Preventing and Treating Powdery Mildew

Preventing and Treating Powdery MildewCredit: Lavender Rose

In central Utah, our growing season is relatively short. Our last spring frost comes rolling through in early June, and our first killing frost can end our vegetable gardening efforts any time after mid August.

To jump the season, many gardeners in our area use greenhouses to start their seeds indoors or buy ready-made plants. Since zucchini plants (also known as courgette or Cucurbita) sprout rapidly, grow quickly, and begin producing in roughly 40 to 50 days, the growing season comes to an end long before we normally have to worry about powdery mildew (Sphgerotheca fuliginea).

A couple of years ago, I decided to buy ready-made plants to make my gardening efforts easier. I was hoping for a larger harvest before the frost hit in August, but that was a big mistake! It turned out to be a relatively cool summer that year with temperatures in the 80s and lots of humidity due to the more-than-average rainfall.

While zucchini plants do well in spring and fall-type temperatures, they are more prone to powdery mildew when the humidity is high. Combine older plants with damp weather and you get a fatal fungus that spreads quickly throughout the garden.

What is Powdery Mildew?

Powdery mildew is a fungal disease that affects plants belonging to the Cucurbita pepo family. That includes vegetables such as:

  • zucchini, courgette, and other summer squashes
  • pumpkins
  • melons
  • gourds

But it can also affect:

Powdery mildew is a white, powdery-looking fungus that grows in a cobwebbed pattern over the top of the leaves, but it can also be seen on the stems and flowers. More likely to occur during hot, humid conditions, such as that experienced in late summer, most vegetable and fruit gardeners only have to contend with it when the season is about to close. At that time, you can just pull up the plants, or cut off the infected leaves when they first appear, to keep the fungus from spreading to the other plants in your garden. 

Untreated Powdery Mildew Causes Leaves to Turn YellowCredit:

However, if you leave the plants untreated, the leaves will curl, turn yellow, wilt, and die within only a few short days. Sometimes, though weakened, the plant will try to survive by sending up new shoots and leaves, but that new growth will almost always be stunted. Overall, the plant’s life will be shortened.

The larger problem is that if it spreads and infects the rest of your garden, your gardening efforts and production can be seriously hindered at best, or at worst, come to a complete hault.

Preventing Powdery Mildew in Zucchini Plants

It’s always best to do what's necessary to prevent powdery mildew from occurring. Zucchini plants need a well-drained, airy place. Don’t plant them too close together. They need lots of air circulation and complete sunshine, so don’t use a damp, shady spot. Mulching the ground at the base of the plant will help keep the fungus spores confined to the dirt and from getting on the leaves.

Don't Place Zucchini Plants Too Close TogetherCredit:

If possible, try not to get water on the leaves if you can help it. Zucchini leaves need to stay as dry as possible. Therefore, don’t use a sprinkler that waters your plants from above. Water the plants from the bottom – either by hand, or with a drip system. Constructing a raised bed to grow your zucchini plants in can make that easier.

In addition, make sure you cut off any infected leaves as soon as you see the mildew appear on them. Waiting even a single day to take care of the problem can seriously risk the plant’s life, as well as the well-being of the rest of your garden.

Organic or All-Natural Fungus Control

While spraying with a chemical fungicide is a viable option, many gardeners are turning to a more organic or all-natural form of fungus and pest control. For powdery mildew, the gardener who wishes to go the more natural route has several options. One of those options is to use a commercial, copper-salt solution that includes a fatty acid to help it adhere to the plant:

This option works well for those who don't have the time to research their own insecticidal soap recipes. However, organic and all-natural solutions are not as difficult as it sounds.

Neem Oil

Neem OilCredit: Public Domain

If caught early enough, when plants are small or when the mildew is just starting to get a foot hold, spraying your zucchini plants with Neem oil works very well. But don't try to use this remedy if the mildew is as wide spread as it was on my own zucchini plant.

Chamomile Tea

Chamomile Tea Can Treat Powdery MildewCredit:

Steep the chamomile tea for at least 10 minutes, and use on zucchini plants when they are young or when the mildew is just starting. Chamomile tea makes a mild, but effective fungicide, so it won't work for well established mildew either.

Baking soda

Oranic Gardeners Use Baking Soda to Treat Powdery MildewCredit: Lavender Rose

Baking soda has been recommended by Organic Gardening magazine since the mid 1980s. Using baking soda against powdery mildew isn’t new. Although their current recommendation is to use about 1 teaspoon of baking soda per gallon of water, many organic gardeners today recommend using more than that. Most gardeners use 1 to 2 tablespoons in a gallon of any insecticidal soap recipe. They don't simply mix it with water.

You can also use baking soda in combination with any pest control recipe that contains something sticky like vegetable oil. You want the baking soda to stick to the leaves, where it will act like a fungicide. Baking soda helps to raise the pH level of your plant, creating an environment that makes it difficult for powdery mildew to grow. In the following video, the gardener's recipe includes Murphy's Oil Soap instead of oil.

Baking Soda and Murphy's Oil Soap Recipe

For Treating Powdery Mildew

Traditional baking soda is made with sodium, but you can also purchase a commercial variety of all-natural powdery mildew treatment solution that’s made from potassium.

Nonfat or Lowfat Milk

Nonfat Milk Can Treat Powdery MildewCredit:

The effectiveness of cow’s milk against powdery mildew was scientifically tested in 1999. Many gardeners use it as a preventative, but it still works well if you begin spraying it on the leaves as soon as the mildew appears. That's why it's a good idea to check on your garden plants regularly, preferably everyday, so you can move quickly to correct any problems. The recommended concentration is about 10 percent. That’s one part milk to nine parts water.

The reason why nonfat milk was used in the study was due to the odor that fat in the milk "might" cause as it breaks down. However, several gardeners on various gardening forums have shared that 2% milk did not cause an odor, and also worked well for them.

Keep on Top of Powdery Mildew to Protect Your Harvest

Powdery Mildew on Zucchini PlantCredit: Lavender Rose

While powdery mildew generally attacks zucchini plants and other squashes later in the season, a wet summer combined with store-purchased plants can hurry the process.

If your garden is basically done producing, the easiest way to get rid of the mildew is to just pull up the affected plants and toss them onto your compost pile. The compost heat will take care of the problem without you having to do anything else. However, many organic gardeners caution against that practise. They believe the spores will survive the heat and contaminate the soil.

If your garden is young, there are many organic and sustainable methods that can help you keep your garden healthy. Above all, don’t ignore the problem. Not only will it cause the affected zucchini plant to wither and die, but it can also destroy the rest of your garden.