Abstract :
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.
