S16 - Session O3 - Apple replant disease - new insights into an old problem

Authors: Traud Winkelmann *

Worldwide, tree nurseries and apple orchards are facing the problem of apple replant disease (ARD) which results in growth depression and fruit yield and quality reduction. The etiology of this complex soil-borne disease which persists at the affected sites for decades is still not fully understood, but can best be described as a dysbiotic state and a negative plant-soil feedback. Although biofumigation with Brassica seed meal, catch cropping and breeding of tolerant rootstocks have been suggested as countermeasures, sustainable and effective management options still need to be developed or optimized. This is the main objective of the consortium BonaRes-ORDIAmur. Important insights were gained from transcriptomic comparisons of apple rootstocks growing in ARD-affected and disinfected soils: genes involved in phytoalexin biosynthesis were strongly upregulated in roots in ARD soils as were the contents of the phytoalexins. Their role in the disease awaits clarification: on the one hand they can support the plant by acting against the ARD-causing organisms, but on the other hand these toxic metabolites could be harmful to the root cells in which they accumulate. Therefore, studies on their exudation are in progress. Regarding ARD-associated organisms, data from ORDIAmur-scientists indicate that (i) members of the Nectriaceae were found enriched in the rhizosphere and isolated from symptomatic root segments and were proven pathogenic, (ii) the relative abundance of Streptomyces was significantly increased in rhizospheres and roots from ARD soils, (iii) a contribution of plant parasitic nematodes to ARD could not be confirmed, but instead, populations of free-living nematodes and their associated microbiome differed between ARD and control soils, and (iv) an increase in microbial genes related to stress sensing in the rhizosphere of ARD-affected soils as well as a lower abundance of genes coding for enzymes involved in the degradation of aromatic compounds was observed in the first metagenomic study.