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

Historically, Cytospora canker of quaking aspen was thought to be caused primarily by Cytospora chrysosperma. However, a new and widely distributed Cytospora species on quaking aspen was recently described (Cytospora notastroma Kepley & F.B. Reeves). Here, we show the relative pathogenicity, abundance, and frequency of both species on quaking aspen in portions of the Rocky Mountain region, and constructed species-level phylogenies to examine possible hybridization among species. We inoculated small-diameter aspen trees with one or two isolates each of C. chrysosperma and C. notastroma in a greenhouse and in environmental growth chambers. Results indicate that both Cytospora species are pathogenic to drought-stressed aspen, and that C. chrysosperma is more aggressive (i.e., caused larger cankers) than C. notastroma, particularly at cool temperatures. Neither species cause significant canker growth on trees that were not drought-stressed. Both C. chrysosperma and C. notastroma are common on quaking aspen, in addition to a third, previously described species, Cytospora nivea. Multiple Cytospora species often co-occur on the same host tree, and evidence of hybridization among species exists.