Powdery Mildew Treatment

Does your plant look like it’s covered with confectioners’ sugar? Chances are these white powdery spots are powdery mildew. Not to fear, this condition is rarely fatal to plants. All you need is our helpful powdery mildew treatment suggestions and prevention tips.

What is Powdery Mildew?

Powdery mildew is a common term for a range of plant fungi found around the globe. Each type affects different groups of plants. For example, the powdery mildew fungus that thrives on cucurbits (think cucumber, squash and melon) isn’t the same one that is found on phlox, sunflowers and zinnias. And neither one is the powdery mildew species that proliferates on roses. Some powdery mildews infect tree and shrub species, including apple, birch, lilac, linden, maple, oak and rhododendron.

What does Powdery Mildew do to plants?

For woody plants and most perennials, powdery mildew is a cosmetic problem that won’t kill established plants.

For annuals, particularly fruiting varieties, powdery mildew can gradually destroy the ability of the plant to produce flowers and fruit as the mycelium (fungal tissue) that develops on both sides of the leaf sends down root-like structures into the top leaf layer to access nutrients. This weakens infected plants, withers the leaves, and stunts fruit growth.

What Causes Powdery Mildew?

Powdery mildew spreads by spores, which can travel dozens of miles in the air or be splashed onto leaves from other plants or debris. Despite the name, powdery mildew does not need water to germinate or infect.

In fact, the spores of some powdery mildew fungi can be killed by extended exposure to water. (But don’t use water as a means to wash off powdery mildew as raising the relative humidity around the plant can encourage mildew growth. Excess water can also cause other problems for the affected plant.)

Conditions need to be right for powdery mildew to proliferate. While high humidity is needed to aid in spore germination, what is crucial are moderate temperatures of 60 to 80 degrees Fahrenheit with crowded, shaded plants that lack air circulation.

Direct sunlight and temperatures above 90 degrees are detrimental to spore growth and spread, which is why powdery mildew tends to appear later in the summer when conditions are generally more favorable.

Powdery Mildew Treatment and Prevention

Prevention is key to combating powdery mildew. Cultural controls include planting resistant varieties and locating plants in open, sunny spots. Pruning to aid with air circulation also can help, as does avoiding overhead watering, which raises the relative humidity. Avoid excessive use of nitrogen-rich fertilizer, which promotes new growth that is more vulnerable to powdery mildew.

If powdery mildew is spotted on just a few leaves, removing those infected leaves may help slow the spread. If the infestation is widespread, even chemical applications are likely to do little to save the infected parts.

One recommendation, especially if there have been prior infections, is to apply preventive treatments on vulnerable plants all through the growing season to help mitigate likely infections.

Commercial fungicides can be used according to label instructions, and we recommend alternating between two different fungicides for best protection.

Non-Chemical Recommended Solutions

• 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.
• Milk: Mix one part milk to two parts water. It’s not fully known why this works, but it’s possible that the compounds in milk work not only as a fungicide but to strengthen the plant itself. Best applied every week and 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.

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’s 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’s a good idea to be proactive.

Your plan to remove the affected leaves and old soil is an excellent first step. Here’s what you need to know and 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’ve already done this, but it’s important to be thorough. When you repot, make sure you clean the old container (if you’re reusing it) with a 10% bleach solution to kill any remaining spores. (If you would prefer not to use bleach, you may scrub with hot water and soap or a mild solution with 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 will give your Coreopsis a clean start and provide it with the nutrients it needs to grow strong.
3. Improve Air Circulation: This is one of the most important preventive measures. When you repot the plant, don’t crowd it. Give it plenty of room to “breathe” in its new container. If you plan to plant it in the ground, ensure there is adequate spacing from other plants.
4. Proper Watering: Avoid watering the plant’s foliage from above. Instead, water the soil directly. The fungus doesn’t 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 on the leaves has time to dry during the day.
5. Location, Location, Location: Coreopsis needs full sun (six to eight hours a day). Make sure you place your plant in a spot where it will get ample sunlight, as this helps prevent the fungus from taking hold.
6. Consider a Fungicide (Preventative): Since ‘Bright Touch’ is susceptible, you may want to consider a preventative treatment. You can use a commercial fungicide specifically for powdery mildew or a home remedy (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 doesn’t 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!

Janine Pineo

Janine Pineo has been gardening all her life in Maine and writing about it for more than two decades. More of her writing can be found on her website, GardenMaine.com.

Peter Geiger

Peter Geiger is the Editor Emeritus of the Farmers' Almanac. Read his full biography.

Farmers' Almanac Staff

This article was published by the staff at Farmers' Almanac. Do you have a question or an idea for an article? Contact us!

• Farmers' Almanac Staff

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  September 15, 2025

  Powdery Mildew Treatment