PHYSIOLOGICAL AND ECOLOGICAL FACTORS INFLUENCING THE EFFECTS OF MYCORRHIZAL INOCULATION

S ummary Inoculations of mycelia known to be mycorrhiza‐formers of certain tree species have frequently been recorded to produce indifferent effects on the development of these species. In a particular heathland soil, the introduction of Boletus scaber was even observed to depress the vigour of birch, a tree which is generally regarded as the mycor‐rhizal host par excellence of this fungus. Pot‐culture experiments revealed that the harmful influence of the inoculum on the higher plant was due to certain fungal interactions. From an analysis of the fungal components concerned in these interactions, it appeared that the interplay of two factors was mainly responsible for bringing about the adverse effect on the young trees: ( a ) the physiological character of the particular form of B. scaber used as inoculum, i.e. its pronounced rhizosphere activity; ( b ) the presence of a deleterious pseudomycorrhiza‐former, Mycelium radicis atrovirens β, in the experimental soil. Introduction of Boletus scaber into cultures of Pinus sylvestris demonstrated that, although this tree species is, generally, not a congenial mycorrhizal partner of the fungus, the formation of mycorrhizal infection and simultaneous growth stimulation of the host plant was achieved under certain ecological conditions.