Effect of soil Pseudomonads on colonization of hazel roots by the ecto-mycorrhizal species Tuber melanosporum and its competitors

Hazel (Corylus avellana L.) seedlings, inoculated with the ecto-mycorrhizal fungus Tuber melanosporum (Vitt.) were grown in pots with soil previously inoculated with fluorescent Pseudomonads. These bacteria have been previously isolated from a typical truffle orchard soil. The effect of Pseudomonads on T. melanosporum synthesis proceeded by a two-stage process. Tuber melanosporum infection first declined and then resumed to almost the original level. Infection by competing mycorrhizal fungi declined after 6 months and was very low after 12 months, at the end of the experiment. The same dynamic process occurred with all bacterial strains tested, regardless of their taxonomical group or iron-chelating power. However, some isolates of Pseudomonads seemed to be more efficient than others. A protective effect of Pseudomonads against soil-borne competitors of mycorrhizae was postulated to occur in orchards, consequently favouring the continuation of the T. melanosporum symbiosis. Prospects for use of Pseudomonads in nurseries are discussed.