Skip to main content

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!


Popular posts from this blog

Peaches and more

Yesterday we handed out the sweetest of peaches thanks to a generous donation from Circle Foot Permaculture .   Along with the quarts of peaches, we also provided Samaritan House Food Pharmacy clients a handout with nutritional information on peaches - how they impact your blood sugar - and a recipe for a Roasted Peach Parfait.  Here are links to the handout: Nutritional Spotlight: Peaches (English) Nutritional Spotlight: Peaches (Spanish) It all seems somewhat routine at this point:  gather donations of locally grown produce quickly pull together helpful nutritional information based on the week's donations In retrospect, this is the culmination of 3 years of effort to create a network of local gardeners, foragers and volunteers - with a goal of becoming a reliable source of fresh produce for lower income families struggling with diabetes and other health disorders.    While it has been known for some time that eating fruits & vegetables can positively impact a person's he

The December Garden

Snap Peas (& a garden gnome!) Beresford Park Community Garden December 15, 2021 It's hard to describe just how good the winter gardening is in San Mateo.  While most gardens across the country have long gone into a frosty hibernation - we're, at long last,  getting the rain we've been without for the last 9 months or more.  It's finally time to turn off the irrigation systems and let Mother Nature do her thing.  The new moisture is creating a full-on nutrient soup that flows through our soil, fueling an explosion of activity within the soil food web.   Fungal activity is pushing mushrooms to the surface.  Our favorite spring flowers of nasturtium, poppies, sweat peas, borage and yarrow are in a race to establish their growing territory.  And, thankfully, the newly moist soil makes pulling up oxalis, mallow and bindweed sprouts a bit easier. As you would expect, plants thriving now in the garden were initially planted out in the fall.  Those crowns of cauliflower and

Getting Started: Wine Barrel Planters

Gardening in a small, urban space requires a bit of creativity.  Our challenges are many: buildings & trees frequently block sunlight;  various critters like to snack on new seedlings and just ripe fruits & vegetables; and here in San Mateo, irrigation is a must.  To increase your probability of gardening success, consider adding recycled wine barrels into your urban garden.    Having tried many types of growing containers over the years, the wine barrel has made it to the top of my list.  My galvanized tubs always seemed to need more water than wooden barrels - perhaps due to the different heating/cool properties of the metal vs. wood?  Aesthetically, a recycled barrel looks like it's been there for years - which I love.  And the smell!  There is no comparison. Cost is definitely a consideration.  A wine barrel at Home Depot is $40 whereas a 35 gallon galvanized tub is $55.  You might say that growing in the ground is $0.00 - but in some locations, the extra 18 inches off