Fungicides are widely used in conventional agriculture to control fungal diseases, but may also affect non-target microorganisms such as arbuscular mycorrhizal (AM) fungi. These root symbionts develop extended mycelial networks within the soil via mechanisms such as anastomosis that indistinctly concerns intact and damaged hyphae, the latter being named hyphal healing mechanism (HHM). The HHM differs between Glomeraceae and Gigasporaceae. However, the effects of fungicides on this mechanism in unknown. Here, the impact of azoxystrobin, pencycuron, flutolanil, and fenpropimorph at 0.02 and 2 mg L–1 were tested in vitro on the HHM of Gigaspora sp. MUCL 52331 and Rhizophagus irregularis MUCL 41833, and repair events visualized carefully under a dissecting bright-field light microscope. Azoxystrobin was the more detrimental for both AM fungi at 2 mg L–1, while fenpropimorph impacted only R. irregularis (stimulating at low and inhibiting at high concentration). Conversely, flutolanil and pencycuron did not impact any of the two AM fungi. The mechanisms involved remains to be elucidated, but perturbation in the still-to-be firmly demonstrated spitzenkörper or in sterols content as well as a process of hormesis are possible avenues that deserve to be explored in view of a rationale management of chemicals to control fungal pathogens without harming the beneficial AM fungi.

Background Azoxystrobin is widely used in crops against several fungal diseases, but may have potential negative effects on soil microorganisms beneficial to plants such as arbuscular mycorrhizal (AM) fungi. To date, the impact of azoxystrobin has been studied on the growth and healing capacity of AM fungi, but not on the inorganic phosphorus (Pi) uptake dynamics. Aims The aim of the study was to investigate the impact of azoxystrobin on the dynamics of Pi uptake by potato plants associated or not to Rhizophagus irregularis. Additionally, fungal root colonization, plant growth, and P accumulation were assessed. Methods A semi‐hydroponic cultivation system was used using mycorrhizal (M) and nonmycorrhizal (NM) plants growing in presence/absence of azoxystrobin. The percentage of Pi depletion in the nutrient solution was determined by inductively coupled plasma atomic emission spectroscopy after 30 and 60 days of circulation. Results Whatever the presence/absence of the AM fungus, azoxystrobin decreased Pi uptake by plants after 30 days of growth. Conversely, after 60 days in absence of azoxystrobin, a larger Pi uptake was measured in the M plants, while the reverse was noticed in presence of azoxystrobin. Conclusions The M plants were more affected by azoxystrobin than those growing in absence of the fungicide, suggesting a direct effect of azoxystrobin on the AM fungus and an indirect effect on the plant–fungus associates via a potential imbalance in the transfer of nutrients. Although it is difficult to extrapolate our results to field, fungicide toxicity on crops inoculated with selected AM fungi deserves to be explored.

Arbuscular mycorrhizal (AM) fungi play key roles in soil fertility of agroecosystems. They develop dense extraradical mycelial (ERM) networks via mechanisms such as hyphal anastomosis. These connections between hyphae can be affected by agricultural practices such as the use of fungicides, but how these compounds affect anastomosis formation within and more importantly between networks of the same AM fungal strain remains poorly unexplored. Here, the impact of azoxystrobin, pencycuron, flutolanil, and fenpropimorph at 0.02 and 2 mg L−1 were tested in vitro on the anastomosis formation within and between networks of Rhizophagus irregularis MUCL41833. Azoxystrobin and fenpropimorph had a particularly detrimental impact, at the highest concentration (2 mg L−1), on the number of anastomosis within and between networks, and for fenpropimorph in particular at both concentrations (0.02 and 2 mg L−1), on the number of anastomosis per length of hyphae. Curiously fenpropimorph at 0.02 mg L−1 significantly stimulated spore production, while with azoxystrobin, the reverse was observed at 2 mg L−1. The two other fungicides, pencycuron and flutolanil had no detrimental effects on spore production or anastomosis formation within and between networks. These results suggest that fungicides with different modes of action and concentrations differentially impact anastomosis possibly by altering the hyphal tips of AM fungi and may thus affect the capacity of AM fungi to develop large hyphae networks exploring and exploiting the soil at the service of plants.

Arbuscular mycorrhizal (AM) fungi play key roles in soil fertility of agroecosystems. They develop dense extraradical mycelial (ERM) networks via mechanisms such as hyphal anastomosis. These connections between hyphae can be affected by agricultural practices such as the use of fungicides, but how these compounds affect anastomosis formation within and more importantly between networks of the same AM fungal strain remains poorly unexplored. Here, the impact of azoxystrobin, pencycuron, flutolanil, and fenpropimorph at 0.02 and 2 mg L−1 were tested in vitro on the anastomosis formation within and between networks of Rhizophagus irregularis MUCL 41833. Azoxystrobin and fenpropimorph had a particularly detrimental impact, at the highest concentration (2 mg L−1), on the number of anastomoses within and between networks, and for fenpropimorph in particular at both concentrations (0.02 and 2 mg L−1) on the number of anastomoses per length of hyphae. Curiously fenpropimorph at 0.02 mg L−1 significantly stimulated spore production, while with azoxystrobin, the reverse was observed at 2 mg L−1. The two other fungicides, pencycuron and flutolanil, had no detrimental effects on spore production or anastomosis formation within and between networks. These results suggest that fungicides with different modes of action and concentrations differentially affect anastomosis possibly by altering the hyphal tips of AM fungi and may thus affect the capacity of AM fungi to develop large hyphal networks exploring and exploiting the soil at the service of plants.