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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 37, Number 2
Spring 1983

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Control of Botrytis Petal Blight of Azalea
Larry D. Smith, Tennessee (Tenn. Tech. Univ.)

Nature of Work: In two years of study the fungicide R-P 2601 9 (iprodione) was evaluated for control of petal blight of Azalea, caused by Botrytis cinerea Pers. ex Fr.
       The 1980 study was conducted on mixed varieties of azaleas. The plants were grown in 6-in. containers under cool temperature (less than 21 C) and high relative humidity (90-95%). Four treatments were utilized: (1) Untreated control (inoculated); (2) R-P 26019 50W, 0.5 lb. ai/100 gal. water; (3) R-P 26019 50W, 1.0 lb. ai/100 gal.; and (4) Benlate 50W, 1.0 lb. ai/100 gal. Each treatment was applied to four plants; the test was replicated four times. Two applications were made on a 14-day schedule prior to inoculation with B. cinerea. Two applications were made following inoculation. The first treatment was made as the flowers were beginning to open; inoculation was made during full bloom; and the final treatment was made after petal fall. Control evaluation was made ten days after inoculation by counting the number of Botrytis-infected flowers. As new growth occurred observations were made for phytotoxicity up to 60 days after inoculation.
       The 1981 study was conducted using similar methods and conditions. However, six treatments were utilized; the same four treatments as in the 1980 study plus R-P 26019 in an oil-base formulation of 2.08 lb. ai/gal. used at the rate of 0.5 ai/100 gal. of water, and R-P 26019 in the oil-base formulation used at the rate of 1.0 lb. ai/100 gal. of water. All of the plants in this study were Rhododendron obtusum (Lindl) Planch, var. Ethel Orme.

Results and Discussion: In the 1980 study, results were expressed as the percent blighted flowers per plant. The disease control by the treatments was significantly different at the 5% level (Fig. 1). R-P 26019, 1.0 lb. ai/100 gal. water showed the greatest control with 18.1% of the flowers infected. This was followed by Benlate 50W (40.7%) R-P 26019, 0.5 lb. ai/100 gal. (67.8%); and the untreated, inoculated control (97.1%). No phytotoxicity was observed with any of the treatments.
       In the 1981 study control was expressed as the percent blighted flowers per plant. The control by the six treatments was significantly different at the 5% level (Fig. 2). R-P 26019 in the oil-base formulation used at the 1.0 lb. rate and 0.5 lb. rate, and the R-P 26019 50W, 1.0 lb. gave the greatest control with 3.40%, 2.40%, and 2.03% blighted flowers, respectively. Benlate 50W, 1.0 lb.; R-P 26019 50W, 0.5 lb. and the control treatment were each in separate classes with 11.90%, 20.55% and 98.83% infected flowers respectively.
       Benlate 50W is commonly used by Azalea producers to control Botrytis petal blight and provides adequate control. The results of these studies indicate that R-P 26019 50W, 1.0 lb. rate; R-P 26019 in an oil-base formulation, 0.5 lb. rate and 1.0 lb. rate also provide good control of the disease.

Figure 1. Control of Botrytis petal blight of Azalea (mixed varieties).
Treatment lb. ai/100 gal. water % blighted flowers*
Control None 97.1 a**
R-P 26019 50W 0.5 67.8 b
Benlate 50W 1.0 40.7 c
R-P 26019 50W 1.0 18.1 d
* Values are mean of 16 plants treated.
** Means followed by the same letter are not significantly different at the .05 level (lsd=5.35%).
 
Figure 2. Control of Botrytis petal blight of 'Ethel Orme' Azaleas.
Treatment lb. ai/100 gal. water % blighted flowers*
Control None 98.831 a**
R-P 26019 50W, 0.5 0.5 20.55 b
Benlate 50W, 1.0 1.0 11.90 c
R-P 26019 oil-base, 1.0 1.0 3.40 d
R-P 26019 oil-base, 0.5 0.5 2.40 d
R-P 26019 50W, 1.0 1.0 2.03 d
* Values are mean of 16 plants treated.
**Means followed by the same letter are not significantly different at the .05 level (1st-6.93%).


Volume 37, Number 2
Spring 1983

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals