COMPARISON OF POWDERY MILDEW CONTROLS ON SNOW WHITE BEE BALM, 2000:
C. Bruce and L. Perry, Dept. of Plant and Soil Sciences, Hills Bldg.,
University of Vermont, Burlington, VT 05405
Monarda plants in six-inch jumbo pots of ProMix BX (Premier Horticulture Inc., Red Hill, PA) were maintained in a glass greenhouse at outdoor temperatures, not to exceed 30C day or fall below 15C nights. Plants were 12-inches on center on greenhouse benches, and were fertilized weekly with 150 ppm 20-10-20 solution, and watered overhead. A randomized complete block design was used with six replications per treatment, four plants per replication. Treatments began 21 Jun at the first sign of mildew development, and were applied every two weeks, for eight weeks, to both abaxial and adaxial leaf surfaces. Treatments, sprayed for total leaf coverage, included SunSpray horticultural oil (98.8% refined petroleum distillate), WiltPruf antidessicant, Neemazid (4.5% azadirachtin), baking soda (sodium bicarbonate), and Remedy (85% potassium bicarbonate). The control group received no treatment. A visual composite percentage of the leaf surface covered with powdery mildew was recorded every two weeks after the first treatment to determine the treatment mean. Treatment mean differences for each date, if significant by ANOVA, were separated by Tukey's procedure.

On all dates, the control of no treatment and Neem had the most disease, with no significant differences between the two
groups. Neem exhibited more disease than the control group on 17 Jul. Remedy provided some of the best control on 5 Jul, but decreased in effectiveness as disease severity increased. Overall, Remedy and baking soda provided poor control, with no significant differences between the two groups on 31 Jul. No significant differences were detected between Remedy, baking soda and Neem on 15 Aug. WiltPruf and SunSpray provided the best control, with no significant differences between the two groups on 5 Jul, 17 Jul and 15 Aug. On 31 Jul, WiltPruf was the most effective treatment. WiltPruf provided slightly better control than SunSpray on all treatment dates, although no significant differences were detected on three of the dates. From the five treatments in this study on Snow White monarda, WiltPruf and SunSpray provided the best control. Remedy and baking soda were moderately effective at first, but became less effective during the study. Neem was not an effective control.
 
Powdery mildew severity (index value*)
Treatment Rate/gal water 5-Jul 17-Jul 31-Jul 15-Aug
No treatment………………. --- 3.88a** 3.67a 4.17a 3.95a
SunSpray Oil………………. 3 Tbsp/gal 1.00 d 2.08 d 2.46 d 2.06 c
WiltPruf antidessicant……. 10 Tbsp/gal 1.00 d 2.00 d 2.20 e 1.88 c
Neem………………………. 1/8 tsp/gal 3.79a 3.83a 4.06ab 3.67ab 
Baking soda……………….. 1.5 Tbsp/gal 3.08 b 3.13 c 3.83 bc 3.25 b
Remedy……………………. 2 Tbsp/gal 1.79 c 3.38 b 3.71 c 3.25 b
Standard Error of Mean
0.063
0.035
0.042
0.233
Significance, p level
<0.001
<0.001
<0.001
<0.001

* Powdery 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.

** Numbers in a column with a letter in common are not significantly different according to Tukey's procedure (a=.05).


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