Proven strategies for identifying and treating powdery mildew.
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, 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.
- Keep the fallen and diseased foliage picked off the plant and from the ground.
- Use a thick layer of mulch or organic compost to cover the soil after you have raked and cleaned it well. The mulch will prevent the fungus spores from splashing back up onto the leaves.
- Washing foliage occasionally in mid-morning may disrupt the daily spore-releasing cycle. Neem leaf shine and PM Wash, used on a 7 day schedule, will prevent fungal attack of indoor grown plants.
- Water only in the morning so plants have a chance to dry during the day. Drip irrigation and soaker hoses will help keep the foliage dry.
- Destroy all plant debris after harvest (see Fall Garden Cleanup).
If disease symptoms are observed, treat plants with one of the following approved organic fungicides:
- Apply sulfur or copper-based fungicides weekly to prevent infection of susceptible plants.
- Green Cure, produced from 85% potassium bicarbonate, is a contact control agent that can be used to establish control once the disease is present. BI-CARB is a similar fungicide containing micro-encapsulated potassium bicarbonate as the active ingredient.
- Safely treat fungal and bacterial diseases with SERENADE Garden. This broad spectrum bio-fungicide uses a patented strain of Bacillus subtilis that is registered for organic use. Best of all, SERENADE is completely non-toxic to honey bees and beneficial insects.
- Organic approved Actinovate uses a patented beneficial microorganism (Streptomyces lyrics) that knocks back diseases yet is proven to be safe around pets, people and the environment.
- Indoor growers may want to consider a Sulfur Burner which turns sulfur prills into a fine dust and changes the pH of leaf surfaces. Fungus and mold can’t get established on this coating.
Photo Credit: University of Maine Cooperative Extension