Common on many plants and easily recognized, powdery mildew is a fungal disease found throughout the United States. It is caused by a variety of closely related fungal species, each with a limited host range. (The fungi attacking your roses are unlikely to spread to your lilacs). Low soil moisture combined with high humidity levels at the plant surface favors this disease.
Symptoms usually appear later in the growing season on outdoor plants. Powdery mildew starts on young leaves as raised blister-like areas that cause leaves to curl, exposing the lower leaf surface. Infected leaves become covered with a white to gray powdery growth, usually on the upper surface; unopened flower buds may be white with mildew and may never open. Leaves of severely infected plants turn brown and drop. The disease prefers young, succulent growth; mature leaves are usually not affected.
Fungal spores overwinter inside leaf buds and other plant debris. Wind, water and insects transmit the spores to other nearby plants. Zucchini, beans, cucumbers, squash, pumpkins, tomatoes, peppers, roses and zinnia are especially susceptible.
- Plant resistant cultivars in sunny locations whenever possible.
- Prune or stake plants to improve air circulation. Make sure to disinfect your pruning tools (one part bleach to 4 parts water) after each cut.
- Remove diseased foliage from the plant and clean up fallen debris on the ground.
- Use a thick layer of mulch or organic compost to cover the soil after you have raked and cleaned it well. Mulch will prevent the disease spores from splashing back up onto the leaves.
- Milk sprays, made with 40% milk and 60% water, are an effective home remedy for use on a wide range of plants. For best results, spray plant leaves as a preventative measure every 10-14 days.
- Wash foliage occasionally to disrupt the daily spore-releasing cycle. Neem oil and PM Wash, used on a 7 day schedule, will prevent fungal attack on plants grown indoors.
- Water in the morning, so plants have a chance to dry during the day. Drip irrigation and soaker hoses will help keep the foliage dry.
- Use a slow-release, organic fertilizer on crops and avoid excess nitrogen. Soft, leafy, new growth is most susceptible.
- Destroy all plant debris after harvest (see Fall Garden Cleanup). Do NOT compost.
If disease symptoms are observed, treat plants with one of the following approved organic fungicides:
- Apply sulfur or copper-based fungicides to prevent infection of susceptible plants. For best results, apply early or at first sign of disease. Spray all plant parts thoroughly and repeat at 7-10 day intervals up to the day of harvest.
- Green Cure Fungicide contains a patented formula of potassium bicarbonate — commonly used in food products — that kills many plant diseases on contact and provides up to 2 weeks of residual protection. At first sign of disease, mix 1-2 Tbsp/ gallon of water and apply to all exposed surfaces of the plant. Monterey® BI-CARB is a similar product containing micro-encapsulated potassium bicarbonate as the active ingredient. Mix 4 tsps in 2 gallons of water to thoroughly cover foliage.
- Effectively treat fungal diseases with SERENADE Garden. This broad spectrum bio-fungicide uses a patented strain of Bacillus subtilis that is approved for organic gardening. Best of all, it’s safe to use — you can treat and pick crops the same day!
- SNS 244 and Zero Tolerance Herbal Fungicide are made from 100% pure, food-grade ingredients that work fast to kill existing plant diseases and prevent new fungal problems from starting.
- Indoor growers may want to consider using a Sulfur Burner/ Vaporizer which turns sulfur prills into a fine dust and changes the pH of leaf surfaces. Fungal spores and mold can’t get established on this plant coating. As an added benefit, studies have shown that this dust will eliminate spider mites.