Factors contributing to disease phenotypes in conifers

This thesis investigates the genetic and environmental factors and mechanisms influencing disease development in conifers, focusing on Scots pine and Norway spruce in Sweden. The thesis contributes new knowledge on the threat that diseases pose to forest ecosystems.I studied two pathogens in pine: the opportunistic necrotroph Diplodia sapinea and the biotrophic rust fungus Melampsora pinitorqua. Drought stress exacerbates dieback, and the pathogen likely acts as an accelerating factor in decline but surviving trees can recover. An investigation into co-infection by M. pinitorqua and D. sapinea in Scots pine found no direct link between the infections but evidence for variation in susceptibility to both pathogens. In a subsequent study, genome-wide association studies (GWAS) were used to identify genetic markers associated with disease symptoms caused by M. pinitorqua and D. sapinea. Markers for reduced susceptibility to D. sapinea were validated in a population of progenies from elite trees in the Swedish Scots pine breeding programme, showing potential for resistance breeding in Scots pine.In the final study, the potential to improve resistance to the necrotrophic root rot fungus H. parviporum in Norway spruce through genomic selection was evaluated. The results indicate sufficient genetic variation within the studied Swedish breeding population. Although genomic selection shows great potential, the findings highlight the need for more data to train the models to improve prediction accuracy.Overall, this thesis demonstrates the importance of understanding host–pathogen interactions and utilising genetic tools to breed disease-resistant conifers, which is essential for ensuring the long-term health and resilience of Nordic forests in the context of climate change.