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Richard Jauron, Horticulture Extension, (515) 294-1871,
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Yard and Garden Column for the Week Beginning July 28
Mildew Resistant Beebalms
By Richard Jauron
Extension horticulturist
Iowa State University Extension
The brightly colored flowers on beebalm (Monarda) are an eye-catching treat in summer. Flowers may be white, pink, red or purple. While beebalms can be beautiful plants, powdery mildew often makes them unsightly. Powdery mildew appears as a grayish white "powder" on the upper leaf surfaces. Severely infected leaves drop prematurely. Disease symptoms are most severe on overcrowded plants and those growing in partial to heavy shade.
Cultural practices can reduce the severity of powdery mildew. When planting beebalms, select a site in full sun. Also, space plants 2 to 2 1/2 feet apart and divide plants every two to three years to prevent overcrowding. Remove and destroy disease-infested plant debris in the fall. The fungal spores of powdery mildew survive the winter on disease-infested plant debris. Removing and destroying this material eliminates the source of next year's infection.
Fungicides also can be used to control powdery mildew. To be effective, fungicides may need to be applied from late spring through summer. For most home gardeners, spraying plants on a regular basis is a tedious, unpleasant chore.
Probably the best way for home gardeners to avoid the annoying problem of powdery mildew is to select mildew-resistant varieties. In 1993, the Chicago Botanic Garden evaluated the mildew resistance of 41 beebalm varieties over a four-year period. They found that many varieties developed serious problems with powdery mildew. Varieties that had very poor resistance to powdery mildew (greater than or equal to 76 percent infection/leaf drop) include 'Beauty of Cobham,' 'Croftway Pink,' 'Mahogany,' 'Mrs. Perry,' 'Prairie Fire' and 'Snow Queen.' 'Adam,' 'Aquarius,' 'Cambridge Scarlet' and 'Prairie Night' are popular varieties with poor powdery mildew resistance (51 to 75 percent infection/leaf drop). While the aforementioned varieties possess attractive flowers, gardeners may want to avoid them because plants often are unsightly by mid- to late summer because of powdery mildew.
The following varieties are mildew resistant and possess good growth habits and attractive flowers. 'Marshall's Delight' was developed at the Agriculture Canada Research Station in Morden, Manitoba. The variety is named for the station's former breeder, Henry M. Marshall. 'Marshall's Delight' grows 2 1/2 to 4 feet tall and produces bright pink flowers. 'Raspberry Wine' bears wine red flowers on 3- to 4-foot-tall plants. Selected by Henry Ross of Gardenview Park in Ohio, 'Gardenview Scarlet' produces scarlet-red flowers on 3- to 4-foot-tall plants. The flower heads of 'Gardenview Scarlet' are 3 to 3 1/2 inches in diameter. 'Violet Queen' grows 3 to 4 feet tall and produces deep purple flowers. Its 2-inch flowers are produced from mid-July to mid-August. 'Colrain Red' has purplish red flowers on 3- to 4-foot-tall plants. Finally, deep purplish red, 2 1/2- to 3 1/2-inch flowers are borne atop 4- to 5-foot-tall plants of 'Rosy-Purple.'
A recent introduction that deserves mention is 'Petite Delight,' the first in a series of dwarf beebalms. It produces lavender rose flowers on 12- to 18-inch-tall plants. 'Petite Delight' has fair resistance to powdery mildew.
The incidence of powdery mildew on beebalm can vary from one location to another. The experiences of some gardeners may differ from those reported by the Chicago Botanic Garden. However, the recommended varieties listed above are an excellent place to begin when selecting beebalms for the home garden.
ml: isugarden
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