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R.S. Roark, B.K. Behe, and K.L. Bowen

Five antitranspirant materials, a horticultural oil, the fungicide chlorothalonil, and an untreated control were applied to rose plots using one of four application schedules. After 12 weeks, treatments were evaluated for their effectiveness in control of blackspot. Disease (P=0.0022) and defoliation (P=0.0008) showed significant treatment differences, while vigor and flowering were unchanged. Two antitranspirants, Stressguard 0.05% applied every 2 weeks and NuFilm17 1% alternated with chlorothalonil around rain events, gave similar disease control to weekly chlorothalonil applications. One antitranspirant, Vapor Gard, 1 % alternated with chlorothalonil around rain events, gave similar defoliation control to weekly chlorothalonil applications. These results indicate that blackspot disease can be effectively managed with fewer applications of chlorothalonil.

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Beth Clendenen, B.K. Behe, and K.L. Bowen

Eleven rose cultivars were field planted and evaluated weekly for disease, defoliation, and overall vigor in order to compare natural resistance to blackspot (Diplocarpon rosae). Alternative treatments were also compared for efficacy in low-maintenance disease control. Treatments included a bimonthly application of chlorothalonil, a bimonthly application of a horticultural oil, an application of chlorothalonil based on rain events, and a no-treatment control. Cultivars showed significant differences in disease severity, defoliation, and overall performance, with old garden rose varieties showing more natural disease resistance than modern susceptible varieties included in the study. Chlorothalonil applied on a 14-day spray schedule did provide a significant decrease in blackspot disease severity when compared to other treatments. A significant incidence of secondary disease including Cercospora rosicola and Botrytis cinerea occurred on old garden rose varieties. No treatment differences were found for these diseases. `Belinda's Dream', `The Fairy', and `Red Mediland' ranked highest in overall performance throughout the season.