MYCORRHIZATION HELPER BACTERIA ASSOCIATED WITH THE DOUGLAS-FIR LACCARIA-LACCATA SYMBIOSIS - EFFECTS IN ASEPTIC AND IN GLASSHOUSE CONDITIONS

A range of bacterial isolates from Laccaria laccata mycorrhizas and sporocarps were tested for their effect on ectomycorrhizal development of Douglas fir with L laccata. The experiments were carried out in aseptic conditions and in the glasshouse under summer and winter conditions. Fourteen isolates increased mycorrhizal development after 16 wk of growth in the summer experiment. Seven bacterial isolates displayed a significant stimulating effect in the winter experiment. All bacterial isolates tested under aseptic conditions displayed a significant stimulating effect. In the winter experiment, the treatments without L laccata inoculation were contaminated by Thelephora terrestris (ectomycorrhizal basidiomycete), a natural contaminant in the glasshouse. Six bacterial isolates displayed a significant inhibiting effect towards ectomycorrhizal infection by T terrestris. Three isolates enhanced the ectomycorrhizal development of Douglas fir with L laccata in all experiments. It is confirmed that the inoculation techniques in forest nurseries could be improved by such mycorrhization helper bacteria (MHB). The results with T terrestris suggest that the mechanisms involved in interactions between bacteria and mycorrhizal establishment are partly fungus-specific. The results of the experiments in aseptic conditions suggest that the MHB act directly on the plant or/and on the fungus. Their stimulating effect is not the result of the suppression of root pathogens or other inhibitors of mycorrhizal infection. MHB could be used both for enhancing the infection by an introduced fungus and for reducing unwanted infection by inefficient symbionts such as T terrestris. Thus, the need for soil disinfection before inoculating might be reduced.