Solutions for Powdery Mildew

By Kerry Ann Mendez

Powdery Mildew is a common problem in the summer garden. It can infect vegetables, roses, perennials, shrubs and trees. Although there are many varieties of this mildew, it usually presents itself as white or powdery gray spots on leaves. However, it can also grow on stems, flowers and even fruit.

Although it may look bad, it rarely kills a plant. The mildew typically first appears on a plant's lower leaves and then quickly spreads. If the leaves are heavily covered by this fungus, then photosynthesis can be negatively impacted, resulting in leaves turning yellow and eventually dropping off.

Why Does This Happen?

Before I suggest how to tackle the problem, it's important to first understand why it occurs. Powdery Mildew is caused by spores located in the soil or garden debris that are then carried by the wind or splashing water to leaves. High humidity coupled with dry conditions often trigger the growth of mildew spores, but there are other conditions that encourage outbreaks (see below).

How to Prevent Powdery Mildew
Here are a few ways to prevent or reduce powdery mildew:

  • BEFORE the problem begins, apply an organic fungicide to plants that are commonly infected. Estabrook's carries a number of great products including Fung-onil and Neem Oil from Bonide among others.
  • Purchase disease resistant plants. Thankfully, there has been extensive breeding resulting in plants that are less prone to this fungus. Some perennials that are particularly susceptible to Powdery Mildew are Beebalm (Monarda), Phlox, Coreopsis, and Black-Eyed Susan (Rudbeckia fulgida). If your garden is prone to mildew, avoid these plants.
  • When watering the garden, water at ground level (i.e. with soaker hoses) versus using overhead sprinklers. Watering in the morning instead of at night is also helpful.
  • Allow plenty of air circulation around your plants. You can also thin out some of the interior to allow more light and airflow.
  • Give plants sufficient sunlight - at least six hours a day is recommended.
  • Don't over-fertilize your plants, which will stimulate lots of new growth that can be more susceptible. Instead, use a slow-release, organic fertilizer like Espoma's Plant-tone or Holly-tone.

If you have additional questions, please stop in and see us at Estabrook's.

About the Author

KERRY ANN MENDEZ is an award-winning garden educator, author and design consultant based in southern Maine. Her latest book is The Budget-Wise Gardener. You'll now find her at Estabrook's consulting on garden design, answering your gardening questions and much more. Plus, check out her Garden Webinar series for more tips and tricks.

Kerry Ann Mendez