Pythium oligandrum induces growth promotion in starch potato without significantly altering the rhizosphere microbiome

Plant health promoting organisms, including microbial biological control agents, are of increasing importance for the development of more sustainable agriculture. To understand the function of these microbes as biological control agents under field conditions and their overall impact on soil and plant health, we need to learn more about the impact of plant beneficial microbes on the rhizosphere microbiome of crops such as potato. The plant beneficial oomycete Pythium oligandrum has previously been reported both as a biocontrol agent and as a plant growth promoter, or biostimulant, in several crop species. To investigate the potential of P. oligandrum as a biostimulant in potato, we performed a series of controlled-environment bioassays in three cultivars. We showed that biostimulation of potato by P. oligandrum is plant genotype-specific. We confirmed the biostimulation by P. oligandrum in the starch potato cultivar Kuras under field conditions. We further investigated the effects of P. oligandrum on the potato rhizosphere microbiome, sampling individual potato plants at three time points over the growing season (representing the vegetative growth phase, flowering, and the onset of senescence). Metabarcoding using ITS and 16S amplicon sequencing revealed no significant overall effect of P. oligandrum application on the bacterial and fungal rhizosphere communities. However, some genera were significantly differentially abundant after P. oligandrum application, including some classified as plant-beneficial microbes. We conclude that P. oligandrum has a cultivar-dependent growth-promoting effect in potato and only minor effects on the rhizosphere microbiome. •The oomycete Pythium oligandrum induces cultivar-dependent biostimulation in potato•Bacterial and fungal microbiomes exhibit temporal changes during the growing season•Specific genera show differential abundance after treatment with P. oligandrum•P. oligandrum does not significantly alter overall microbiome diversity•Thus P. oligandrum promotes growth with only minor effects on the potato microbiome