Thursday, August 27, 2009

powdery mildew arrives

Powdery mildew on zucchini.
It has happened. Powdery Mildew (PM) has come to the Edible Landscape. Thankfully we have many resistant varieties in the edible landscape, so plants such as petunias and zinnias (which are commonly susceptible) have gone unharmed. The trouble is primarily with the squash. The winter squash and melons have it the worst. Zucchini is next. I also noticed it on the Red Lake currant. The cucumbers look ok so far, as do the yellow summer squash.

Good cultural practices can help prevent an outbreak. These include:
  • planting resistant varieties (not too helpful if you like to grow heirlooms)

  • locating plants in a sunny area (minimizing shade)

  • maintaining good air circulation around the plants to keep humidity low

  • avoiding excess fertilizing (too much and you'll have so much foliage that air circulation will be reduced)

  • removing infected material to reduce spread (although once powdery mildew is out there, it seems impossible to stop it)

  • using drip irrigation, and avoiding excessive irrigation

Many extension bulletins go on to say that regular applications of fungicide may be necessary, starting at the first sign of infection and every 10 days thereafter. This is rarely necessary in the home garden, and should be avoided. PM generally doesn't arrive until late in the season when plants are declining anyway. It is also very season-dependent, so just because you had it this year, doesn't mean you will next year.

There are some organic home remedies and a couple of commercial products out there that I have not personally tried, but may offer prevention and in some cases control of a powdery mildew infection. As one reader found, a mixture of milk and water is often recommended on various gardening websites. One of the few commercial products for organic control of PM is Green Cure, a product developed by a plant pathologist at Cornell University. It is a potassium bicarbonate-based substance, and the manufacturers claim it can be used as a preventative and also will control existing infections. Be sure to do your homework before applying any type of treatment to your plants.


  1. Hey there....just found this organic control. 1 part milk (skim from the store) in 9 parts water as a foliar spray. I'm about to try it myself. good luck!

  2. What plants are you treating? Is the infection just beginning or is it already well-established? Be sure to report and let us know how it goes.

  3. Are the vegetables and fruits safe to eat if the plants have powdery mildew? No chemicals have been used yet.

  4. The vegetables and fruits from plants with powdery mildew are safe to eat, provided you have not sprayed them with any chemicals. If the fruits themselves are mildly infected with powdery mildew, you could thoroughly scrub and cut out any rotted areas.

  5. Is it safe to eat collard greens that are affected by a mild case of powdery mildew? I am growing them without any chemicals or fertilizers in my home garden. Thanks!

  6. I just wanted to put in my 2 cents here. I have both eaten kale and chard with PM and survived; I also have used Green Cure on ornamentals. It's fantastic!! Yes, it works on existing infections in about a day. Great stuff!