- Since around 1923 Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murray) Parl.) has been affected by Phytophthora root disease caused by the virulent introduced pathogen Phytophthora lateralis. A systematic resistance testing and breeding program located at the USDA Forest Service Dorena Genetic Resource Center in Cottage Grove, Oregon was initiated in 1997 with the goal of producing Phytophthora root disease resistant planting stock suitable for replacing wild native POC killed by the disease. Although families of Port-Orford-cedar have been identified as having a heritable form of resistance to Phytophthora lateralis, there is concern that susceptibility to other pathogens of Port-Orford-cedar, such as Seiridium sp. and Stigmina thujina, could compromise the success of deploying root disease resistant planting stock. This thesis examines methods used to evaluate the heritability of susceptibility of Port-Orford-cedar families in the Phytophthora root disease resistance breeding program to infection and disease caused by species of Seiridium and Stigmina thujina. Inoculation studies were undertaken to investigate the feasibility of developing protocols for screening Phytophthora root disease resistant POC for resistance to cypress canker caused by species of Seiridium and foliage blight caused by Stigmina thujina. Procedures were developed for culturing the pathogens, producing viable spores, and developing inoculation and disease rating procedures. In addition, the species of Seiridium associated with cypress canker of Port-Orford-cedar in Oregon was examined by observing morphological characteristics and comparing ß-tubulin and histone gene sequences. Cypress canker symptom severity was compared among 30 POC families with differing susceptibility to Phytophthora root disease. Seedlings were wound inoculated with cultures of a Seiridium sp. isolated from symptomatic POC and resulting symptoms scored monthly over the following 12 months. Symptoms of infection began to develop within two months after inoculation. Symptoms began as slight chlorosis of foliage above the site of inoculation, progressed to a darker browning of foliage, and finally resulted in seedling death. One year post inoculation about 71% of seedlings had been killed. Average between-family mortality ranged from 46 to 89% with a grand mean mortality of 52%. A multifactor ANOVA showed a significant variation in mortality among families. No correlation between resistance to Seiridium and resistance to P. lateralis was found. Variation in susceptibility to Stigmina foliage blight was also examined for the same 30 families. Port-Orford-cedar seedlings were inoculated by exposure to natural inoculum at a field site or by aerosol application of macerated mycelium of S. thujina cultures to seedling foliage in a greenhouse study and evaluated for Stigmina blight symptoms after 12 months. Stigmina symptoms were also compared in a naturally infected POC common garden plantation representing a range-wide collection of Port- Orford-cedar. Differences in symptom severity between families were found for all three assays. However, with one exception, no correlation was found between resistance to infection by Stigmina thujina and resistance to P. lateralis. Seedstock collected from watersheds that differed in elevation, precipitation, latitude and longitude were found to have significant variation in levels of disease severity caused by S. thujina. Families from watersheds nearest to the coast, from more northern latitudes and lower elevations showed lower levels of S. thujina disease symptoms. POC seedlings from parent trees located in watersheds in the southernmost, most inland provenances, higher elevations and lower mean annual precipitation (mm) had higher Stigmina disease ratings. In order to identify the species of Seiridium associated with cypress canker in Oregon, eight isolates of Seiridium were collected from three host species in various plantations in southwest Oregon. The diagnostic features of the Seiridium isolates were morphologically most similar to those described for S. cardinale. However, phylogenetic analysis based on ß-tubulin and histone gene sequences grouped the Oregon isolates in a separate clade from S. cardinale isolates. The Oregon Seiridium isolates occurred in a sister relationship with S. unicorne isolates from Portugal. The lack of agreement between the morphological characteristics of the Oregon Port-Orford cedar isolates and the phylogenetic analysis suggests that there may be greater variation in the morphological characters of S. unicorne than previously recognized, or that the Seiridium species from Oregon represents a hitherto unrecognized species. Previously reported analyses of pathogenic Seiridium species did not include isolates from North America, which appear more variable than isolates from Europe, New Zealand and Africa. The results of this study indicate that further analyses of Seiridium isolates from North America, including additional putative specimens from C. macrocarpa are needed to resolve this question. A number of challenges affected the success of inoculation testing procedures in this study. These included seasonal differences and variation in seedling diameter among inoculation groups. The results of this study provide a limited understanding of variation in susceptibility of Port-Orford-cedar families to natural populations of Seiridium due to the constraint of using a single Seiridium isolate, and should be interpreted accordingly. Despite these limitations, this study presents evidence for heritable variability in susceptibility within POC families to both cypress canker and Stigmina foliage blight, and suggests that there is high potential for improvement by selective breeding. The methods described in this thesis should be useful for future testing. Variation in susceptibility to these diseases should be considered in evaluating Phytophthora root disease resistance breeding efforts.