Invasive plant pathogens are often recognized as serious threats to the maintenance of biodiversity affecting both structure and function of ecosystems. Here, we investigate the potential impact of the invasive pathogen Phytophthora ramorum Werres, de Cock & Man in't Veld by using physical girdling of tanoak, Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S. H. Oh (Fagaceae), as a surrogate for the disease and to test for changes on the hyphal abundance of ectomycorrhizal fungi. In this study, the flow of phloem to the roots of girdled trees was severed by cutting two narrow incisions (about 10 cm distant) through the inner bark around the circumference of the stem of each tree (fully-girdled), or by cutting two narrow incisions half of the circumference of the tree (half-girdled), to compare with untreated (non-girdled) trees. The hyphal abundance of two common and ecologically important ectomycorrhizal genera (Cenococcum and Tricholoma) was estimated from the roots and surrounding soil using real-time PCR quantification (TaqMan) assay. A significant decrease in the hyphal abundance from soil was observed in girdled tree plots. In contrast, no similar decrease in the root hyphal abundance was observed. Ectomycorrhizal fungi have a major impact on ecosystem function through their control over decomposition, nutrient acquisition, and mobilization and regulation of succession in plant communities. Given their important function, the decline in EM abundance of tanoak infected by P. ramorum will likely disrupt the function and structure of these forests.