When it comes to diseases that can affect your plants, powdery mildew tops the list as one of the most common culprits. While almost no type of plant is immune—unless they're hybrids bred specifically for mildew resistance—certain species are more susceptible than others. Plants that are notably susceptible include lilacs, flowering crab apple trees, tall garden phlox, bee balm, roses, squash, cucumbers, and zinnias.
Powdery mildew fungi can be found anywhere, but conditions favoring it include high relative humidity at night, low relative humidity during the day, and temperatures of 70-80 degrees. These conditions prevail in spring and fall.
The spores are carried by air currents and germinate on the leaf surfaces when there are extended periods of warm temperatures paired with dry conditions. The fungi spores reside in plant buds. They can also overwinter in plant debris and become transported to your plants via wind, insects, and splashing water.
What Is Powdery Mildew?
Powdery mildew is a fungal disease in plants that is commonly seen in warm, dry climates. Several different species of fungi can cause powdery mildew.
Recognizing Powdery Mildew
As the name implies, powdery mildew presents itself as dusty splotches of white or gray powder on the leaves and stems of infected plants. Splotches on the surface of leaves are often the most obvious sign of powdery mildew, but the disease typically starts on the undersides of leaves, often also appearing on the stems, flower buds, and even fruit.
Although powdery mildew can impact a variety of different kinds of plants, each fungal infection is "host-specific," meaning the breed of fungi infecting that plant is specific to that varietal. Translation: the powdery mildew on your lilacs will not spread to different varieties of plants in your garden.
Symptoms of Powdery Mildew
The good news: although powdery mildew is an unattractive nuisance, it's rarely fatal to your plants. That being said, it does stress the plant, and severe or repetitive infections can weaken the plant, making it more prone to other diseases and insect damage.
Additionally, powdery mildew can leech important nutrients from the plant, causing its leaves to wither and yellow. If enough of the leaf surface becomes covered with powdery mildew, photosynthesis is impaired, and the infected leaves will drop from the plant prematurely. This can be an even bigger issue for edible plants, like fruits or vegetables because insufficient photosynthesis can diminish the amount of sugars produced, ultimately affecting flavor.
Treating Powdery Mildew
While removing and destroying all infected plants is the ideal solution, it's not very practical. Understandably, few gardeners are willing to sacrifice their peonies or squash for every powdery mildew outbreak. Luckily, there are less drastic measures you can take to rid your garden of these pesky fungi.
Remove Infected Portions of Plants
To start, use plant clippers to remove or cut back the portions of your plants that have visible powdery mildew on them. If you notice it on a few leaves, remove them from the plant and do not compost them (which can allow the spores to spread). Wash your hands and clean your clippers with alcohol wipes to further prevent spread of the disease.
Apply a Fungicide
Many fungicides are available—look for one containing potassium bicarbonate, neem oil, sulfur, or copper. You can also create home remedies—one made from baking soda (see below) and one made from milk—that can help to prevent powdery mildew before it starts.
For continuous protection, reapply fungicides every seven to 14 days and be sure to follow the label instructions for both application and the waiting period before harvesting fruits and vegetables. While fungicides won't cure powdery mildew, it can help stem the spread of the fungi 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 the air circulation in your garden by thinning and pruning until each plant appears to have "room to breathe." This practice can help prevent the spread and growth of any fungi already present among your plants.
- Do not fertilize affected plants until you think the powdery mildew outbreak is under control—the spores favor young, succulent growth, so fertilizing a plant while an infection is still present can actually increase the spread.
- Avoid watering plants from above (with the exception of rain, of course) because wet leaves might encourage more mildew growth. Some powdery mildew are inhibited by moisture on leaves while others are favored by wetness on leaf surfaces.
Controlling Powdery Mildew With Baking Soda
Baking soda alone isn't effective in controlling powdery mildew, but when combined with liquid, non-detergent soap and water, it works well as a preventative. It is less effective as a cure once the fungus has taken hold. If you know a plant is affected by powdery mildew year after year, as is the case with many monarda, phlox, and lilacs, spraying early in the season, as well as weekly applications (and reapplying after rain), might prevent mildew that year. At the first signs of infection on a plant, remove the leaves that are infected with powdery mildew, if there aren't too many, and spray the rest of the plant. Spray any susceptible plants located nearby, too.
To control powdery mildew on plants, mix together:
- 1 tablespoon of baking soda
- 1/2 teaspoon of liquid, non-detergent soap
- 1 gallon of water
Pour the mix into a sprayer, and evenly coat all areas of the plant, including the underside of leaves and stems. The soap helps the mix spread and cling to the leaf surface. Discard any unused mixture; it loses effectiveness over time.
While this recipe has been known to be effective, it can burn the leaves of some plants. It is recommended that you water your infected plants well a couple of days before applying this mixture, and do not apply it in full sun. Spray it on a small area first to test the plant’s response before spraying the entire plant. Do not treat plants if they are already under stress from drought or excessive humidity or when temperatures exceed 90 degrees.
Preventing Powdery Mildew
The best defense against powdery mildew is to follow these best practices to maintain an environment that does not encourage mildew growth:
- Choose healthy plants and work to keep them that way. Stressed plants (from drought, overwatering, or other poor growing conditions) are an invitation to disease
- Purchase powdery mildew-resistant cultivars—this is especially important if you garden in an area that is known to be susceptible to an annual attack of powdery mildew
- Avoid planting susceptible plant varieties in the shade where they might remain damp and offer the spores an ideal place to grow