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The Dreaded Powdery Mildew

March 8, 2012
Anne Gachuhi - Founder/CEO • Home Gardening Support Network , The Maui Weekly

Dear Anne,

I am struggling to keep my squash plants alive. They are no longer sending out new runners or flowering. I have never found any bad bugs on the plants, but they do have white, powdery spots on them.

Ha'iku Homeowner

Article Photos

Powdery mildew is a fungal disease that affects a wide variety of vegetables, but it’s most prevalent amongst the curcubits group of vegetables such as squash (as shown here), cucumber, pumpkin, melon and cantaloupe.


Dear Ha'iku Homeowner,

Powdery mildew is a fungal disease that affects a wide variety of vegetables. It's most prevalent amongst the curcubits group of vegetables such as cucumber, squash, pumpkin, melon and cantaloupe.

Powdery mildew will show up as white, powdery spots that form on both surfaces of leaves, on shoots, and sometimes on flowers and fruit. These spots eventually spread over a large area of the leaves, causing them to gradually turn completely yellow, die, and fall off. On some plants, powdery mildew may cause the leaves to twist, buckle or otherwise distort.

Unlike most diseases, powdery mildew does not require wetness on the leaf surface for infection to occur. Powdery mildew is most likely to occur in areas with temperatures between 60 to 80F and shady conditions. Spores and fungal growth will not occur where temperatures are above 90F and in direct sunlight.

Powdery mildew is best controlled by the following methods:

Prevention is the best cure. Plant resistant varieties when possible and avoid varieties susceptible to powdery mildew

Plant cucurbits in full sun and in areas with good air circulation

Avoid applying excess fertilizers and use slow release fertilizers where possible

Overhead sprinkler irrigation could help wash off powdery mildew spores. Unfortunately, use of overhead irrigation could contribute to other diseases.

Use horticultural oils such as Saf-T-Side Spray Oil, Sunspray, Ultra-Fine Spray Oil (don't use oils when temperatures are above 90F (consult the label for directions).

Use plant-based oils such as neem or jojoba for protection.

Use wettable sulfur sprays as preventative measures in the early days of the disease or before symptoms appear.

Copper fungicides have been used in some instances, though not very effective at times.

As a last measure, use a biological fungicide called Serenade. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew.


Contact Anne Gachuhi, founder/CEO of Home Gardening Support Network LLC, at (808) 446-236 or email questions to



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