L. Perry, Dept. of Plant and Soil Sciences, Hills Bldg.,

University of Vermont, Burlington, VT 05405

Bee balm (Monarda didyma cultivars) plants in 4-in. pots from various commercial sources were transplanted in summers of 1994 and 1995 into a field plot of Windsor loamy sand at the Horticulture Research Center, South Burlington, VT. Bee balm were planted in a 3-ft wide row with 2 ft between plants in the row. A randomized complete block design was used with six replications per cultivar. Plants were fertilized with organic 5-3-4 Pro Gro (North Country Organics, Bradford, VT) at the rate of 3 lb/100 sq ft in May each year. Plants were weeded by hand and with targeted applications of glyphosate as needed. Overhead irrigation was supplied in afternoons for 2-hr intervals, three times per week as needed to supplement rainfall. A visual composite percentage of the upper foliage surface covered with powdery mildew (Erysiphe cichoracearum) was estimated at 3-wk intervals from onset of disease incidence for each plant by two individuals to determine the cultivar mean, with mean differences if significant by ANOVA separated by Tukey's procedure.


Generally each year, the standard errors of the means were between 10% and 30% of the means, and means were highly significantly different. The first year (1994), least mildew occurred on Violet Queen, Marshall's Delight, Gardenview Scarlet and Blue Stocking. Data not shown reflects cultivars planted the following year. The second year (1995) there was little to no incidence of mildew so data was not taken. The third year (1996) cultivars with least mildew included the above except for Gardenview Scarlet, and additionally with Stone's Throw Pink. The fourth and final year (1997) of the study, the cultivars with least mildew were the same three common to the previous ratings. Stone's Throw Pink also had little mildew as in the previous year. Ratings were only taken this last year late in the season as no mildew was apparent before that time. The bee balm cultivars most resistant to this species of powdery mildew appear to be Blue Stocking, Violet Queen, and Marshall's Delight. The least resistant cultivars, generally with 80-100% of foliage showing mildew each year, included Adam, Cambridge Scarlet, Croftway Pink and Souris.

Powdery mildew severity (index value) 1

Cultivar color year in 8/24/94 8/19/96 8/24/97 Overall  
Adam red 94 4.5d2 5.0e 3.0e 4.2  
Blue Stocking purple 94 2.0ab 1.0a 1.0a 1.3  
Cambridge Scarlet red 94 3.0c 5.0e 4.5g 4.2  
Croftway Pink pink 95 --3 4.7e 5.0g 4.9  
Gardenview Scarlet red 94 1.8ab 2.8c 2.8e 2.5  
Kardinal red 94 2.5bc -- -- 2.5  
Mahogany purple 94 3.0c 4.0d 3.8f 3.6  
Marshall's Delight pink 94 1.7a 1.3a 1.0a 1.3  
Mrs. Perry red 95 -- 2.0b 1.5abc 1.8  
Ohio Glow pink 95 -- 2.5c 2.0cd 2.3  
Prairie Night purple 95 -- 3.8d 1.3ab 2.6  
Purple Crown purple 95 -- 4.2d 1.8bcd 2.0  
Red Stocking red 95 -- 2.7c 2.0cd 2.4  
Snow White white 94 3.2e -- -- 3.2  
Souris pink 95 -- 4.8e 4.8g 4.8  
Stone's Throw Pink pink 94 3.2c 1.2a 1.8bcd 2.1  
Violet Queen purple 94 1.5a 1.0a 1.0a 1.2  


1Powdery mildew was rated on a scale of 0-5, with 0=none, 1=1-20%, 2=21-40%, 3=41-60%, 4=61-80%, and 5=81-100% of the upper leaf surface covered with powdery mildew.

2Numbers in a column with a letter in common are not significantly different according to Tukey's procedure.

3Missing data indicates cultivar not present in study that year.


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