Interspecific hybridization of Belgian pot azaleas (Rhododendron simsii) with tropical Vireya genotypes

Belgian pot azaleas are hybrids of 4 Rhododendron species belonging to the evergreen Tsutsusi subgenus. Unfortunately, intersubgeneric crosses with genotypes from other subgenera having traits such as new flower colours, scent, new flower types or altered growth habitus are mostly unsuccessful. The aim of the current research is to evaluate the possible use of Vireya genotypes for interspecific sexual hybridization with Belgian pot azaleas. To this end, 5 Vireya genotypes, R. laetum, R. ‘Sunny’, R. ‘Silver Thimbles’, R. ‘Jazz Band’ and R. ‘Our Marcia’ were used as seed parents in crosses with 13 pot azalea cultivars. Altogether, 188 flowers were pollinated. After 3 months, the number of non-aborted fruits was counted and the seeds were dissected and sown. The final efficiency of each cross was determined as the ratio between the number of vigorously growing hybrids originating from a cross and the number of flowers that was pollinated. All Vireya genotypes set fruit; the success rate varied between 41% (R. ‘Silver Thimbles’) and 100% (R. laetum). The final efficiency varied between 0.2 hybrid plants / pollinated flower for the seed parent R. ‘Jazz Band’ and 6 for R. ‘Sunny’. For R. laetum, 1.4 plants were obtained per pollinated flower; for R. ‘Silver Thimbles’ and R. ‘Our Marcia’ this proportion was 0.22 and 0.44, respectively. Viable hybrid plants were obtained from each Vireya seed parent. Many of the hybrids displayed an intermediate plant and flower morphology. Nevertheless, none of the hybrids produced fertile pollen. Experiments with other hybrid groups from the subgenus Rhododendron as breeding partners for Belgian pot azaleas were unsuccessful. Our results are in accordance with earlier findings and indicate a significant potential for Vireya types for the induction of interspecific F1 hybrids.