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Vf scab resistance of Malus

Gessler, Cesare, Pertot, Ilaria
Trees 2012 v.26 no.1 pp. 95-108
Malus floribunda, Venturia inaequalis, apples, breeding, crop production, cultivars, disease resistance, fungi, genes, heterozygosity, mechanism of action, pathotypes, spring, temperate zones
The apple production in temperate regions with spring rains, the Scab caused by the fungus Venturia inaequalis is the most important constraint. To produce spotless apples and avoid damage that develops during storage, growers apply fungicide on a regular or weather-determined basis. All major apple cultivars are highly susceptible to this disease. To limit the need for fungicide applications, apple breeders are currently introgressing disease resistance from wild Malus accessions into commercial lines. The first attempts to do this were made 100 years ago. As apples are self-incompatible, pseudo-backcrossing is used to eliminate unwanted traits from wild Malus and select new cultivars that are attractive to both producers and consumers. This process, from the first cross of a commercial cultivar with a wild, disease-resistant Malus, is extremely long due to apple’s long juvenile phase, the need for more than seven backcross steps and the high heterozygosity of this genus. Therefore, most of today’s scab-resistant cultivars rely on a single introduction of scab resistance from Malus floribunda 821, referred to as Vf. In this paper, we trace the history of Vf from its initial identification through its use in breeding and commercial production. We sum up the literature describing how and where Vf resistance has been overcome by new pathotypes of V. inaequalis. Finally, we describe the current knowledge of the genes behind Vf resistance, its mode of action and the use of Vf genes in gene technology.