By Joe Lamp'l
I’VE learned three things over the years from your questions about powdery mildew -- it’s everywhere, you don’t like it and you want to know how to get rid of it. So, here’s what you need to know to prevent, control and even eliminate it.
What to look for
White or gray powdery spots appear, often covering most if not the entire leaf surface. It’s also found on plant stems, flowers and fruit. This fungus is host specific, meaning just because you find it on one plant species it does not threaten other plants in your landscape. Fortunately, the symptoms of powdery mildew are usually worse than the actual damage.
Mildew is rarely fatal to a plant but advanced stages can cause plant foliage to yellow, curl or turn brown and eventually cause the plant to defoliate prematurely. On flowering plants and trees, the fungus can lead to early bud drop or reduce the flower quality.
So Now What
Conditions that favor mildew formation include dry foliage, high humidity, low light and moderate temperatures. Active steps to avoid or minimize this risk include:
•Looking for disease resistant varieties.
• Providing adequate air circulation by not crowding plants.
• Putting plants where they will get sufficient light; at least six hours each day.
Avoid over fertilization. Rapid new growth is more susceptible. Instead, apply a slow-release fertilizer that provides 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 active ingredients used for control is chlorothalonil. Although effective, it coats the leaf surface with a white milky film that is quite noticeable.
Lesser known options:
• Neem oil is an effective organic disease control and a broad spectrum, natural insecticide that is kinder to beneficial insects and mammals.
• Baking soda is possibly 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, it works well if applied in the early stages or before an outbreak occurs. Mix one tablespoon of baking soda with a teaspoon of dormant oil and one teaspoon of insecticide or liquid soap (not detergent) to a gallon of water. Spray on plants every one to two weeks.
• Potassium bicarbonate is similar to baking soda. It is a contact fungicide that actually eliminates the disease. In addition, it’s approved for use in organic growing.
• Generic, ethanol-based mouthwash can be an effective control. Tests using one part mouthwash to three parts water worked well; just be careful when mixing and applying mouthwash as new foliage can be damaged.
Scripps Howard News Service
By Joe Lamp'l