Evaluation of Malus genetic resources for tolerance to apple replant disease (ARD)

Replant disease, also described as soil decline, is known for centuries; yet the ultimate causes remain unknown. After repeated growing of the same plant species, the soil loses its capacity to support the growth of plants of the respective species. Replant disease is characterized by a poor vegetative development, stunted growth and reduced yield in terms of quantity and quality. A growing concern for tree nurseries and orchards is that most dwarfing apple rootstocks used in modern apple cultivation are susceptible to apple replant disease (ARD). Thus, the use of ARD-tolerant rootstocks would be a sustainable and economically advantageous solution. Currently, only a few Malus genotypes less susceptible to ARD are described. Identifying ARD-tolerant genotypes and understanding the molecular and physiological basis underpinning such tolerance are prerequisites for future rootstock breeding. Hence, in a greenhouse bio-test, the responses to ARD were evaluated for 48 Malus genotypes, including 41 accessions of 18 wild apple species and 7 rootstock genotypes. Susceptibility to ARD was classified by an ARD-susceptibility index (ASI) calculated based on biomass and increase in shoot length increase, respectively, after cultivation of each genotype in both ARD-soil and disinfected ARD soil by γ-irradiation. In total, six accessions were categorized as less susceptible based on either the ASI_biomass or the ASI_shoot. These accessions are available for future breeding of less susceptible rootstocks.