MARKER ASSISTED SELECTION FOR SCAB RESISTANT APPLE

Abstract

Apple scab, caused by the fungus Venturia inaequalis (Cke.) Wint., is of major economic importance in European commercial production. Many apple breeding programs focus on the development of high quality disease-resistant cultivars, since the pathogen requires between 15 and 25 chemical treatments, effecting both the environment and the economics of apple production. Since 1974, the section of Trento Experimental Institute of Fruit Culture, has been running a traditional breeding programme, which released, in 1997, seven scab resistant varieties. In order to reduce the long time that traditional methods require, and to verify the molecular nature of the observed resistance, we directed our effort on marker selection, supported by ISMA, where the technique has so far been developed. A population of 110 individuals, derived from a cross between the apple variety 'Chahla' (susceptible) x 'Priscilla' (resistant) was scored for response to the disease in both field trials and with marker assisted selection. The molecular analysis, conducted with the markers AM19, AL07 and M18 (Tartarini et al., 1999; Gianfranceschi et al., 1996; King et al., 1998), detects the presence of the Vf gene, introgressed from Malus floribunda 821, which confers resistance to the pathogen. In the present work, the classification of the progeny, obtained using the three markers of the Vf gene, conformed very well to the classification done by the foliar symptoms test, between resistant and susceptible individuals. The same assisted analysis was carried out on the resistant varieties released by our Institute: Golden Lasa; Golden Mira; Golden Orange; Red Earlib; Summerfree; Brina and Nova, in order to prove, by molecular approach, that the resistance source is associated to Vf. Another aspect of our research is directed towards the identification of other sources of resistance, different from Vf, like Vr (introgressed from Malus pumila R12740-7A) and Va (from Antonowka PI172612). In order to pyramid these different genes, we crossed several lines carrying the resistance sources and different quality traits: a population (TN 91 21), of about 200 individuals, selected at ISF for scab resistance in the field and derived from the cross between 'PRI1641-100' (carrying the Vr gene) x 'Priscilla' (carrying Vf) gave very promising results. A previous molecular analysis, conducted randomly on a part of the population, showed that the Vf resistance gene and the scab resistance gene present in the cultivar TN 91 21, supposed to be Vr, seem to be located in the same genomic region. The analysis of the whole population TN 91 21 allowed us to establish whether Vf and Vr present in TN 91 21 are allelic or only tightly linked and to verify if the TN 91 21 genotypes are homozygous or heterozygous for the alleles which are in coupling with the resistance gene Vf.