In Mediterranean Europe and the United States, oak species (Quercus spp.) have been in various states of decline for the past several decades. Several insect pests and pathogens contribute to this decline to varying degrees, including Phytophthora cinnamomi, Armillaria spp., various insect defoliators, and, in the United States, the oak wilt pathogen Bretziella fagacearum. More recently, two emerging canker pathogens, Diplodia corticola and D. quercivora, have been implicated in causing dieback and mortality of oak species in Europe and in several regions in the United States. In 2019, a fungal survey was conducted in the Mid-Atlantic region of the eastern United States, including Maryland, Pennsylvania, Virginia, and West Virginia, to determine the range and impact of D. corticola and D. quercivora on forest health in the United States. A total of 563 oak trees between red and white oak family members were evaluated across 33 forests spanning 18 counties. A total of 32 Diplodia isolates encompassing three Diplodia spp. were recovered from 5,335 total plugs collected from the 13 of 18 sampled counties. Recovered Diplodia species included D. corticola, D. quercivora, and D. sapinea, as well as Botryosphaeria dothidea, a closely related canker pathogen in the Botryosphaeriaceae. Both D. corticola and D. sapinea were recovered from red and white oak family members, whereas D. quercivora was exclusive to white oak family members and B. dothidea to red oak family members. Of these species, D. corticola was most frequently isolated, followed by D. quercivora, D. sapinea, and B. dothidea. Overall, mortality was low across all sampled counties, indicating that these fungi, at the levels that were detected, are not widely inciting oak decline across the region, but probably are acting opportunistically when the environment is conducive to disease. To better understand the relationships between D. corticola and potentially their geographic origins, a multigene phylogenetic study and corresponding morphological study were conducted. A total of 49 Diplodia isolates from Spain, France, Italy, and the United States were assessed. Across all isolates and geographic regions, D. corticola formed a strongly supported monophyletic clade sister to D. quercivora and included two strongly supported subclades, one that included isolates from Spain and California and a second that included isolates from Italy, Maryland, and West Virginia. Both subclades also exhibited overlapping spore measurements. These results support D. corticola as a cosmopolitan pathogen, native to both Europe and the United States, with the possibility of secondary introductions.

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