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Powdery Mildew

Powdery Mildew

powdery mildew on rose leaf

Powdery mildew is a fungus disease of many plants. It is easily recognized by its white powdery growth on both sides of leaves, and sometimes on blossoms, fruit and stems. The first sign of infection is often white circular spots on leaves. Later the leaves often curl, and take on a distorted shape, turn yellow or brown, and may fall from the plant prematurely. This may cause the plant to weaken, and, in some cases, die.1

Powdery mildew can be a problem on many plants. The list includes many fruit trees, grapes, berries, tomatoes, peas, cucumbers, roses, African violets, begonias, asters, delphiniums phlox and many others!2,3

Powdery mildew can be confused with other powdery-type plant diseases. Downy mildew is one. It is usually more often brownish-gray than white and tends to attack the undersides of leaves.1 The phrase, "Downy mildew on the 'down' side," can help you remember this. Downy mildew is more active at cooler temperatures and higher moisture conditions than powdery mildew.

Gray mold (Botrytis) is also sometimes confused with powdery mildew. It typically attacks berries, fruit, blossoms, especially during extended periods of mild drizzly weather.3 Gray mold often finishes off over-ripe berries at the end of the growing season.

Life Cycle

Small white powdery spots on leaves are usually the first sign of powdery mildew. This white thready material is the mycelial stage of the fungus and is the stage that does the damage. As the weather turns cold, the mycelium may go dormant and overwinter in buds or other sheltered parts of the plant, or may form spore structures that burst open in late summer or fall in advance of winter rains. Spores can be released anytime during the growing season that conditions are favorable. This is usually between the 60 to 80 degree range. Temperatures much above 90 degrees can kill powdery mildew.1

Technically, there are many species of powdery mildew, each one attacking only certain plants. Most species have similar life cycles so backyard growers don't need to fuss about which one they have.1

powdery mildew on squash

Monitoring

Keep an eye out for the first appearance of powdery mildew, as it is easier to prevent than eradicate once it is established. Caught early, removing a few infected leaves may keep it from getting a toehold. Seriously infected plants should be removed as soon as possible too.1

Watch the thermometer. Grape researchers have determined that powdery mildew is most likely to become epidemic when temperatures between 70 and 85 degrees are maintained for six hours or more at a time, for three days in a row. Temperatures above 95 degrees for 12 continuous hours will often stop infestations.4 When the weather is mild, which is not uncommon during the mild summers of the Pacific Northwest, many plants will be vulnerable to attack. Stay vigilant!

Resistant Varieties

Bar none, the best way to control powdery mildew is by using disease-resistant varieties. Whenever there is a choice, and there usually is, select disease-resistant varieties of whatever you are planning to grow. Resistant varieties of tomatoes, cucumbers, green beans and many other garden crops rarely get serious cases of powdery mildew. Resistant varieties of ornamentals including roses, crape myrtle, phlox and rhododendrons are also readily available. Look for them in seed catalogs, at nurseries, or on the Web.

  • Roses - Lists of resistant varieties can be easily found on the Internet and often at your local nursery too. The Berkeley Horticultural Nursery web site offers a nice chart of resistant varieties.5,6
  • Apples - Red Delicious, Stayman, Winesap7
  • Strawberries - Resistant varieties include Hood, Totem, Benton, Chandler.9,10
  • Caneberries - Resistant varieties include Chief, Marcy, Malling Orion, Logan.10
  • Vegetables - The Cornell University Vegetable MD Online website lists many tables of disease resistant vegetables.11

Use Good Growing Techniques

Keeping plants healthy helps them to fight off fungal attacks. Some of the key cultural practices include the following:1

  • Provide full sun for most plants.
  • Remove prunings from the yard, especially infected prunings.
  • Don't crowd garden plants. Allow for good spacing.
  • Prune ornamentals to allow good air circulation.
  • Keep gardens well weeded to help reduce humidity levels.
  • Avoid excess fertilization, which may cause susceptible tender growth.
  • Overhead watering can be helpful in that it can wash off spores. But try to water early enough so that plants can dry before nightfall.
  • On apples and other fruit trees, prune infected shoots during dormancy. Powdery mildew can cause stunted, russeted fruit. For susceptible varieties consider spraying lime and sulfur or a highly purified oil at "pink bud" stage.6 Consult with your local Cooperative Extension for timing.
  • Don't forget about resistant varieties!

What about Spraying?

A generation ago, it seems everyone who grew roses or tomatoes, sprayed them all season long. (Of course, a generation or two before that, nobody sprayed anything!) Fortunately, with resistant varieties and good growing methods, there should be no need to ever spray most plants.

If you couldn't resist a non-resistant veriety, you may have decided that you need to spray. Backyard growers are fortunate in that there are several products registered for use in organic agriculture that backyard growers can try. Among them are products containing potassium bicarbonate (Kaligreen, MilStop), a highly purified horticultural oil used in organic agriculture (Organic JMS Stylet-Oil), an insecticidal soap (M-Pede) as well as the old standby, sulfur.12

Timing is Everything...

All powdery mildew products – with the exception of horticultural oils – are best used as protectants. This means they should be used before there is a serious problem. Therein lies the rub: How do you know ahead of time that you will be having a serious mildew problem?

This limits the usefulness of most fungicides unless you are prepared to spray regularly through the growing season (so not recommended...!). Also each of these products has certain limitations. For example, sulfur use may lead to mite problems, and both sufur and oil can be toxic to plants during hot weather or in combination with other products.3 Read the product labels carefully!

If You are Still Thinking About Spraying, Consider This:

Remember that powdery mildew tends to be a problem at the beginning and end of the growing season.1 At the beginning of the year, conditions may change to slow or stop an infestation in its tracks. Many plants will be able to outgrow it, or live through it with only somewhat lower yields or quality. At the end of the growing season as the weather turns cold, powdery mildew may be there to help finish off plants in your garden as they weaken. No amount of spraying will save them at this stage.

Finally, sometimes it helps to be philosophical about garden setbacks. A favorite quote: "Remember that failure is the price of tuition for the opportunity to learn a new lesson."13 Instead of making heroic efforts to save what may be a lost cause anyway, think ahead to what you can do next year to avoid powdery mildew problems.

References

  1. Buchinski, A. 2003. Powdery mildew. Master Gardeners of Santa Clara County Online. U.C. Cooperative Extension. http://www.mastergardeners.org/publications/powderyMildew.html
  2. Garden Organic [UK]. 2007. Powdery mildew: Factsheet DC15. http://www.organicgardening.org.uk/factsheets/dc15.php
  3. Raabe, W.D. et al. 2001. Powdery mildew on ornamentals. U.C. Agriculture and Natural Resources. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7493.html
  4. Cornell Univ. Plant Disease Diagnostic Clinic. 2007. Botrytis blight (Botrytis cinerea) fact sheet. http://plantclinic.cornell.edu/FactSheets/botrytis/botrytis_blight.htm
  5. Gubler, W.D. et al. 2006. Grape powdery mildew. UC IPM Online. http://www.ipm.ucdavis.edu/PMG/r302100311.html
  6. Berkeley Horticultural Nursery. 2006. Disease resistant roses. http://www.berkeleyhort.com/roses/r_diseaseresistant.html [no longer available]
  7. For a similar list see Oregon State University Extension's Rose Cultivar Resistance http://plant-disease.ippc.orst.edu/articles.cfm?article_id=24
  8. Savonen, C. 2007. Plant these disease resistant roses, tested in PNW. Oregon State Univ. Extension Service Garden Hints. http://extension.oregonstate.edu/news/story.php?S_No=777&storyType=garden
  9. Gubler, W.D. 2006. Apple powdery mildew. UC IPM Online. http://www.ipm.ucdavis.edu/PMG/r4100311.html
  10. DeFrancesco, J. et al. 2002. Crop profile for strawberries in Oregon. Oregon State Univ. http://www.ipmcenters.org/cropprofiles/docs/ORstrawberries.html
  11. Teviotdale, B.L. et al. 2007. Powdery mildew on fruits and berries. U.C. Agriculture and Natural Resources. [rev. Mar. 26, 2007] http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7494.html
  12. Cornell University Department of Plant Pathology. 2006. Vegetable MD online. Tables of disease resistant varieties. http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm
  13. Organics Materials Review Institute. 2007. The OMRI products list. http://www.omri.org/OMRI_about_list.html
  14. Stamets, Paul. 2005. Mycelium Running. p 122. Ten Speed Press. Berkely, CA.

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Stein, Dan. "Powdery Mildew."
Eugene, OR: Northwest Coalition for Alternatives to Pesticides, April 2007.

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