What Is Powdery Mildew?

Does it look like someone came into your garden and sprinkled talcum powder on the leaves of your plants? That's powdery mildew. It's a super common fungal disease, especially if you're growing certain plants like squash or gardening in a greenhouse.

We use the term powdery mildew to label a number of fungi in the Erysiphales family. These guys thrive in warm, humid conditions and can affect everything from ornamental plants like roses to fruiting plants like strawberries.

Powdery mildew basically coats the leaves of your plants, which can hinder photosynthesis. In other words, your plants won't be able to feed themselves and get energy as easily, which obviously impacts the overall health and productivity of your garden. If left untreated, the fungus will sap all the nutrients from the leaves of your plant.

But we're not going to let that happen! Powdery mildew is pretty easy to treat—you just have to be consistent. Let's look at how to spot powdery mildew, effective ways to treat powdery mildew organically, and then how to prevent powdery mildew in the future.

Signs of Powdery Mildew

Powdery mildew is usually pretty easy to spot. Symptoms include:

• white or gray spots on the surface of the leaves (most common), stems, flowers, and fruit
• leaves turning yellow or brown, curling, and falling off
• flowers dropping or failing to open

You'll typically notice the white fungal growth on the top of your leaves first. Powdery mildew tends to take hold on the lower leaves of the plant and then spread from there. It's most common in mid to late summer.

Don't freak out if you suddenly see white patches all over your plants. The early stage of powdery mildew looks a lot worse than it really is. It's only in the more advanced stages—after the fungus has had a long time to penetrate the plant cells and pull out all the nutrients—that powdery mildew can kill your plants.

Your goal is to routinely check your plants so that you can spot it early and treat it before it causes any real damage.

Steps to Treat Powdery Mildew

Powdery mildew is like the garden's version of the common cold. We've all gotten it, and it's typically not very life threatening. Just as you wouldn't treat a cold with antibiotics or surgery, we're not going to start busting out the strongest chemical fungicides at the first sign of white on some leaves.

All you have to do is follow these three simple steps to treat powdery mildew in your vegetable garden.

Step One: Prune the Affected Leaves

I bust out my pruners at the first sign of any type of pest or disease in the garden. Use a clean pair of pruners to remove the plant parts most visibly affected by the fungus. Avoid pruning more than a third of the plant. If you've caught it early, that shouldn't be necessary.

Once you're done pruning, pick up any fallen leaves, flowers, or fruit from the soil around the affected plants. These plant parts need to go in the trash, not the compost pile (your pile likely doesn't get hot enough to kill the spores).

Wipe your pruners down with rubbing alcohol afterwards.

Believe it or not, this first step can go a long way in preventing the disease from spreading.

Step Two: Make a Powdery Mildew Spray

I don't recommend treating powdery mildew with Neem oil, even though it's natural and organic. It's super easy to whip up a homemade spray with more-eco-friendly antifungals to treat the remaining leaves of any plants affected by powdery mildew.

There are several different types of sprays you can make, depending on what you have at home.

• BAKING SODA SPRAY - Mix 1 tablespoon of baking soda (sodium bicarbonate) with a teaspoon of liquid castile soap and a gallon of water.
• VINEGAR SPRAY - Mix 3 tablespoons of apple cider vinegar or distilled white vinegar with a gallon of water. Test this one on a small section first since acetic acid can burn your plants.
• POTASSIUM BICARBONATE - Mix 1 tablespoon of potassium bicarbonate with a teaspoon of liquid soap and a gallon of water.

Whichever solution you're using, spray it thoroughly onto the affected plant parts. Don't forget the undersides of the leaves. Repeat every one to two weeks.

Tip: If you're spraying flowering plants like squash or cucumbers, it's best to wait until the evening to spray to minimize your impact on pollinators that visit the plant during the day.

Observe your plants over the next couple of weeks. Make sure you're collecting any fallen leaves from infected plants so that the spores can't hop to new plants.

If your baking soda or vinegar spray doesn't seem to be working, switch to the mixture of potassium bicarbonate instead. Potassium bicarbonate has been shown to kill fungal spores better than sodium bicarbonate.

STEP THREE: Remove the Plant, if Needed

If the fungus seems to be spreading, then it's best to pull the affected plants from the garden. Again, you'll throw these plants in the trash, not the compost. Clean the garden space of any fallen debris and then plant something that hasn't been stressed out by disease in the empty spots.

The good news is that powdery mildew is plant-specific, so new plants shouldn't be affected by any lingering spores, as long as it's something different than what was growing before. If you've just ripped up your zucchini plants because of powdery mildew, you can still grow some tomatoes or peppers there.

Ways to Prevent Powdery Mildew

If you're growing certain types of plants or getting a lot of rain, then powdery mildew is all but inevitable. That being said, here are some best practices to follow in your garden to keep your plants healthy and generally slow the spread of fungal disease.

Buy Resistant Varieties

Plants in the squash family (zucchini, pumpkins, gourds, cucumbers, and of course, squash) are particularly susceptible to powdery mildew, but certain varieties have been bred to be resistant. The next time you're shopping for squash seeds, look for varieties that say they're resistant to powdery mildew.

Maintain Good Air Flow

Proper air circulation around your plants can prevent a number of fungal diseases. I practice what's called intensive planting, which means I pack my plants in my raised beds instead of following the plant spacing rules. The way I prevent overcrowding is by regular pruning. That's key.

At least every other week, prune some of the lower, outer leaves of your plants. This goes a long way to inhibiting fungal growth by ensuring there's air circulating around each and every plant. Bonus: those lower, outer leaves are the oldest leaves on the plant, and guess what? They're also the most vulnerable to powdery mildew (along with the youngest leaves).

Regular pruning is also key to ensure your plants are getting enough direct sunlight. You know how you put mildewy towels under the sun to kill the odor? Sunlight has a similar effect on powdery mildew.

Water Wisely

Water your garden in the early morning so that plants have time to dry out a bit before nighttime. A dark and humid environment is most conducive to fungal spores (aka disease) developing.

When you're watering, it's also a good idea to aim water at the roots of the plant, not the leaves. A wet canopy of leaves is basically an invitation for fungi. Avoid using a hard spray, which will splash water, along with spores, from the soil onto your plants.

Drip irrigation is an ideal watering method because it delivers slow but consistent water right over the root zone of your plants, keeping all the leaves nice and dry.

Add Compost

Apply a quarterly installment of organic compost around the base of your plants. In addition to maintaining soil moisture and regulating soil temperature, it also prevents fungal spores from splashing up onto your plants.

Clean Up Fallen Leaves & Soil Debris

Before you close up your garden for the winter, use a hand rake to clean up any leaves and debris on your soil surface. The fungus can actually hang out all winter long on affected plant parts left behind.

Don't Let Powdery Mildew on Plants Get You Down

Try not to let pests and fungal disease ruin your gardening fun. Honestly, they're just a natural part of gardening, especially if you're growing organically and avoiding harmful herbicides, fungicides, and pesticides.

The key is just to keep a lookout for early signs of trouble. That way, you can catch problems early on, handle them, and still end up with tons of organic produce in your harvest basket. If an issue ever gets out of hand, remove the affected plants from your garden, toss them in the trash, and start over. Thank the garden for another lesson learned.

Thanks for being here and making gardening ordinary again (even the not-so-fun parts of gardening like dealing with fungal disease)!