Fungicide seed treatments delay arbuscular mycorrhizal fungi colonization of winter wheat in the greenhouse, but the effect is attenuated in the field.

Seed-applied fungicides support agricultural production by controlling seed- or soil-borne diseases. However, they can impact non-target soil organisms. In this study, we investigated the effect of eight seed treatments (including two authorized for organic farming) on root colonization of winter wheat (Triticum aestivum L.) by arbuscular mycorrhizal (AM) fungi. One experiment was conducted in greenhouse conditions, on a sterile substrate inoculated with the AM fungus Rhizophagus irregularis MUCL 41833 and one in field conditions, where winter wheat was colonized by native soil AM fungi. In greenhouse conditions, the six conventional seed treatments reduced root colonization five weeks after sowing. No difference with the control treatment was measured thereafter for a product containing triazole alone. In contrast, seed treatments containing fludioxonil (fungicide molecule alone or formulated with the triazole difenoconazole), and prochloraz formulated with the triazole triticonazole significantly reduced root colonization until 11 weeks after sowing. Notably, when formulated with sedaxane, the adverse effect of fludioxonil was reduced. The negative effect of seed treatments on AM fungal root colonization in field was smaller than in the greenhouse and generally not significant, with disparate results from one timestep to another. This may be related to the dilution or the degradation of the active ingredients in the soil during the winter period or AM fungal species/strain involved in symbiosis. Overall, our results outline that the direct effect of seed treatment is highly variable depending on the modes of action, half-lives and interactions between active ingredients. By contributing to highlight the undesired effects of pesticides on AM fungi (i.e., by delaying root colonization), this study pleads for a reduction of pesticide applications to encourage the rapid and efficient establishment of functional mycorrhizal symbioses.