The volume of water supplied to mycorrhizal hazelnut trees (fig 2) induces variations in the composition and dynamic of the mycorrhizospheric bacteria (fig 3). The higher proportion of fluorescent Pseudomonas is observed in very humid conditions when the temperature is not a restricting factor (fig 4a). On the contrary, actinomycetes are strongly affected by water and very resistant to dryness (fig 4b). The iron chelating capacity of fluorescent Pseudomonas increases with soil water content (fig 5). Taxonomic studies (fig 7) have shown that, after a 150 d treatment, dryness and high irrigation are responsible for a decrease in P fluorescens levels. P putida populations increase with all treatments and especially with high irrigation. Intermediate taxon l1 evels (i1 and i2) remain low. Strong iron chelating capacities (CMI>32 ppm) are connected with to P putida and P fluorescens (fig 8). The incidence of irrigation on tree development and mycorrhization is observed. A linear correlation between the surface and the length and width of the leaves of hazelnut trees was previously obtained (fig 1). Dryness has no effect on shoot development (fig 9b) but the sum of the leaf surfaces is correlated with the water supply (fig 9c). Root development and mycorrhization are evaluated as described in table I. The total mycorrhization level is the same in the 3 treatments but the Tuber to competitors ratio depends on the water supply (fig 10). No root development occurs in dry conditions but Tuber mycorrhiza are not affected. On the contrary high irrigation results in numerous young roots essentially colonized by competitors which also act as Tuber substitutes on older roots. A moderate irrigation induces a good root development, an expansion of the Tuber mycorrhization and very few competitors. The results indicate that T melanosporum and the associated ecosystem are well adaptated to dryness. The incidence of irrigation on the perenniallity of Tuber symbiosis and the fructification is discussed.