Assessing the effect of commercial arbuscular mycorrhizal fungal inoculum on potato plant disease incidence, yield and the indigenous root fungal community composition

Plant growth-promoting and biocontrol microorganisms have emerged as safe alternatives to chemical pesticides. For instance, retail markets offer an extensive selection of commercial arbuscular mycorrhizal fungal inoculums, but most of them are tested only under greenhouse conditions. The present field study evaluated the effect of two commercially available arbuscular mycorrhizal fungal inoculums on potato yield, root overall fungal and pathogen community, and its ability to control soil-borne diseases. The inoculums were tested on an open field site for two consecutive years. Fungal community structure in potato roots was assessed using Illumina MiSeq sequencing. Simultaneously with root samples, the fungal community was evaluated from the packages of commercial inoculum used in the field. Sequencing revealed that the inoculums did not include almost any sequences belonging to arbuscular mycorrhizal fungi, but these contained several taxa belonging to plant pathogenic fungi. In both years the inoculum composition was different. There was no effect of arbuscular mycorrhizal granule treatment, whereas arbuscular mycorrhizal liquid inoculum significantly decreased black scurf and silver scurf incidence. Furthermore, plots treated with liquid inoculum increased yield in the second year of the experiment. These results may rely on the potential effects of other fungal species present in the liquid inoculum. No statistically significant differences were observed in the root overall fungal and pathogen richness and community composition after the commercial inoculum treatment. The results of this study bring attention to the importance of the legislation and quality control of products marketed as biofertilizers. Future studies should also include an assessment of the product composition to assure their effectiveness.