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Fri, March 22

Deal with powdery mildew before damage occurs

Powdery mildew is unattractive but is rarely fatal. It does stress plants, and severe or repetitive infections will weaken a plant’s health. (Ken Lain/Courtesy)

Powdery mildew is unattractive but is rarely fatal. It does stress plants, and severe or repetitive infections will weaken a plant’s health. (Ken Lain/Courtesy)

Powdery mildew is one of the most prevalent and easily recognized plant diseases of our rainy season, from July to the end of October. Virtually no plant is immune, but some are more susceptible than others. Lilacs, crab apples, phlox, monardas, roses, grapes, squashes and cucumbers are likely targets for this powdery pest.

Recognizing powdery mildew: Take a close look at this photo of a squash leaf in my garden. Powdery mildew looks like splotches of white or gray powder on the leaves and stems of plants. There are several types of powdery mildew fungi, but they all look the same. You may not notice a problem until the top surfaces of the leaves turn powdery, but powdery mildew can also affect under- leaf surfaces, stems, flowers, buds, and even the fruits of plants.

Although powdery mildew is unattractive, it rarely is fatal. It does stress the plant, and severe or repetitive infections will weaken a plant’s health. If enough of the leaf surface becomes covered with powdery mildew, photosynthesis is impaired. Infected leaves often drop prematurely. This can be a particular problem on edible crops since insufficient photosynthesis can diminish the flavor of the fruit or vegetable. If buds become infected, they may not mature or open at all.

Mildew is host specific: owdery mildew fungi are host specific, meaning that different powdery mildew fungi infect different plants. The powdery mildew on your lilacs will not spread to your grapes or your roses ...

Cause and effect: Powdery mildew fungi seem to be everywhere. They overwinter in plant debris, and begin producing spores in the spring. These spores are carried to our plants by wind, birds, insects, and splashing water. All powdery mildews thrive in the same conditions. The growth and spread of powdery mildew is encouraged by dampness, high humidity, poor air circulation, and crowded plants in the garden. In the mountains of Arizona all of these conditions appear at the start of our monsoon season!

Controls: First and foremost choose healthy plants. Healthy plants are less likely to become diseased and are able to fend better for themselves if an attack happens. Good to know that there are plants being bred to be more resistive to mildew. Whatever you do, don’t plant non-resistant varieties in shaded areas with poor air circulation!

Infected plants: Just a little attention and care will fend off pesky powdery mildew. Remove and destroy all infected plant parts, especially yellow leaves and those that are mostly covered by the white spores. Because powdery mildew favors young, succulent growth, don’t fertilize until the problem is corrected. Try to water infected plants at ground level to keep leaves as dry as possible.

Fungicides simplified: Spray the entire plant from top to bottom with a good fungicide. For continuous protection most fungicides will need repeat applications every 7 - 14 days. Always follow the label instructions for both implementation and the waiting period before harvest.

Organic controls: Early prevention is best done with Triple Action organic spray by ferti-lome. Spraying an infested plant with this neem-based liquid not only kills that specific spore and prevents its spread, it also eliminates small insects like aphids, thrips, and mealybugs.

Edibles coated in white mildew will need something stronger. Clean up yellowed, infected leaves of fruit and vegetable plants as best you can, then spray with 100 percent organic copper fungicide. It may be applied moments before harvest.

Seriously infected plants need Bonide’s Infuse, the most powerful fungicide you can buy over the counter. Use it on plants like roses, lilacs, vincas, and privets for quick relief. Be aware that it is not rated for garden edibles.

Spray at least every other week until new leaves emerge clean and disease free. At this stage feed your plants and they should take off with new growth.

If you just aren’t sure about treating powdery mildew, consult one of the plant experts here at Watters Garden Center.

All products mentioned in this article are available here at the garden center with a knowledgable staff on hand to guide you in their use.

Please: Powdery mildew spreads very easily to other plants. If you are bringing a leaf sample with you to Watters, please place it in a ziplock bag or a jar to contain the spores and prevent spreading here at the garden center. Thank you.

See samples of Powdery Mildew from local gardens on Watters Pinterest Board, Follow our board for local updates.


Gardening for Newcomers: 9:30-11 a.m. Sept. 1

New to northern Arizona or just new to gardening? This class is Gardening 101 for everyone hoping to cultivate a green thumb in our area. Learn all the mountain secrets to local gardening success from soil preparation and planting to watering and fertilizing. This is an information-packed class guaranteed to increase blooms and fruit this year and reduce frustrating gardening mistakes!

Secret Gardens with Hedges & Privacy Screens — 9:30–11 a.m. Sept. 8

Beautiful mountain views on one side, neighbor’s RV on the other? This class shows off the best, fastest growing plants to fill in for privacy screens! You can screen neighbors’ unsightly yards, enhance your view, block pesky traffic sounds, cut other noise and light pollution. Watters’ experts will be on hand to help individuals with unique situations.

If you can’t attend this class, watch the Livestream on Facebook. Like our page to be notified when we go Live.

Until next week, I’ll be helping local gardeners with plant problems here at Watters Garden Center.

Ken Lain can be found throughout the week at Watters Garden Center, 1815 W. Iron Springs Rd in Prescott, or contacted through his web site at


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