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.
What Is Powdery Mildew?
Powdery mildew is a fungal disease in plants that is prevalent in warm, dry weather. Several different species of fungi can cause it.
Recognizing Powdery Mildew
As the name implies, powdery mildew looks like powdery splotches of white or gray that appear on the leaves and stems of plants. You may not notice a problem until the top surfaces of the leaves turn powdery, but powdery mildew often starts on the undersides of leaves and can also take hold on the stems, flowers, buds, and even the fruit.
Although powdery mildew affects a great number of plants, the different powdery mildew fungi are host specific, meaning each of the powdery mildew fungi infects only specific plants. Thus, the powdery mildew on your lilacs will not spread to your grapes or your roses. However, all powdery mildews favor the same weather conditions.
Symptoms of Powdery Mildew
Although powdery mildew is unattractive, it is rarely fatal. However, it does stress the plant and severe or repetitive infections will weaken the plant and stressed plants are more prone to other diseases and insect damage. Powdery mildew can leech nutrients from the plant, causing leaves to wither and yellow.
Additionally, if enough of the leaf surface becomes covered with powdery mildew, photosynthesis is impaired and infected leaves will often fall prematurely. This can be a particular problem on edible plants because insufficient photosynthesis can diminish the number of sugars produced and affect the flavor of the fruit or vegetable. If buds become infected before the flowers are even allowed to open, the plants may not bloom or set fruit at all.
Causes of Powdery Mildew
Powdery mildew fungi seem to be everywhere. The spores overwinter in plant debris and begin producing more spores in the spring. These spores are carried to your plants via wind, insects, and splashing water. Conditions that encourage the growth and spread of powdery mildew include:
- Periods of warm temperatures and dry conditions: Powdery mildew is less common during prolonged rainy seasons and in extreme heat.
- Crowded plantings where the air circulation is poor and the plants remain wet: The spores are also able to spread faster when they appear in clumps of their host plant.
What You'll Need
- Plant clippers
- Fungicide (look for ingredients such as potassium bicarbonate, neem oil, sulfur, or copper)
- Alcohol wipes (to remove powdery mildew from hands and clippers)
While removing and destroying all infected plants is the ideal solution, it is not very practical. Few gardeners are willing to sacrifice their peonies or squash every time there is a powdery mildew outbreak. Luckily there are some less drastic measures you can take.
Remove Infected Portions of Plants
Remove or cut back the portions of your plants that have powdery mildew on them. If you see it on a few leaves, remove those leaves and do not compost them. Carefully clean your hands and clippers after finishing the job.
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. A home remedy made from baking soda is also an effective preventative. One made from milk is thought to prevent powdery mildew. For continuous protection, most fungicides will need to repeat applications every 7 to 14 days. Always follow the label instructions for both application and waiting period before harvest. While the fungicide won't cure powdery mildew on leaves, it can help stem the spread to other leaves or plants.
Manage Your Garden
Now that you know your plants are susceptible to powdery mildew, you'll need to take a few steps to prevent its spread or reoccurrence.
- Improve air circulation by thinning and pruning.
- Don't fertilize the affected plants until the problem is corrected. Powdery mildew favors young, succulent growth.
- Try not to water plants from above. This will spread the spores. Of course, there is not much you can do about this if it rains.
Preventing Powdery Mildew
The best defense against powdery mildew is preventing the ideal conditions it requires to take hold. That's not always possible, though, and some seasons will be better than others. To gain an advantage:
- Choose healthy plants and keep them growing healthy. Stressed plants are an invitation to disease. Don't let your plants become stressed from drought or other poor growing conditions.
- Try to find a powdery mildew-resistant cultivar. This is especially important if you garden in an area that is known to be susceptible to an annual attack of powdery mildew.
- Don't plant non-resistant varieties in the shade where they might remain damp and offer the spores an ideal place to grow.
Powdery Mildews. Colorado State University Extension
Powdery Mildew on Ornamentals Management Guidelines. University of California Integrated Pest Management