Powdery mildew attacks in the fall

By Marianne C. Ophardt, Special to the Herald

I can't believe all the powdery mildew in my garden this fall.

It's all over summer squash, zinnias, coreopsis, purple coneflower and my Wave petunias.

It's easy to identify powdery mildew with the characteristic white powdery patches of mycelium on the top surface of leaves and stems.

While often referred to in the singular, powdery mildew is not just one fungus disease, but it's actually several different closely related fungi. A specific powdery mildew fungus will only attack a specific species or specific plant family.

Area gardeners are lucky when it comes to plant-attacking fungal diseases. Most fungal diseases require the presence of free water on plant tissues for an infection to occur. Our arid climate on this side of the Cascades makes it easier to garden because our plants are troubled by few fungal diseases.

Powdery mildew is an exception. It doesn't need free water for infection, just high humidity. For infection to occur, mildew spores must be present on a susceptible plant along with the right conditions for infection to occur. High humidity, poor air circulation, moderate (60 to 80 degrees) temperatures, and shade are all favorable conditions for powdery mildew infections.

I'm seeing so much powdery mildew now because the weather has cooled off and the humidity probably is higher. Also, my container garden plants are very crowded. In addition, I fertilized the plants in mid-August, stimulating newer, younger growth that's more susceptible to attack.

Plus, some of my planters only get sun for part of the day.

So how worried am I about all this powdery mildew? Not much. It's late in the season and the annual flowers and vegetables will be gone with the impending hard frost. Applications of fungicide would be wasted.

However, I am planning on taking steps to avoid it next year.

First, a thorough garden cleanup is planned. It's important to remove and dispose of all infected plant material from the garden. (Composting will not kill all the fungus, so these plants will go into the garbage.) If I leave the infected dead plants and debris in the garden, it would be a source of spores for reinfection next season.

To improve air circulation next year, I'll try to refrain from "over planting" my container gardens. That will be difficult since I love crowded containers overflowing with plants. I also plan to watch mildew prone plants, like zinnias, and may consider applications of fungicide if the disease appears early in the season.

It's important to note that many garden fungicides only work as protectants, protecting new plant growth from getting powdery mildew, not killing it on already infected parts of the plant. There are a few garden fungicides, such as plant oils, that work both as protectants and eradicants, killing the disease on plants with mild infections. They won't kill the disease on heavily infected plants, like the ones in my garden this fall. If powdery mildew is a recurring problem on woody or perennial plants in your garden, you should plan on applying protectant fungicides before the disease appears each year.

Go out in the garden today and check your plants. You might find evidence of a powdery mildew attack too.

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.

tool name

Saturday, Oct. 03, 2009

Powdery mildew attacks in the fall

By Marianne C. Ophardt, Special to the Herald