For a species rhododendron such as Rhododendron maximum, it is written: R. maximum.
For a hybrid rhododendron such as Rhododendron 'Nova Zembla', it is written Rcv. 'Nova Zembla' (Rcv. meaning rhododendron cultivar).
For a selection of a species such as Rhododendron yakushimanum 'Koichiro Wada', it is called R. yakushimanum 'Koichiro Wada' although you may see it written Rcv. 'Koichiro Wada'.