Estimating combining ability for fire blight resistance in apple progenies

Resistance to fire blight (Erwinia amylovora) is one goal of many apple breeding programs. The objectives of this experiment were: 1) to determine whether segregating populations can be screened reliably as seedlings in the greenhouse and 2) to estimate general combining ability (GCA) and specific combining ability (SCA) in crosses representing the range of susceptibility in commercial apple cultivars. A 4 x 4 half-diallel mating design was used with 'Braeburn', 'Fuji', 'Spartan' and 'Splendour' as females and 'Delicious (Bisbee)', 'Golden Delicious', 'Cripp's Pink' and 'Gala' as pollen parents. Shoots of ca. 130 seedlings per family were inoculated in the greenhouse. Another 100 plants per family, plus parent clones and survivors of the greenhouse screening, were inoculated in the field two years later. For both tests, inoculum was comprised of mixed virulent strains of E. amylovora. The severity of infection (% shoot blighted) and a familial index of severity were calculated separately for greenhouse and field-grown trees. The severity of infection was uniformly high for all families in the greenhouse trial. In the field trial, none of the families showed a normal distribution with respect to infection severity, and in all crosses transgressive segregation was observed. Inter-family differences in severity index were greater in the field than in the greenhouse. Amongst survivors of the greenhouse screening, the correlation between fire blight severity in the greenhouse and field tests was not significant (R=0.10, n=154, p=0.23). GCA values indicated that all parents except 'Cripp's Pink' (Pink Lady (R)), 'Gala' and 'Braeburn' contributed to reducing susceptibility, but SCA was also observed. The results suggest greenhouse screening overestimates susceptibility in many instances and it was not a reliable predictor of the response of individual plants or families in this trial. Tests of elite apple clones prior to commercial release remain a prudent practice.