According to OrganicGardening.about.com, “Powdery mildew fungi are host-specific, meaning the different powdery mildew fungi infect different plants. The powdery mildew on your lilacs will not spread to your grapes or your roses.” Conditions that encourage the fungi’s growth and spread include dampness or high humidity (watering in the morning rather than after noon will help), crowded plantings and poor air circulation.
Once plants are infected with powdery mildew, OrganicGardening.about.com recommends the following:
• Remove and destroy all infected plant parts.
• Improve air circulation by thinning and pruning.
• Don’t fertilize until the problem is corrected. Powdery mildew favors young, succulent growth.
• Don’t water plants from above.
• Apply a fungicide: There are many fungicides available. Check the label to be sure they are safe and effective on the type of plant that is infected. Look for ingredients such as: potassium bicarbonate, neem oil, sulfur or copper, or use home remedies using milk, baking soda or other readily available kitchen ingredients.
Researchers in Bazil found that weekly sprays of milk controlled powdery mildew in zucchini just as effectively as synthetic fungicides. They also found that milk acted as a foliar fertilizer, boosting the plant’s immune systems. A concentration of 10-30% significantly reduced the severity of powdery mildew by 90%. Scientists aren’t 100% sure how milk works to control powdery mildew; it appears to be a natural germicide and possibly boosts the plant’s immune system to prevent the disease.
Sharon Sweeny wrote in “Dealing with Powdery Mildew – 7 Home Made Remedies that Really Work” that before using any natural remedies, plants should be hosed off “to remove as much mildew from the surface as you can. Be sure to cover all parts of the plant with the treatment spray, including the undersides of the leaves and the stems. Spray some on the soil around the plant as well. Finally, spray for mildew early in the morning before the sun rises too high in the sky. Spray plants once or twice a week in dry weather and more often if rainy.”
Milk-based recipe: Real Simple magazine (Sept. 2011) – 1 part skim milk to 9 parts water. Spray on plants dotted with powdery mildew. Repeat weekly to keep leaves fungus-free. [Note: Skim milk has no fat content so there is less chance of odors.]
Baking soda recipe: Mix 1 gallon of water, 1 tablespoon of baking soda, 1 tablespoon of vegetable oil and 1 tablespoon of dishwashing liquid or castile soap (e.g., Dr. Bronner’s). Mix the ingredients together and add them to a spray bottle. Spray weekly.