Preventing powdery mildew on squash leaves

Credit: flickr /evensongnw

Q: Is there anything I can do to stop the mildew on my squash leaves?

I’m quite sure your fungus is powdery mildew, which I’ve been struggling with myself. Squash is particularly susceptible to it and unfortunately it is difficult to control. I’ve taken to not growing plants that are really vulnerable to powdery mildew, such as bee balm and lupines, but I do continue to grow summer and winter squash because it’s a valuable food crop and still produces despite this fungus. My advice is to do what you can to control powdery mildew but don’t let it ruin the experience of your garden. It will come and go without doing much harm.

Here’s some advice about preventing powdery mildew on squash from Michelle Buchanan:

Powdery mildew is a very common problem with squash. Once the plants have it, the priority is to prevent it from affecting the fruit. Placing a board or other flat surface underneath the fruit to keep it off the soil usually does the trick. There isn’t much else you can do once they have it. The key, as you already suspect, is prevention.

Superflow, a natural surfactant, is very effective in preventing powdery mildew. Though it is not sold for this purpose, Cannor Nursery in Victoria has been using it on their roses for the past two years and it has proven to be very effective. It is sold by


Other things you can do:

  • Water at the base of the plant rather than overhead. This will make the plants less susceptible to mildew.
  • Practice a four year rotation in your garden beds for disease prevention of all sorts.
  • Burn or bury all infected plants away from the garden.
  • Give your squash plants a lot of room to promote air circulation.
  • Plant resistant varieties of squash whenever possible. Westcoast Seeds may be able to give you some suggestions as to names.