What Is This Powdery Mildew on My Plants' Leaves?
As a former landscaper, I have seen a lot of plant diseases. One of the easiest ones to recognize is powdery mildew; it looks just like its name. The mildew will form a white or gray powdery film on a plant's leaves, stems, and fruit.
Powdery mildew is a fungal disease caused by the many varieties of the fungi that belong to the order Erysiphales. The fungus thrives in warm, humid environments, and overwinters in the soil. The mildew forms spores that spread through wind, insects, and water run-off, which carries the disease to other plants. According to the Arizona Cooperative Extension, "Unlike most fungi, spores germinate on the surface of plant parts without the presence of free water."
8 Organic Treatments to Get Rid of Powdery Mildew
- Potassium bicarbonate
- Neem oil
- Baking soda
- Copper fungicides
1. Potassium Bicarbonate
Potassium bicarbonate is a safe, effective fungicide that kills spores on contact. Like baking soda, it is also a great preventative treatment because it raises the pH level above 8.3—an alkaline environment that is not ideal for fungal growth.
How to Use:
Mix 3 tbsp. of potassium bicarbonate, 3 tbsp. vegetable oil, and 1/2 tsp. soap into a gallon of water. Spray onto affected plants.
Numerous studies have shown milk and/or whey to be even more effective at killing powdery mildew than chemical fungicides. In a 2009 study by the University of Connecticut, which tested a milk treatment of 40% milk and 60% water on plants infected with powdery mildew, "the milk treatment provided significantly less disease than the untreated control, and the chemical treatment had equal or significantly less disease than the milk." Scientists are not sure why milk is so effective, but they believe that when milk interacts with the sun, it produces free radicals that are toxic to the fungus.
2 Ways to Use:
- Mix 60 parts water with 40 parts milk or whey, and spray onto the affected plants bi-weekly. You can even use whole milk without dilution for a strong effect.
- Mix 1 oz. powdered milk to 2 liters of water, and spray onto affected plants bi-weekly.
3. Neem Oil
Neem oil is made from the seeds and fruit of the evergreen neem tree, and it is powerful enough to kill powdery mildew in less than 24 hours. The oil works by disrupting the plant's metabolism and stopping spore production. Neem oil is also a great insecticide and since spores can be carried by bugs, this oil is a great preventative treatment as well.
How to Use:
Mix 3 tbsp. of neem oil to one gallon of water, and spray onto affected plants every 7-14 days. Take precautions to avoid sunburning the leaves, and avoid spraying the plant's buds and flowers.
The acetic acid in apple cider vinegar is very effective in killing powdery mildew. Take care to not make the mixture too strong as the acidity of the vinegar can burn plant leaves.
Mix 4 tbsp. of vinegar (5% solution) with 1 gallon of water. Reapply every three days.
5. Baking Soda
Baking soda has a pH of 9, which is very high! Treating with baking soda raises the pH level on the plants and creates a very alkaline environment that kills fungus. There have been mixed reports of success when using baking soda to treat severe cases, so it may be better as a preventative treatment than a fungicide.
How to Use:
- Mix 1 tbsp. of baking soda and 1/2 tsp. liquid hand soap with one gallon of water.
- Spray solution on affected leaves, and dispose of any remaining solution.
- Do not apply during daylight hours. It may be best to test one or two leaves to see if the solution will cause the plant to suffer sunburn.
Garlic has a high sulfur content and is an effective anti-fungicide. Garlic oil can be bought commercially if you do not wish to make the solution at home. It works best when added to organic oil mixtures.
How to Use:
- Crush six cloves of garlic and add to one ounce of an organic oil such as neem oil and one ounce of rubbing alcohol. Let set for two days
- Strain and retain the liquid and crushed garlic.
- Soak the garlic again (this time in one cup of water for a day). Strain out and dispose of the crushed garlic.
- Add the oil and alcohol mixture and garlic water to one gallon of water.
- Spray your plants, coating only the leaves.
Sulfur is a natural product that is very effective at preventing and controlling powdery mildew. Sulfur can be bought as a dust or as a liquid and can be added to sulfur vaporizers.
How to Use:
Follow the dosing instructions closely and wear gloves, eye protection, and a face mask. Avoid inhaling or coming into contact with the sulfur.
8. Copper Fungicides
Copper is a very effective fungicide, but it is very important to follow label directions closely. Too much copper will be detrimental to the plant and the soil.
How Can I Prevent Powdery Mildew?
Preventing the spread and/or severity of powdery mildew is the most cost-effective way of dealing with the fungus. Powdery mildew thrives in temperatures 50-65 degrees Fahrenheit with humidity levels of 80-90 percent. To prevent powdery mildew from forming in the first place, avoid low-temperature, high-humidity environments.
Do Not Crowd Plants
Good air circulation ensures lower humidity levels, inhibiting the growth of powdery mildew. Crowded plants also provides too much shade for the lower leaves, which encourages fungi growth.
Do Not Grow Susceptible Plants in the Shade
Powdery mildew does not tolerate high temperatures. Direct sunlight helps stem the growth of mildew because the sun's strong rays kill spores before they can spread. Plants that are shaded much of the day will stay cooler, thus encouraging the growth of mildew.
Dispose of Infected Leaves and Stems
Never use infected plant leaves or fruit as mulch or compost. Trim off infected leaves and stems and dispose of them properly. If your municipality allows backyard fires, then burn the debris. If not, dispose of the debris according to your local plant disposal regulations.
Water the Soil, Not the Plants
While water itself will not encourage mildew growth, splashing the leaves with water will spread the spores. Run a hose to the base of your plants instead of using a sprinkler system.
Buy Mildew-Resistant Varieties
There are a large variety of hybrid plants that are resistant or tolerant to the growth of powdery mildew. The resistant plants will be less likely to develop the mildew. The tolerant plants will show fewer ill-effects of an infestation of the fungi.
Plants and Vegetables Most Susceptible to Powdery Mildew
Are All Forms of This Fungus the Same?
There are many forms of powdery mildew, and each is species specific. Grapes will suffer from the powdery mildew that affects only grapes, roses suffer from rose powdery mildew, and so on.
Although all plants can get the fungus, certain species are more susceptible to it. If your plant has a black sooty substance on its leaves, it may be sooty mold.
Where Does Powdery Mildew Start?
The mildew usually starts on a plant's lower leaves, and if the fungus is not treated it will spread over the entire plant. When the leaves become severely covered with the fungus, photosynthesis will be affected and leaves will yellow and drop off. As a result, the plant may become so stressed it will not flower and/or fruit with any vigor.
For More Information
- Powdery Mildew
Most powdery mildews are recognized by the white to gray,powdery spots or large blotches on the surface of leaves,stems and fruits of host plants. T
Questions & Answers
Question: I discovered powdery mold on my cucumber plants today. I've already picked seven of them. Many have already started. Will this kill the ones that are started?
Answer: No, but powdery mildew can affect the growth and vigor of the cucumber plant. Treat the powdery mildew now, and then weekly until the fungus is gone.
Question: Can I treat the soil before I replace the current plant with a new shrub?
Answer: You can, but the mildew spores do not reside in the soil. Remove any plant debris from an infected plant from the area and dispose of. If you are having a chronic problem with mildew, ensure you have adequate circulation between plants, and water plants at ground level to avoid spreading the spores. Also look to see if there is enough natural sunlight, which helps control the growth of the mildew.
Question: Can I still wash and use the cucumbers if a plant has signs of Powdery mildew?
Answer: Unless you have an allergy to the fungal spores, washing it would make it safe to eat, although if the cucumbers themselves are heavily mildewed, they may taste moldy.
If there is any doubt, DO NOT eat the fruit as a severe allergic reaction could be fatal.
Question: How often should I apply baking soda spray to kill mildew?
Answer: Reapply once a week, or after a heavy rain.
Question: Will a weak solution of bleach and water kill powdery mildew on lilac leaves?
Answer: Yes, it will, but I would make the solution very diluted and test it on one leaf first. Do not drench the leaves or soil with the mixture.
Question: I have kale with powdery mildew. Is it safe to eat the leaves if I wash them well?
Answer: Most people have no problem with powdery mildew, but some will have an allergic reaction to the spores.
Question: I'm effectively using the baking soda solution in my vineyard this year to prevent white powdery mildew, though I am a bit concerned. Will it have a negative effect in making the wine? There is a slight film on the grapes now. We will harvest within the next 30 days.
Answer: Just wash the grapes well before making wine. The baking soda film will easily wash off the grapes, and will not penetrate the thick skin of the fruit.
Question: My mango tree is a sapling and has powdery mold. How should I treat it?
Answer: Cut off the affected leaves and remove any diseased fruit or blossoms. Remove the infected plant parts as the fungus can still be carried to healthy parts of the plant.
Spray with any of the products mentioned in my article, concentrating on the leaves and their undersides; repeat every two weeks. In the spring, a preventive spraying will reduce fruit loss.
Question: Do you treat small houseplants the same way?
Answer: Yes, houseplants can be treated the same way. I would suggest treating the plant and changing the soil.
Question: We grow vine leaves for making dolma to eat. Which treatment would be most effective?
Answer: I would use the neem oil on your leaves. Remember to wash leaves well before consuming.
Question: What plants (shrubs) are resistant to powdery mildew fungus?
Answer: There are many varieties of shrubs bred to be resistant to powdery mildew. If you know what shrubs you want, research which varieties are available, or ask your local nursery or local county extension agent which varieties are good for your climate.
Question: Can I reuse the soil after it has been treated for powdery mildew?
Answer: Yes, but I would either pretreat the soil or otherwise sterilize it.
Question: Can I treat the soil prior to planting to prohibit the mildew from growing?
Answer: Yes, you can. There are several ways to do this.
Terraclor, Iprodione, and Mefenoxam are examples of chemicals that can treat the soil. Check the labels to see what fungi it controls and how to apply.
Some organic gardeners pretreat large gardens areas by heavily soaking the ground with water, then covering the garden area for 4-6 months. The heat and humidity kill spores as well as weeds.
Jo Ellen Serum on August 20, 2020:
Great information. I do container gardening & powdery mildew has been an invader for 3 years on my zuchinni & cucumber.
Gable Rhoads (author) from North Dakota on July 27, 2020:
Old Bean, a biweekly application would be every two weeks.
You will need to spray every leaf that has mildew. I would also recommend watering at the base of your plants rather than using the fan sprayer. Watering the leaves helps spread the mildew.
Eileen on July 25, 2020:
I recently purchased a home with citrus trees. We put grass down and used a fan sprinkler. We now have white powder on citrus leaves. After doing research it seems it could be powdery mildew from the fan sprinkler watering the leaves. Do I need to spray all the leaves (with milk/neem, etc) to rid the mildew? I don't want to miss our crop or have it ruined. And, I'm not sure what type of oranges and lemons and for sure if it is a mildew.
OldBean on June 28, 2020:
Thank you for all of these great alternatives! I used milk last summer on some Coreopsis and it did work, however I started using it too late for the plants to survive. Could you please clarify your usage of biweekly? It is a most confusing term (as is bimonthly!) as it has two very different meanings, both of which are correct! Twice a week or every two weeks avoids the confusion! Thank you!—and again, for all of the suggestions!
Yitayew Belete on June 01, 2020:
Thanks to you for posting this significant remedies for aggressive powdery mildew diseases!!!
Zara on May 04, 2020:
What causes the powdery mildew?
Amanuel on April 12, 2020:
Thank you...but I have one question..is it working on mango's tree if I use the above ways?
Diane on April 08, 2020:
I had the same experience as the previous writer using the baking soda mixture. I carefully mixed it and applied to my shad bush. Every leaf fell of the tree. It did survive and is starting to leaf this spring. Now wondering which prevention to try next...
D K Morgan on March 31, 2020:
Powdery Mildew is my most difficult garden problem. Last spring, I tried the Potassium Bicarbonate treatment listed above on my infected tomato plants which were about 2-3 ft. in height,... spraying them after the sun was off the plants. The experience taught me another painful garden lesson... experiment on 1 plant 1st. The treatment killed all 10 of my plants, each plant limp with crinkled leaves the next morning. And yes, I did double check each ingredient measurement before mixing to be sure of accuracy. The Potassium Bicarbonate used was of highest organic food grade PB, so I am still at a loss. Perhaps just paying the extra $ for GreenCure is the safer alternative as the sting of disappointment still haunts me.
Girlie on March 23, 2020:
I have custard apple plants and the fruits are usually covered in a white fungus. How do I remove the fungus and still have the fruit safe for eating? Thanks :)
Vanessa on March 12, 2020:
I have a small rose plant at home and it began to get covered by white powder like stuff. It was growing and then it got infested. Can i apply just regular milk all over it? Or just in soil? And is it daily?
Gerry on January 23, 2020:
Great advice. Been lucky so far, but will be trying the household stuff first if I spot it at any time. Neem oil here (UK) is expensive.
Gable Rhoads (author) from North Dakota on July 28, 2019:
Yes, the weekly application will be enough barring heavy rain. Remember to remove dropped leaves and fruit.
Is it possible that the mango trees have something other than powdery mildew? Check out this link to mango diseases. https://www.gardeningknowhow.com/edible/fruits/man...
Have your rambutan trees ever borne fruit? Is it possible you have all male or female trees, or that the trees may be too immature to fruit?
John Thomas on July 28, 2019:
We have rambuttan trees and mango trees at home. The mango trees have an odium attack which has developed dark patches on the fruit which affects the fruit quality.
The rambuttan trees have not flowered ( whither away ) for 2 years. I see it’s leaves affected too. Today I sprayed neem oil mixed with water....to be done once a week. Is this enough ?
Magdalena F. Algura on February 14, 2019:
I try it to my grapes affected severely with powdery mildew. Thank you so much for the idea.
Suzie HQ on October 13, 2018:
Thanks for a really interesting article and I am all for organic solutions. At present I find a number of plants in your list in need of some treatment. I am living in Southern Italy where high summer temps and high humidity play havoc with plants on one hand and give amazing growth on the other. I have a big sunflower plant in a container with about 80 flowers quite a showpiece but it does seem to have alot of mildew . I have tried your vinegar recipe so fingers crossed! Many thanks again I will be saving and using it again.
Paula on August 25, 2018:
...ive had this powdery on my houseplants...ive been
using...an organic purchased product..doesnt seem to do the job..i love baking soda/vinegar..gt products 4 many uses..i will try this..and also milk..thanku
Chris Nddie from Dana Point on August 16, 2018:
Organic is Natural else all are Chemicalize. Great post
Doug on July 27, 2018:
Thanks for this article, Gable. Great information! I am testing some of the treatments you suggest in my garden.
As regards using milk to combat powdery mildew, it may be true, as you write, that "Numerous studies have shown milk and/or whey to be even more effective at killing powdery mildew than chemical fungicides." That is not true of the study you cite, however.
That paper compared milk treatment to chemical treatment at two outdoor locations (Storrs field and Windsor) and concluded that "Storrs field showed milk was as effective as the chemical control [chlorothalonil)]....At the Windsor location, however, the milk treatment was not as effective as the chemical control."
The study also compared milk to chlorothalonil in a greenhouse environment and found that "the milk and chemical treatments... did not differ from one another...in their effectiveness."
The very good news for organic gardeners is that this study found milk to be very effective: just as good as the chemical fungicide at two out of three test sites! It did not find, however, that milk is "even more effective...than chemical fungicides."
Gable Rhoads (author) from North Dakota on June 10, 2018:
Typically the leaves do die because of the infection. Remove them to stop the spread of any spores which may still be living. As to the entire plant dying, it could be the result of a very bad case of powdery mildew or many other causes. It may be best to start over with new plants and soil.
Alex tesfaye on June 10, 2018:
The powdery mildew reduced. But now i have much problem the leaf of the sage curl upward and turn in to brown and the plants are dying
Gable Rhoads (author) from North Dakota on June 08, 2018:
Alex, neem oil is an organic substance. Do you continue to have powdery mildew after spraying?
Alex tesfaye on June 07, 2018:
I am interested in agriculture field in Production of vegetables, fruit and herb. And i have problems with powdery mildew my sage plant. I am growing them organic. I spray neem oil but it is the same?
Anne H on February 23, 2018:
What is this “Mitey sauce “you speak of? Coconut oil?
Patricia Bagshaw on November 09, 2017:
Wonderful article. My plants have stunted growth. No blossoms. Geraniums have very small leaves and no flowers. I water and fertilize so I am not sure what my
Problem is. Thank you.
Jasmine Flower on October 09, 2017:
That is a really good idea John.
John on August 12, 2017:
Try "mitey sauce" a local company- the coconut oil in it works on Powdery mildew but does not block the stomata like neem oil does.
Gable Rhoads (author) from North Dakota on August 09, 2017:
Liz, according to this website, the rosemary is safe to eat: https://extension.umd.edu/growit/downy-mildew-basi...
Other sites say it may affect the flavor. If you are treating the plants, wash the leaves thoroughly.
Liz on August 09, 2017:
I have some on my rosemary plant, what happens if I eat it?
Carmine on July 31, 2017:
I think if powdery mildew on grapes turns the skin dark color and some skins crack it is too late those clusters should be cut off and discarded i trash bin. Then the infected vines should be sprayed with one of the articles recommended materials
Vishal on April 20, 2017:
In India Maharashtra the powdery mildew have largely affected grapes...
Mike on August 12, 2016:
Fist site I found that gave the when to reapply info. Thanks.
Gable Rhoads (author) from North Dakota on February 13, 2016:
Thanks for the comments Kristen and breathing. It is encouraging to get positive feedback. :)
TANJIM ARAFAT SAJIB from Bangladesh on February 12, 2016:
Plant lovers will be greatly benefited from this post. Indeed plants are subjected to many diseases. Powdery mildew is a very common disease among the plants. Many people who have their own plants don’t know how to treat powdery mildew. This post can be the cornerstone for them. The steps described in this hub are really worthwhile. I’ve tried a few ones myself and find them to be really effective. The organic ways are very much helpful. That’s why I advise the plant lovers to use the teachings of this post to exterminate powdery mildew. Also none of the processes are costly.
Kristen Howe from Northeast Ohio on October 22, 2015:
Great hub, Gable. This is so useful to know for next season, when I watch over my container garden. I'll keep it in mind to use milk or baking soda, if I do have that powdery mildew on my plants. Two green thumbs up!
JANARDHAN N on January 16, 2014:
this as given me some assistance to fight against powdery mildew
thanks to rentokil
Gable Rhoads (author) from North Dakota on April 25, 2013:
I'm glad I could help. :)
RTalloni on April 25, 2013:
Thanks for this helpful look at eliminating powdery mildew from plants. I have one that needs attention…