By Charlie Nardozzi

In summer, certain plant diseases really take off in the garden. One is powdery mildew. This fungal disease loves the cooler nights we can get in the middle to end of summer. It can quickly spread to infect a broad range of common garden plants, such as roses, impatiens, begonias, bee balm, phlox, lilacs, squash, and pumpkins.


It often starts as a whitish film on the leaf surface. As the disease progresses, it turns the leaves yellow and they die. If severe, plants can be defoliated, and flower and fruit production can be curtailed. While this disease is ubiquitous in the environment, its severity is somewhat dependent on the weather. During periods of 70 to 80 degree F temperatures with low daytime humidity and high nighttime humidity, the disease will flourish. Here are some steps you can take to slow down this disease:

  • Resistant Varieties: Plant breeders have worked hard to develop a number of powdery mildew resistant varieties to grow in our gardens. Look for varieties such as ‘Jacob Cline’ bee balm, ‘David’ phlox, and ‘Dunja’ zucchini.
  • Watering: While many other diseases thrive when leaves are wet, water on leaves actually thwarts powdery mildew disease. During periods of dry, cool weather with low-humidity, consider washing your leaves with water to prevent the disease spores from germinating.
  • Sprays: Try a variety of organic sprays to stop this disease. They all work best when sprayed early in the disease cycle. Serenade (Bacillus subtilis) is a bacteria that fights a variety of fungal diseases. Neem oil has also been known to thwart powdery mildew disease. You can make your own disease spray by mixing 1 tablespoon of baking soda, 2.5 tablespoons of horticultural oil, and 1 gallon of water. Another home mixture that can benefit squash and melon leaves is made with milk. Mix 1 part milk with 9 parts water, and spray the stems and tops of leaves with this solution. With all these sprays, reapply after a rain.