Apple cultivar Regia possessing both Rvi2 and Rvi4 resistance genes is the source of a new race of Venturia inaequalis

The apple cultivar Regia, bred in Dresden-Pillnitz, Germany, is one of the few cultivars originating from a different scab (Venturia inaequalis) resistance background than Malus floribunda 821, the source of Rvi6. Cv. Regia is a descendant of Russian apple R12740-7A and has been proved to contain the two scab resistance genes Rvi2 and Rvi4. The cultivar itself has been grown in Dresden-Pillnitz since the early 1970s, while seedling populations derived from cv. Regia have been raised in fungicide-free plots since at least 1982. In 2011, small scab lesions were found for the first time on leaves of cv. Regia trees in an experimental unsprayed orchard in Dresden-Pillnitz. Single spore isolate Regia2 (R2) was cultured from these scab lesions and propagated, and then inoculated on scions of the VINQUEST scab differential host set comprising host (0) to host (15), host (17) and cv. Regia, grafted on rootstock M9, in a greenhouse. Strong sporulation was observed on hosts (0), (1), (2), (8), (9), (10) and cv. Regia, and weak sporulation on hosts (3), (4), (13) and (17), suggesting that the new isolate possesses the respective virulences. The implications of these results are discussed in relation to the natural infection of the differential hosts in the orchard and the artificial inoculation of the grafted differential hosts with inoculum gained from in vitro culture of R2.