Lifestyle, mating type and mitochondrial genome features of the plant pathogen Calonectria hawksworthii (Hypocreales, Nectriaceae) as revealed by genome analyses

In 2019, fungal necrotic spots were observed on the cotyledons of grafted avocado (Persea americana) seedlings from a nursery in Ventura County, California. Morphology and comparison of DNA sequences from eight loci identified the isolate as Calonectria hawksworthii, a species in the Nectriaceae not yet recorded in the United States. Calonectria hawksworthii is a necrotrophic fungal pathogen in the C. cylindrospora species complex. Most species in this group are associated with Eucalyptus, but the type host for C. hawksworthii is waterlilies (Nelumbo nucifera). In this study, the C. hawksworthii genome was sequenced using Illumina technologies. The draft assembly of 64.8 Mb contained 18 703 predicted gene models, of which 70% could be assigned to a GO functional category. The mating type loci indicated this species is heterothallic. Approximately, 3.79% of the draft genome consists of transposable elements (TE), and close to 36% of the predicted proteins were homologous to those known to be involved in pathogenicity in other fungal species. The C. hawksworthii genome displays elements typical of a necrotrophic lifestyle based on the composition of predicted genes encoding for carbohydrate-active enzymes, number of predicted secretory proteins including effectors, secondary metabolite biosynthesis clusters and cytochrome oxidase P450 (CYP) genes. The annotated mitochondrial genome was found to be 35.7 kb with very few intra or intergenic introns. This report constitutes the first draft genome of a Calonectria species belonging to the C. cylindrospora species complex, as well as the first report of C. hawksworthii species occurring in the United States on avocado.