Since the first report of the stain canker agent Ceratocystis platani in 2001 in Geneva, dieback of London plane trees (Platanus × acerifolia) has focused greater attention and an epidemiological monitoring has been implemented, as part of a compelling state directive for stain canker management. Genetic identification was carried out in order to ascertain the presence or absence of C. platani. We report here observations recorded between 2011 and 2013, of samplings from a total of 6 plane trees in 4 locations. Identification of bacteria and fungi was performed by sequencing of the rDNA ITS region for fungi, and of the 16S rRNA gene for bacteria. Fungi belonging to the Botryosphaeriaceae species occurred in almost every sampling, whereas Ceratocystis platani was only isolated in 2 trees. The 4 botryosphaeriaceous species, Diplodia mutila, Dothiorella sp., Diplodia seratia and Neofusicoccum parvum, could be responsible for the observed plane cankers, while other fungi could participate in the dieback symptoms. Since these species have been reported as canker agents on other tree species in Europe, artificial infections were carried out with pure cultures of Dothiorella sp., Diplodia mutila and Neofusicoccum parvum on young plane trees. Only Neofusicoccum parvum managed to provoke cork canker symptoms after a few months, but D. mutila and N. parvum were found in internal necrotic tissues. This is the first report of Dothiorella sp., Diplodia mutila and Neofusicoccum parvum associated with plane tree dieback in Switzerland and the first report of pathogenicity of Neofusicoccum parvum in plane trees. This survey showed that most cases of plane tree dieback in the Geneva region were not caused by C. platani and that other fungi could be responsible for similar symptoms.