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Mildew and Blackspot Control Options
By Paulette Mouchet

Originally published in "The Rose Garden" newsletter, March 2003. Revised February 2007

rose disease: powdery mildew

rose disease: powdery mildew
Powdery mildew

One of the easiest ways to control powdery mildew is to give your roses a thorough wash at least two mornings a week. A morning bath raises the humidity so powdery mildew spores can't mature. Free water left on the plants also discourages spore maturation. Plants must be dry before nightfall so residual moisture does not provide favorable conditions for spore germination.

Powdery mildew is attracted to lush, new growth. If you have a serious problem year after year, you might want to reduce the quantity of fertilizer applied in January and/or postpone fertilizing until your plants start growing vigorously in the spring.

Solution #1 - The Cornell Baking Soda Formula
(for Powdery Mildew AND Blackspot)

4 teaspoonsbaking soda
2 tablespoonshorticultural oil such as SunSpray brand UltraFine Year-Round Pesticidal Oil
1 gallonwater

Mix well then pour into a no-clog type hose-end sprayer. If you only have a couple of roses to treat use a hand-held spray bottle. Thoroughly soak the entire plant making sure to get the undersides of the leaves. Apply in the morning, approximately every 7 days as needed.

In the late 1980s, Dr. R. Kenneth Horst of Cornell University began testing the use of baking soda and insecticidal soap to control powdery mildew and blackspot on roses. The soap helped the baking soda solution spread over, and stick to, the plant leaves, but had no effect in suppressing disease. Roses were sprayed every 3 to 4 days and Dr. Horst found the mixture was most effective in preventing blackspot, but was also effective on powdery mildew.

Other research at Cornell focused on controlling fungal diseases, including powdery mildew, on members of the gourd family (i.e. cucumbers and pumpkins). Researchers discovered that a single spray of baking soda and SunSpray brand horticultural oil almost completely inhibited powdery mildew on heavily infected pumpkin foliage. Baking soda without any oil was completely ineffective.

The above baking soda formula is based on the Cornell research. According to Dr. Horst, his formula is now available commercially under the name of Remedy. Remedy by Bonide available online at Gardener's Supply and fine garden centers or contact Bonide, 800-424-9300

Solution #2 - The Baking Soda-Vinegar Formula

1½ tablespoonsbaking soda
2 tablespoonsvegetable oil
1½ tablespoonsSafer Insecticidal Soap (or liquid, not dish, soap)
1 gallonwater
1 tablespoonvinegar

Mix together the baking soda, oil, soap, and water. STIR in the vinegar last - don't shake! Pour into a no-clog type hose-end sprayer. If you only have a couple of roses to treat use a hand-held spray bottle. Spray in the morning and when the temperature is less than 80 degrees. Thoroughly soak all parts of the plant. Apply weekly as needed.

If you remember your high school chemistry, when you mix an acid (vinegar) and a base (baking soda) they cancel each other out and you get water that has a neutral pH. So the question is, how does this formula work to kill mildew?

When baking soda and vinegar are mixed they produce water, carbon dioxide (which bubbles away), and sodium acetate (which stays in solution). I found many references to sodium diacetate as a mold and mildew inhibitor in baked goods and some references to sodium acetate as a fungicide. Apparently, the active ingredient of the baking soda-vinegar solution is the sodium acetate.

Solution #3 - The Vinegar Rinse

2 tablespoonswhite or cider vinegar (5 percent)
1 gallonwater

Pour undiluted vinegar into the container of a dial-a-spray hose-end sprayer. Set the dial to deliver 2 tablespoons per gallon and start spraying. Soak the entire plant, making sure to get the undersides of the leaves. Spray only in the morning. Apply once a week as needed until the weather warms up.

by J.K. Clark
Copyright 2000-2001 Regents of the University of California

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