A seedling of Rhododendron calendulaceum x occidentale, which is a Oliver hybrid documented as R. calendulaceum 'Colossus' x R. occidentale, has been determined to be a diploid using flow cytometry by
Dr. João Loureiro, Dr. Silvia Castro, José Cerca, and Mariana Castro
Plant Ecology and Evolution Group,
Centre for Functional Ecology,
Department of Life Sciences,
Faculty of Science and Technology,
University of Coimbra, Portugal.
A seedling of Rhododendron calendulaceum x occidentale being a diploid may indicate the seed parent was R. cumberlandense instead of R. calendulaceum. This seedling was determined to be both seed and pollen fertile by John and Sally Perkins. Moreover, this deciduous azalea seedling pollen tested as a diploid by accepting pollen from confirmed diploids.
John and Sally Perkins suggest in the Rules of Engagement that tetraploid X diploid crosses in deciduous azaleas are seldom result in seedpods.
Rhododendron cumberlandense and Rhododendron occidentale are diploid species. Rhododendron calendulaceum is a tetraploid species.
Summary of Ploidy Levels
Diploid R. cumberlandense
Diploid R. occidentale
Diploid calendulaceum x occidentale
Tetraploid R. calendulaceum
Source: Dick Cavender
John and Sally Perkins
