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 – 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 inhibiting photosynthesis resulting in 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) Use crop rotation. The University of Illinois says, “crop rotation have no effect on powdery mildew infection.”
6) 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!
Getting Rid of Powdery Mildew
Once leaves are infected, you can’t get rid of the infection. At best you can stop further spread on infected leaves and on un-infected leaves, but it is critical to act fas as soon as you see the infection.
Bicarbonate solutions do work, and are commonly called the Cornell formula.
If you do a search for this formula you quickly realize that there is no ‘one’ formula. In fact, there are many versions of the formula but they are mostly similar and consist of the following key ingredients.
- 1 gallon water
- 1 tablespoon backing soda
- 1 tablespoon vegetable oil
- 2 drops dish washing liquid
These ingredients are mixed together and then sprayed on the plant that is under fungal attack. It is popular because it’s simple and most people have easy access to the ingredients.
Other ingredients can also be added and various types of oils and soaps are recommended. Many recipes do not include the oil.
Baking soda is sodium bicarbonate, a simple salt that can be found in the grocery store. Some people substitute potassium bicarbonate (available on Amazon).
A solution of milk and water will also work. Try using a 25% solution of 2% milk. Non-fat milk does not seem to work as well.
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.
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