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Powdery Mildew

Controlling and Preventing Powdery Mildew on Plants

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Powdery Mildew on Phlox

Powdery Mildew on Phlox

Marie Iannotti

Powdery mildew is one of the most common and easily recognized plant diseases. Almost no type of plant is immune, however some are more susceptible than others. Lilacs, crab apples, phlox, monarda, roses, grapes, squash and cucumbers are all likely targets for powdery mildew.


Recognizing Powdery Mildew

As the name implies, powdery mildew looks like powdery splotches of white or gray, on the leaves and stems of plants. There are actually several types of powdery mildew fungi, but they all look basically the same. You may not notice a problem until the top surfaces of the leaves turn powdery, but powdery mildew can also affect the lower leaf surface, stems, flowers, buds and even the fruit.

Although powdery mildew is unattractive, it is rarely fatal. However it does stress the plant and severe or repetitive infections will weaken the plant. If enough of the leaf surface becomes covered with powdery mildew, photosynthesis is impaired. Infected leaves often fall 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 open and mature at all.

Powdery mildew fungi are host specific, meaning the different powdery mildew fungi infect different plants. The powdery mildew on your lilacs will not spread to your grapes or your roses. However all powdery mildews favor the same conditions.


What Causes Powdery Mildew?

Powdery mildew fungi seem to be everywhere. They overwinter in plant debris begin producing spores in the spring. These spores are carried to your plants by wind, insects and splashing water. Conditions that encourage the growth and spread of powdery mildew include:
  • Dampness or high humidity (Not common during rainy seasons or in extreme heat)
  • Crowded plantings
  • Poor air circulation


Controlling Powdery Mildew

  • Choose healthy plants and keep they growing healthy
  • Try and find a powdery mildew resistant cultivar, if your area is susceptible
  • Don't plant non-resistant varieties in the shade

Once Your Plants are Infected:

  • Remove and destroy all infected plant parts
  • Improve air circulation by thinning and pruning
  • Don't fertilized until the problem is corrected. Powdery mildew favors young, succulent growth
  • Don't water plants from above

  • Apply a fungicide: There are many fungicides available. Check the label to be sure they are safe and effective on the type of plant that is infected. Look for ingredients such as: potassium bicarbonate, neem oil, sulfur or copper. There are also chemical fungicides, such as triforine, that can be used on ornamental plants. There is also a home remedy made from baking soda that is effective.

    Most fungicides will need repeat applications every 7 - 14 days, for continuous protection. Always follow the label instructions for both application and waiting period before harvest

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