'Betty Hume' is a Diploid

Rhododendron 'Betty Hume', which is a C.O. Dexter hybrid, 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.

The nursery literature often mentions the following as suspected polyploids:

'August Lamkin': suspected polyploid - tested triploid,
'Betty Hume': suspected tetraploid - tested diploid,
'Gargantua': suspected triploid - tested tetraploid,
'Hallelujah!': suspected tetraploid - tested triploid,
'Marie Hoffman': suspected tetraploid - tested diploid

Summary of Ploidy Levels

Diploid Betty Hume

Source: Werner Brack
Source: Don Hyatt

John and Sally Perkins