'Swerti' spelling

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Hello all,

A question for those into bearded species and historics. The cultivar 'Swerti' or 'Swertii' - which is the preferred spelling? My understanding of the conventions of Latinizing non-Latin names says it should be 'Swertii', but perhaps there is a tradition of usage behind the other spelling, which seems quite prevalent. Do earliest references favor one over the other? Any comments welcome.

Thanks, Tom