The objective of this research was to investigate the relationship between the aggressiveness, assessed by nursery screening tests, of Ganoderma with a pathogenic factor which is quantifiable in the laboratory. This should lead to a rapid test to screen for isolate aggressiveness. A Czapek modified liquid medium was first improved for the culturing of Ganoderma boninense. The culture filtrate was used to study metabolites secreted by the fungus which could be involved in pathogenicity, as the production of toxin(s). As a first step a wilting leaf bioassay, using the fungus filtrate, was set up to develop this approach. This tool was used in another pathosystem (Breton et al., 2000) and allowed a quantitative estimate of the production of toxin(s) which was estimated by water loss from leaves (wilting). In parallel, the ability of isolates to degrade rubber wood blocks (RWB) was studied in vitro. Preliminary results led to the hypothesis that (i) there is no correlation between RWB degradation by isolates with their natural aggressiveness and (ii) Ganoderma isolates seem to produce toxic compound(s) which lead to leaf wilting in vitro. The chemical nature of these compounds is unknown and it is too early to assert whether (i) there is a role of these compounds in Ganoderma pathogenicity and (ii) there is a positive correlation between the production of toxic compound(s) with isolate aggressiveness observed in the nursery. However, preliminary results indicate that such relationships may exist.