A novel Cryphonectria species (Cryphonectriaceae, Diaporthales) discovered on Eucalyptus, including insights into the mating biology of the genus

The Cryphonectriaceae (Diaporthales, Ascomycota) accommodates numerous important fungal pathogens that cause branch and stem canker diseases on woody plants worldwide. These include, for example, Cryphonectria ( Cry. ) parasitica , the causal agent of chestnut blight, which caused the near eradication of the American chestnut ( Castanea dentata ) in North America. The profound ecological and economic impact of Cry. parasitica has prompted considerable research interest in the diversity, biology, and pathogenicity of other species in the Cryphonectriaceae globally. During a 2023 disease survey in North Sumatra, trees of a single Eucalyptus clone dying as result of infection by the vascular wilt pathogen Ceratocystis ( Cer. ) manginecans were also found to have structures typical of the Cryphonectriaceae on their bark. The aim of this study was to identify this fungus, to test its pathogenicity and to consider its mating biology in comparison to other related species. DNA sequence comparisons for six loci (ITS, LSU, TUB1, TUB2, TEF1, and RPB2 ), phylogenomic analyses, as well as morphological observations, revealed that the fungus was an undescribed species of Cryphonectria , for which the name Cry. eucalypticola is provided. Inoculation trials on four eucalypt genotypes showed that Cry. eucalypticola is only mildly pathogenic and different eucalypt genotypes differed in their susceptibility to infection. Mating-type locus analysis based on whole-genome data showed that the fungus is homothallic. Analyses of the genomes of six other Cryphonectria species showed for the first time that, the genus includes both homothallic and heterothallic taxa. This is the first confirmed report of a Cryphonectria species associated with trees in the Myrtales, whereas all previously described species have been reported from the Northern Hemisphere on trees in the Fagales. The low level of aggressiveness of the fungus, and its occurrence on trees infected by Cer. manginecans, suggest that it is likely an endophyte on healthy trees that sporulates on their bark as they die.