Date: August 1999

Monarda Mildew Resistance

Richard E. Bir
NC State University

Richard Hawke
Chicago Botanic Garden

"UNDER OUR CONDITIONS" should be at the top of every plant disease resistance trial report. We learn in our first plant pathology class that the amount of disease on a plant depends upon disease pressure. Disease pressure depends upon local growing conditions. However, it has become all too convenient to view the results of a plant disease resistance study . . . often because their results were conveniently printed in a table . . . and cite these results as if the same plant will respond the same way everywhere.

The bee balm resistance studies referred to here are the result of similar tests at two dramatically different sites, the Chicago Botanic Garden in Illinois and in the North Carolina Mountains. The tests were similar except that plants in Chicago were irrigated "as needed" while those in NC received over an inch of rainfall per week as well as almost daily morning fog during June, July and August.

Part of the difference in plant responses can be attributed to rainfall and humidity since high humidity but a lack of free water on a leaf surface favors the development of powdery mildew when temperatures are between 59 and 86 F. Temperatures rarely exceed 86F in summer in the mountains of NC while summer temperatures above 86 F are common near Chicago.

Winter temperatures are also a major factor in both survival and vigor of Monarda. Cultivars that performed well under the severe mildew pressure of NC which did not perform well in Chicago all sustained winter injury in Chicago whereas 'Marshall's Delight,' which displayed excellent mildew resistance in both locations did not show winter injury.

Conclusions: 1. On the rare occasion when similar research can be compared on different sites, pay close attention to what is being measured. The Chicago study shows a multi-year summary while the NC data is from one year with tables showing response at different times during the growing season.
2. When research is similar enough, always refer to the research done where growing conditions are most similar to local conditions when choosing which cultivars to grow.

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