Fungi in the Cryphonectriaceae are important canker pathogens of plants in the Melastomataceae and Myrtaceae (Myrtales). These fungi are known to undergo host jumps or shifts. In this study, fruiting structures resembling those of Cryphonectriaceae were collected and isolated from dying branches of Syzygium cordatum and root collars of Heteropyxis natalensis in South Africa, and from cankers on the bark of Tibouchina grandifolia in La Réunion. A phylogenetic species concept was used to identify the fungi using partial sequences of the large subunit and internal transcribed spacer regions of the nuclear ribosomal DNA, and two regions of the β-tubulin gene. The results revealed a new genus and species in the Cryphonectriaceae from South Africa that is provided with the name Myrtonectria myrtacearum gen. et sp. nov. Two new species of Celoporthe (Cel.) were recognised from La Réunion and these are described as Cel. borbonica sp. nov. and Cel. tibouchinae sp. nov. The new taxa were mildly pathogenic in pathogenicity tests on a clone of Eucalyptus grandis. Similar to other related taxa in the Cryphonectriaceae, they appear to be endophytes and latent pathogens that could threaten Eucalyptus forestry in the future.