September 27, 2007 at 7:59 a.m.

Powdery mildew on tomatoes and bee balm

Powdery mildew on tomatoes and bee balm
Powdery mildew on tomatoes and bee balm

Due to the hot, dry summer the Chisago County Master Gardeners have received few calls on blight of any kind. However, this is not true for powdery mildew. Unlike most plant pathogens, powdery mildews do not need water to germinate, and can germinate and infect when the only available moisture is high humidity. Powdery mildew is a generic term covering a wide range of pathogens that produce similar white powdery coating on leaves, flowers, and stems. For the most part, powdery mildew is very host specific. This means that the mildew on the lilacs is not the same as the powdery mildew on the roses, bee balm, etc.

Powdery mildew is one of the most common plant diseases. I have seen it on hollyhocks, bittersweet, lilacs, roses, sweet corn, and tomatoes. According to Janna Beckerman, University of Minnesota Extension Pathologist, powdery mildew has been a problem on tomatoes in the garden for years. It has even spread to greenhouse tomatoes.

Since an outbreak in the Netherlands in 1986, powdery mildew of tomatoes has spread to green house tomatoes throughout Europe and North America. In 1995, there was a disease outbreak reported in eastern and mid-western North America that was due to one distinct fungus. Two other fungi also cause powdery mildew on tomatoes, but have not yet been found in North America.

In another article by Beckerman, she wrote about powdery mildew on bee balm. I have come to appreciate bee balm as one of my favorite fall flowers. Its vibrant shades of red continue to be admired until a killing frost. I planted bee balm because it's a magnet for hummingbirds. In addition to beautiful flowers, bee balm has wonderful lemon-scented foliage that has been used in tea.

According to Janna, there is a misconception that the native bee balm is more resistant to mildew than the newer varieties. This is not true. Since the 1990's there have been more than 60 new cultivars of bee balm released with some progress in the battle against powdery mildew.

The story is similar with tomatoes. Most varieties available have no resistance to the disease. However, the resistant cultivars have recently been released for commercial production.

Symptoms of powdery mildew on bee balm occur only on the leaves. They initially appear as light green to yellow blotches ranging from 1/8" to 1/4" in diameter on the upper leaf surface. As the disease progresses, white mycelia develop, resembling powder. Eventually, the leaves turn yellow, dry up and drop off.

There are some things you can do to minimize the chances of powdery mildew attacking your plants. Plants should be in full sun to light shade. Keeping plants well watered to reduce drought stress will minimize severe infection and defoliation. The disease does not warrant chemical control in most cases. Fungicides prevent infections from occurring but don't cure existing infections. If you can't stand this disease, it is important to treat it as soon as it appears to prevent additional infections. Some of the least toxic fungicides labeled for control of powdery mildew include potassium salts of bicarbonate, horticulture oil and sulfur. Apply additional sprays as needed according to product label directions.

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PLANT CLINICS: The Monday night plant clinics at the office are done for this year, but you can still talk to a Volunteer Master Gardener at the Lindstrom Farmer's Market, on Saturdays from 8 a.m.-noon on Highway 8 in the St. Bridget's Catholic Church parking lot. Please note -- there is no longer staff at the North Branch Office who can answer gardening questions.

VOICE MAIL: You can leave a question for a volunteer Master Gardener at 651-674-4417. Depending on the volume of calls, they try to respond within a couple of days. During office hours ask for the Master Gardener voicemail, after hours, select ext. 18. You can also get your question answered on the web at: www.extension.umn. edu/askmg.

UPCOMING CLASSES: Join the Master Gardeners Saturday, Oct. 13 for fall clean-up to control insects from 10-11a.m. Dr. Vera Krischik, Associate Professor, Department of Entomology, U of M will talk about what steps you can take now and in early spring to control insects in your yard and garden. You are invited to join us after the class for a harvest potluck lunch. This would also be a good opportunity for anyone thinking about becoming a Master Gardener to see what we do and get more information about the program.

The class will be held at the Senior Center in North Branch. You can call 651-674-4417 for more info. This would also be a good opportunity for anyone thinking about becoming a Master Gardener to see what we do and get more information about the program.

A cutback in staffing has made it necessary for the Extension Office to limit hours of operation. We would suggest that you call ahead to verify, that the office is open if you plan to stop by.


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