The brightly colored flowers on beebalm (Monarda) are an eye-catching treat in summer. The 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 2 to 3 years to prevent overcrowding. Remove and destroy disease-infested plant debris in the fall. Fungicides can also be used to control powdery mildew.
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 4 year period. They found that many varieties developed serious problems with powdery mildew. Varieties that have very poor resistance to powdery mildew ( =76% 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% infection/leaf drop). While the aforementioned varieties possess attractive flowers, gardeners may want to avoid them because plants are often 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 flowers are borne atop 4- to 5-foot-tall plants of 'Rosy-Purple.'
An new variety that deserves mention is 'Petite Delight.' 'Petite Delight' is 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.
This article originally appeared in the July 2, 1999 issue, p. 91.
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