What's This White Stuff on My Squash Leaves? • Gardenary (2024)

Are the Leaves of Your Squash Plant Turning White? That's Powdery Mildew

If it looks like someone sprinkled baby powder on the leaves of your squash plant, you've got powdery mildew.

This is a very common, albeit frustrating, problem to have in the vegetable garden, especially if you're partial to growing your own summer squash and zucchini.

What Is Powdery Mildew?

Powdery mildew is a fungal disease caused by various species of the Erysiphales family. It mostly shows up as a white or gray coating on the surface of the plant's leaves, but it can sometimes spread to the stems and even fruit. It thrives in warm and humid conditions.

This pesky little fungal disease can affect a number of plants—everything from beans to strawberries—but plants in the Cucurbit plant family are particularly susceptible. Think cucumbers, melons, squash, pumpkins, and zucchini.

Because powdery mildew coats the leaves of the plant, it can hinder photosynthesis, which is how a plant feeds itself and gets energy. Over time, this negatively impacts the plant's health and productivity.

There are lots of harsh chemicals out there to treat powdery mildew, but I prefer to follow organic gardening practices. Let's look at some effective, eco-friendly methods to prevent and treat powdery mildew.

Powdery mildew is almost inevitable if you choose to grow certain types of plants, squash especially. Even so, there are some best practices to follow to keep your garden healthy and slow the spread of fungal diseases.

Buy Resistant Varieties

Select squash varieties known for their resistance to powdery mildew and other diseases common in Cucurbits. One of my favorite seed companies, Botanical Interests, has disease resistant varieties.

Ensure Good Air Circulation

If you practice intensive planting in a raised bed like I do, it's critical that you prune your squash plants regularly (at least every other week). Remove lower leaves and extra stems that aren't involved in fruit production. This will ensure there's air circulation between and around plants and inhibit fungal growth.

The leaves that are most vulnerable to powdery mildew are the oldest and the youngest leaves. That's another reason it's a great idea to just remove those lower, older leaves.

Water in the Morning

Water your squash plants in the morning, if possible. Watering in the evening can create a moist environment overnight, which is more conducive to powdery mildew development. Also, make sure to aim your water at the roots of the plant, not the leaves.

If you don't already have an irrigation system installed, consider setting up an automated drip irrigation system. Not only does drip irrigation give plants the deep watering they prefer, it also keeps the foliage dry.

Add Compost

Apply a layer of organic compost around the base of your plants. This helps maintain soil moisture, prevent splashing of fungal spores, and regulate soil temperature.

Steps to Treat Powdery Mildew

The best thing to do is to start treatment as soon as possible to prevent the disease from spreading. This is why it's important to regularly check on your garden so you can catch small problems before they become major issues.

Follow these steps to treat powdery mildew in your vegetable garden.

Step One

My first response to any type of disease or pest in the garden space is to bust out my pruners. Use a clean pair of pruners to remove the leaves that are most affected by the fungus, but don't prune more than a third of the plant. Pick up fallen leaves and plant debris in the soil around the plant.

Remove and dispose of any infected plant material, including fallen leaves and fruits. Do not compost infected plant parts, or you'll just perpetuate the disease.

Make sure to wipe your pruners down with rubbing alcohol after.

Step Two

Make a baking soda spray to apply to the remaining leaves of the plant. Baking soda, AKA sodium bicarbonate, is an eco-friendly antifungal that's long been used to treat powdery mildew.

Mix one tablespoon of baking soda with a teaspoon of castile soap and a gallon of water. Spray the mixture onto the affected plant parts, including the tops and undersides of the leaves, ensuring thorough coverage. Repeat every one to two weeks. The best time to spray is in the evening, once the squash flowers have closed, so that your solution doesn't affect the bees.

Step Three

Keep a close eye on your plants. If the baking soda spray doesn't seem to be doing the trick, you could switch to a mixture of potassium bicarbonate instead.Mix two teaspoons of potassium bicarbonate with one gallon of water and spray the solution on the affected plants. This can disrupt fungal cell membranes and inhibit powdery mildew growth.

Step Four

A lot of gardeners would turn to Neem oil, a natural fungicide and insecticide, by this point. I hesitate to use Neem oil, even though it's considered organic. If the problem is persisting, I think it's best to just pull the plant from the garden (make sure to throw it in the trash, not the compost) and start over with fresh plants that haven't been super stressed out by disease.

Plant something that's not in the Cucurbit family in that space for the next warm/hot growing seasons.

Powdery Mildew vs Leaf Blight

Leaf blight is another fungal disease that commonly affects plants in the Cucurbit family like squash and melons. It also thrives in warm and humid conditions and causes leaves to die. Instead of seeing white spots, you'll likely see yellowy-brown spots on the leaves of your squash and zucchini plants (see picture below). These spots will eventually cause the leaves to die and fall off.

The steps to prevent and treat leaf blight are the same as powdery mildew. Remove damaged leaves and spray the remaining leaves with a baking soda solution.

Don't Let Powdery Mildew Squash Your Garden Dreams

Try not to let pests and disease get you down. They're just a natural part of gardening, especially if you're following organic gardening practices and avoiding harmful herbicides, fungicides, and pesticides.

If you're able to catch powdery mildew early and treat it, you can still end up with loads of squash in your harvest basket this season. If not, the very worst outcome of powdery mildew is you have to remove a plant or two from your garden. Should that happen, thank the plant for the fruits it gave you or the gardening lessons it taught you, toss it in the trash, and move on.

Thanks for being here and making gardening ordinary again (even the less glamorous side of gardening like dealing with fungal infections)!