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Grape Powdery Mildew: Diagnosis & Treatment

Grape Powdery Mildew (Uncinula necator)

AKA (Erisiphe necator)

If you grow grapes in coastal California, (or almost anywhere in the world for that matter), you are probably going to see a white powdery substance on the leaves.  This is grape powdery mildew.  While this is common, it is a disease that will damage your grape plants and your crop if left untreated.

White powdery mildew

Grape powdery mildew (white)

 

What is powdery mildew?

Powdery mildew is a fungal disease that affects a wide range of plants. The specific fungus that causes powdery mildew on grapes is Uncinula necator.

Powdery Mildew

Grape powdery Mildew

How do you get grape powdery mildew?

  • The fungus that causes grape powdery mildew is all around us and it is therefore close to impossible to avoid.  It is spread by the wind.
  • The fungus is thought to have originated in North America.  Therefore, North American grapes that co-evolved with it are more resistant than European grapes.
  • A small amount of the fungus stays dormant on grapes in the winter, and when conditions are right it spreads rapidly over the plant.
  • Like any fungus, it does best in cool moist environments.  This is why the disease is particularly problematic in coastal areas.  A wet growing season and/or excessive marine layer will exacerbate the disease.
  • Ideal temperatures for the growth of grape powdery mildew is between 70° and 85°F.  Welcome to California.
  • High temperatures and sunlight are inhibitory to powdery mildew.
Powdery Mildew

Grape powdery Mildew dehydrating and curling grape leaf

Where can you see powdery mildew first?

  • Because the disease is a fungus, you will first find the disease on the undersides of leaves.  This is because the undersides of leaves are protected from the sun.   The fungus is also more common in the deeper parts of the plant where there is poor air circulation.

 

What will powdery mildew do?

  • Grape powdery mildew will suck the life out of your plant, damage leaves and spoil your grapes.
  • The disease can and will cover the entire plant if not controlled.
  • It can cause leaf curling, withering, red blotchy areas and distortion of leaves.

 

How do you treat powdery mildew?

  • Because the disease lives on the surface of the plant, topical spray treatments are very effective.
  • I recommend that you get (and label) a separate garden sprayer for each of your different garden needs.  That way you don’t have to worry about spraying residue of something like an herbicide on a plant that you would actually like to keep around.  It is also nice to have one with an angled spray nozzle so you can easily get the bottoms of the leaves.
  • The first few treatments of the season are the most important, and should be applied at bud break and during early growth.
  • There are many different anti-fungal sprays that can be used to fight powdery mildew.
  • A few of the organically acceptable methods include sulfur, Serenade, Sonata, and Stylet oil.  There are many other good options out there, I happen to use a copper based spray.
  • Many professional wine growers rotate through different types of fungal sprays to prevent the fungus from developing a resistance to one specific treatment.
  • Most spray treatments for grape powdery mildew are on a near weekly basis depending on your micro-climate.  Complete spray coverage is important to get the hard to reach parts of the plant that are also the most susceptible to fungal disease.
  • A Powdery Mildew Index (PMI) was created to determine how frequently you need to spray grapes for powdery mildew.  The University of California, Davis has the one most widely available models, which requires data entry for leaf wetness and temperature.  However, for the home grower, it is a lot easier to just spray on a regular interval as suggested on the fungicide label.

My method:

  • I found that the most important element for success is to spray the buds in the dormant season with a mix of horticultural oil and anti-fungal spray.  This is process is kind of like treating for peach leaf curl.  For example, if you wait till the leaves are already out and infected then it is an uphill battle all season long.
  • That being said, you can also spray all season long for powdery mildew and it will help while in the case of peach leaf curl, spraying after bud break doesn’t reverse the damage.
  • Specifically: I am using a mix of “Bonide All Seasons oil”  and Bonide Copper Fungicide.  I just use the recommended concentrations of both in the same spray bottle.

 

More on horticultural oil:

  • Overall, horticultural oil is an awesome  alternative to systemic pesticides and it kills all kinds of sap sucking pests.
  • If you are really interested in learning more about the fine details of oil insecticides, the “Using Oils as Pesticides” article is a rather complete outline from “agrilife extension” of Texas A&M University.

 

Wine Industry Experience:

  • My good  friend Stasi Seay has extensive experience in the wine growing business as a viticulturalist.  She has recently contributed some farming perspective to the issue of grape powdery mildew.  Please read her 6/18/13 insight in the comment section below.

 

Prevention of powdery mildew?

  • While you can’t change the weather, there are some other things that you can do to battle this disease.
  • Planting grape rows in a North to South orientation will result in a more even sun exposure on the plant.  This will reduce the areas where fungus will thrive.
  • Pruning out overlapping and clumping branches will help with aeration and sun exposure.
  • Use drip irrigation and keep your plants clear of sprinkler irrigation.
  • Plant grapes away from humid micro-climates such as close to a pool or hot tub.

About Thomas Osborne, MD

Dr. Osborne is a Harvard trained Radiologist and Neuroradiologist who loves to share his insight about medicine and gardening.

24 comments

  1. There is a very easy way to prevent powdery mildew from every happening and, if you get it, to treat grapes, squash and cuke plants. Just mix 1 part whole milk with 9 parts water and spray, weekly. It is a proven treatment for vegetables and actually got it’s biggest field test with grapes. Wrote about it here
    http://growsoeasyorganic.com/2013/01/19/grow-so-easy-organic-bugs-that-bug-summer-squash/

    and there is a link in my post to the information on Bettiol’s research! Cheap, easy and organic. Love those solutions.

    • Thomas Osborne, MD

      Thanks Pat!
      Very interesting.
      Do you know how the whole milk works against powdery mildew (the mechanism of action)?
      Thanks, Tom

      • I don’t! But I would love to know. It seems so simple just to spray milk and water on your vining plants. Can you share?

      • Dear all Greetings! I have working on Grape Vine in Ethiopian Institute of Agricultural Research. I have faced powdery mildew in grape research. Can you suggest any fungicide chemical which control powdery mildew? Please send any suggestion with my E-mail: mulukend04@gmail.com

    • I am a viticulturalist in California. I haven’t heard of the milk treatment (interesting!) but know that the key to controlling powdery mildew on grape vines is disruption of the spore cycle, followed by use of elemental sulfur. To do this, you need to spray with high volumes of water and a light detergent. The high volumes of water, coupled with a bit of soap, breaks up the spore cells.

      For home use, you can put a few drops of dish soap in a spray bottle of water. For several plants, consider using a 2-3 gallon sprayer and add 2 ounces of laundry soap. Cover each plant thoroughly.

      For high infection, as soon as the leaves dry from the soap treatment, use dusting sulfur, which is an organic product. You can buy it at your local home and garden store or farm center. Give the leaves a light dusting and always follow the label directions.

      Cheers! Stasi
      http://www.unlockwine.com

    • thanks for that i,ll give it a go AD gilberdyke

  2. Stasi, when applying the sulfur after the dish soap application, can the sulfur be ‘wettable’? I have 80 vines and don’t know how to apply dry, but can easily apply through a wet sprayer.

    Thanks … John

  3. Your website won’t let me leave the page.
    My browser stopped it from opening 6 pop-ups.
    this is what is called unprincipled use of the web.
    Annoying, unnecessary and it has just lost you me as
    a potential reader.

    • Thomas Osborne, MD

      Hi Gerry

      Sorry and thank you:
      Sorry to hear about your experience.
      It does sound annoying and this would never be my intention.
      But thank you very much for sharing this important information.

      Overall:
      I am very surprised and a bit alarmed to hear about your popup issue.
      I have never added any advertisements to this site, so I dont know how popups could be associated with this site.

      Testing:
      After your note, I have tested this site on multiple different computers and devices and there were no popups or other issues leaving the page.

      Favor:
      If you could be so kind and provide me additional information so I can further troubleshoot, that would be great.
      * Did you get a specific type of popup or specific adds?
      * Did you experience this with a particular browser (Internet Explorer, Firefox, Chrome, etc)?
      * Have you had similar experiences with other websites?

      Thank you very much,
      Tom

  4. How long into the growing season can I apply sulfur?

  5. I have 3 grape plants out of many that have this powdery mold on it real bad this summer. Now in Oct, I have cut all the branches off the plant to try and rid it of the infection. I just have the main mother stalk now. Would now, in the fall, be a good time to spray with soap or sulfur? What should I do with the old branches and fruit? Will it spread on other things if I try to use the debris for a hugo culture or something?

    • Thomas Osborne, MD

      Hi Diana
      Great questions.

      Winter treatment:
      There are a lot of potential anti fungal treatments you can use. I happen to like copper-based sprays but others work great too. Some people only spray when they see the powdery mildew on the leaves. But in my opinion, once you see it your really behind on things. Spraying before the buds open seems to make a big difference much like spraying for peach leaf curl. For reference, see article on peach leaf curl treatment

      What to do with cuttings:
      The disease is spread by tiny spores and the spores are probably everywhere.
      However, the spores are in the highest concentration where you see the “powdery mildew” which is basically a dense concentration of fungal parts and spores. Therefore, anything that you can actually see powdery stuff on has a huge volume of infectious material on it. Thats trouble. In addition, the more you move these infected leaves and branches, the more you spread the disease. So, if I was you… I would carefully move all the infected plant parts to a trash bin. Collect all the dead grape leaves and throw them away too. Once you have cleaned up everything and trimmed everything then do the fungal spray. Spraying again in late winter/early spring before buds break is also a good idea.

      Good luck,
      Tom

  6. I have few questions;
    1. When to apply spray?
    2. Shall we apply it on winter or when grape bloom in to 3 to 4 inches?
    3. Bud break?
    4. How is lime sulfure?

    • Thomas Osborne, MD

      Hey Shafiq
      There are differing opinions regarding the best time to apply treatment for powdery mildew.
      In my experience, it is important to spray before bud break (much like you would treat for peach leaf curl).
      Some people will spray several times in the dormant season. However, if you are going to spray once, I would time it for about a week or two before bud break. For me that is usually late winter, early spring.

      I have not tried lime sulfate. I would apply any antifungal with the same timing protocol.

      Happy holidays!

      • We have many gardens of grapes in Afghanistan/Kandahar.
        The grapes got powdry mildew now we don’t knw the exact time to spray…
        How if the 1 to 2 inch bloom, we spray…?

        • Thomas Osborne, MD

          Hi Shafiq
          The best time to spray (in my opinion) is before buds break.
          Of course you can spray later but that will be less effective.
          It will also be more expensive because you will need to cover more surface area (spraying all the leaves vs just branches).

          Spraying sooner is better than waiting till later.

          Best,
          Tom

          • Do u mean that now we should start spray…?
            If u could please tell us the month or date of it…?
            Thank you

          • Thomas Osborne, MD

            Hey Shafiq

            The best time to spray is before bud break.
            If you miss that, then the best time to spray is sooner than later (as soon as possible).
            The longer you wait, the worse the infection gets.

            The problem is that it is really hard to deal with the infection after untreated leaves open up. When this happens the leaves are inoculated with the fungus. You will then need to spray more during the growing season and the spraying will have less of an effect.

            Prevention is a really important part of the equation (as detailed in the article).

            Hope this clarifies things for you.

            Best of luck,
            Tom

          • Thank you Dear Tom!
            But I have one presentation of University of Kentucky about fungicide they gave details about to spray early season… 1 to 3 inch growth through 3 to 4 weeks after bloom…
            I want to share it with you give me ur email address.

          • Thomas Osborne, MD

            Hey Shafiq
            Thanks for the info.
            Feel free to send the link if you like so everyone can benefit.

            My suggestions come from personal research and personal experience.
            I have found it is best to do the first spray before bud break and then again after that as needed (per fungicide brand-label recommendations).
            If you feel more comfortable with the University of Kentucky’s suggestions then that’s great too.

            Best,
            Tom

  7. Interesting discussion of use of oils. I had mites on my lemon and tried a gentle finger-rub of coconut oil on the infected leaves and it worked like a charm. I also gave it a few weeks of semi-shade to prevent sun burn. That solution worked because the tree is small and the infestation was limited. I will try same on my grape fungus.

    • Thomas Osborne, MD

      Hi Ann
      Great idea with the coconut oil.
      Looking forward to hear how it works on your grape fungus.
      Thanks,
      Tom

  8. Great Post. Thanks for sharing this informative post with readers. This was very helpful and I enjoyed it reading.

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