Powdery Mildew Treatment: Home Sprays and Prevention

Does your plant look like it got dusted with confectioners’ sugar? Those white powdery spots are almost certainly powdery mildew, and they tend to show up in the second half of summer just as the garden hits its stride. Do not panic. This fungus is rarely fatal to plants, and most gardens shake it off. All you need are our powdery mildew treatment suggestions and a few prevention habits to keep it from spreading.

What is Powdery Mildew?

Powdery mildew is a common term for a range of plant fungi found around the globe. Each type affects a different group of plants. The powdery mildew fungus that thrives on cucurbits, meaning cucumber, squash, and melon, is not the same one you find on phlox, sunflowers, and zinnias. And neither of those is the species that proliferates on roses. Some powdery mildews go after trees and shrubs instead, including apple, birch, lilac, linden, maple, oak, and rhododendron.

That specialization is good news for a mixed garden. A mildew running wild on your squash will not automatically jump to your roses, so an outbreak in one bed does not doom the rest of the yard.

What does Powdery Mildew do to plants?

For woody plants and most perennials, powdery mildew is a cosmetic problem that will not kill an established plant. The leaves look dusty and tired, but the roots and stems keep right on going.

For annuals, especially fruiting varieties, powdery mildew can gradually destroy the plant’s ability to produce flowers and fruit. The mycelium, which is the fungal tissue, develops on both sides of the leaf and sends root-like structures down into the top leaf layer to steal nutrients. This weakens infected plants, withers the leaves, and stunts fruit growth. On a cucumber or squash vine, that can be the difference between a full harvest and a handful of runts.

Plants Most Prone to Powdery Mildew

Some plants seem to invite trouble year after year. Knowing the usual targets lets you scout them first and start a preventive routine before the white spots ever appear.

• Vegetables: cucumbers, squash, pumpkins, melons, and other cucurbits, along with beans, peas, and tomatoes.
• Flowers: phlox, zinnias, sunflowers, bee balm, dahlias, and roses.
• Trees and shrubs: apple, birch, lilac, linden, maple, oak, and rhododendron.

When you shop, read the plant tags. Many seed and nursery catalogs now list mildew-resistant varieties of cucumbers, phlox, and bee balm, and choosing one of those is the cheapest insurance you can buy. Clemson University’s Home and Garden Information Center keeps a helpful rundown of susceptible plants and resistant picks if you want to check a specific species before you plant.

What Causes Powdery Mildew?

Powdery mildew spreads by spores, which can travel dozens of miles on the wind or get splashed onto leaves from nearby plants and debris. Despite the name, powdery mildew does not need standing water to germinate or infect.

In fact, the spores of some powdery mildew fungi can be killed by extended exposure to water. But do not try to wash the mildew off with the hose. Raising the humidity around the plant only encourages more growth, and the extra water can bring on other problems for the leaves and roots.

Conditions have to line up for powdery mildew to take hold. High humidity helps the spores germinate, but the real triggers are moderate temperatures of 60 to 80 degrees Fahrenheit paired with crowded, shaded plants that lack air circulation.

Direct sunlight and temperatures above 90 degrees are hard on spore growth and spread, which is why powdery mildew tends to show up later in the summer, when nights cool off and mornings turn muggy. That seasonal timing is your cue to start scouting the usual suspects in July and August.

Powdery Mildew Treatment and Prevention

Prevention is the heart of any powdery mildew treatment. Cultural controls come first: plant resistant varieties and set them in open, sunny spots. Pruning to open up air circulation also helps, as does skipping overhead watering, which raises the humidity right where the fungus wants it. Go easy on nitrogen-rich fertilizer too, since it pushes out tender new growth that mildew loves.

If powdery mildew turns up on just a few leaves, pulling those infected leaves may slow the spread. If the infestation is already widespread, even chemical sprays will do little to save the parts that are already coated.

One good habit, especially if a plant has been hit in past seasons, is to apply preventive treatments on vulnerable plants all through the growing season to head off likely infections. Mildew is far easier to prevent than to cure.

Commercial fungicides can be used according to the label instructions, and we recommend alternating between two different fungicides for the best protection, since the fungus adapts less quickly when you rotate them.

Non-Chemical Recommended Solutions

Plenty of gardeners never reach for a commercial product at all. These kitchen-cupboard remedies are gentle on beneficial insects and cheap to mix up at home.

• Baking soda: Perhaps the best known “home remedy” for powdery mildew treatment is baking soda. Make your own spray by mixing one tablespoon of baking soda with 1/2 teaspoon of liquid dishwashing soap in one gallon of water.
• Vinegar: In one gallon of water, add two to three tablespoons of apple cider vinegar (with an acidity of 5 percent). The acid in the vinegar controls powdery mildew. For more ways to put this pantry staple to work, see our guide to the many powers of vinegar.
• Milk: Mix one part milk to two parts water. It is not fully known why this works, but it is possible that the compounds in milk act not only as a fungicide but also to strengthen the plant itself. Best applied every week, and it works well on cucurbits.
• Mouthwash: Mix one part mouthwash with three parts water. A generic brand of mouthwash with an ethanol base works best.
• Neem oil: Follow the instructions on the label for application.
• Jojoba oil: Less toxic than neem oil, jojoba oil has antifungal properties. Mix one to two tablespoons of oil in a gallon of water.

Homemade Spray Recipes at a Glance

Keep this chart handy when you are mixing a batch. Whichever recipe you pick, apply it in the evening and coat the top and underside of every leaf.

Important Notes

• Fungicides may harm beneficial insects, such as bees, and therefore should either not be used or used sparingly. Less toxic pesticides can be effective against powdery mildew but are best applied throughout the season to help prevent an outbreak.
• Any spray should be applied in the evening to avoid sunshine and beneficial insects. Spray on the top and underside of the leaves every one to two weeks. Please note: tender, new foliage may be damaged by any spray application.
• Always test a spray on a few leaves before spraying the full plant or plants. Also, do not apply a powdery mildew treatment to drought-stressed plants or if the temperature is 90 degrees or warmer.

FA Community Question Answered

In September 2025, we received a question about powdery mildew in the comments section of our Gardening Calendar.

Question

“I bought a plant at a nursery and didn’t notice until home that all the leaves were covered with mildew. I sprayed it and tried to pick all the mildewed leaves off. It is a herbaceous perennial, Coreopsis (Bright Touch). I tried to remove the fallen leaves in the container also. I will remove it from the container and remove as much of the soil as possible. Giving it good fresh potting soil and a new container. Will it still have mildewy leaves on its new growth? Is it worth trying to rescue?” -Marti

Answer

Dear Marti,

It is definitely worth trying to rescue your Coreopsis. While it is a frustrating discovery, powdery mildew is a very common fungal disease, and a plant can often recover with proper care. Coreopsis ‘Bright Touch’ is known to have a “low resistance” to powdery mildew, so it is a good idea to be proactive.

Your plan to remove the affected leaves and old soil is an excellent first step. Here is what you need to know, plus a few more things you can do to give your plant the best chance:

Steps to Take Now and in the Future

1. Sanitation is Key: You have already done this, but it pays to be thorough. When you repot, clean the old container (if you are reusing it) with a 10% bleach solution to kill any remaining spores. (If you would rather not use bleach, scrub with hot water and soap or a mild solution of vinegar and water. Ask us follow-up questions if you like.) Dispose of the old soil and the mildewed leaves in the trash, not your compost pile.
2. Repot with Fresh Soil: Using new, high-quality potting soil gives your Coreopsis a clean start and the nutrients it needs to grow strong.
3. Improve Air Circulation: This is one of the most important preventive measures. When you repot, do not crowd the plant. Give it plenty of room to “breathe” in its new container. If you plan to set it in the ground, keep adequate spacing from other plants.
4. Proper Watering: Avoid watering the foliage from above. Instead, water the soil directly. The fungus does not need wet leaves to germinate, but wet leaves can still spread the spores. Watering in the morning is a good practice, so any splashed water has time to dry during the day.
5. Location, Location, Location: Coreopsis needs full sun, six to eight hours a day. Place your plant where it will get ample sunlight, which helps keep the fungus from taking hold.
6. Consider a Fungicide (Preventative): Since ‘Bright Touch’ is susceptible, you may want a preventive treatment. You can use a commercial fungicide made for powdery mildew or one of the home remedies mentioned above.

Is It Worth Trying to Rescue?

Yes, absolutely. Powdery mildew is rarely fatal to a plant. While it can stress the plant and cause leaves to yellow, twist, or drop, it usually does not kill it.

By following the steps above, you can help your Coreopsis recover and thrive. A strong, healthy plant is always better equipped to fight off disease.

Join The Discussion

Do you know an effective way to treat powdery mildew not mentioned here?

Share your tips with your community here in the comments below!

Any other questions about plant diseases? Let us know!

Powdery Mildew Treatment: Frequently Asked Questions

What is the best homemade powdery mildew treatment?

The best known home remedy is a baking soda spray: mix one tablespoon of baking soda with 1/2 teaspoon of liquid dish soap in one gallon of water. Apple cider vinegar, milk diluted one part to two parts water, and diluted ethanol mouthwash also work. Apply any of them in the evening, every one to two weeks, coating the top and underside of the leaves.

Will powdery mildew kill my plant?

Usually not. Powdery mildew is rarely fatal to woody plants and perennials, where it is mostly a cosmetic problem. On annuals, especially fruiting types like squash and cucumbers, it can stunt fruit and flower production if left unchecked, so treat those early.

What conditions cause powdery mildew?

Powdery mildew thrives at moderate temperatures of 60 to 80 degrees Fahrenheit with high humidity, crowded plants, and shade that blocks air circulation. Direct sunlight and temperatures above 90 degrees slow it down, which is why it usually appears later in the summer.

Should I remove leaves affected by powdery mildew?

Yes, if only a few leaves are affected. Removing infected leaves early can slow the spread. Dispose of them in the trash rather than the compost pile so the spores do not travel. Once the infestation is widespread, even chemical sprays do little for the parts already coated.

How do I prevent powdery mildew from coming back?

Plant resistant varieties, give plants full sun and open spacing, prune for air circulation, and avoid overhead watering. Go easy on nitrogen-rich fertilizer, which pushes tender growth. If a plant was hit last year, apply a preventive spray all through the growing season.

Can I wash powdery mildew off with water?

No. Although extended water exposure can kill some mildew spores, hosing the plant raises the humidity around it and encourages more growth. The extra water can also cause other problems for the leaves and roots. Use one of the sprays above instead.