Apple replant disease stresses plant health by rhizosphere carbon bleeding
Aims  Apple replant disease (ARD) is a worldwide problem causing severe crop and seedling losses. We investigated potential biotic factors involved in the development of ARD at the rhizosphere-soil interface.   Methods  M26 apple seedlings were planted in either untreated ARD soil, disinfected gamma-irradiated ARD soil, or ARD-free soil from adjacent grassland. Plants were labelled with a 13CO2 pulse to track carbon fluxes from plants to the mycorhizosphere and its microbial community. Allocation of 13C to different microbial groups was tracked using phospholipid fatty acids (PLFAs) and neutral lipid fatty acids (NLFAs). DNA extraction and amplicon sequencing were used to analyse possible shifts in the soil microbiome.   Results d13C measurements showed that ARD plants allocated more photo-assimilates into arbuscular mycorrhizal (AM) fungi and the soil than plants growing in the disinfected soil. ITS sequencing showed a shift in relative abundances especially of Basidio-, Asco-, and Mortierellomycota between ARD, disinfected and ARD-free soils.     Conclusion The high 13C enrichment in the arbuscular mycorrhizal (AM) fungal biomarker NLFAs indicates that plants interacted increasingly with AM fungi under ARD conditions, although without any visible benefit for the plant itself. We refer to this as 'rhizosphere carbon bleeding' (RCB). Supported by correlation analyses, we infer that RCB strongly altered the fungal community, supporting antagonistic root-colonising microorganisms (e.g. AM fungi) against pathogens causing ARD.