Abstract
M. ulei remains the main obstacle to the development of rubber cultivation in Latin America. As early as 1960, M. ulei overcame the supposed resistance of the planting material derived from interspecific crosses, and revealed its variability. Studies of its variability began in Brazil and are continuing in French Guiana, where the inoculation of a range of 10 differential clones with 16 M. ulei isolates revealed the existence of 7 virulence factors and 12 Microcyclus races, hence the substantial diversification capacity of the fungus with respect to the host population.M. ulei remains the main obstacle to the development of rubber cultivation in Latin America. As early as 1960, M. ulei overcame the supposed resistance of the planting material derived from interspecific crosses, and revealed its variability. Studies of its variability began in Brazil and are continuing in French Guiana, where the inoculation of a range of 10 differential clones with 16 M. ulei isolates revealed the existence of 7 virulence factors and 12 Microcyclus races, hence the substantial diversification capacity of the fungus with respect to the host population.