and are canker pathogens involved in an insect-fungus disease complex of American beech () in North America commonly known as beech bark disease (BBD). In Europe, both and are involved in BBD on European beech (). Field observations across the range of BBD indicate ascospores to be the dominant spore type in the environment. Several studies report a heterothallic (self-sterile) mating strategy for fungi, but one study reported homothallism (self-fertility) for . As such, investigations into mating strategy are important for understanding both the disease cycle and population genetics of . This is particularly important in the United States given that over time dominates the BBD pathosystem despite high densities of nonbeech hosts for . This study utilized whole-genome sequences of BBD-associated spp. along with other publicly available and genomes and in vitro mating assays to characterize mating type (MAT) locus and confirm thallism for select members of and . MAT gene-specific primer pairs were developed to efficiently characterize the mating types of additional single-ascospore strains of , and and several other related species lacking genomic data. These assays also confirmed the sexual compatibility among strains from different plant hosts. Maximum likelihood phylogenetic analyses of both and sequences recovered trees with similar topology to previously published phylogenies of and . The results of this study indicate that all and tested are heterothallic based on our limited sampling and, as such, thallism cannot help explain the inevitable dominance of in the BBD pathosystem.