Variability and genetic structure of a novel Turnip mosaic virus (TuMV) population from horseradish (Cochlearia armoracia L.) were examined. Over 60 horseradish plants were tested to identify a total of 28 TuMV isolates, constituting the Cochlearia ARmoracia (CAR) TuMV population. Two subgroups of the CAR TuMV isolates could be distinguished: subgroup N did not infect oilseed rape (Brassica napus var. oleifera) cv. Westar plants, while subgroup A infected these plants systemically. Two types of infection of oilseed rape plants were induced by inoculation with the CAR TuMV isolates: systemic mosaic infection and systemic necrotic lesions. The complete sequences of isolates CAR37 (subgroup N) and CAR37A (subgroup A) were determined and compared. The sequences of HC-Pro and CP genes of CAR37 and CAR37A and other isolates of TuMV from other countries were compared to provide some insight into their relatedness. CAR37A, initially regarded as a variant, proved to be very different from CAR37. Re-sequencing after repeated passages confirmed the genetic stability of both isolates.