I have been seeing a lot of, and been receiving lots of calls about, a common disease of multiple garden plants including squash, cucumber, pumpkin and melon.  Perhaps you have noticed recent decline of some of these garden plants and seen a white powdery substance on leaves?  If so, you were probably looking at powdery mildew.

Q: So what is powdery mildew & what can you do about it?

Powdery Mildew

A:  Powdery mildew is a fungus that has developed multiple races. 

  • Powdery mildew is a common plant disease that can affect multiple garden plants including vegetables, tree fruit, ornamental plants, and even weeds!
  • Powdery mildew takes on the appearance of white flour dusted on leaf surfaces and can cause decline of affected plants due to a decrease in the ability of the plants to adequately photosynthesis (produce energy).
  • Once leaves of affected plants are covered with the white mycelium of the fungus, they may not be able to absorb enough sunlight to sustain plant and fruit growth, hence the plant declines.
  • The time of year that powdery mildew occurs depends on the race of the disease organism. For cucurbit family members, Podosphaera xanthii powdery mildew occurs during warm months while Erysiphe cichoracearum occurs in the spring and early summer during cooler temperatures.

plant disease


A:  Powdery mildew, like many plant diseases, can be managed through multiple techniques.

  • Look for resistant varieties when purchasing new seeds or plants (especially if you have seen evidence of powdery mildew in your garden in the past), bearing in mind that powdery mildew has developed multiple races. Read the fact sheet powdery mildew on cucurbits for more information about resistant race selection.
  • Use sulfur products at first appearance of mycelium on leaves (white spots), but only use sulfur when air temperatures do not exceed 90°F and read and follow the label of the product you are using to ensure the product is labeled for use against powdery mildew. For a listing of products permitted for use on USDA Certified Organic produce, see omri.org.
  • Frequently apply potassium bicarbonate to affected leaves as it does not provide residual control (it only impacts mycelium during time of treatment and hence does not provide future control of mycelium development).
  • Pull and completely remove diseased plants so spores are not left to cause new infections in the garden. Do not compost plants with evidence of powdery mildew in your backyard compost bin – throw all diseased plants away or place them in your green waste bin instead.

And remember, there is no need to get the blues over a case of powdery mildew– it happens to the best of gardeners. Our Meals Plus Harvest Garden which is run by some of the best Master Gardeners around reported a decline in the melons and squash just this last week……

The harvest was huge, but the garden has a bad case of powdery mildew. We took some entire plants out and cut back a bunch. The squash and melons were the most effected. The Thursday group should decide if more needs to be taken out. We filled 4 big garbage cans. — Garden Manager Sharon