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ask-ruth-pictureDear Ruth,

The leaves on my squash have whitish spots and are starting to look a little shriveled. They were looking great, and now all of the sudden they look bad. It is on the stems too, and it seems to be spreading. What is this and what can I do?

West Asheville


Dear Tyler,

That is a fungus called powdery mildew. There are lots of strains of powdery mildew, but the symptoms basically look the same. The leaves get white or greyish spots that look “powdery”, and sometimes the leaves begin to look distorted. Eventually, the mildew will cover most of the leaf, it can spread to the stem and buds, and finally take over the whole plant. If it gets bad enough, it can actually impair photosynthesis.

First, check the air circulation in your garden area. Are your garden plants far enough apart to allow for air-flow through the garden? Can you increase air-flow around the plants by staking or trellising them? Can you thin within the plant to encourage air-flow without compromising your harvest? If weeds have grown up in surrounding areas or the grass is high, consider weed-eating/mowing. Are the plants in full sun? (Shade contributes to powdery mildew.) Remove diseased leaves from the plant and burn them or put them in your garbage. You don’t want the spores to winter over on your property or in your compost pile.

As always, a healthy plant is less susceptible to disease and insects than a stressed plant. Make sure that your plants are receiving the nutrients and the water that they need on a regular basis. Many veggies are prone to powdery mildew, especially those in the cucurbit family ~ including summer and winter squash. Here is a list of some of them:

  • artichoke
  • beans
  • beets
  • carrot
  • cucumber
  • eggplant
  • lettuce
  • melons
  • parsnips
  • peas
  • peppers
  • pumpkins
  • radicchio
  • radishes
  • squash
  • tomatillo
  • tomatoes
  • turnips

Bee balm, grapes, and hops are susceptible to powdery mildew, and so are many familiar ornamentals like columbine, dogwoods, lilacs, phlox, and roses. As mentioned earlier, there are many different strains of powdery mildew and each strain is plant specific ~ the powdery mildew on your lilac will not spread to your squash plants. Seek out powdery mildew-resistant cultivars, and plant in full sun to help prevent this disease.

Powdery mildew comes on quickly if conditions are right ~ that’s when the humidity is high and temperatures are between 68 and 80 degrees. The fungus stops spreading when temperatures hit 100 degrees. It spreads more quickly in dry, but humid weather than when it is rainy. Tender, succulent growth is very attractive to powdery mildew, so avoid applying nitrogen fertilizer while trying to combat powdery mildew.

There are a number of fungicides you can use to stop the spread of powdery mildew. Here are a few:

  • Actinovate
  • Bicarbonate concoctions
  • Copper
  • Neem Oil
  • Safer Soap Fungicide (contains Sulfur)
  • Serenade
  • Sulfur (do not apply sulfur within two weeks of an oil spray).

Spray the plant thoroughly on all sides of the leaves, the stems, and the buds to the point of dripping. For most products, spray every 7 days when combatting a disease and every two weeks when preventing a disease ~ BUT read and follow all directions and safety precautions on the labeling of the specific product you will be using. Sprayers with wands are especially helpful when trying to coat all sides of the leaves and for reaching the undersides of leaves. Spray early in the morning or late in the day to avoid sunscald (if using Neem Oil, spray late in the day and avoid the flowers since Neem is toxic to bees). When you spray late in the day, the plant has all night to soak in the fungicide before the sun hits it the next morning. Well-made compost tea should work too, especially when sprayed early as a prevention.

There are different bicarbonate mixtures utilized to stop powdery mildew. Milstop®, Armicarb 100®, and Kaligreen® are commercially produced potassium bicarbonate-based products. Baking Soda (sodium bicarbonate) is used in the recipe below.

Here is a spray you can make at home with ingredients you probably have on hand. Safety hint: If you do not have baking soda on hand, you should buy some immediately and store it near your stove for emergency use in case of a grease/oil fire when you are frying those squash blossoms.

Recipe for Homemade Powdery Mildew Spray:
With ingredients most of us have in our kitchens.

  • 1 heaping Tablespoon of Baking Soda (Sodium Bicarbonate)
  • 1 Tablespoon of Horticultural Oil (like All Seasons Oil) or Vegetable Oil
  • ½ Teaspoon of Insecticidal Soap or liquid soap (use plain liquid soap and not detergent; some recipes called for 1 T.)
  • 1 Gallon Water

Mix ingredients well and keep well mixed when spraying.

Planting your squash early and having succession plantings can go a long way to assuring a decent harvest. On her farm, Meredith Mckissick plants early, and utilizes floating row cover. Many commercial growers plant in succession using row covers at planting, and only plan on having each succession of plants in the ground for 6 weeks before the next succession is ready to go ~ ensuring a steady harvest despite the presence of mildew.

Tyler, I hope you enjoy lots of squash this summer. Squash Casserole sounds delicious right now!

Best wishes,

Ask Ruth © 2013 Ruth Gonzalez & Organic Growers School

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Ruth Gonzalez

Author: Ruth Gonzalez

Ruth Gonzalez is a former market farmer, gardener, and local food advocate who wants to see organic farms proliferate and organic gardens in every yard. She serves on the Organic Growers School Board of Directors, and in her job at Reems Creek Nursery, Ruth offers advice on all sorts of gardening questions, and benefits daily from the wisdom of local gardeners.

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