Controlling or Eliminating Powdery Mildew

Providing adequate air circulation will help avoid powdery  mildew in your garden

Judging from the number of questions I get every summer about the plant fungus known as powdery mildew, I’ve learned three things over the years; it’s everywhere, you don’t like it and you want to know how to get rid of it. So, here’s what you need to know to prevent, control and even eliminate it after it appears.

You’re not alone

Powdery mildew is one of the most widespread and easily identifiable plant fungal diseases. From vegetable gardens to rose gardens, ornamental trees and shrubs, almost no type of plant is immune.

If you find that some of your plants or trees have powdery mildew, don’t worry. This fungus is host specific, meaning just because you find it on one plant species, does not make it a threat to other type plants in your landscape. Although there are many different species of powdery mildew, the symptoms all look about the same from one to another.

What to look for

You’ve likely seen it many times. White or gray powdery spots appear, often times covering most if not the entire leaf surface. It’s also found on plant stems, flowers and even fruit. Fortunately, the symptoms of powdery mildew are usually worse than the actual damage. Rarely is it fatal to the plant.

Advanced stages can cause plant foliage to yellow, curl or turn brown and eventually cause the plant to defoliate prematurely. On flowering plants and trees, the fungus can lead to early bud drop or reduce the flower quality.

So Now What?

Conditions that favor mildew formation include dry foliage, high humidity, low light and moderate temperatures. Proactive steps to avoid or minimize this risk include:

· Look for disease resistant varieties. Contact your local county extension service for named varieties and cultivars.

· Provide adequate air circulation by not crowding plants.

· Site plants where they will get sufficient light of six hours or more each day. Minimize shade and trim trees and shrubs that block the light.

· Avoid over fertilization. New growth is more susceptible. Instead, apply a slow-release fertilizer that provides more controlled growth.

Controlling an existing problem

Should you find the need to react to an existing condition of powdery mildew, early detection provides the best way to contain and potentially eliminate the problem. There are many commercial products that are effective at containing the spread. However, eliminating an existing problem is not a given.

Most conventional products are made for prevention and control, not elimination of an existing infection. That’s why it’s important to start a control program before powdery mildew occurs or at least at the earliest sign of detection.

There are many retail, off-the-shelf fungicide products that are effective at treating mildew. One of the most common active ingredients used for control is “chlorothalonil”. Although effective, it coats the leaf surface with a white milky film that is quite noticeable.

Lesser know options include:

Neem oil - This is a readily available organic option to disease and pest control. Neem oil is extracted from the neem tree, native to India. This is an effective disease control and a broad spectrum, natural insecticide that is kinder to beneficial insects and mammals.

Water – Ironically, dry conditions and high humidity are the most favorable conditions for powdery mildew to form. But straight water is its enemy because it washes off the spores before they have time to embed. However, water isn’t something that I promote for control because wet foliage is friend to many other plant diseases.

Baking Soda (sodium bicarbonate) -This is possibly the best known of the home-made, organic solutions for powdery mildew. Although studies indicate that baking soda alone is not all that effective, when combined with horticultural grade or dormant oil and liquid soap, efficacy is very good if applied in the early stages or before an outbreak occurs.

Use this recipe to make your own solution—mix one tablespoon of baking soda with a teaspoon of dormant oil and one teaspoon of insecticidal or liquid soap (not detergent) to a gallon of water. Spray on plants every one to two weeks.

Potassium bicarbonate- Similar to baking soda, this has the unique advantage of actually eliminating powdery mildew once it’s there. Potassium bicarbonate is a contact fungicide which kills the powdery mildew spores quickly. In addition, it’s approved for use in organic growing.

Mouthwash - If it can kill the germs in your mouth, certainly the fungal spores of powdery mildew are no match. And that’s the premise. Generic, ethanol based mouthwash can be very effective at control. Tests using one part mouthwash to three parts water worked for well for Jeff Gillman, Ph.D and Associate Professor at the University of Minnesota, Department of Horticulture. Just be careful when mixing and applying mouthwash as new foliage can be damaged.

Vinegar – Similar to mouthwash, the acetic acid of vinegar can control powdery mildew. A mixture of 2-3 tablespoons of common apple cider vinegar, containing 5% acetic acid mixed with a gallon of water does job. However, too much vinegar can burn plants but at the same time, higher concentrations (above 5%) are more effective.

Sulfur and Lime/Sulfur - Direct contact by sulfur prevents disease spores from developing. When mixed with hydrated lime, the solution will penetrate leaves for even greater effectiveness. A widely available version of this combination includes copper sulphate and hydrated lime, known as Bordeaux mix. However, all of these solutions can burn plant tissue and is damaging to microorganisms in the soil and harmful to beneficial insects. It is also considered moderately toxic to mammals and humans. Use sparingly and with caution if at all.

Milk - The latest player in the fight against powdery mildew is milk. It’s not clear yet why it works so well, but it is believed that naturally occurring compounds in the milk are at work to combat the disease while also boosting the plant’s immune system. One experiment showed good results by applying a weekly dose of one part milk to two parts water. This is yet another case when more is not better. Concentrations above three parts water had adverse side effects.

Even with many choices for control, prevention is still the best medicine, not only with powdery mildew, but with other diseases as well.

About

Joe Lamp'l is the Host and Executive Producer of the award winning PBS television series Growing A Greener World. Off camera, Joe dedicates his time to promoting sustainability through his popular books, Compost Confidential blog, podcast series, and nationally syndicated newspaper columns. Follow Joe on Google+

Comments

  1. Susan says

    I have two columnar oaks already quite high. They have brownish bumps on the bottom of leaves and fungus on the top. This was the case last year as well. A professional did deep root feeding. The leaves came back this spring looking very healthy but now in July we have the same problem. I could use the milk idea but it would be a big job. I am wondering if one does nothing will the tree always look great in the spring and then in July develop this disease but not really kill the trees.

    • says

      Hi Susan. You likely have a leaf gall which is very common to oak trees. It’s cosmetic only and will not harm your tree. I get them all the time and do nothing. My trees are fine and continue to thrive year after year. If you want to confirm this condition, take a sample to your county extension service for verification.

  2. Karen says

    I applied milk water solution to my squash and cucumber that both have powdery mildew yesterday and they already look better, how often should I do this? I saw mention every 2 weeks but was that just for prevention or also for present mildew problem? Thanks for article!

  3. Jennifer says

    This morning I sprayed a 50/50 solution (milk/water) on my squash, pumpkin and cucumber leaves… and then I just came across your site and read, “Concentrations above three parts water had adverse side effects.” What are the side effects?

  4. Ariel says

    I am a first-time gardener this year in Florida, and soon found my glorified dreams of a successful apartment balcony container garden under seige by that nasty powdery mildew crap I had only read about in passing and while skipping over lots of reading since I assumed nothing could threaten my screened-in plants. I was so frustrated I gave up on some of my plants, worried food from these plants wouldn’t be edible because the leaves become quite disgusting. Can food be harvested from a withering moldy plant? Bleh

    Anyway, this webpage is a lifesaver. Thanks for giving me several eco-friendly options to combat the horrifying powdery mildew and giving me renewed resolve in learning to garden!!

    • says

      Thank you Ariel! So glad you found our information helpful and that you took the time to write us about it. Never give up. Just try and learn from your experience. That will make you an awesome gardener!

  5. Susan Gelderbloom says

    Would Diatomaceous Earth also work? Since I have a two fold problem in my garden (broccoli and cabbage being eaten to bits) I was hoping to be able to use a single product. I just puffed my veggies with the DE but it only works if dry.

    Thanks!

    • says

      Nice idea Susan but I’m pretty sure that won’t work. DE is used to cut into the tissue of soft bodied pests like slugs. It has sharp edges so when a slug or snail passes over a dusting of DE, it can cut them and they dry out and die. But theres nothing about DE that will kill Powdery Mildew.

  6. Jami says

    HI Joe… My name is Jami and I am kinda new at this whole plant thing. My front porch was looking a little bare so my Mom and I did a little day project about 4 months ago and I have fallen in love with plants and can not stop myself from planting more. but in the last month I have notice this powder mold you are talking about. I am going to try the mouth wash thing tomorrow during the day. But I was wondering if I covered my plants at night? if it would help with the problem. I live in Monterey,Ca and I am a stones throw away from the beach so I am wondering if the night foggy air would be blocked from a thin sheet over the flowers? Please help because I am so disappointed when I look at them because I feel like they are suffocating!

    • says

      Hi Jami. I totally get your newfound addiction to plants. Good for you. As for your idea of covering your plants at night, I’d advise against that. One of the most important things your plants need to keep diseases such as mildew from getting worse is air circulation. By covering your plants at night, you’re actually trapping moisture and preventing air from helping your plants.
      I would suggest you try spraying the leaves with a solution of 2 parts water to 1 part milk. Do it every week and get all the leaf surface, on top and underneath. Also, keep in mind, some varieties of plants are far more susceptible to powdery mildew than others. But treating them the way I propose will do far more to kill the spores than covering your plants at night. Good luck.

  7. Cookie says

    I’m gardening this summer in a “community garden”.
    My garden neighbor has a huge powdery mildew problem. The neighbor is not treating his garden.

    My squash, zucchini , and cucumbers have been severely damaged by the powdery mildew.

    What I’ve done about the problem:
    First I trellised my plants.
    Then I pulled off all the badly infected leaves.
    I’ve removed 2plants that were beyond help, determining it was better to concentrate on the healthier plants and remove as much of the fungus from my space as possible.

    I started with the neem oil – it definitely works. I’ve applied all my solutions in the heat of the afternoon. I have quite a few herbs near my infected plants and felt uncomfortable that the neem oil mist was drifting over to my mints and lemon verbena . So I switched to organic apple cider vinegar – I did not think the vinegar was affective . But I could have tried different concentrations but decided to Switch to milk..
    I’ve applied 3 sprayings of buttermilk, 1 part milk to 2 parts water. My plants love it! I think the leaves actually grow faster and the leaves are greener looking.
    –my theory — I’m actually consistently spraying and I’m actually pretreating the new leaves and pulling the infected and I’m producing a new healthier plant — just my theory.

    During the past 5 weeks I’ve removed all the badly infected leaves on my plants. At one point, one of my cucumber plants probably had 2/3 of the leaves removed.
    I’ve been desparate to keep the fungus under control. Most of the gardeners in the community garden have been infected with this powdery mildew and it’s my opinion the gardeners who leave their plants on the ground seem to have the quickest infestation and the gardeners who trellis are better able to control the powdery mildew.
    I find it easier to spray the plants on the trellis – I can get to all sides of the plant and walk around and spray more effectively.
    If I grow squash, cukes, and zucchini. Next year I will definitely trellis, prune, and spray early to prevent the fungus.
    I also wanted to mention that it’s my opinion the gardeners who water in the evening have the worst fungus problems. I water in the early morning – with a strong dispersed spray – I DON’T “gently shower” my infected plants. I’ve been giving them a good heathy strong spray in the mornings.

    My plants have been spending a lot of energy growing more leaves and I’m not getting the yield I should but the plants are healthier now and have been producing.
    Hope this helps other gardeners.

  8. Roberta says

    Your info on powderly mildew was most helpful!! I live in the rainy state of Washington and have been noticing this on my petunias and pansies primarily. I appreciate the help!

  9. Anne Garcia says

    Hello to you, Joe Lamp’l. Your site was the first to pop up when I typed in powdery mildew and the one I clicked on. I live in Illinois about an hr. NW of Chicago. I would like to thank you for your excellent information on this subject. The plant attacked with this mildew is the beautiful Bee Balm. I’m going to try the mouthwash since it has spread all over the plants. I’m hoping to eradicate it. I will let you know. Also what I found quite nice was that you take time to reply to everyone. I also was confused about the Neem Oil and the Dormant Oil. Thank you for explaining what they are. We have had nothing but hot humid weather here. Do you offer a site that shows pics of the most common ailments of plants? Thanks for the great info. Very glad I found your site.

    • says

      Hi Anne and thanks for the kind words. I don’t have a sight that has what you’re asking for, but it’s a great idea. Glad you found our site too. All the best.

  10. mary says

    my apple tree has white powdery leaves can i wash them with the soap and baking soda its not planted yet

    • says

      It shouldn’t hurt Mary if you don’t use too much soap. Soap can dry out leaves to excess and result in burning. The baking soda is good though. But try the spray solution of two parts water to one part milk. That’s safe and effective. Spray about once a week or more often if rain washes it off. Once you have mildew, it’s hard to reverse what’s there, but you can reduce the spread with spray.

  11. Valerie Torres says

    Hi there! I am new to the whole Powdery Mildew thing and just sprayed a 1:5 milk solution to my baby squash. Flowers were falling off prematurely and initially, I just cut off all of the affected leaves. Seemed to work, because now I see squash and the flowers are staying on! But, the powdery mildew came back with a vengeance! So, I sprayed tonight, but it was around 6:30pm. Now I am reading it should have been during the day? And the plants are not in Full Sun. Should i move them to get at least 6 hours of sunlight? I live in Italy, near Venice, and it is super humid here!! 80% humidity! Yikes! Help please… ;)

    • says

      Don’t move them now Valerie unless they’re in containers. I think your humidity is the biggest culprit and there’s nothing you can do about that. The mildew on squash is almost a given. I too struggle with it in my Atlanta garden. It’s more of a race to see how much harvest I can get before the plants finally succumb to the mildew. The key is air circulation. So if in containers, give them full sun and lots of air circulation. Or, next time you plant. Good luck.

  12. Traci W. says

    HI Joe! Love your site. I have recurrent powdery mildew on my tall phlox. I have spaced my plants to provide plenty of air circulation and the milk remedy with no success. I am going to try the Neem oil or one of the other treatments you suggested . Should I also treat the soil?

  13. Joan Keller says

    Thank you for the suggestion to use Neem Oil on powdery mildew. We use Neem Oil on minor cuts, bites, poison ivy etc. It is great. I will be applying a weak spray today to my petunias. I will get back to you in a week or so.

    • Joan Keller says

      It has been 2 weeks. Instead of Neem Oil, I sprinkled my petunias with a weak baking soda solution; one level teaspoon household baking soda to one gallon of water. After the second application, seven days apart, the powdery mildew seems to be under control. After each application the petunia leaves curled a bit, but, did not wilt. The flowers appear strong and cascading and the fragrance is lovely. I still water and deadhead the petunias daily. They hang with ferns just outside our front door. Thank you very much for the info about baking soda. If the mildew returns I will leave another comment.

  14. Marlene says

    I used the baking soda solution, and also the vinegar solution on my Tuscan Blue Rosemary greenhouse grown. In addition, I used a general fungicide called 3336 which is a milky-like solution with water. Anyway, I placed all the flats outside and are growing beautifully. There remains a white residue on the leaves. I cannot sell these plants with the white residue…or is it mildew (would they continue to grow if the mildew was still present)? Can these plants still be used or do I have to discard them all. Is there a way to remove the white powdery-like residue?

    • says

      I would pick a leaf or two that is covered in this white residue. Examine it under a strong magnifying glass. If the residue is smooth and possibly shiny, it’s the coating from the fungicide spray. It does that. If it’s this, it will go away eventually. If it’s mildew, you should be able to see that it is not just a coating on the surface but also has depth. If it’s mildew, the treatments will help stop new mildew but won’t erase what’s there already.

  15. Kirsten says

    I have a mini rose plant that is only kept indoors. I noticed the mildew just today and I want to try the milk treatment. Does the type of milk matter? I have skim milk, but I didn’t know if that was as effective.

  16. says

    Hello there. I sprayed at the 1:10 mix having quickly read your article before the correction. I am spraying lettuce, so any taste or residue is important. I’d like to know how long the reaction takes. Is the milk’s effectiveness stopped at drying or should it sit for a day or two?
    I plan to rinse with water instead of the normal irrigation cycle. I will post any interesting findins and also use the 1:2 ratio for a test plot.

    • says

      In university trials, the spray was effective for about a week with good results. Dr. Jeff Gillman who appeared on our episode where we addressed this says he thinks you can go 2 weeks between spraying.

  17. Gul Hassan says

    Hi JOE LAMP i am Gul Hassan from Afghanistan i am Studies you are all topic about the powdery mildew it use full for me but I am not understand on neem tree oil and dormant oil, if you have time kindly explain it with image or other synonyms which we will find it in market.

    • says

      Hi Gul. Neem oil should be sold as just that. It should say “Neem Oil” on the bottle. Dormant oil on the other hand, it a highly refined grade of horticultural oil that may be sold as “Horticultural Oil”. It is used to spray on trees and shrubs during their dormant period to suffocate overwintering pests. Need oil is used during active growth to help control plant diseases and to repel certain pests. They are used for completely different reasons so one would not be a substitute for the other. Hope this helps.

  18. says

    I’m a bit confused about the “milk” solution to fungus. It suggests 1 part milk to 9 parts water as being effective…however, it goes on to say that anything above 3 parts water can have adverse side effects! So, where is the recommendation? Did I misunderstand something?

    • says

      Hi Charles and thanks for catching this. No wonder you were confused. The “1 part milk to 9 parts water” was an error. The correct ratio is 1 part milk to 2 parts water. That should also clear up the confusion. It has been corrected in the post. Thanks again.

      • mark kimball says

        You say to “apply the milk and water solution weekly” what does apply mean? as watering or as a spray?

        mark

        • says

          spray it on Mark. Get a cheap spray bottle at the hardware store, mark it with a sharpie and coat the leaves on both sides each week. Watering on the other hand will do no good. You have to coat the leaves.

  19. Pam Bradberry says

    This was helpful. I’m going to try the baking soda remedy first.
    1 Tablespoon baking soda to 1 gallon of water

  20. Kim Winstead says

    Hi, i live in North Alabama & I have a terrible case of the black & white mold, it’s on my trees & it has spread to every flower, shrub etc.. That I have. It killed my Lilly tree. I really need help ASAP. I have cut back the branches in the trees that it was on & raked all the leaves in the area, but my elephant ear & other flowers haven’t started coming up & I’m afraid they have been killed due to this mildew problem. I really need your help.
    Thanks
    Kim

    • says

      Hi Kim. I can’t say with certainty on how to deal with this widespread outbreak and have never experienced this problem so I cannot offer any firsthand solutions. But if I were you, I would contact your county extension office and speak with the agent there. They are far more up to speed on local problems and solutions backed by university data and research. If anyone can help you, I would start there. You may need to take samples of infected and semi-infected plants so they can conduct a thorough analysis. Make sure you discuss this when you call. Good luck.

  21. bernadette says

    Hi, this summer 2013, I had the worst case of mildew. It spread to shrubs, and flowers (clematis), all round the garden. It may have come from plants I overwintered in the shed, (unsuccessfully), or for the shrubs I think I was too zealous over watering every day. I cut it down to once a week and never watered the privits as they started to rust.
    My question is, I was thinking of using potassium bicarb, or plain bicarb (I use this on the fish). Do I wait for the leaves to grow or can I spray whilst plant is dormant?
    Also, my experience with shop bought fungus treatments is its lethal to wildlife. I lost a fish spraying a magnolia, even though pond was covered apart from furthest 6 inches. Later, bees started to die in the garden. I left the plum tree to ‘survive’ as my rabbits lived underneath. Is potassium safe for wildlife?

    • says

      You need to spray it while the plant is actively growing to coat the target “host” area of the disease but you need to do this before you encounter an outbreak. Prevention is key. Once you have the problem, it’s more about keeping it in control and from spreading.
      Your point about the consequences of using this to wildlife and aquatic creatures is important! People fail to consider this all too often. I’d rather have a bit of cosmetic damage and protect the creatures. Thanks for making that point.

      • T.A. says

        I’ve got the mildew. Live in east Ala. and it’s Dec. What can I do to get through the winter and fight the mildew this spring or do I need to fight now. Help!!

        • teeb says

          Milk saved my entire spagetti squash crop last year! I was spending $25 a week on fungicide that barely was effective. I read about using milk and tried it with huge success! MILK!!! MILK!!! MILK!!! WORKS!!!

    • says

      Good question. I suppose you could use the neem as a way to help the baking soda stick to the foliage in place of detergent or soap. And you might get the bonus of a pest repellent also. I don’t know of a downside here but don’t hold it against me if something does happen. But I’d love to hear out this plays out if you do end up trying it. Keep us posted please.

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