Nomenclature of Venturia inaequalis races

Nomenclature of Venturia inaequalis races

Gene-for-gene (GfG) relationships had been demonstrated for the V. inaequalis-Malus pathosystem by the time Flor reported their existence for flax rust. By the late 1960s, 19 pathogenicity genes had been named (Boone 1971), but the researchers at the same time realised that most of the accessions for which these GfG relationships were demonstrated, were apple accessions that were regarded susceptible. No nomenclature system was set up until V. inaequalis races were identified that could overcome major genes, such as those in ‘Dolgo’, Russian apple R12740-7A and ‘Geneva’, that were deemed useful for scab resistance breeding in apple, provided that they were effective to a large proportion of the V. inaequalis population, i.e. were broad spectrum. This criterion is still a major determinant for new resistance genes and QTLs to be included in the naming of new GfG relationships. An exception was made for the Rvi1 (Vg) gene from ‘Golden Delicious’, which is overcome by most pathotypes of V. inaequalis. As a consequence, all the published research to date reporting the ingrained use of race (1) as the assumed completely avirulent reference isolate (race (0) in the new nomenclature) will remain relevant. New genes/QTLs also have to be demonstrably different from other scab resistance genes based on molecular genetic (genome mapping) and/or phytopathological experiments. For genes mapping to the same genomic region, differential interactions will have to be demonstrated with the aid of V. inaequalis pathotypes. Repositories of both the Malus differential hosts and the reference V. inaequalis pathotypes have been set up for distribution among researchers investigating host-pathogen interactions of scab on apple.

The nomenclature system used to date allowed the assignment of single numbers to complex races, which in the long-term will complicate the system when an increasing number of new resistance genes and associated avirulence genes are identified. A new naming system was proposed recently (Bus et al. 2008) that is based on the definition of single GfG relationships, but accommodates complex races in a simple and clear way. To date, 19 differential Malus hosts with major scab resistance genes have been named. (Bus et al. 2011) Researchers can submit new resistance genes and QTLs in the nomenclature system by providing the details of the gene/QTL or preferably the full GfG relationship. Material of the Malus hosts carrying new resistance genes and, if available, their complementary virulent and avirulent reference isolates of V. inaequalis will be requested to be maintained at the respective repositories and to be distributed freely among researchers.