Suppression of common root pathogens by helper bacteria and ectomycorrhizal fungi in vitro

Root pathogens cause considerable loss of tree seedlings in nurseries and are generally difficult to control using conventional methods. Inoculation with ectomycorrhizal fungi may provide some suppression of pathogens. Bacteria (so-called mycorrhization helper bacteria) have been isolated that stimulate mycorrhiza formation on seedling roots and enhance seedling growth; however, their role in pathogen inhibition has not been explored. Four strains of helper bacteria were inoculated together with the ectomycorrhizal fungal species Laccaria bicolor, L. proxima and Suillus granulatus on culture plates to determine inhibition of the pathogens Fusarium oxysporum and Cylindrocarpon sp. Buffered medium was used to rule out acidification of the medium as a mechanism of inhibition. None of the ectomycorrhizal fungal species alone inhibited the growth of Fusarium but all showed slight inhibition of Cylindrocarpon growth. Helper bacterium strain MB3 (Bacillus subtilis) was effective in inhibiting both pathogens and, when inoculated with either L. proxima or S. granulatus, inhibition of Fusarium growth was enhanced over MB3 alone. With Cylindrocarpon, however, only S. granulatus inoculated along with MB3 showed enhanced inhibition over MB3 alone. The other three bacterial strains had little effect on the growth of Fusarium or Cylindrocarpon. More research is necessary to determine if these inhibitory effects are reproducible in situ. © Springer-Verlag 1996.