Inheritance of apple scab resistance from polygenic sources based on greenhouse and field evaluation

Resistance of 15 progenies of apple (Malus x domestica Borkh.), three females crossed with five males, to apple scab [Venturia inaequalis (Cooke) Winter] was determined by greenhouse inoculation and natural field infection. Five of the parents were reported to carry polygenic resistance ('Antonovka Monasir', 'Discovery', 'Akane', 'Rosu de Cluj', and 'Spartan') and two were susceptible to scab ('Royal Gala' and 'Summerred'). The field test was conducted at a site with heavy scab pressure in the Fraser Valley of British Columbia (Agassiz). Estimates of general and specific combining ability (GCA and SCA) of categorical data were provided by chi squared analysis using the SAS procedure CATMOD. GCA was low and SCA absent in the greenhouse, but GCA was high and SCA present in the field. Chi squared analyses indicated that the greenhouse scab resistance scores were related to the field scores in only three of the progeny. After three years in the field, only 'Antonovka Monasir' and 'Discovery' positively contributed to the inheritance of complete resistance. In the third year, the resistance of five 'Antonovka Monasir' progenies segregated in a way consistent with control by a single dominant gene. The resistance (% leaves with sporulating lesions) of infected offspring of 'Antonovka Monasir' or 'Discovery' was also greater than those of other parents, indicating the possible presence of minor resistance genes.