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Powdery mildew signs: How to keep your plants healthy

Aug 2nd, 2013 | Category: Growing

by Chuck McClung

All along the West Coast from Mexico to Canada, powdery mildew is one of the most common summer plant diseases. There are many different types of powdery mildew, and each one infects a different plant.


Powdery mildew commonly looks as if baby powder has been sprinkled or smeared on leaves and sometimes stems. Oldest leaves are the first to show damage, and while it often does not kill the plant, it will affect plant growth and decrease the yield of certain vegetable crops.

Many plants rarely, if ever, get powdery mildew, while other plants are notoriously susceptible to it, such as shrubs like Lilacs, Roses, Spiraea, Nandina; annuals and perennials like Zinnias, Cosmos, Delphinium, Columbine, Dahlias, and Verbena. Many vegetable gardeners are very familiar with powdery mildew on crops like cucumbers, squash, and tomatoes.

One of the common misconceptions about powdery mildew relates to the difference between why a plant contracts the disease versus what spreads the disease.

Most often we hear that a plant “gets” powdery mildew from overhead watering or water on the foliage. While water on foliage can spread spores of powdery mildew, a plant that is lacking water is more likely to contract the disease. Spores of all types of powdery mildew will germinate and grow in the absence of water. One evidence of this is that powdery mildew is more prevalent during the drier summer months than in winter. The native Vine Maple (Acer circinatum) and Big Leaf Maple (A. macrophyllum) do not show signs of powdery mildew until after the rains stops and summer begins.

A simple demonstration further illustrates this point. Plant a cucumber or squash in one pot and plant another in a second pot, but place a saucer filled with water under the second pot. Once vigorous growth begins and the plants require more water, the first one without the water-filled saucer will be far more likely to get powdery mildew. A plant lacking water or that has wilted between waterings is far more likely to become infected with powdery mildew.

Preventing powdery mildew

First off, be sure your plants are well watered. During summer, vigorously growing squash and tomatoes may require daily waterings, especially when grown in containers or shallow raised beds.

Water the soil not the plant. Keep water off the foliage and avoid overhead waterings. Water in the morning instead of evening so that splashed water does remain on the plant overnight.

Provide good air movement for your plants. Many fungal diseases are promoted by damp, still air. Also give your plants as much sun as they will tolerate; again, damp shade can promote development of mildew spores.

Remove severely infected leaves, especially on vegetable crops and roses. Also be sure to remove plant debris on the ground under your plants.

Use organic, slow-release fertilizers. The beneficial microbes in Dr. Earth brand fertilizers help plant roots access water and nutrients in the soil creating a healthier, more disease resistant plant.

As a preventative use a Neem Oil-based fungicide. Be sure to read the entire label before application. You will be instructed to apply it in early morning or evening, i.e. not in direct sun.

If you think you have powdery mildew on your plant and would like more assistance, take a sample of your plant to your favorite independently owned nursery; they’ll have the knowledge to happily to assist you in finding the right remedy, so that you are successful in all your gardening endeavors. Keep your plants watered through the summer months!

Chuck has a Master’s Degree in Botany and helps others solve their gardening dilemmas. He may be reached at

Published in the August 2013 issue of Grow Northwest

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