Aromatic Plants and Their Associated Arbuscular Mycorrhizal Fungi Outcompete Tuber melanosporum in Compatibility Assays with Truffle-Oaks

SIMPLE SUMMARY: Truffle culture is a fairly profitable agricultural practice, yet there is a long waiting period to reach peak sporocarp production from the point when ectomycorrhizal truffle-oak seedlings are planted in the field. Adding a secondary crop, such as medicinal and aromatic plants, could enhance the sustainability of truffle agro-forest systems. In this work, we study the relationships between oaks and aromatic plants and their associated mycorrhizal fungi (either ectomycorrhizal or arbuscular mycorrhizal, respectively) under controlled conditions. A reciprocal competition effect is revealed between oaks and aromatic plant species, as well as between the different types of mycorrhizal fungi. Our results indicate that managing arbuscular mycorrhizal fungi in truffle plantations is a relevant factor to be considered when establishing the dual cultures of plant species and mycorrhizal types in intercropping systems. ABSTRACT: The high value of black truffle recompenses the slow growth of the fungus when established in the field. Adding a secondary crop, such as medicinal and aromatic plants (MAPs), could further enhance the sustainability of truffle production agro-forest systems. The dual cultures of ectomycorrhizal truffle-oak seedlings and MAPs (lavender, thyme, and sage) previously inoculated and non-inoculated with native arbuscular mycorrhizal fungi (AMF), were established to evaluate plant–fungi relationships. After 12 months in a shadehouse, plants’ growth, mycorrhizal colonization, and extraradical soil mycelium (both of Tuber melanosporum and AMF) were measured. Overall, truffle-oaks’ growth was negatively affected by the presence of MAPs, especially when inoculated with AMF. In turn, the presence of truffle-oaks barely affected the co-cultured MAPs, and only lavenders showed a significant growth reduction. All AMF-inoculated MAPs showed higher shoot and root biomass than non-inoculated ones. Compared to truffle-oaks growing alone, the presence of co-cultured MAPs, especially when they were AMF-inoculated, significantly decreased both the ectomycorrhizas and soil mycelium of T. melanosporum. These results reveal the strong competition between AMF and T. melanosporum and warn about the need for the protection of intercropping plants and their associated symbiotic fungi to avoid reciprocal counterproductive effects in mixed truffle-oak–AMF–MAP plantations.