Inheritance of Virulence of Venturia inaequalis Toward Mains × domestica Cultivars

The resistances or the three apple cult iva rs ʹ Boskoop-, ʹSparta n-, and ʹGolden Delicious were dilTe rentia l to the isolates Bo-7 and Sp-7 or Venlllria illaequalis. Isolate 80- 7 was virulent towa rds ʹBoskoopʹ and ʹGolden Deliciousʹ no t to ʹSpartanʹ, and Sp-7 was onl y virulent toward ʹSpartanʹ, suggesting thai a gene- for-gene relationship is working in this palhosystem. The two isolates lVe re hybridized in order to determine the genetic basis or virulence. All theo retically possible phenotypes were ro und in 33 ascospore progenies from two selected strains carrying virulence towards ʹBoskoopʹ and "Spartanʹ, with 14 progenies virulent towards ʹBoskoopʹ, 9 virulent (Q ʹSpartanʹ, and 15 virulent to ʹGolden Deliciousʹ. Seven progeny were avirulent to all three culti va rs, which is in agreement wi th the theoretical number of isolates with three avirulenccs. The segregation of virulence and avirlllence among the progeny towards ʹBoskoopʹ, ʹSpa rtanʹ , a nd ʹGolden Delicious fi t best to the hypothesis that one gene is responsi ble ror vir ulence lO ʹBoskoopʹ a nd ʹGolden Deliciousʹ and lhat two genes are responsible ror virulence towards ʹSpa rtanʹ, assuming th at each gene segrega ted independently rrom the other genes. The tlVO genes for viru lence to ʹSpa rtanʹ werecomplcmen tary and were inherited independentl y. Virulence or V. illaequalis is governed by rew herita ble genes ror each culti var. Combinations of virulences are observed, indicating thal the building-up or complex races is possi ble.