Plant Rx: How to Treat + Prevent Powdery Mildew


Powdery mildew on a rose bush. Image by Paul Maguire courtesy iStock.
Powdery mildew on a rose bush. Image by Paul Maguire courtesy iStock.

Rising temperatures and high humidity are perfect conditions for one of the garden's most common and insidious foes: Powdery mildew, a fungal blight that wreaks havoc on all kinds of flowers and shrubs. Here's how to identify, treat, and prevent it.


What is powdery mildew?

Powdery mildew looks appears as spots early on in its infection. Image by User2547783c_812 courtesy iStock.
Powdery mildew appears as spots early on in its infection. Image by User2547783c_812 courtesy iStock.

Powdery mildew is a fungal disease that presents itself on the foliage of a wide range of plantings. It thrives in warm (60 to 80 degree) temperatures and spreads quickly in more humid regions, though it can also manifest in dry climates. It can be especially aggressive in shadier gardens where it's protected from direct sun exposure. Common plantings that are effected by powdery mildew are:

  • Peony

  • Monarda

  • Verbena

  • Sunflowers

  • Dahlias

  • Roses

  • Zinnias

  • Melons

  • Squash + zucchini

  • Pumpkins

  • Parsley

  • Tomatoes

  • Peppers

Note: Most of the time, powdery mildew isn't lethal. However, it can seriously hinder a plant's ability to grow and to yield any fruit, vegetables, or flowers it is supposed to produce.


How to treat it?

Fortunately, powdery mildew is relatively easy to treat with a variety of organic methods.

  • Milk + Water Solution: Mix 1 part milk to 2 parts water and spray liberally on affected areas.

  • Potassium Bicarbonate + Soap Solution: Mix 1 TBSP potassium bicarbonate with 1/2 TSP of liquid soap (Castile soap, such as Dr. Bronner's, is best) with 1 gallon of water. Spray liberally on affected areas.

  • Neem Oil: Derived from the seeds of the neem tree (Azadirachta indica), this oil can be bought as a pre-mixed solution at your local garden center or hardware store. Bonide offers a spray that is widely available and effective for treating a number of different garden issues.

  • Prune: In some cases, pruning off the affected parts of the plant is the best way to contain the mildew's spread. New foliage should quickly and cleanly grow back. This is a good option if the mildew hasn't spread throughout the whole plant as you want to avoid removing too much foliage during growing season.

Treating powdery mildew on zucchini plant with spray. Image by Fokusiert courtesy iStock.
Image by Fokusiert courtesy iStock.

Note: Never put mildewy clippings in your compost bin. The spores can lay dormant and reawaken when you use your compost out in the garden and will infect new plants.


How to prevent it?

Ample sunlight and good airflow are key to helping prevent powdery mildew, so plant sun-loving varieties in areas where they can get the maximum amount of sunlight and in configurations that give them plenty of breathing room. Reducing foliage density will also help. It's important to focus all watering on the base of the plantings, rather than letting it spray on the foliage and blooms. This prevents powdery mildew from forming and deters other fungal issues as well as leaf scorch. Watering in the early morning instead of the evening may help, too. Lastly, nurseries have begun offering new cultivars of especially sensitive plantings that are more resistant to powdery mildew and other diseases, so aim to use these options if your space is particularly prone to infections.