Pyraclostrobin impairs metabolic activity and phosphorus uptake of the extraradical mycelium of Rhizophagus intraradices more severely than iprodione.

Pesticide residues are widespread in agricultural soils and may adversely affect arbuscular mycorrhizal (AM) fungi, key symbionts involved in plant phosphorus (P) acquisition. Most studies to date have focused on pesticide effects either on spores (asymbiotic phase) or on the mycorrhizal plant as a whole. Here, we investigated the effects of two fungicides with contrasting modes of action-pyraclostrobin (quinone-outside inhibitor) and iprodione (dicarboximide)-applied specifically to the extraradical mycelium (ERM) of Rhizophagus intraradices MUCL 49410 associated with Medicago truncatula. To this end, a bi-compartmented pot system was developed, allowing fungicide application at the recommended field dose directly to the ERM, while preventing direct root exposure. Treatments were applied for 30 days (T1) or 3 days (T2) to assess time-dependent responses. Pyraclostrobin markedly reduced ERM biomass (by up to 75%), hyphal alkaline phosphatase activity, and root colonization, particularly arbuscule abundance, indicating severe impairment of mitochondrial function. In contrast, iprodione slightly increased ERM biomass and the proportion of metabolically active spores, and the increase in Pi depletion within the in-growth tube after prolonged exposure, reflecting a possible higher Pi uptake and suggesting a compensatory or mild hormetic response. Overall, pyraclostrobin exerted pronounced inhibitory effects on AM fungal structures and function, whereas iprodione showed neutral to mildly stimulatory effects under comparable conditions. These contrasting responses likely reflect differences in fungicide mode of action and exposure duration. Our findings demonstrate that AM fungal sensitivity to fungicides is compound-specific and underscore the importance of integrating functional and physiological endpoints into pesticide risk assessments frameworks.