Differential resistance in apple against scab and its use in breeding and in orchard planting strategies to control the disease

Abstract

Early and recent studies demonstrate in many commercial cultivars the presence of functional different resistances overcome by specific races of the scab fungus. It is postulated therefore that the composition of the pathogen population is due to selection by host resistance genes. Those ephemeral resistances are of no effect on the scab populations in homogeneous orchards since they are composed of cultivar adapted races. On the other hand, this information could be used to develop new cultivars which carry several functionally different resistances. The consequences for breeding strategies and for orchard planting to minimise fungicide input are discussed as well as the use of genomic markers to help in adopting the proposed strategies. Secondly, strategies to control apple scab by planting orchards with a mixture of selected cultivars carrying different functional resistances can be developed. To study the effect of cultivar mixtures in apple orchards, results of a simulation model are presented and discussed.