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Dealing with the inevitable: Powdery Mildew

A healthy squash leaf

Now is the time of year to be on the lookout for powdery mildew.  It will show up initially as small round spots on the leaves of squash, cucumber, pumpkin, green beans and even fruit crops like apples, peaches, tomatoes and strawberries.  If you're growing in one of the San Mateo community gardens - it's not a question of if you'll see PM (powdery mildew) - it's when? 

Fortunately, there are some things you can do to disrupt the progression of this type of fungus.  Here are a few tips:

1) Diligently prune healthy leaves to allow a good amount of air & light into the plant.  

2) Avoid overly moist soil conditions.  

Water adequately and, preferably, earlier in day to avoid having wet plant surfaces overnight.  Splashing water from infected plants can transport the mildew spores to nearby plants - making drip irrigation optimal.

3) Many gardeners spray a mixture of baking soda (sodium bicarbonate), cooking oil and dish soap upon discovering PM. Here's one such recipe:

  • 3 table spoons of baking soda
  • 3 tables spoons of Wesson oil
  • 1/4 teaspoon of dish wash soap
  • add all of the above to 1 gallon of water
Note that sodium is toxic to plants - so the amount and frequency of application will impact your plant's health.  Best to test spray a small area before applying more broadly.

Neem oil can also be mixed into a spray and used to kill PM.  Here's a recipe from to make a foliar spray:

  • mix 2 tablespoons of dish wash soap into a gallon of water
  • add 4 teaspoons of 1% clarified hydrophobic neem oil
  • be sure to test the spray on a small portion of the infected plant

Yet another option is to use a fungicide like potassium carbonate.  Kaligreen is a product recommended by Pam Pierce in Golden Gate Gardening.  While it is a OMRI certified organic product - reading the packaging (below) you see that protective clothing must be worn and care taken not to come in contact with your skin, or be inhaled or swallowed.  Perhaps a little sodium isn't so bad, after all.

Kaligreen produce image from

4) Prune infected leaves.  

Be sure to carefully carry the leaves out of the garden to dispose of them in your green waste can- as you don't want to spread the mildew spores onto other plants.  (recycled Lyngso bags are perfect to get infected plant material from the garden to the green bin).

Do not put infected cuttings in the compost bin unless you have a compost system that successfully achieves & sustains a high temperature in the thermophilic phase (at least 135 degrees F).  Most home composting systems do not get this hot.  Our compost bin at Los Prados community garden does not.

5) Remove plants that have become completely infected.

6) This winter when you are shopping the seed catalogs, look for varieties that are resistant to PM

Hope your summer harvest is a success!


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