Gardener Master: “Disease Triangle” key to attack Powdery Mildew | Home & Garden

Tom Ingram Ask a master gardener

‘TThe leaves on my tall garden are covered with something white and they look bad. What can I do about it?” —NK

Sounds like your phlox has contracted a disease called powdery mildew. I fight this on my garden phlox every year. She seems to like my peonies a lot too. In reality, powdery mildew isn’t too picky. Many plants are susceptible to powdery mildew including azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, english ivy, photinia, blueberry, Pecan, Cucumber and Squash.

The severity of a powdery mildew infestation depends on the overall health of the plant and weather conditions. We had a lot of rain and relatively mild temperatures in the spring, so we should be prepared for powdery mildew.

Powdery mildew is a fungus that gets its nutrients from small, root-like appendages called haustoria. It is the haustoria that invade the leaves and give it access to the nutrients contained in the leaves. Left untreated, powdery mildew will cause your plant’s leaves to turn brown, die, and fall off.

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Although there are a variety of powdery mildew species, their life cycle is the same. Small, black, spore-bearing structures, called cleistothae, overwinter and become active in spring when temperatures rise above 60 degrees. These now-active structures begin to produce spores that travel through the air and eventually land on a suitable host. Splashing rain can also contribute to the spread of these spores.

High humidity can contribute to its development, but plants that are crowded or growing in clusters with poor air circulation are also good hosts for powdery mildew. Once it finds a home, it spreads quickly. Moist, shady areas are also good incubators for powdery mildew.

For this reason, giving your plants room to breathe through air circulation is a good first line of defense against powdery mildew. If you’ve had a powdery mildew problem in previous years, removing last year’s leaf litter will also help reduce its ability to spread.

The way you water your plants can also make a difference. Since we know that moisture contributes to the spread of powdery mildew, it’s best to water your plants in the morning rather than at night. Plants that are watered in the evening often stay wet all night, providing a perfect breeding ground for the spread of powdery mildew. The leaves of your plants do not need water either. It’s the roots that need the water. So try to water only the roots. This will help minimize the conditions needed for powdery mildew to thrive.

This might be a good time to rethink what we call the “disease triangle.” The disease triangle is a way to remember what it takes for plant diseases to thrive. Three things are needed for plant disease to occur: a suitable host, an active pathogen, and the right environment. If you remove even one side of the triangle, you’re well on your way to minimizing plant disease in your garden. In this case, if you can minimize environmental conditions like crowded plants or wet, humid conditions, you’re removing a necessary element for powdery mildew to thrive.

One of the disadvantages of powdery mildew is that there is no cure for the infected leaves once your plant is symptomatic. At this point, your main strategy becomes preventing the spread of the disease. Removing the infected leaves is a good start. When you remove the leaves, you remove one of the necessary elements of the disease triangle – the pathogen. Unfortunately, if you don’t catch it early, removing all leaves is not an effective strategy for long-term plant health.

Once you’ve removed the infected leaves, you need to start a fungicide treatment program that includes multiple applications of the fungicide on a manufacturer-recommended schedule. It’s also a good idea to alternate between a few different fungicides to prevent the pathogen from developing resistance to the fungicide. As summer gets warmer, the heat also helps to minimize the fungus’ spore production.

If powdery mildew is a recurring problem in your garden every year, you should start your fungicide treatment program as soon as your plant leaves in spring.

If the growing season is nearing its end in the fall and then powdery mildew sets in, you may not need to treat at all as the plant is likely at the end of its seasonal life cycle. If this is the case, you should eliminate the infected plant debris in the fall to minimize the overwintering spore population. See you in the garden!

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You can get answers to all of your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, stopping by our Diagnostic Center at 4116 E. 15th St., or emailing us at .


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