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



Powdery Mildew = Take Action!

P1040721This is powdery mildew, a fungal disease, and I’ve spotted it in our garden.

Not only is it unappealing, it can reduce a plant’s production of vegetables and impact flavor.

What can we do?

We can control it, but not cure it once it appears. To start, remove the leaves most affected on the pumpkins, zucchini and squash. Throw them in the trash bin next to the rec building not the compost bins.

Don’t compost diseased leaves of any kind.

Next,  spread the vine or leaves so the air circulates around the plant. Good sun exposure and good air circulation should inhibit spore germination.

An Experiment

Last year participants in the Family Gardening Program tried an experiment and compared two methods touted online as slowing the spread of powdery mildew.

In one plot, we sprayed the remaining leaves and stem with a cow’s milk spray made with 3 parts whole milk and 7 parts water.

In another plot, we sprayed with a mix of 1 teaspoon baking soda in 1 quart water.

Both methods reportedly create an environment that inhibits the spread of this disease and are best started before powdery mildew appears.  In our experience last year, the milk was effective is slowing the progression of the disease.

For Future Reference: Gardeners can purchase resistant varieties at the start of the season. While resistant doesn’t mean the plant won’t get powdery mildew, it does mean they are less susceptible.

Grand Morning in the Garden

MCGWe harvested tomatoes, green beans and peppers. That was the first task. And after we put the bags of fresh vegetables on the picnic table to go back to the community center for snack, we rolled up our sleeves and set to work.

Our job for the day was to treat the pumpkins, zucchini, yellow squash and cucumbers that are infected with powdery mildew, a fungal disease. Before everyone arrived for a morning of gardening, I prepared two remedies for powdery mildew so we could conduct an experiment. The participants in the Family Gardening Program were divided into two teams.

In some plots Team A sprayed the leaves of the susceptible plants with a milk and water mix. In other plots Team B used baking soda and water. Now we wait and watch to see if one group of plants does better than another.

The dozen or so children were curious about the disease. They wanted information on how the plant got the disease, what it does to the plant and if it had any impact on the vegetables since the problem appears to be just on the leaves. Good questions! Smart Kids!

P1040721Powdery Mildew is a widespread problem for gardeners and there are lots of different forms of this disease. The fungi germinate spores when the humidity is high. Remember the weather the week of July 18th?

The disease thrives where plants are crowded and air circulation is poor. And when it gets hold, powdery mildew will coat leaves a whitish gray, then leaves turn yellow and wither. Buds often fail to open. It can impact the flavor of any vegetables on the plant.

So what can we do? Gardeners can space plants for good air flow, buy seeds that are disease resistant and if they see a problem, remove infested leaves quickly. Wash your hands so you don’t transport spores and then treat the plants like we are doing in the Moreau Community Garden.

All diseased materials should go in the trash bin and not the compost pile.

We put signs up in the garden were we sprayed. If you’re curious, take a look.

swallowtail1Oh, just a side note – we spotted a Giant Swallowtail butterfly enjoying the nectar of our zinnias today. We are all delighted to see that!

Many gardeners were out this morning enjoying the flowers and the feast that is coming along. They happily worked at weeding, watering and other garden chores. eric

If you haven’t visited the garden lately, here’s a sample of the garden’s beauty for you to enjoy.