Controlling or Eliminating Powdery Mildew

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

By JOE LAMP’L

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 nine 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. 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

  2. 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.

  3. 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!!

    • 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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