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Powdery Mildew Treatments

Powdery mildew is a very common infection on plants and most books that discuss plant diseases will discuss this one. But do the suggestions for reducing the disease work or are they a myth?

powdery mildew treatment

Powdery mildew

Powdery Mildew – What is it?

Powdery mildew is a fungus that attaches itself to leaf surfaces. It is easily identified and looks like a white power on the surface of stems and leaves. The above picture is a scanning electron micrograph of powdery mildew. It will not help you identify the fungus, but it is a real cool picture by Marco Todesco.

The fungus rarely kills plants. It is unsightly and does weaken the plant by both extracting nutrients from the leaves, and reducing the amount of light that reaches the leaf. Less light results in less photosynthesis and less food production by the leaf. When fungus spores land on a leaf, they produce feeding tubes and insert them into the leaf. The fungus then extracts food through these feeding tubes similar to a mosquito sucking blood on your arm.

As gardeners we talk about powdery mildew as one disease but that is not correct. There are many kinds of powdery mildew and most are fairly specific for a particular type of plant. For example the fungus on your lilacs will not infect your grapes or your roses. All three of these plants have their own type of powdery mildew. As a gardener you can think of this disease as one type of fungus since they are all very similar.

How Do You Prevent Powdery Mildew?

There is much written about preventing the fungus. Let’s have a look at some of the advice.

1)      Keep plants well watered. This makes sense for most diseases. A healthy plant is less susceptible to infection.

2)      Avoid overhead watering. The idea here is that overhead watering gets the leaves wet, increasing the humidity around the leaves, and a high humidity encourages the growth of the fungus. According to the Penn State Department of Agriculture (ref #1), the conditions that favor powdery mildew are high humidity at night, and low humidity during the day. Which means that overhead watering in the morning would actually help prevent the fungus, not encourage it’s growth–the opposite of the recommendation. Besides, watering would only increase humidity for a very short period of time. A bit of wind dries the leaves quickly.

3)      Prune to open up the interior of shrubs and trees. Opening up the space between branches will allow better air movement, which in turn reduces the humidity. Given the fact that low humidity during the day favors the powdery mildew, this may not have much effect on powdery mildew. It is however a good idea for preventing other diseases.

4)      Remove infected plant material. This will reduce the number of new fungus spores produced on the infected plant and should keep the fungus from spreading too much. There are a couple of problems with this logic. No matter how well you remove the infested parts you will not get them all–the spores are there even if you clean up the vegetation. Infection of other types of plants is not a problem since the fungus is type specific. When you remove an infected leaf, you remove both the fungus and the healthy part of the leaf. Remember that the fungus rarely covers the whole leaf surface. This means that the plant has less green leaf surface to make food, making it even more difficult for the plant to survive the attack. It is probably a good idea to remove heavily infected leaves, but I would not remove leaves that are slightly infected. This is just my opinion and not validated or disproved by research that I know of.

5)      Plant resistant varieties. This is probably your best defense but it does not always work. Summer phlox is notorious for powdery mildew infection, and the variety ‘David’ is reported to be resistant – not in my garden!

Powdery Mildew Facts

Here are some other interesting facts about the fungus. Some powdery mildew varieties are inhibited by moisture on leaves while others are favored by moisture. Unless you know the specific biology of your current or future infection– you don’t know if the leaves should be kept wet or dry!

High humidity favors spore formation while low humidity favors spore dispersal. Soooo … if you keep humidity high you will be creating more spores in the future, but if you keep the humidity low the spores that have already developed are more likely to be dispersed causing more infection. Seems like a no win situation–for you–not the fungus.  I don’t think that you can control humidity in the garden to have much effect on powdery mildew.

The bottom line: learn to live with the disease. Plants that get it a lot can be replaced with better plants.

Resources:

1) List of Plants Affected by Powdery Mildew: http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/powdery-mildew

2) Photo Source:  Marco Todesco, Max Planck Institute for Developmental Biology

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

8 Responses to 'Powdery Mildew Treatments'

  1. New Gardener says:

    I have read that spraying the leaves with diluted cows milk kills powdery mildew. Please advise.

  2. Tap says:

    Through research I found many articles on grafting pumpkins for multi-problems including wilt. However, no root plant/stock was mentioned.
    One article mentioned Lagenaria siceraria. Any suggestions? Also, root stock for tomatoes that is relatively common. Have heard of Maxifort but very expensive.

  3. johnoot says:

    It’s that time of year that many people “clean” their garden in preparation for winter. Your “chop and drop” and other approaches suggest that this is of no benefit, and perhaps is harmful. Today I came across this article (http://davesgarden.com/guides/articles/3-important-garden-tasks-to-do-before-first-the-frost) that states the old myth yet again. In particular it says:

    “Composting the debris is far better than allowing it to stay in the garden over the winter, because a hot compost pile will kill off disease spores, insects and weed seeds that would otherwise cause problems next year.”

    It would seem to me that eliminating disease causing spores, etc, also eliminates the natural controls such as beneficial bacteria and other pathogen reducing microbiota. Yet I can find little factual information.

    An overview by you of the garden myth that one should “clean up” the garden before winter would be appreciated

    • Good questions. I find several problems with the link you posted.

      I don’t agree that fall cleanup benefits the garden – it is a complex topic, and one that would make a good post as you suggest. Your point about also destroying beneficial microbiota is quite valid. The other issue is that in colder climates, I doubt that many compost piles get hot enough to kill spores. In fall we have too many browns, and not enough greens to get it hot and who turns it in winter? I certainly don’t.

      I have also been doing some experiments to see the effect of covering plants with a sheet – preliminary numbers show very limited benefits in terms of keeping plants warm. I’ll report on this in spring.

      I also don’t agree with the comment that ice harms plants under plastic. The fruit growers spray water on their crops to prevent frost damage – and they get covered in ice. The process of forming the ice take cold out of the air, and keeps the air around the plants warmer.

  4. jose mejia says:

    I have had cucumber plants under greenhouse conditions with “complete” coverage. Including fruit and stems. I do believe that removal of photosynthetic area is less damaging that the removal of millions of spores that will be released on the plant.

    • Robert Pavlis says:

      As I said in the post “It is probably a good idea to remove heavily infected leaves, but I would not remove leaves that are slightly infected”. With complete coverage I agree with you. But if all the leaves are covered and you remove all of them, you really don’t have a plant left that will produce cucumbers.

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