- Douglas-fir provides social, economic, and ecological benefits in the Pacific Northwest (PNW). In addition to timber, forests support abundant plant and animal biodiversity and provide socioeconomic viability for many rural communities. Products derived from Douglas-fir account for approximately 17% of the U.S. lumber output with an estimated value of $1.9 billion dollars. Employment related to wood production accounts for approximately 61,000 jobs in Oregon. Timberland also supports water resources, recreation, and wildlife habitat. Minor defoliation has previously been linked to Swiss Needle Cast, associated with the fungus Phaeocryptopus gaeumannii, however, unprecedented large-scale defoliation began in the 1990s and has increased since, leading to decreased growth and yield. Areas affected areas by SNC exceed 500,000 acres in Oregon. Defoliation symptoms are inconsistent with predicted effects of P. gaeumannii, and targeted chemical control has had mixed results. While the microbiome community of conifer needles is poorly described to date, we hypothesize the full interstitial microbiome complex is involved in disease response in conifers. There are at least three known pathogenic endophytes of Douglas-fir, in addition to a large number of endophytes with undetermined host relationships. In order to understand the mechanistic dynamics of needle cast, it is necessary to uncover the underlying cause of the symptoms. The first step is establishing a baseline inventory of endophytes in PNW Douglas-fir needles. Two-year old needles were collected from two sites each in Oregon and Washington, for a total of four sites. Study sites varied in elevation (185 – 860m) and mean annual precipitation (250 – 1,575 mm year). DNA extraction and sequencing revealed 46 unique isolates from 39 taxa in Douglas-fir needles gathered from four test sites in Oregon and Washington. Rates of infection for all needles was 39%, with infection ranging from 5% to 60% among seed sources. The results suggest that the probability of endophyte occurrence at the cool, wet site, was 2.8 (p < 0.0001) times higher than the warm, dry site, and 2.2 (p = 0.0007) times higher than the warm, wet site. Non-local needles at the warm, dry site were 13.6 (p < 0.0001) times more likely to be infected than needles from the local seed source. The environmental variable that seemed most positively correlated with endophyte presence was winter relative humidity. There was strong evidence for the influence of seed source type on the relationship between continentality (the difference between mean warmest month and mean coldest month temperatures) and mean average number of endophytes. Long-term silvicultural management of PNW forests will benefit as a result of this study. It is the first study to use modern molecular techniques to uncover the fungal endophytic communities of West-Coast Douglas-fir foliage. It is vital to understand the complete etiology of the needle cast disease affecting Douglas-fir, and whether to focus on mitigating a single-species pathogen, or a number of different pathogens. Because forests are a long-term strategy, it is critical to understand the ecological implications of disease in this important tree species.