Relative infectivity of two Pisolithus spp. strains inoculated to the nitrogen-fixing legume tree Acacia crassicarpa A. Cunn. ex Benth. in a field experiment in Madagascar

A field behaviour trial was set up in Mahela (Madagascar) to evaluate the ecological competence of two ectomycorrhizal fungal strains on Acacia crassicarpa. This fast-growing Australasian species is very well appreciated locally by farmers as multipurpose wood. However, its symbiotic associations with nitrogen fixing bacteria, ectomycorrhizas, and arbuscular mycorrhizas have been poorly studied so far. The inoculated fungal strains used in the present experiment belonged to two different species of the genus Pisolithus, strain 441 originating from Pisolithus microcarpus collected in Brazil under Eucalyptus citriodora and strain COI.007 from Pisolithus albus collected in Senegal under Acacia mangium. A survey of sporocarps fruiting in the inoculation trial and the molecular identification of the fungal partners present within the ectomycorrhizal roots showed the exclusive occurrence of the inoculant strain 441 in all the experimental plots of A. crassicarpa after 19 months of tree growth. The inoculant strain COI.007 was not detected in any plot. We confirmed the ability of A. crassicarpa to form ectomycorrhizal associations but exclusively with P. microcarpus in the field conditions tested here. The lack of growth response to inoculation that we observed could be attributed to the contamination of all plots by strain 441.