The Effects of Seed Source and Planting Environment on Douglas-fir (Pseudotsuga menziesii) Foliage Diseases

Graduation date: 2017Douglas-fir (Pseudotsuga menziesii) is a commercially and ecologically important tree species native to western North America. The foliar pathogens Phaeocryptopus gaeumannii, the causal agent of Swiss Needle Cast, and Rhabdocline species, the causal agents of Rhabdocline needle cast, are two important pathogens specific to Douglas-fir. These pathogens are highly influenced by climatic conditions and have been shown to disproportionately affect genetically maladapted populations, causing decreased productivity and sometimes mortality. The relationship between the levels of resistance/tolerance to these foliar pathogens and climate of the population source is a key component in the identification of proper seed sources for reforestation. The Douglas-fir Seed Source Movement Trials (SSMT) is a large provenance study of Westside Douglas-fir which consists of populations and test sites chosen to represent the range of climatic conditions experienced by Douglas-fir west of the Cascade and Sierra Nevada Mountains. We used the SSMT to assess variation in the probability of moderate to severe infection by Rhabdocline spp. and P. gaeumannii, as well as expression of disease symptoms, in relation to climatic differences between test sites and population sources. Using generalized linear mixed models and the probit link function, probabilities were calculated using differences between population source and test sites with respect to the climate variables May through September precipitation (MSP), continentality and mean winter temperature (MWT). Stark differences were observed in side-by-side trees from different seed zones, especially in relation to the resistance to Rhadocline needle cast. No patterns were observed in the infection levels of P. gaeumannii based on number of stomates occluded by fungal reproductive structures. However, there was variation in disease symptom expression such as crown density which was used as a surrogate for Swiss needle cast impacts in this analysis. The transfers of populations from regions of low MSP to test sites of high MSP, cool to warm MWT and high to low continentality are associated with high probabilities of moderate to severe Rhabdocline spp. infection and SNC disease symptoms (low crown density). Trends in high probabilities of disease symptoms and Rhabdocline spp. infection were associated with the movement of populations from south to north. Predictions of disease occurrence based on climate-disease-host seed source interaction indicates danger in choosing trees for assisted migration from drier regions and moving them to wetter areas in anticipation of climate change. This project exemplifies the importance of seed source movement trials in understanding trophic interactions as well as environmental controls on tree growth and adaptatio