The Iowa Gardener: Prevention is key to avoid powdery mildew

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In late summer, many of our favorite shrubs and flowers develop a funny, grayish-white powder-like coating on the top of their leaves. Lilacs, roses, bee balm, tall garden phlox, peonies, asters and zinnia are especially susceptible to this fungal disease. It’s common in regions like Iowa, which has warm humid days and cool nights.

Mild cases simply make the plant look bad. But severe cases can weaken and even destroy the plant, especially perennials and annuals.

In late summer, there’s not much you can do to get rid of powdery mildew. It’s a disease that must be prevented rather than cured.

The time to prevent powdery mildew is in the spring. Take note now of which plants are affected. Then, in March and April, do repeat sprays of a fungicide that specifies control of powdery mildew.

If you prefer an organic (and less expensive) solution, here’s an easy organic spray recipe to prevent black spot, powdery mildew and other fungal diseases in your garden.

Unlike some home remedy recipes, this one actually has university research behind it to demonstrate that it works. I feel it’s not as effective as commercial chemical fungicides, but it definitely reduces fungal diseases.

Here’s how to make it and use it:

l Mix together 2 tablespoons baking soda, 1 tablespoon liquid soap, and 2 quarts water. Use a large spray bottle if you have just a few plants; invest in a small landscape sprayer if you have dozens of larger plants to do.

l Spray plants in the early spring, just a few weeks after plants start active growth. Whenever the lilacs start to leaf out significantly is a good time to start.

l Repeat the spray at least three times, about 10 days apart. This should be accomplished before daytime highs regularly hit 80 degrees.

You can minimize the effects of the disease now by tearing out any badly diseased annuals now. Cut back perennials, if they’re done blooming or if they are severely damaged, to a few inches high. Don’t put the trimmings in the compost heap or any other place where the disease spores will be able to spread in your garden.

Also consider how much light an affected plant is getting. Sometimes moving it to a spot with less shade will remedy the problem next year.

And in the future, avoid watering overhead in the afternoon. Instead, water in the morning or on the ground only so leaves don’t stay wet for long periods of time, encouraging fungal disease.

Veronica Lorson Fowler is co-publisher of The Iowa Gardener website at

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