Distribution, diversity, and pathogenicity of Oomycete root pathogens affecting Fraser fir (Abies fraseri) Christmas tree production in the Southern Appalachian Mountains.

Fraser fir (Abies fraseri) is the most economically important Christmas tree species in the Southern Appalachian Mountains, especially in North Carolina. However, its production is increasingly threatened by Phytophthora root rot (PRR), caused primarily by Phytophthora cinnamomi and several other Phytophthora species, resulting in significant losses for growers annually. This study surveyed symptomatic Fraser firs across the region and recovered 151 Phytophthora isolates. Six species were identified including: P. cinnamomi (88.7%), P. cryptogea (6%), P. europaea (2%), P. cactorum (1.3%), P. citricola (1.3%), and P. plurivora (0.7%). P. cinnamomi was the most prevalent. Phylogenetic and population statistical analysis of P. cinnamomi populations showed minimal genetic variation and no correlation between genetic and geographic distance, likely reflecting clonal lineage diversity, low marker resolution, or both. A greenhouse trial was conducted to assess the pathogenicity of several Phytophthora species on Fraser fir seedlings. Phytophthora cinnamomi caused the most severe disease symptoms and highest mortality, while other species showed less severe disease progression. Control seedlings and those inoculated with non-Phytophthora oomycetes remained healthy. These findings improve our understanding of Phytophthora diversity affecting Fraser fir in North Carolina, highlight the dominant threat posed by P. cinnamomi, and inform monitoring and management strategies.