Rhizosphere microbial communities associated to rose replant disease: links to plant growth and root metabolites.

Growth depression of Rosa plants at sites previously used to cultivate the same or closely related species is a typical symptom of rose replant disease (RRD). Currently, limited information is available on the causes and the etiology of RRD compared to apple replant disease (ARD). Thus, this study aimed at analyzing growth characteristics, root morphology, and root metabolites, as well as microbial communities in the rhizosphere of the susceptible rootstock Rosacorymbifera 'Laxa' grown in RRD-affected soil from two sites (Heidgraben and Sangerhausen), either untreated or disinfected by γ-irradiation. In a greenhouse bioassay, plants developed significantly more biomass in the γ-irradiated than in the untreated soils of both sites. Several plant metabolites detected in R. corymbifera 'Laxa' roots were site- and treatment-dependent. Although aloesin was recorded in significantly higher concentrations in untreated than in γ-irradiated soils from Heidgraben, the concentrations of phenylalanine were significantly lower in roots from untreated soil of both sites. Rhizosphere microbial communities of 8-week-old plants were studied by sequencing of 16S rRNA, ITS, and cox gene fragments amplified from total community DNA. Supported by microscopic observations, sequences affiliated to the bacterial genus Streptomyces and the fungal genus Nectria were identified as potential causal agents of RRD in the soils investigated. The relative abundance of oomycetes belonging to the genus Pythiogeton showed a negative correlation to the growth of the plants. Overall, the RRD symptoms, the effects of soil treatments on the composition of the rhizosphere microbial community revealed striking similarities to findings related to ARD.