The Effect of Rotating Apple Rootstock Genotypes on Apple Replant Disease and Rhizosphere Microbiome

Continuous plantation of apple trees (Malus pumila) at the same sites where the same or a closely related species were grown previously leads to poor establishment, reduced growth vigor, and subsequent crop losses; this phenomenon is termed apple replant disease (ARD). Management of ARD is a continual challenge due to (i) restrictions on the use of broad-spectrum soil fumigants, (ii) putative causal agents of ARD can be site-specific, and (iii) more than one causal agent can be present at a site. We conducted an experimental study at two orchard sites to investigate the succession effect of rootstock genotypes on apple tree establishment with three specific objectives: (i) whether ARD in newly planted apple trees is more severe if the same rootstock as the previous one is replanted, (ii) whether trees develop better in the aisle than in original tree stations, and (iii) the extent of association of rootstock genotypes with rhizosphere microbiome in relation to ARD. Tree growth and rhizosphere microbiome were assessed within 30 months of replanting. In one orchard, replanting trees with a rootstock genotype different from the previous one can be effective in reducing ARD development, susceptibility to ARD is likely to be genetically controlled, and replanting trees in the previous grass aisle can reduce ARD development. However, the opposite results were obtained in the other orchard, possibly due to the lack of weed management in the grass aisle affecting tree establishment in the grass aisle. Rhizosphere microbiota associated with specific rootstock genotypes reached a stable state within 7 months of replanting. An arbuscular mycorrhizal fungi operational taxonomic unit (OTU) had reduced abundance with ARD trees; however many other OTUs associated with ARD cannot be identified to low taxonomic ranks and hence their roles cannot be easily interpreted. In conclusion, replanting trees with rootstocks that are genetically differently from the previous one can reduce ARD development. However, in adopting this strategy, the extent of genetic relationships among rootstock genotypes needs to be considered.