Gardening in Alaska : A Cure For Powdery Mildew?

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Gardening in Alaska presents big challenges, whether it's the extra effort in finding plants tough enough to survive our Zone 2-4 climate, communicating with like-minded Alaska gardeners, or keeping up with the latest trends, issues and solutions. We'll try to help with that. We'll also tour gardens from Homer to Anchorage to Wasilla to Willow whenever we get the chance, and post the best garden photos around. Presenting a forum about cold-weather gardening and for cold-weather gardeners is what we are all about. We hope you'll join us on the Talk Dirt garden blog.

Photographer and gardener Fran Durner (fdurner@adn.com) writes the blog.

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A Cure For Powdery Mildew?

Posted by talkdirt

Posted: July 19, 2007 - 9:01 am

Fran Durner/ADN Powdery mildew on Siberian pea shrubs is caused by a fungus.
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I'm wondering what causes the powdery mildew stuff all over the Siberian Pea Shrubs? What can I do to get rid of it? Thanks for your help, Chris

According to IPM of Alaska, powdery mildew is a fungus that forms a white, powdery coating on the tops of leaves and sometimes on buds and flowers. Different powdery mildew fungus’ cause the disease on different plants. Some plants are more susceptible than others such as some varieties of rose, monarda, polemonium and caragana - the Siberian pea shrub.

Powdery mildew needs live plant tissue to grow. Once infected, it is unlikely you will ever rid your plant of the fungus though you can try to control it with good horticultural practices and fungicides, either commercial or organic. A handout from the Cooperative Extension Service recommends some of the things you can do to try and control the fungus. Trim off as much of the diseased tissue as you can and clean your pruner with a 1:4 solution of beach and water after each cut. Clean up and dispose of all weeds and debris under and around your shrubs and give the shrubs as much ventilation as possible. Avoid high nitrogen fertilizer, which encourages tender new growth.

Contrary to popular belief, powdery mildew does not need water to grow. Infection can occur in warm, dry weather. It is actually recommended for the Siberian pea shrubs that already have powdery mildew, to hose down all the infected leaves with plain water prior to a fungicidal treatment. This helps to dislodge as many of the spores as possibly to help you get better results. Hosing with water can also kill spores and inhibit germination. But do it in the morning so your shrub can dry by nightfall. IPM of Alaska says sodium bicarbonate, commonly known as baking soda, has been found to possess fungicidal properties and a baking soda spray has been used successfully on Siberian pea shrubs as a general non-chemical control. Use the baking soda spray as prevention or as treatment at first signs of the disease.

Baking soda is non-toxic, effective, readily available and inexpensive. It must be applied weekly to control powdery mildew and an emulsifier must be applied with it to keep it on the plant and evenly distributed. IPM provides this recipe: Mix 1 tablespoon baking soda, 2 1/2 tablespoons vegetable oil with one gallon of water. Shake this up very thoroughly. To this mix add 1/2 teaspoon of pure castille soap and spray. Be sure to agitate your sprayer while you work to keep the ingredients from separating. Cover upper and lower leaf surfaces and spray some on the soil. Repeat every 5-7 days as needed.

The Cooperative Extension Service provides this recipe, which is a little different: Use 1 teaspoon of baking soda in a quart of water. Add a few drops of liquid soap or detergent to the solution. Be careful, as detergent can cause plant damage if too much is used. Always test the solution on a small portion of the plant a day in advance to check for any negative reactions.

And Camille Williams of the Anchorage Garden Club swears by a mixture of 1 part milk (any kind) to 3 parts water. “I’ve been using it for years, even on my blackberries. It works on anything,” she said.

As always, prevention is the best method of combating any plant disease. If you are interested in growing any plant that is susceptible to disease, pick the most disease-resistant variety available. Plant in a sunny spot with good drainage and give your plants some room to grow.
--Fran

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A Cure For Powdery Mildew?