COMPARISON OF POWDERY MILDEW CONTROLS ON MT. FUJI PHLOX, 1999

C. Bruce and L. Perry, Dept. of Plant and Soil Sciences, Hills Bldg.,
University of Vermont, Burlington, VT 05405

Phlox plants in 52-cell flats, obtained from a commercial nursery, were transplanted in early July 1999 into six-inch jumbo pots of ProMix BX at the University greenhouse, Burlington, VT. Plants were established outdoors for one month on benches, then brought into a glass greenhouse maintained at outdoor day temperatures, not to exceed 30C day nor 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 were begun 26 July at first signs 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) applied at 3 Tbsp/gal water; WiltPruf antidessicant; Neemazid (4.5% azadirachtin) applied at 1/8 tsp/gal water; baking soda applied at 1.5 Tbsp/gal water; and Remedy (85% potassium bicarbonate) applied at 2 Tbsp/gal water. 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.

Even though significant, differences among treatments were less on the first and last treatment dates. On the first, the disease was not well-established, so treatments resulted in relatively good control. On the last date, with severe disease on most treatment plants, only the WiltPruf afforded much control. Data within treatments became less variable as the study progressed. On all dates the control of no treatment had the most disease, with similar disease on the Neem treated plants on 23 Aug and 20 Sept. Although Neem provided the most control on 9 Aug, not significantly different from WiltPruf, it provided among the least control on the remaining dates. There was no significant difference between SunSpray horticultural oil and baking soda on all dates, both providing moderate control at the start and little control at the end. Except on 9 Aug when there was no difference in control with Sunspray or baking soda, Remedy provided some of the best control. WiltPruf was the most effective treatment, its control being significantly better than all other treatments on the last three dates. From the five treatments in this study on Mt. Fuji garden phlox, best control of powdery mildew was provided by WiltPruf, followed next by Remedy. Moderate control was provided by Sunspray or baking soda, with little control from Neem.

Powdery mildew severity (index value*)

Treatment 9 Aug 99 23 Aug 99 6 Sept 99 20 Sept 99
         
No treatment..... 4.0a** 4.7a 5.0a 5.0a
SunSpray ..................... 2.9 b 3.9 b 4.0 cd 4.7 b
WiltPruf antidessicant....... 2.1 c 2.5 d 2.2 e 2.2 d
Neem................................. 1.9 c 4.4a 4.5 b 4.9a
Baking soda...................... 2.6 b 3.9 b 4.3 bc 4.8 b
Remedy........................... 3.1 b 3.3 c 3.8 d 4.4 c
Standard Error of Mean 0.23 0.11 0.09 0.12
Significance, p level 0.01 0.01 0.01 0.01

*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.

Appreciation is expressed to the New England Greenhouse Conference for support of this study.


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