Treat powdery mildew fast by removing infected leaves, improving airflow, and spraying remaining foliage with a potassium bicarbonate, milk, baking soda, or hydrogen peroxide mix on a strict weekly schedule.
For the full breakdown, see our best Horticultural Oil For Powdery Mildew guide.
Remove Infected Growth First
Start with the worst-hit leaves and stems. Prune out heavily coated foliage and dispose of it in the trash — never the compost pile, because spores survive and reinfect your garden. If the infection is severe on a houseplant, isolate it from your other plants. For outdoor beds, open up the plant structure: remove crowded interior branches and any shoots touching the ground to let air move through.
After pruning, clean your shears with rubbing alcohol between plants so you do not spread spores to healthy ones.
Choose and Apply the Right Spray
Once infected material is gone, treat the remaining foliage with a spray that kills surface spores. Cover every leaf surface — tops, bottoms, stems — until the spray drips off.
| Ingredient | Mix Per Gallon of Water | Spray Schedule |
|---|---|---|
| Potassium bicarbonate | 4 teaspoons Monterey Bi-Carb or 1–2 tablespoons MilStop, plus ½ teaspoon liquid soap | Every 7 to 14 days |
| Baking soda | 1 tablespoon baking soda + ½ teaspoon liquid dish soap | Every 7 to 10 days |
| Whole milk | 1 part milk to 2–3 parts water (40% milk) | Every 7 to 14 days |
| 3% hydrogen peroxide | 1 part peroxide to 9 parts water | Every 3 days for 2 weeks |
| Vinegar (5% acetic acid) | 2–3 tablespoons white vinegar | Test first — can burn leaves |
Important: test any home spray on a few leaves first and wait 24 hours. Baking soda and vinegar can scorch tender foliage, especially in hot sun. Spray in the morning so leaves dry before nightfall.
Prevention That Keeps Mildew Away
Powdery mildew thrives in crowded, humid conditions with poor airflow. For indoor plants, run a small fan near your pots and keep leaves dry at all times.
Calcium polysulfide (lime sulfur) applied as a dormant spray on fruit trees also kills overwintering spores, but follow label directions exactly.
When to Use a Commercial Fungicide
For indoor plants, use a product labeled for indoor use (preferably systemic) and follow every label instruction.
FAQs
References & Sources
- Royal Horticultural Society. “Powdery Mildews.” Guide to identification, treatment, and prevention for garden plants.
- UC IPM. “Powdery Mildew on Fruits and Berries.” Fungicide timing and cultural control guidance for home gardens.
- Missouri Botanical Garden. “Powdery Mildew — Indoor Plants.” Indoor-specific treatment and prevention recommendations.