Pooja Singh, J. Bradley St.Clair, Brandon M. Lind, Richard Cronn, Nicholas P. Wilhelmi, Mengmeng Lu, Dragana Obreht Vidakovic, Richard C. Hamelin, David C. Shaw, Sally Aitken

Understanding the genetic architecture of tolerance and resistance to pathogens is important to monitor and maintain resilient tree populations. Here we investigate the genetic basis of tolerance and resistance and to needle cast disease in Douglas-fir (Pseudotsuga menziesii) caused by two fungal pathogens: Swiss needle cast (SNC) caused by Nothophaeocryptopus gaeumannii, and Rhabdocline needle cast (RNC) caused by Rhabdocline pseudotsugae). We performed a case-control genome-wide association analysis (GWAS) and found these traits to be polygenic. Significant associations with SNC resistance were found for SNPs in genes for stomatal regulation and ethylene and jasmonic acid pathways, which are known for their roles in plant defense and immunity. Top-associated SNPs for SNC tolerance were found in genes of secondary metabolite pathways. We identified a key upstream transcription factor of plant defence, ERF1, as the main candidate for RNC resistance. Our findings contribute to the understanding of the highly polygenic architectures underlying disease resistance and tolerance in Douglas-fir and have important implications for forestry and conservation as the climate changes.