Evaluation of tolerance to apple replant disease (ARD) in Malus germplasm

Apple replant disease (ARD), also described as soil decline, is known for centuries; however, up to now the ultimate causes are unknown. After replanting plants of the same species, the plants are characterized by a poor vegetative development, stunted growth and reduced yield in terms of quantity and quality. For tree nurseries and orchards, replant disease is increasingly problematic and a sustainable and economically feasible solution would be the use of ARD-tolerant rootstocks. Up to now, only a few Malus genotypes with tolerance to ARD have been described and hardly any information is available about the genetic and physiological mechanisms underlying these tolerances. The discrimination of ARD tolerant and susceptible genotypes is a prerequisite for future rootstock breeding, but also for molecular studies aiming at improving the breeding efficiency. For this reason, 48 genotypes of wild apple species and rootstock cultivars were evaluated for their tolerance/susceptibility to ARD in a greenhouse bio-test. The susceptibility to ARD was classified by an ARD susceptibility index (ASI) calculated based on biomass and the increase in shoot length, respectively. The development of genetic linkage maps enables the detection of markers linked to agronomical important traits. Such associated markers can be used for an early and rapid selection of eligible plants during the breeding process. In order to map quantitative trait loci (QTL) for ARD tolerance a cross between the genotype Malus × robusta 5 (tolerant to ARD) and the rootstock M.9 (susceptible to ARD) was established and used for the construction of a first genetic linkage map including 78 SSR anchor markers. In addition, a comparative transcriptome study using mRNA from roots of M.9 and M.×robusta 5, which were cultivated on both ARD and disinfected ARD soil will provide information about potential candidate genes, which are related to ARD-tolerance.