Every year about midsummer, I start getting a lot of questions from people wanting to know what those white or gray powdery spots are on their plants. They often cover most if not the entire leaf surface, plant stems, flowers and even fruit. Some of the most hospitable plants for this fungal disease known as powdery mildew include zinnias, phlox and hydrangeas. Fortunately, this fungus is host-specific, so it is not an automatic threat to other plants in your landscape. The symptoms are usually worse than the actual damage; rarely is it fatal to the plant.
In advanced stages, however, powdery mildew can cause foliage to yellow, curl or turn brown, and eventually defoliate the plant prematurely. On flowering plants and trees, the fungus can lead to early bud drop or reduced flower quality.
Causes and prevention of powdery mildew
Conditions that favor mildew formation include dry foliage, high humidity, low light and moderate temperatures. Proactive steps to avoid or minimize this risk include:
-- Buy disease-resistant varieties.
-- Provide adequate air circulation by not crowding plants.
-- Site plants where they will get sufficient light -- at least six hours or more each day.
-- Avoid over-fertilization. Rapid new growth is more susceptible. Instead, apply a slow-release fertilizer that provides more controlled growth.
Controlling an existing problem
Early detection provides the best way to contain and potentially eliminate the problem. Most conventional, off-the-shelf products are made for prevention and control, not elimination of an existing infection. One of the most common multipurpose fungicides for control contains the active ingredient chlorothalonil. Daconil 2787 is probably the most well-known brand containing this ingredient. Although effective, it coats the leaf surface with a white, milky film that is quite noticeable.
-- Neem oil -- This is an effective organic disease control that doubles as a broad-spectrum, natural insecticide that is kinder to beneficial insects and mammals than many other options.
-- Baking soda (sodium bicarbonate) -- This is the best known of the homemade, organic solutions. Although studies indicate that baking soda alone is not all that effective, when combined with horticultural-grade or dormant oil and liquid soap, efficacy is very good if applied in the early stages or before an outbreak occurs. Mix 1 tablespoon of baking soda with 1 teaspoon of dormant oil and 1 teaspoon of insecticidal or liquid soap (not detergent) to 1 gallon of water. Spray on plants every one to two weeks.
-- Potassium bicarbonate -- Similar to baking soda, it is a contact fungicide, which kills the powdery mildew, eliminating the disease once it's there. In addition, it's approved for use in organic growing. It is currently sold as GreenCure through garden centers and online at www.greencure.net.
-- Mouthwash -- Generic, ethanol-based mouthwash can be very effective. Use one part mouthwash to three parts water; just be careful when mixing and applying mouthwash, as new foliage can be damaged.
-- Vinegar -- The acetic acid of vinegar can control powdery mildew. A mixture of 2 to 3 tablespoons of common apple cider vinegar, mixed with 1 gallon of water, does the job -- but concentrations of acetic acid above 5 percent are more effective. However, use with caution. It is commonly used as a natural herbicide and too much vinegar can burn plants.
-- Milk -- It is believed that naturally occurring compounds in milk work to combat the disease while also boosting the plant's immune system. One experiment showed good results by applying a weekly dose of one part milk to nine parts water. Concentrations above three parts water had adverse side effects.
The best strategies for preventing or minimizing the impact of powdery mildew are listed above. Once detected on your plants, removal of infected leaves will help to minimize its spread.