How to Get Rid of Powdery Mildew, a Common Disease That Affects Most Plants

When tending to your garden, you'll want to make sure that your plants are in good, healthy standing. One thing to monitor your plants for is powdery mildew, a fungal disease that can easily infect most plant varieties, from flowers and vegetables to trees. We spoke with an expert about what powdery mildew is, what causes it, and how you can treat and prevent it in your garden this season.

What Is Powdery Mildew?

Powdery mildew is a fungal plant disease, says Allen Tate, an ISA-certified arborist at Blooma Tree Experts. "It is a foliar fungal infection caused by many different bacterial species in many genera," he says. It's easily identifiable by its powdery, patchy white or gray splotches on plant leaves or stems. It can affect most plants and trees, but thankfully it doesn't tend to kill them—it just might make them look disfigured if not treated in time.

Powdery mildew needs a host plant to germinate and flourish, and most powdery mildew fungi tend to only infect one type of host. "Many are host-specific and do not jump from one plant species to another. However, most can move around quite freely," says Tate. This means that if your lilac plants have powdery mildew, it likely won't infect other flowers or bushes nearby—just your lilacs.

Powdery Mildew Symptoms

You can identify powdery mildew by the following symptoms:

• Patchy white or gray powdery growths
• Yellowed, twisted leaves
• Dropped leaves
• Distorted shoots and buds

Causes of Powdery Mildew

Powdery mildew often occurs during warmer times of the year and can be common in crowded plantings that don't have good air circulation between them. High relative humidity, low soil moisture, and warmer air temperatures can all play a role in powdery mildew's ability to germinate spores on its host plant. "Powdery mildew most often forms on foliage of susceptible plants in areas of the canopy dense in foliage, where humid and stagnant air can get trapped (and on foliage that is consistently wetted by irrigation)," says Tate. Most often, these plants are experiencing stress from drought, poor pruning and/or other disease issues, he says.

Powdery mildew spores can find their way to your garden by the wind, via insects, or even water, landing on host plants and infecting them when the conditions (temperature, relative humidity, and fairly dry soil and air) are right.

Powdery mildew also tends to favor young, immature plants—fully mature plants and leaves are less susceptible to a powdery mildew infection.

How to Get Rid of Powdery Mildew

If you find that your plants have the tell-tale fuzzy, powdery patches on their leaves or stems, there are some things you can do to get rid of the infection before it continues to spread.

First, make sure that the needs of the plant are well-addressed. "Preventing powdery mildew means meeting the needs of your plants regarding proper watering, nourishment, pruning, and other cultural practices such as mulching," says Tate. Also, make sure your plants are planted in the proper place—Tate suggests avoiding putting plants under large eaves, for instance, because it can be hard for the plants to get the necessary water. After you've made sure your plant's conditions are correct, you can move on to ridding the plant of the disease.

• Prune infected leaves: You'll want to remove disease-ridden leaves to halt the spread of the fungus to other parts of the plant. Use clean snippers, and don't compost infected leaves—this can cause the fungus to continue spreading.
• Clear infected debris: Once you've pruned and snipped infected leaves and stems, make sure to promptly clean up the debris—don't compost this either.
• Use a fungicide: Oftentimes, the only way to really get rid of powdery mildew is to use either a biological or chemical fungicide, says Tate. However, he warns that other factors need to be addressed first before applying fungicide. "These products are most effective when the stress factors have been addressed in the environment," he says. "If chemicals are applied without the stressors and needs of the plant being addressed, these chemicals can actually cause more damage." Neem oil, potassium bicarbonate, and sulfur are examples of fungicides that can work against powdery mildew. You'll want to apply fungicide every seven to 14 days to make sure it's doing its job.
• Limit nitrogen fertilizers late in the season: Extra nitrogen encourages new plant growth, which powdery mildew loves to attack.

How to Prevent Powdery Mildew

To prevent a powdery mildew infection, keep these tips in mind.

• Don't water your plants from above: Water your plants directly at the soil level so that leaves stay free of excess moisture that can encourage mildew growth.
• Prune plants to improve air circulation: You want to make sure that plenty of air can get around your plants. Overcrowded plants can increase the relative humidity around them, giving powdery mildew optimal conditions to thrive.
• Grow resistant plant varieties: You can choose to grow plants that are resistant to powdery mildew, such as Largeleaf phlox, Jeana garden phlox, or Kousa dogwood tree. This helps lessen the need for fungicides in your garden.
• Avoid the shade: If you're planting non-resistant plants, take care to grow them in spots where they will have the chance to dry out—planting some of these plants in the shade can mean longer drying times when wet and an increase in the likelihood of spore germination.