Occurrence of Diplodia corticola, including new oak host records, in Wisconsin, USA

Summary In the United States, diseased oaks (Quercus species) exhibit tip blight, branch and stem cankers, and dieback often attributed to Diplodia species or related fungi. Emergence of Diplodia corticola as a pathogen of European oaks, and reports of this fungus in the eastern and western United States, prompted re‐examination of strains from Wisconsin. These had been obtained in the late 1990s and early 2000s and previously identified only as Diplodia species. Nuclear ribosomal ITS sequences were obtained from the Wisconsin strains and analysed with other sequences from GenBank. Wisconsin strains confirmed as D. corticola were from northern red oak (Q. rubra), black oak (Q. velutina), white oak (Q. alba) and bur oak (Q. macrocarpa). Other strains from oaks in Wisconsin were D. mutila and D. seriata. Wound inoculation of northern red, white and bur oak seedlings with D. corticola in a greenhouse resulted in shoot death and stem lesions, from which the pathogen was reisolated. We conclude that D. corticola has been present in the northcentral United States for at least two decades and report two previously unrecognized hosts of this pathogen: white oak and bur oak. The roles of D. corticola, related fungi and influences of other environmental factors in deterioration of oak health in North America merit additional investigation.