Master Gardener: the key to the “disease triangle” for attacking powdery mildew | Home & Garden


Tom Ingram Ask a Master Gardener

‘Tit leaves on my big garden the phloxes are covered in something white and they look bad. What can I do about this? —NK

It looks like your phlox has a disease called powdery mildew. I fight this one every year on my garden phlox. He also seems to like my peonies a lot. In reality, powdery mildew is not 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 have to be prepared for powdery mildew.

Powdery mildew is a fungus that obtains its nutrients via small root-like appendages called haustoria. It is the haustoria that penetrate the leaves, giving it access to the nutrients present in the leaves. Left untreated, powdery mildew will cause your plant’s leaves to turn brown, die, and drop.

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Although there are a variety of powdery mildews, their life cycle is the same. Small black spore-bearing structures called cleistothecia overwinter and begin to become active in the spring when temperatures exceed 60 degrees. These now active structures begin to produce spores which are moved through the air, eventually landing on a suitable host. Splashing rain can also help circulate these spores.

High humidity can be a contributing factor 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 tends to spread quickly. Moist, shady areas are also good incubators for powdery mildew.

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

How you water your plants can also make a difference. As we know that humidity contributes to the spread of powdery mildew, be sure to water your plants in the morning rather than in the evening. Plants watered in the evening often stay moist overnight, making them an ideal breeding ground for the spread of powdery mildew. In addition, the leaves of your plants do not need water. It is the roots that need water. So try to water only the roots. This will help minimize the conditions necessary for the development of powdery mildew.

Now might be a good time to revisit what we call the “disease triangle”. The disease triangle is a way to remember what is needed for plant diseases to thrive. For a plant disease to occur, you need three things: a suitable host, an active pathogen, and a suitable environment. If you remove even one side of the triangle, you are well on your way to minimizing plant disease in your garden. In this case, if you can minimize environmental conditions such as overcrowded plants or damp, damp conditions, you are removing an element necessary for the development of powdery mildew.

One of the downsides of powdery mildew is that once your plant is symptomatic, there is no cure for infected leaves. At this point, your primary strategy becomes one of preventing the spread of disease. Removing infected leaves is a good place to 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 the leaves isn’t an effective strategy for long-term plant health.

Once you have removed the infected leaves, you will need to begin a fungicide treatment program which will involve multiple applications of the fungicide on a schedule recommended by the manufacturer. It is also a good idea to alternate between two different fungicides to prevent the pathogen from developing resistance to the fungicide. Also, as the summer heats up, the heat helps to minimize the spore production of the fungus.

If powdery mildew is a recurring problem in your garden every year, you might consider starting your fungicide treatment program as soon as your plant sprouts leaves in the spring.

As the growing season draws to a close in the fall, if powdery mildew begins to appear, you may not need to treat at all, as the plant is likely at the end of its life cycle. seasonal. If so, be sure to clean up infected plant debris in the fall to minimize the population of overwintering spores. See you soon in the garden!

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You can get all your gardening questions answered by calling the Tulsa Master Gardeners Helpline at 918-746-3701, visiting our Diagnostic Center at 4116 E. 15th St., or sending us a email to


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