Wednesday, February 11, 2009

Rosemary in Winter (Continued): Dealing with Powdery Mildew

Rosemary Stems

Above: A closer look at rosemary (Rosmarinus officinalis).

Most people who try to overwinter rosemary (Rosmarinus officinalis) inside are familiar with powdery mildew, Erysiphe sp. Similar to the whitefly I discussed in the earlier post, this fungus is favored by the indoor conditions that are typically provided in attempt to overwinter rosemary.

There are several different types of fungi that share the common name “powdery mildew.” Although they may appear similar, these different species affect different plants. For example, the fungus that you find on your rosemary is different than the one you find on your zinnias. When powdery mildew is problematic outdoors, seasonal crop rotation is recommended. This breaks the relationship between the host plant and parasite.

Powdery mildew is favored by warm days, cool nights, humidity above 90%, and lower light conditions with ideal temperatures between 68 to 96 degrees Fahrenheit. It appears as a white or gray dust on the leaf surface. The fungus is a parasite, which survives solely on the nutrients it takes from the host plant. If allowed to progress, this can lead to a rapid decline in health. Symptoms include yellowing, distorted growth, leaf drop, and decreased flower production.

Breeders have worked diligently to produce powdery mildew???resistant plants. Unfortunately, there has yet to be any success with rosemary. However, plants in good health are generally more resistant to any parasites and have a better chance of defending themselves until a parasite is detected and treated.

There are many safe and effective treatments for powdery mildew, all of which will have greater success the earlier they are applied. Bicarbonates have become a popular ingredient in many fungicides treating powdery mildew. Home gardeners???including my late grandmother???have used this for years in the form of baking soda and dish soap. Recent research is showing that this practice is not only preventative, but also helps eliminate existing fungi. The only problem with this treatment is the appearance of the residue: it leaves behind a white powdery film that looks similar to the original fungus. For that reason, we have been treating the powdery mildew on our rosemary with neem oil.

Neem oil has been our most successful treatment for powdery mildew to date. After some experimentation to find the most effective concentration, it seems to control the fungus completely after one to three treatments.

???Kevin Wiecks


Ball, Jeff, and Liz Ball. Rodale???s Flower Garden Problem Solver. New York: Rodale Press, 1990.

Yepsen, Roger. The Encyclopedia of Natural Insect and Disease Control. New York: Rodale Press, 1984.

Smith, Miranda, and Anna Carr. Rodale???s Garden Insect, Disease, and Weed Identification Guide. New York: Rodale Press, 1988.

“Powdery Mildew Factsheet,” Cornell University (February 11, 2009).

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Comments (4)

  1. thea mcginnis Says:

    last year on the blog i mentioned a hardy rosemary overwintering quite well in my garden. living in d.c. area my garden was under 2 ft of snow for a couple of weeks. today i peeked out to find the snow had melted away and that same rosemary plant was FINE. amazing! thea

  2. Barclay A. Dunn Says:

    Oops! You left out the most helpful part! What was the recipe you arrived at with the neem oil? Is it just neem oil and water? At what concentration?

  3. Deirdre Larkin Says:

    Dear Barclay,

    I’m answering on Kevin’s behalf, as he is no longer with the Museum. The neem oil was mixed with water and a little liquid castile soap added as an adjuvant. As I recall, the proportion used was an ounce to the gallon, but you will find doubtless find recommendations on any neem oil purchased. I have become wary of an indiscriminate use of neem oil, as it does have a residual effect and interferes with the program of beneficial insect release we now employ at The Cloisters. Our biggest pest problem in winter is with hard scale on woody plants, such as bay laurel and olive. We now release a tiny parasitic wasp with good effect. Once the wasps have thoroughly parasitized the scale, they do perish of starvation, and have to be re-introduced in case of re-infestation. Rather than applying neem oil to nearby rosemarys, I am experimenting with applying a pine sap-based anti-transpirant as a physical barrier against mildew and fungi. This is not a proven method—merely an experiment. Our rosemarys are not mildewed as yet, but it is early in the season. I find that they often hold their own until March, only to succumb.

    If I were to use neem oil on a plant in the collection, I would isolate it from our other plants until I was reasonably sure that any residual effect that could effect beneficial insect populations had worn off.

  4. bill Says:

    Try concentrated lemonjuice. I put the juice in a clean gen purpose spray bottle and sprayed my rosemary entirely. I repeated once more two days later and then just in stubborn spots. Two weeks later- no fungus and safe to consume.

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