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Signs of Powdery Mildew

It starts as white patches on the leaves of squash, lilacs, phlox, bee balm, and other plants, making them look like they have been dusted with baby powder. Early on, it wipes off or washes away, only to return again.

Eventually, the affected leaves turn yellow and die on many plants while others continue to soldier on.

So far this year, only our squash has been affected.

Thankfully, there is no sign of it on other susceptible plants such as phlox.

Causes of Powdery Mildew

The lack of powdery mildew on my phlox is probably due to the fact that it is not caused by just one fungus but by several different species that affect different kinds of plants. The powdery mildew that you find on your squash is not the same as the mildew on your beans or roses. 

Cucurbits such as pumpkins, squash, cukes, and melons have three different powdery mildew fungi gunning for them that can thrive in both humid and dry weather. The spores of the fungi are windborne and can’t really be avoided. No wonder the squash gets hit every year!

Remedies for Powdery Mildew

There are lots of home remedies, but researchers have found that simply spraying with plain water weekly can be effective. The spores like humidity but hate rain and water. They can’t germinate or grow if the leaves are wet, which is the opposite of what most gardeners, including me, think. 

One season, I tried to defeat powdery mildew by planting squash in our high tunnel, thinking that if I could keep the leaves dry, they would not be affected. Naturally, they got the worst case of powdery mildew I have ever seen! It was the perfect place for it to thrive: high humidity and no rain hitting the leaves. Another lesson learned the hard way!

If you decide to try a home remedy rather than plain water, there are several that have been proven to be effective if used early. They can slow or stop the spread early on, but once the fungi are established in the leaves, they won’t eliminate it.

Homemade Baking Soda Spray for Powdery Mildew

Many of these remedies include baking soda. Just be aware that baking soda can burn plants, and it can build up in your soil, potentially causing deficiencies in calcium, magnesium, and iron. Potassium bicarbonate can be substituted for baking soda. Test these sprays on a small area first to ensure they do not damage your plants.

• Mix 1 tablespoon baking soda with 1 tablespoon vegetable oil and 1 teaspoon dish soap in 1 gallon of water.
• Mix 4 tablespoons baking soda with 2 tablespoons of Murphy’s oil soap in 1 gallon of water.
• Mix 2 to 3 tablespoons vinegar with 1 gallon of water. Be sure to test this first because vinegar can burn plants.
• Neem is an organic fungicide. Follow the instructions on the label.
• Mix 1 part milk with 10 parts water.
• Some folks swear by mouthwash as an effective fungicide, but it is not organic. They recommend 1 cup of mouthwash to 3 cups of water.

To keep the fungi from developing a resistance to your homemade spray, it is recommended that you alternate remedies each week. Use baking soda one week and milk the next. Whether spraying with water or a home remedy, do it early in the day so the leaves can have a chance to dry before evening.

When adding new plants to your flower beds, look for mildew-resistant varieties. Don’t over-fertilize with high-nitrogen fertilizers since soft new growth is very susceptible to infection. Space plants are far enough apart to promote good air circulation. If the infection starts in the lower leaves, snip them off. Make sure plants get enough direct sunlight. Prevention is the best medicine!

→ See more about Powdery Mildew Control.

Have you experienced powdery mildew? How do you manage it? Tell us in the comments!