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Powdery Mildew Control Is Easy
by John Harmon
November 17, 2002

Now that the outdoor gardening season is over for at least a while many folks have moved plants that have enjoyed a few months of the great outdoors back indoors for the winter. Unfortunately the conditions inside most homes during a northern winter are perfect to encourage the growth of powdery mildew.

It is generally believed that powdery mildew requires free water to get established and grow. This however is not true. Powdery mildew infection can get started even on dry leaves. Warm temperatures and low light levels in the winter can encourage the fungus to grow and spread.

The optimum temperature for powdery mildew infection is between 68 to 77 degrees F. with a relative humidity between 40 to 100%. Low diffuse light, (like florescent light indoors) also seems to favor powdery mildew development. Powdery mildew can spread rapidly since the disease cycle can be completed in as little as 72 hours if the conditions are optimal. Most of the time it takes seven to ten days from the time of infection to the development of symptoms.

Powdery mildew is a common disease on many types of plants including houseplants. In fact it's one of the oldest plant diseases mentioned in written records. A fellow by the name of Theophrastis wrote about powdery mildew destroying his roses in 300 BC! There are a number of different strains of fungi that cause powdery mildew but they are all characterized by a powdery white to gray fungal growth on leaves and stems. It can also attack flower heads and buds. If you have a fine white to gray powder on any part of your plant it is probably some kind of powdery mildew.

Whatever strain you have the treatment is the same. The best treatment is direct sunlight but that's not an option for most of us in the north over the winter. The spores and mycelium are also sensitive to extreme heat but so are the host plants so forget about using the tiger torch.

Good cultural practices will prevent powdery mildew but all those "good cultural practices" pretty much go out the window when you have to move plants indoors for the winter. Some of those good cultural practices are just not possible like "plant in full sunlight in a well-drained area" or "do not crowd plants". I wish I had the space indoors to do that. Good air flow and ventilation will help so use a small fan to keep air moving around your plants and give them as much room as you can.

Another factor that contributes to powdery mildew infection is high rates of nitrogen. New growth from fertilizing is tender and susceptible to infection. Lower the rate at which you fertilize in the winter. The lower light levels in winter mean less growth and less need for fertilizer. Another easy step is to water in the morning so the plants have all day, what little of it there is, to dry off. Be careful about misting plants. Damp cool leaves in the dark are prime targets for fungus.

If you do get an infection going remember that quick action is necessary to keep it from spreading. You can prune off affected leaves or get rid of the entire plant if it's replaceable. If you get Powdery mildew on large plants you don't want to prune or destroy you can mix up a simple spray to kill the fungus off. Add one teaspoon of baking soda to a quart of water. Shake it up to get it mixed well and then add a few drops of liquid dish soap. Don't use the "anti-bacterial dish soaps". If you have Safers Soap you can use that instead of dish soap. Besides killing the fungus it will also discourage many kinds of bugs. Baking soda increases the surface pH of the leaf making it harder for powdery mildew spores to get started or keep growing. Spray the infected leaves with the solution once a week for three weeks.

Sulfur is also effective against powdery mildew. You can make a solution by crushing up a few cloves of garlic, which is naturally high in sulfur, and mixing it in a quart of water. Strain the solids out and use a spray bottle to apply to the infected parts and don't forget the underside of the leaves.

As with any treatment test it first on just one leaf. Some tropical plants are sensitive and even homemade natural sprays can cause damage. If the plant shows signs of damage dilute the spray with more water and try again. You can also buy commercial sprays for treating powdery mildew but why buy one if the homemade spray works?

With indoor conditions in the winter it's easy to get a powdery mildew infection going but with a little care and a natural spray it's just as easy to get rid of.

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