Possibilities to improve apple scab (Venturia inaequalis (Cke.) Wint.) and powdery mildew [Podosphaera leucotricha (Ell. et Everh.) Salm.] resistance on apple by increasing genetic diversity using potentials of wild species

Creating new apple cultivars largely depends on the availability of sufficient genetic diversity, while apple breeding has eroded in time the genetic base of domestic cultivars. The possibility to induce and exploit useful variability for selecting hybrids resistant to apple scab and powdery mildew attack was evaluated in interspecific descendants derived from five wild apple species Malus coronaria, M. floribunda, M. niedzwetzkyana, M. zumi and M. prunifolia). A number of 1650 F^sub 1^ hybrids were analyzed, belonging to fifteen combinations, framed in five cyclic models of hybridization, in which crab apple species were used as maternal or paternal tester, each being crossed with two-four different apple cultivars (namely Cluj 218/2, Frumos de Voinesti, Reinette Baumann, Rosu de Cluj, Jonathan, and Golden Delicious). M. coronaria and M. prunifolia proved to have descendents with valuable response to scab attack (based on genetic gain and expected selection response), compared to M. zumi offsprings. The offsprings of M. coronaria offer a good background for an efficient selection of resistant individuals for powdery mildew. The additive effects proved inferior compared to genetic effects of dominance and epistasis in several analyzed crosses. Phenotypic correlations between responses to apple scab and powdery mildew attack on F^sub 1^ hybrids were identified in three out of the five cyclic combinations. The results reveal the potential of wild apple species as resources of genes for resistance to both scab and powdery mildew. Keywords: expected selection response, heritability, hybridization, genetic gain, seedlings. Abbreviations: PRI-Purdue, Rutgers and Illinois Universities cooperative apple breeding program, started in 1948; CGV-Coefficient of Genetic Variability; H^sup 2^-broad sense heritability; h^sup 2^-narrow sense heritability; σ^sub G^^sup 2^-genotypic variance; σ^sub P^^sup 2^-phenotypic variance; σ^sub Ga^^sup 2^-additive variance; GG or ΔG-Genetic Gain; σ-square root of the total variance among families; σG-square root of the genotypic variance among families; x-mean of trait; σ^sub P^-square root of the phenotypic variance among families or populations represented by F^sub 1^ hybrids from each cyclic combinations of half-siblings; R-expected selection response.

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