- A series of studies, in three western Oregon Douglas-fir plantations, was conducted to understand the physiological impacts of Swiss needle cast on Douglas-fir physiology. Four aspects of the disease complex were investigated: fungal colonization and assessment, plant-water relations, carbon assimilation and interaction with climate. Several techniques were developed and used to assess the colonization of Phaeocryptopus gaeumannii, the causal fungus, in foliage (i.e., ergosterol concentration, quantitative PCR, and visual estimates of fruiting bodies). All measures of fungal colonization were significantly correlated with each other (r [greater than or equal to] 0.733) and with the amount of visible symptoms present (i.e., needle chlorosis and retention) (r [greater than or equal to] 0.578). Furthermore, removal of P. gaeumannii with fungicide applications reduced visible symptoms and increased tree growth. Upon sporulation, P. gaeumannii produces fruiting bodies (pseudothecia) that emerge from needle stomata, significantly reducing gas exchange in Douglas-fir needles by physically impeding gaseous diffusion. Maximum rates of needle gas exchange (CO₂ and H₂O) were inversely proportional to the presence of P. gaeurnannii in needle stomata. Anatomical and biochemical changes, such as reduced sapwood permeability and reduced rubisco activity, associated with prolonged disease presence further reduced the capacity and duration of needle gas exchange. Climate was also shown to play a significant role in disease development. Differences associated with site topography (i.e., slope and aspect), influenced both fungal colonization and symptom development. For example, an increase in fungal colonization and symptom development was observed on foliage from south slopes, which typically experienced greater evaporative demands (i.e., increased temperature and/or 1ower relative humidity). The cumulative effects of P. gaeumannii infection were integrated into a process-based model of photosynthesis. Modeled estimates of stomatal conductance and photosynthesis were well correlated with observed values (R²= 0.777, R² = 0.820, respectively). Yearly estimates of whole-canopy carbon assimilation, accounting for P. gaeumannii infection and site climate differences, were significantly correlated with tree height growth (R² = 0.792).