White, dusty patches on the leaves of a begonia or African violet are often powdery mildew. It looks alarming, but it is one of the more manageable houseplant diseases. UC IPM says it usually will not kill a plant, although the fungus and the leaf drop that can follow spoil its looks.
What makes it unusual is the weather it likes. Most leaf diseases need wet leaves. Powdery mildew does not, which is why it turns up in heated rooms in winter.
Recognise powdery mildew
Maryland Extension describes small spots of powdery white growth that merge into a white coating over leaves and stems. Missouri Botanical Garden says it can look white to gray, thread-like or dusty, and can also cover buds and flowers.
Check these points:
- Does it rub off? Missouri Botanical Garden notes that rubbing removes some of the growth, but this does not eliminate the fungus.
- Which surface? UC IPM says the spores form on both upper and lower leaf surfaces.
- Is the leaf changing? Iowa State says severely affected leaves may be distorted. Clemson HGIC adds that some houseplant infections show as dry, brown, papery spots.
- On succulents. RHS notes that on some Crassula species the infection scars the leaf surface, with very variable amounts of white growth.
Look-alikes
| What you see | More likely | How to tell | Source |
|---|---|---|---|
| Growth mostly on leaf undersides after cool, very wet conditions | Downy mildew | Spores grow on branched stalks, mostly on lower surfaces; needs free moisture | UC IPM |
| White cottony tufts in leaf joints and along veins | Mealybugs | Cottony wax covering small oval insects | UC IPM |
| White crust on the soil or pot rim | Salt buildup | On the mix and pot, not on leaves | Clemson HGIC |
| Fuzzy white growth on the potting mix | Saprophytic soil fungi | Grows on the mix, not the plant | Clemson HGIC |
White spots on a jade plant can also be mineral deposits; see our jade mineral deposits guide. For fuzz on the potting mix, read mold on houseplant soil.
Why it appears indoors
UC IPM states that, unlike many diseases, powdery mildew does not require moist conditions. All powdery mildew species can germinate and infect without water on the leaf surface. Water left on the leaf for long periods actually inhibits spore germination and kills the spores of most of them.
RHS explains why. The airborne spores have an unusually high water content, so they can infect in drier conditions than most fungi. High humidity in the air, though not free water, favours spore production and infection.
The conditions that favour it, by source:
| Condition | Source |
|---|---|
| Moderate temperatures of 16–27 °C (60–80 °F) and shade | UC IPM |
| Growth suppressed above 35 °C (95 °F) and by strong sunlight | UC IPM |
| High relative humidity and poor air circulation | Iowa State, Maryland |
| Still, stagnant air, such as against walls | RHS |
| Cool, damp conditions; cold drafts | Missouri Botanical Garden, Iowa State |
| Lush growth from too much nitrogen fertilizer | UC IPM, RHS, Missouri Botanical Garden |
| Intermittent drought stress | RHS, Iowa State |
Iowa State says outbreaks on houseplants typically occur in winter or early spring.
Plants commonly affected
Each powdery mildew fungus has a narrow host range. Missouri Botanical Garden explains that the species on African violet is not the one on rosemary, so mildew on one plant may not spread to a dissimilar plant.
Houseplants the sources name:
- African violet: Iowa State, Missouri Botanical Garden
- Begonia: Iowa State, Missouri Botanical Garden, UC IPM
- Jade plant: Iowa State, Missouri Botanical Garden
- Kalanchoe: Iowa State, Missouri Botanical Garden, UC IPM
- Ivy and grape ivy: Missouri Botanical Garden, UC IPM
- Poinsettia: Iowa State, Missouri Botanical Garden
- Pilea: UC IPM
- Rosemary grown indoors: Missouri Botanical Garden
Control, from least toxic up
Isolate the plant first. Missouri Botanical Garden advises doing this at the first sign of infection.
1. Remove infected leaves
Maryland and Iowa State say that if only a few leaves are affected, pick them off and discard them. Missouri Botanical Garden says to remove all infected parts. Clemson focuses on removing severely infected leaves.
2. Change the conditions
This is the core of control indoors. Iowa State calls altering the environment the best way to control powdery mildew on houseplants.
- More airflow. Iowa State suggests spacing plants further apart and running a small fan in the room. Maryland also stresses spacing.
- More light. UC IPM suggests moving plants into brighter light.
- No cold drafts. UC IPM and Iowa State say to keep plants away from cold drafts near windows and doors.
- Water early in the day. UC IPM and Iowa State both suggest this so foliage dries. Missouri Botanical Garden says to avoid wetting leaves.
- Ease off nitrogen. Missouri Botanical Garden says powdery mildew attacks new succulent growth.
- Avoid drought stress. Iowa State says to keep plants in good vigour this way.
3. Products, only as labeled
Fungicides work best early. UC IPM says most are preventive, and once mildew growth is mild to moderate it is generally too late for effective control. Iowa State warns that most fungicides for powdery mildew are not labeled for indoor houseplants. Read the label for houseplant use before you buy.
- Neem oil and horticultural oils. UC IPM lists them for mild to moderate infections and says oils work best as eradicants. Do not apply oils above 32 °C (90 °F), on water-stressed plants, or within 2 weeks of sulfur.
- Potassium bicarbonate. UC IPM says it has some eradicant activity but can injure infected plants. Do not use it on advanced infections, where it says phytotoxicity will be severe.
- Sulfur. UC IPM says sulfur is effective only before the disease appears.
- Other actives. Clemson lists sulfur, chlorothalonil, myclobutanil, tebuconazole and Bacillus amyloliquefaciens sprays after removing infected parts. It advises spraying houseplants outdoors in shade, in mild weather.
RHS takes a firmer line and does not recommend fungicides in gardens. It also advises against homemade products, as they are unregulated and usually untested. For a worked example on one plant, see begonia powdery mildew and neem oil.
4. When to discard
Missouri Botanical Garden and Iowa State say severely infected plants may need to be discarded. Missouri Botanical Garden adds that you may be able to start a new plant from a cutting of healthy tissue.
Prevention
- Inspect before buying. Iowa State says to check plants before purchase.
- Give susceptible plants room. Spacing and a fan keep air moving around begonias and African violets.
- Watch in winter. That is when Iowa State says outbreaks typically start.
- Fertilize moderately. Excess nitrogen produces the soft growth mildew favours (UC IPM, RHS).
For care basics on two of the most susceptible plants, see our African violet care guide and jade plant care guide. For other houseplant pests, read our houseplant pest treatment guide.
References
- UC Statewide IPM Program, Pest Notes 7493: Powdery Mildew on Ornamentals
- UC Statewide IPM Program: Pest Notes: Houseplant Problems
- Iowa State University Extension and Outreach: What are the white blotches on my African violet leaves?
- Iowa State University Extension and Outreach, C. Engelbrecht: Two Common Houseplant Diseases
- Missouri Botanical Garden: Powdery mildew – Indoors
- Clemson Cooperative Extension HGIC 2251: Houseplant Diseases & Disorders
- University of Maryland Extension: Powdery Mildew on Indoor Plants
- Royal Horticultural Society: Powdery mildews