Keeping ahead of powdery mildew

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If you've ever lost a nice zucchini plant or a crop of cucumbers or melons to this white fungus, then you know the heartbreak of powdery mildew.

It is a plant disease that looks like its name. It starts as small white circles that look like talcum powder, circles that will spread and eventually cover your plant, reducing the amount of photosynthesis and fruit production if left untreated.

Powdery mildew is actually not one single fungus, but a family of closely related fungal species that affect a range of trees, flowers and vegetables, including apple, rose, ash, birch, grapes, zinnia, lilac, beans and tomatoes, but are particularly fond of cucurbits. Fortunately, the different family members are fairly host specific, so that the powdery mildew on your rose won't spread to your cukes.

This disease is persistent. It loves to grow in humid conditions, but the spores also will spread in dry conditions via breeze or insects.

The best prevention, as usual, starts with good cultural practices. If you have mildew-resistant strains available, plant those. Make sure that you plant in a sunny location, water in the morning so the plants have time to dry, and space the plants far enough apart so there is good air circulation. (I am often guilty of overplanting myself, but leaves laying on top of leaves is the perfect environment for fungal growth.)

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Inspect your plants regularly. If you see any small white circles start to form, remove the affected leaves immediately. You can then treat the rest of the plant with one of a number of organic (OMRI-listed) fungicides, including sulfur, copper and others, as well as horticultural oil.

Milk solutions have been studied to some degree and are especially intriguing. It is thought that these work as a powdery mildew deterrent by changing the basic pH of the leaf surface to something inhospitable to the fungus--like vinegar but in the opposite direction--although other studies suggest that other special properties of milk might be responsible for its effect.

However, keep in mind that using a milk solution would actually be an illegal use of the product, since it has not been reviewed and approved by the Environmental Protection Agency. Instead, it is always best to use the fungicides that have been reviewed and tested for efficacy. These formulated fungicides also may contain additives (spreaders and stickers) that help the efficacy of the spray.

Whichever spray you use, the key is to start before you see the circles forming and to use it regularly. It works best as a preventative, not a cure.

Spray the plants thoroughly (undersides too) once a week, but not before it rains, and not in midday sun. If the disease does appear, clip off affected leaves and keep spraying regularly. Your plants may still succumb eventually, but you will give them weeks of extra life and productivity.

Gordon Clark is a master gardener with the University of Vermont.


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