Tracking mycorrhizas and extraradical mycelium of the edible fungus Lactarius deliciosus under field competition with Rhizopogon spp.

The objective of this study is to evaluate the field persistence of the edible ectomycorrhizal fungus Lactarius deliciosus in competition with two ubiquitous soil fungi. Couples of plants inoculated with either L. deliciosus, Rhizopogon roseolus, or R. luteolus were transplanted, 10 cm apart, in two different sites at the following combinations: L. deliciosus-R. roseolus, L. deliciosus-R. luteolus, L. deliciosus-control (non-inoculated), control-R. roseolus, control-R. luteolus, and control-control. Eight months after transplantation, root colonization and extraradical soil mycelium for each fungal species were quantified. For mycelium quantification, soil cores equidistant to the two plants in each couple were taken, and total deoxyribonucleic acid (DNA) was extracted. Real-time polymerase chain reaction analysis was performed using specific primers and TaqMan Minor groove binding (MGB) probes designed in the ribosomal DNA internal transcribed spacer region of each fungal species. Field site significantly influenced persistence of both mycorrhizas and extraradical mycelium of L. deliciosus. Extraradical mycelium quantity was positively correlated with the final percentage of ectomycorrhizas for the three fungal species. Different competitive pressure between the two Rhizopogon species on L. deliciosus persistence was observed, with R. luteolus having no effect on L. deliciosus survival. Negative correlation between the final percentage of mycorrhizas of L. deliciosus and R. roseolus was observed. However, no relationship was determined between extraradical mycelia of both fungal species. The results obtained suggest that competition between L. deliciosus and R. roseolus takes place in the root system, for ectomycorrhiza formation in available roots, rather than in the extraradical phase.