AbstractThe pathogenicity of an isolate ofCryphonectria cubensiswas tested in artificial inoculations onEucalyptus grandis, Psidium guajavaandSyzygium cordatmn. The isolate was able to colonize host tissue of all three species following inoculations of branch sections. It rapidly colonized the tissue of potted guava plants of the important commercial cultivar, Fan Relief and resulted in the death of a number of inoculated branches. Pathogenicity was also proven on mature guava trees in the field. An important implication of these findings is that the cultivation of guavas in South Africa could be confronted with a potentially severe disease problem in the future.