Herbarium Specimens Provide Evidence for the Early Introduction of Erysiphe quercicola Into the United States and Document New Hosts

ABSTRACT The powdery mildew fungus Erysiphe quercicola ( Erysiphaceae ) has a complex taxonomic history that has long complicated assessments of its geographic distribution and host associations. Although knowledge of the global host range of this species has expanded substantially over the past two decades, sequencing‐confirmed records from North America remain scarce, and despite its recognition as an introduced pathogen, the timing of introduction and the geographic extent and host range of E. quercicola in the United States are poorly understood. In this study, herbarium specimens of E. quercicola from North America, spanning both historical and contemporary collections, were examined using molecular phylogenetic approaches. Specimens collected from multiple Quercus species across several regions of the United States, as well as from related hosts within the Fagaceae , were evaluated. Sequence data confirm the presence of E. quercicola in North America on Quercus species based on herbarium specimens collected as early as 1944, as well as on mango ( Mangifera indica ; Anacardiaceae ) in Florida from material collected in 1935. These records raise the possibility of historical introduction pathways associated with cultivated hosts, potentially from the horticulturally important mango tree ( M. indica ); however, additional multilocus phylogenetic analyses and host range inoculation experiments will be required to determine whether powdery mildew populations infecting mango and Quercus in North America represent the same lineage. Additional sequencing‐confirmed records document the species on multiple native oak species representing different sections of Quercus including Q. bicolor , Q. gambelii , Q. garryana , Q. geminata , Q. kelloggii , Quercus × jolonensis and Q. macrocarpa as well as on Notholithocarpus densiflorus . Together, these findings clarify the long‐term presence of E. quercicola in the United States, expand knowledge of its North American host range, and demonstrate the value of herbaria for reconstructing the invasion history of forest pathogens.