Black spot disease, incited by the fungus Diplocarpon rosae Wolf, is the most important disease of roses (Rosa hybrida L.) in the outdoor landscape. Though partial resistance exists in cultivated germplasm, the genetic basis of this trait has not yet been elucidated. Six diploid and six tetraploid rose cultivars were crossed in two factorial combining ability arrays. Whole plant and detached leaf inoculation methods were used to assess partial resistance under two different disease pressures using a characterized single-spore isolate. Parents from both arrays had significant general combining ability effects across multiple inoculation methods and environments. Specific combining ability was not significant for either array. Parent per se performance was highly correlated with progeny performance on a family mean basis. High positive correlations among whole plant and detached leaf inoculation methods indicate that detached leaf assays can substitute for whole plant assays. Based on these results, a breeding strategy including parental selection and early, among-family selection is proposed. To our knowledge, this is the first investigation of combining ability for disease resistance in rose.
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Acknowledgments The authors thank Kathy Zuzek for assistance with controlled crosses and David Zlesak for providing rose genotype 95-1 and offering critical comments on the manuscript. Thanks also to Laci High, Amber Halberg, and Ryan Crowe for assistance with inoculations and data management. The authors also acknowledge Steve McKay and Jim Luby for contributing statistical expertise. This manuscript is Scientific Journal Series No. 091210183 of the Department of Horticultural Science, University of Minnesota and has been supported by the Minnesota Agricultural Experiment Station and the University of Minnesota Graduate School.
- Black spot
- Combining ability
- Partial resistance