Seedling_Growth_Increment

· Invasive pathogens threaten the stability of forest ecosystems worldwide. Trees possess intrinsic pathogen defense mechanisms, like major-gene resistance (MGR) and quantitative-disease resistance (QDR). Symbiotic fungi can enhance this resistance, influencing entire ecosystems. Yet, how fungal symbionts influence different host resistance genotypes, and their phenotypic expression is poorly understood.· We inoculated <i>Pinus monticola</i> seedling families exhibiting resistance (MGR or QDR) or high susceptibility to <i>Cronartium ribicola</i>, the causative agent of white pine blister rust (WPBR), with endophytic and ectomycorrhizal (EM) fungi individually and in combination. We monitored growth, foliar defenses, and disease progression before and after <i>C. ribicola</i> infection and in controls without <i>C. ribicola</i> infection.· We observed prolonged changes in inducible defenses and immune priming across all treatments. In the non-infected control treatments, shifts in terpene composition coincided with increased seedling growth rate. In WPBR-infected seedlings, shifts in terpenes correlated with reduced symptoms in QDR, and to a lesser extent, in susceptible, while MGR seedlings appeared less affected.· These results underscore the role of endophytic and EM fungi to improve tree growth and defense against pathogens, reducing disease symptoms. We also show that these fungi can play a critical role in latent and long-lasting effects on conifer foliar terpenes, expanding our understanding of their ecological significance.