Powdery Mildew and What to Do

Coffee in hand, I wandered the gardens very early this morning.

I looked for animal tracks near our beds, there were none.

I checked on the tiny pumpkin plants. They are still a little droopy but coming around. They will be fine.

Then I walked the pathways checking on the health and well being of the plants growing.  All was well but one.

A single zucchini plant has powdery mildew. While this is not surprising because of the wet weather, it needs to be tended to at once. I removed the diseased leaf and will connect with the bed’s gardener to let her know.


This photo is from another garden, but shows what PM looks like. The plant at PMCG was not heavily infested. 

What is Powdery Mildew?

The name is appropriate as the leaves and stems develop a white, powdery fungal growth that is made up of asexual spores called conidia. Conidia are airborne, can travel long distances and can reproduce rapidly under favorable conditions such as the high humidity we have been experiencing. The length of time between infection and visible symptoms is 3 days to a week, which is not long at all.

PM typically begins on leaves that are tender, the undersides of a leaf and lower leaves.  In short time, the infected leaves develop white areas that some say look like a plant was dusted with flour.

In the future, if you are buying seed or plants, look for varieties with genetic resistance. Resistance doesn’t mean the plant won’t get fungal issues. Think of it like humans having a strong immune system. Those with a strong immune system are better able to fight off maladies.

 What to Do Now

If you are growing susceptible plants such as zucchini, summer squash, cucumbers, pumpkins and other cucurbits. The same copper fungicide that we used for septoria leaf blight works for powdery mildew and should be applied every 7-10 days. Spray even the undersides of the leaves. Inspect your garden plants every time you are in the garden and be quick to get in touch if you think something is wrong.

The most critical time is when the plants begin fruiting. If you are growing the plants mentioned above, you may want to apply a fungicide or a home-made brew now.

Home-made PM Fungicide

Baking soda is an effective control but beware how much you use and what it is mixed with. Research at Cornell University found that baking soda mixed with horticultural oil “almost completely inhibited PM on heavily infected pumpkin foliage. Baking soda without spray oil was ineffective, and a 2% (wt./vol. of water) solution of baking soda damaged the leaves.” So follow the recipe. More is not better.

In one gallon of water, mix

  • 1 tablespoon baking soda
  • 1 tablespoon oil (horticultural oil is thought best, but vegetable oil works, too)
  • 1 or 2 drops dishwashing liquid

Shake well and keep shaking between sprays. Apply to plants being diligent to spray leaves near the soil and the undersides of leaves.

Never apply any fungicides when the temperatures are above 80 degrees or in direct sun.


If you are curious about PM and want more information visit the Cornell University website: http://www.neon.cornell.edu/training/ppts/McGrathpmnotes.pdf



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