Powdery mildew bark worse than bite

I’ve learned three things over the years from questions I’ve received from readers 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. Proactive 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 or more 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.

* Similar to mouthwash, the acetic acid of vinegar can control powdery mildew. A mixture of two to three tablespoons of common apple cider vinegar mixed with a gallon of water does the job. While too much vinegar can burn plants, concentrations of acetic acid above 5 percent are more effective.

* Direct contact with sulfur prevents disease spores from developing. When mixed with hydrated lime, the solution will penetrate leaves for even greater effectiveness. One version includes copper sulphate and hydrated lime, known as Bordeaux mix. These solutions can burn plant tissue and are damaging to soil microorganisms and harmful to beneficial insects. It is also considered moderately toxic to mammals and humans. Use sparingly and with caution.

* The latest player in the fight is milk. It is believed that naturally occurring compounds in the 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.

Even with many choices for control, prevention is still the best medicine, not only with powdery mildew, but with other diseases as well.

Joe Lamp’l, host of “Fresh From the Garden” on the DIY Network and GardenSMART on PBS, is a master gardener and author. Contact him at www.joegardener.com.