Segregation and transmission of host resistance to scab (Venturia pirina) in pear breeding progeny under natural infection in an orchard

The introduction of scab (Venturia pirina) resistant pear varieties will increase the economic return of fruit growers and benefit food safety and the environment through a reduction of fungicide use. Seedling progenies of 11 families derived from the controlled cross-pollination of 10 pear varieties: Ya Li, Hood, Buttira Precoce Morettini (BPM), Guyot, Howell, Corella, Rogue Red, Comice, Williams’ Bon Chretien (WBC) and Eldorado, were examined for their response to scab infection in a diseased orchard during the 2004/05 and 2005/06 cropping seasons. The infection was rated by a 9-point scale on diseased leaves of seedling trees with nine being most severe infection. The weather conditions in both seasons were conducive to the disease epidemics with a higher mean disease severity in 2005/06. When progenies with a score of 4 or less were classified as resistant all progeny of the two resistant (R) × resistant (R) families, Ya Li × BPM and Guyot × Hood, were resistant, and those of the R × susceptible (S) family, Eldorado × BPM, were susceptible. In the other families, however, segregation was observed with a proportion of 26–66% progeny being resistant. The segregation patterns observed in all the families can be interpreted by a model of two triallelic genes. The gene, designated as Vpa, has an allele (Vpa2) conferring resistance and two alleles (Vpa1 and Vpa3) conferring susceptibility with Vpa1 being dominant to Vpa2 and Vpa2 being dominant to Vpa3. Similarly, the gene, designated as Vpb, has an allele (Vpb2) conferring moderate resistance and two alleles (Vpb1 and Vpb3) conferring susceptibility with Vpb1 being dominant to Vpb2 and Vpb2 being dominant to Vpb3. Breeding strategies for enhancing scab resistance in pears were discussed.