Organic Control of Powdery Mildew on Squash

Hello fellow gardening enthusiasts!

Spring is actually here, even though the weather has been spring-like since January. Our fruit trees need thinning, still harvesting huge heads of cauliflower and broccoli, tomato plants setting fruit like crazy and we are cutting green and yellow summer squash three times a week.

Our cut flower program is also yielding many beautiful and fragrant rose and alstromeria blooms with our first sunflowers to be snatched this week , to brighten up the homes and put smiles on all of our residents faces here at Sunshine Care, A Community of Assisted Living Homes in Poway. Protea and pincushions are right around the corner.

One of our big items that we grow in our organic gardens, for use in the menus from the kitchen, is our bounty of assorted summer squashes. In early February, when the fear of frost was no more, we planted over a hundred seedlings of green and yellow zucchini. After tomatoes, zucchini is one of our most heavy producers. We have been cutting these squashes since mid March and will continue to pull them, plant more and harvest through November.

We try to get an early start so that we can get lots of zucchini through May. When June comes around, the kitchen is in full swing and focused on the onslaught of tomatoes.

We actually will pull most of our zucchini around June 1st and plant a late crop in early July to end the year with vigorous new plants producing heavy yields of new zucchini from August to frost.

For a backyard grower, this would seem rather silly to pull plants that are still producing for your dinner table. I agree, but my scenario is different and you will need to be extra special wary of the dreaded Powdery Mildew.

Erisyphe cichoracearum is the powdery mildew most commonly associated with members of the cucurbit family (squashes, melons, cucumbers and pumpkins). This fungal problem likes warm weather and thrives on mean temperatures from 68 degrees F to 80 degree F and will persist and be controlled at temperatures above 95 degrees F. So basically in Southern California , it is around from spring through early fall.

White spots of mycelial growth will first become visible on the leaf surface and can spread to the stems and eventually the fruit. Infestations will reduce yield, quality and can eventually kill your squash plants.

Powdery mildew is our biggest fungal nemesis in the garden. The best control of this disease organically is prevention. You will still probably have outbreaks but you will get a crop, if you follow the following steps.

  1. When purchasing seed, acquire varieties with powdery mildew resistance or at least tolerance. The varieties we use at Sunshine Care have been selected from Johnny’s Selected Seed Co. and have the letters PM after the description, indicating powdery mildew resistance. LOOK FOR THE PM!

We grow Dunja, Spineless Perfection, Alexandria, Golden Glory and Yellow Fin- all from Johnny’s and all with PM disease resistance.

  1. Plant your squash in sunny areas as much as possible but more importantly GOOD AIR CIRCULATION is critical. So avoid shady areas in the corner of your garden or patio. Avoid overcrowding of plants. We plant on a line 18” apart which is as close as you want to get. Most people plant at a 24” spacing. When harvesting a fruit, there is generally a leaf associated with that fruit. That leaf kept it well fed and growing. Take the fruit and also cut off the leaf. That leaf has served its purpose and when you continually cut fruit and then leaves, you are opening up the plant and taking off old desirable powdery mildew matter. This disease likes the old tissue first. Many people say to avoid heavy fertilizations to help curb the problem. Again, this is to avoid big bushy plants where air circulation is difficult. I have found that fertilizing every couple of weeks and taking off leaves at harvest time to be successful.


  2. WASH YOUR TOOLS. Don’t spread disease around your garden! A quick splash of bleach, 1 part to 5 parts water will suffice.
  1. PREVENTATIVE ORGANIC SPRAYS work well but not a 100% lethal kill in the organic world. Starting early before disease pressure is noticeable is the key. Serenade is an organic material with an active ingredient of a strain of bacteria- Bacillus subtilis and it is nontoxic. Sulfur and horticultural oils are very effective but I tend to shy away from their use because both can burn at temperatures in the 90’s. Homemade solutions can be very effective on a 7-10 spray program, by mixing 1 qt milk, 1 tablespoon baking soda and a drop of liquid dish detergent. No matter what you use, coverage is the key.
  2. Don’t irrigate at night. Even though you can wash PM spores off with water, you can be setting yourself up for other disease issues by having a wet plant. Water in the morning and preferably use some type of drip irrigation.

Try these tips and you should have plenty of squash coming out of your ears!

Call or email me anytime with questions, comments or ask for a tour –

Roy Wilburn

Horticulture Manager at Sunshine Care, A Community of Assisted Living Homes in Poway, CA 92064

(858) 472-6059 or

Check our website, for info on our free monthly garden lectures held on the third Saturday of each month at 10:30 am. Informative speakers will dazzle you with their knowledge and we wrap things up with a walk through the gardens, greenhouse and orchards.

Happy gardening!!

Categories: Horticulture

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