Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
Powdery mildew is a scourge of gardens, both vegetable and ornamental. There are some simple steps you can take to prevent powdery mildew or get rid of it once it infects your plants.
What is Powdery Mildew?
Powdery mildew is caused by fungus that lives in the soil. Although it looks the same throughout your yard, powdery mildew is caused by many different types of fungus, each specific to a particular plant. Fortunately, different species of fungus cannot spread to different kinds of plants. For instance, the powdery mildew infecting your lilacs cannot infect your zucchini. They are caused by two different fungi.
Fungi grow best in humid weather and shady locations. It infects your plants but doesn't kill them. It is unattractive in flowers and shrubbery. It is especially a problem in the vegetable garden because once infected, the leaves no longer photosynthesize (make food for the plant) so any fruit that is produced will be malformed and less flavorful.
How to Prevent Powdery Mildew
Plant flower and vegetable varieties that are resistant to powdery mildew. Plant breeders are always working on new plant varieties to better resist diseases such as powdery mildew. Check the labels on plants before you buy them or look for powdery mildew resistance in catalog descriptions when ordering seeds or plants.
Do not plant susceptible plants in the shade. Growing plants which are susceptible to powdery mildew in the shade encourages the growth of the fungi. Shade encourages moisture which in turn encourages the growth of fungi. Sunlight tends to dry out moisture, rendering the environment less hospitable to fungi.
Do not crowd your plants. Proper spacing encourages good air circulation between the plants. Air that is moving between plants carries the fungi away from the plants. When plants are crowded, the air does not move between the plants and the fungi is able to stay on the leaves and infect them.
Apply fertilizer or compost in steady quantities rather than in periodic large amounts because the fungi prefers the new fast growing shoots. Slow release fertilizer is best because it supplies the plants with small amounts of fertilizer each throughout the growing season each time it rains or you water.
Avoid watering from overhead with sprinklers or handheld hoses. The force of the falling water will cause soil to splash onto your plants possibly infecting them if the fungi is present in the soil. Always water at the roots with a long handled watering wand or use drip irrigation.
Thoroughly clean up your garden in the fall. Fungi will over-winter in any garden debris that is left on the ground, emerging in the spring to infect your plants again. Remove all plant debris from your garden in the fall.
Sprays That Prevent Powdery Mildew
You can purchase sprays at your local nursery to prevent powdery mildew. They contain sulfur or potassium bicarbonate which will prevent fungi from growing on your plants.
You can make your own spray by mixing 1 teaspoon of baking soda and 1 quart of water. The baking soda raises the pH of the leaves so that the fungus can't grow.
Whether you purchase a spray or make your own, you will need to reapply after it rains.
How to Get Rid of Powdery Mildew
Once your plants become infected, it is difficult to get rid of the powdery mildew. Try these methods:
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Remove any infected plant material so that the infection doesn't spread to other plants. Either throw it out or compost it.
Use fungicide sprays made up of horticultural oils or plant based oils such as jojoba or neem oils, copper or potassium bicarbonate. They can be purchased at your local nursery or you can make your own fungicide spray. Mix 1 tablespoon of baking soda in a gallon of water and add ½ teaspoon of liquid soap.
Use a milk spray. Studies have shown that milk is effective in slowing the spread of powdery mildew on plants that are already infected. Mix 1 part milk to 9 parts water and spray on plants showing signs of powdery mildew. As with any spray, you will need to re-apply after any rain.
Powdery mildew is ugly but it won't kill your plants. Prevention is always best because once it infects your plants it is difficult to get rid of it as well as stop it from spreading to other plants.
Questions & Answers
Question: If the powdery white mildew on roses is not spotted but evenly distributed over leaves is it powdery mildew or something else?
Answer: It is powdery mildew. Other diseases produce colored or black spots.
Question: What if the mildew is on the soil?
Answer: The mildew is always in the soil. The issue is making sure that the soil does not come in contact with the leaves. When you water from overhead, the water hits the soil with force and splashes up on to the leaves spreading the fungus. Always water your plants at the roots or use drip irrigation and keep the fungus in the soil and off of your plants.
Question: My Pride of India trees are infected with mildew no matter whats pray on the tree it does not get rid of it. Is there something I can put in the soil so that it can clean the inside of the tree through absorption?
Answer: I don't know of any systemic fungicide that you can use. There are sprays that you can purchase or make that are effective in preventing powdery mildew as mentioned in my article.
© 2014 Caren White
Janet Tanner-Tremaine on August 02, 2019:
Hello Caren - This is more about brassica caterpillars (little green, major munching little beasts) and an organic homemade solution to get rid of the. I think that they are laid on the leaves of cauliflower etc by a white butterfly. It is too late for my crop this year and we have pulled out all the plants affected and will start again - maybe next year!
I would be most grateful if you could recommend something not too complicated we could mix at home and spray the plants as they grow.
Caren White (author) on September 04, 2014:
Your welcome Pawpaw! The older zinnia varieties are prone to powder mildew no matter how careful you are. One of the features on the newer hybrids is their resistance to powdery mildew. The same with lilacs. I treasured my old-fashioned lilac despite its annual attack of powdery mildew. The new ones are pretty and the leaves stay green rather than turning gray, but they just don't have the same sentimental value for me. Thank you for reading and commenting.
Jim from Kansas on September 04, 2014:
I have a little on some of my Zinnias, which catch a little spray, at the edge of where our lawn sprinklers reach. Thanks for the excellent information.
Caren White (author) on June 18, 2014:
Flourish, so glad that you found it useful. Thanks for reading and pinning.
FlourishAnyway from USA on June 18, 2014:
This is very useful. Voted up and pinning to my gardening and outdoors board.
Caren White (author) on June 12, 2014:
Peggy, I was surprised about the milk too! Thanks for reading and pinning.
Peggy Woods from Houston, Texas on June 12, 2014:
Thanks for this information about how to treat powdery mildew. That is interesting that milk works in treating it. Good old baking soda to the rescue mixed with water is certainly an easy approach. Living in humid Houston, it can sometimes be a challenge to combat powdery mildew especially since we have a sprinkler system that hits plants with water from above. Pinning to my gardening board.