Scale-dependent effects of fungicide seed coating on arbuscular mycorrhizal fungi: from spore germination to field performance

Modern agriculture faces the challenge of feeding a growing global population while simultaneously addressing climate change and biodiversity loss. It is therefore essential to transition from high-input systems to more sustainable yet productive agricultural practices. Beneficial soil microorganisms, such as arbuscular mycorrhizal fungi (AMF), contribute to this transition by enhancing plant nutrient uptake and stress resilience. However, seed coatings with fungicides, which are commonly used to protect seedlings in conventional farming, may unintentionally interfere with AMF-plant symbioses. Here, we investigated the off-target effects of seed-applied fungicides on mycorrhizal symbiosis using a combination of in vitro , greenhouse, and field experiments using maize ( Zea mays ) and the AMF species Rhizoglomus irregulare as model organisms. In vitro assays showed that the fungicide metalaxyl-M significantly inhibited AMF spore germination, while the fungicide fludioxonil had a marginal inhibitory effect. These effects were observed even below the recommended application rate. In a greenhouse experiment with sterilised soil and AMF inoculation, seed coating negatively affected plant biomass and mycorrhizal growth response, with the strongest effects observed four weeks after planting. A second greenhouse experiment using natural soils from nine field sites showed that effects of soil type on plant biomass and AMF colonisation were much stronger than effects of seed coating. Quantitative PCR detected no significant differences in AMF root colonisation between coated and non-coated seeds, suggesting limited effects on overall AMF abundance. Interestingly, PacBio sequencing revealed shifts in AMF community composition at the onset of symbiosis, suggesting that AMF taxa vary in sensitivity to seed coating. Field experiments showed significant early effects of seed coating on plant biomass that diminished at later growth stages, alongside a marginal effect on total AMF abundance. Overall, this study demonstrates that seed coating with the two fungicides can negatively affect AMF, particularly during early plant development. However, these effects are highly scale-dependent, as they are buffered in natural soils. These findings highlight the need to consider AMF interactions in seed treatment design to support sustainable and resilient agricultural systems.