Pathogenicity and distribution of two species of Cytospora on Populus tremuloides in portions of the Rocky Mountains and midwest in the United States

•C. chrysosperma is more aggressive than C. notastroma on drought-stressed aspen.•A third species, Cytospora nivea, was also detected on sampled aspen.•Cytospora species often co-occur on the same host tree.•Evidence of recombination or possible hybridization between Cytospora species exists.