This was not the question I set out to answer, rather what was the genetic susceptibility of various bee balm (Monarda) cultivars to powdery mildew. Fifteen other cultivars had previously been rated for mildew over a several year period, with results previously reported. However, as often happens in our microclimate (an open USDA zone 4b), this study yielded different but useful results.

Plants were established in summer 1998, with no data taken during that first year. (Plants were spaced three feet apart, in a diagonal pattern in a row, in a randomized complete block design with six replicates.) Nor were data taken during summer 1999 due to growth stunted by record drought in the loamy sand soil. Although plants were located at the Horticulture Research Center, in South Burlington, Vermont, where there was overhead irrigation from ponds, due to the drought this was rationed and minimal. With minimal possibility for watering, plants were weeded only early in the season and at the end, resulting in severe weed pressure on growth as well. Finally, there was no snow cover during the first half of the 1999-2000 winter, until mid-January, resulting in low crown temperatures during this period (on occasion dropping below 20 degrees F, one time for over 3 days). So the results of this study are a reflection of the vigor of these bee balm cultivars with drought, weed and cold stress.

The most vigorous cultivar, with no reduction in vigor was Elsie's Lavender, followed by Cherokee and then Vintage Wine. Next and tied were Loddon Crown and Marshall's Delight. Rounding out the top ten, similar in vigor so listed in alphabetical order were Aquarius, Colrain Red, Dark Ponticum, Pisces and Squaw. Those that survived poorly, if at all, included in alphabetical order Jacob Cline, Oriental Lace, Petite Delight, Scorpio and Twins. Jacob Cline was a surprise, being so vigorous and surviving fine in an even colder trial site in USDA zone 4a, but in better stony loam soil, less weed pressure, and less drought pressure. Snow White had not even survived the first winter.

The above top ten cultivars, with the addition of new plants of Jacob Cline and Sagittarius, have been potted with hopefully several years of mildew comparisons possible under more controlled conditions at the UVM greenhouse and adjacent nursery.
 

Comparison of Monarda cultivar vigor.
 

Cultivar	Flower color	Rating*
Aquarius	light pink	2.0
Cherokee	rose pink	4.5
Colrain Red	red	3.0
Dark Ponticum	purple	2.8
Elsie's Lavender	lavender	5.0
Jacob Cline	red	1.2
Loddon Crown	purple-red	3.8
Marshall's Delight	pink	3.8
Petite Delight	pink	1.0
Pisces	rose pink	2.7
Scorpio	purple	1.5
Squaw	red	3.3
Twins	dark pink	1.2
Vintage Wine	red-purple	3.8

*Rating 5/15/00: 1=dead, 2=0-5 shoots over 2", 3=6-10 shoots, 4=11-15 shoots, 5=over 15 shoots.

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