Neonectria ditissimaandN. faginataare canker pathogens involved in an insect-fungus disease complex of American beech (Fagus grandifolia) commonly known as beech bark disease (BBD). In Europe, bothN. ditissimaandN. coccineaare involved in BBD on European beech (Fagus sylvatica). Field observations across the range of BBD indicate that new infections occur primarily via ascospores. Both heterothallic (self-sterile) and homothallic (self-fertile) mating strategies have been reported forNeonectriafungi. As such, investigations into mating strategy are important for understanding both the disease cycle and population genetics ofNeonectria. This is particularly important in the U.S. given that over timeN. faginatadominates the BBD pathosystem despite high densities of non-beech hosts forN. ditissima. This study utilized whole-genome sequences of BBD-associatedNeonectriaspp. along with other publicly availableNeonectriaandCorinectriagenomes andin vitromating assays to characterize mating type (MAT) loci and confirm thallism for select members ofNeonectriaandCorinectria. MAT gene-specific primer pairs were developed to efficiently characterize the mating types of additional single ascospore strains ofN. ditissima,N. faginata, andN. coccineaand several other related species lacking genomic data.In vitromating assays were used in combination with molecular results to confirm thallism. These assays also comfirmed the sexual compatibility amongN. ditissimastrains from different plant hosts. Maximum likelihood phylogenetic analysis of both MAT1-1-1 and MAT1-2-1 sequences recovered trees with similar topology to previously published phylogenies ofNeonectriaandCorinectria. The results of this study indicate that allNeonectriaandCorinectriatested are heterothallic based on our limited sampling and, as such, thallism cannot help explain the inevitable dominance ofN. faginatain the BBD pathosystem.