Genetic analysis of resistance to swede mildew

The incidence of swede mildew (Erysiphe cruciferarum) was analysed on three occasions in five swede varieties Crifel, Doon Major, Harvester, Ne Plus Ultra and Ruta Øtofte and their F₁ hybrids. Overall levels of infection were greater in Ne Plus Ultra and Harvester and least in Ruta Øtofte. Genetic analysis by the diallel method showed that initially resistance was recessive with Ruta Øtofte possessing most recessive genes but results from the final analysis showed partial dominance, with Ne Plus Ultra having most recessive and Harvester most dominant genes. The changing pattern of inheritance with time was due to variation in the rates of increase in disease incidence and diallel analyses of these rates, calculated by regression analyses over five observations, showed in almost entirely additive control. Although no variety was immune it was considered that except on soils of high pH or in the event of a marked change in the genetic composition of the pathogen the level of resistance in Ruta Øtofte was satisfactory.