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

Abstract 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 Vp a , has an allele ( V p a 2 ) conferring resistance and two alleles ( V p a 1 and V p a 3 ) conferring susceptibility with V p a 1 being dominant to V p a 2 and V p a 2 being dominant to V p a 3 . Similarly, the gene, designated as Vp b , has an allele ( V p b 2 ) conferring moderate resistance and two alleles ( V p b 1 and V p b 3 ) conferring susceptibility with V p b 1 being dominant to V p b 2 and V p b 2 being dominant to V p b 3 . Breeding strategies for enhancing scab resistance in pears were discussed.