cucumbers, or zinnias outdoors
When to use AI diagnosis
If the white coating looks different from what is described here — or there are also brown spots or webbing — it might be downy mildew, botrytis, or spider mites instead. Open the AI Plant Doctor app, snap the affected leaf, and the AI will tell you which pathogen you are dealing with plus the exact treatment.
Frequently asked questions
Can powdery mildew spread to other plants?
Yes. Spores travel on air currents and on your hands and tools, so isolate the infected plant as soon as you spot it and wash your hands after handling it. It spreads fastest between plants of the same family, such as cucumbers and squash.
Does baking soda really kill powdery mildew?
Baking soda raises the pH of the leaf surface so the fungus cannot keep growing, and the dish soap helps the spray stick. It stops the spread rather than erasing existing patches, so remove heavily coated leaves first and repeat the spray weekly.
Is powdery mildew harmful to humans or pets?
Powdery mildew fungi are plant-specific and do not infect people or pets. Some people with mold allergies react to the spores, so wear a mask if you are sensitive while removing infected leaves.
Should I throw away a plant with powdery mildew?
No. Powdery mildew is one of the most treatable plant diseases. Only discard the plant if more than two thirds of the leaves are coated and it is dropping them, or if it keeps reinfecting a collection of susceptible plants.
Read next
White spots on leaves
Dusty, cottony, crusty, or bumpy? The texture of a white spot tells you whether it is a fungus, a pest, hard water, or harmless edema, and each needs a different fix.
Leaf spot
Spots with dark rims or yellow halos that keep multiplying are leaf spot disease. Here is how to tell fungal from bacterial and stop it spreading.
Mealybugs
That bit of white fluff in the leaf joint is a mealybug colony, and there are always more than you can see. Here is how to clear them with a cotton swab, repeated sprays, and patience.