| Invasive forest pathogens currently threaten forest health and biodiversity worldwide. In my dissertation, I address various aspects of the ecology and evolution of invasive pathogens in two pathosystems: the basidiomycete fungus, Heterobasidion annosum sensu lato, causal agent of a lethal root and butt rot in northern coniferous forests, and three sympatric oomycete pathogens: Phytophthora ramorum, P. nemorosa and P. pseudosyringae within and beyond the range of P. ramorum -caused "sudden oak death" in the coastal mixed-evergreen forests of California and Oregon.Using molecular phylogenetic methods, I identified eastern North America as the most likely source for genetically anomalous H. annosum isolates recovered in conjunction with high stone-pine mortality in Italy. I also identified an instance of introgression between two sympatric Heterobasidion taxa in North America. A comparison of specific ecological and evolutionary factors affecting invasiveness of the North American taxon in Italy did not show evidence that lack of co-evolution led to increased aggressiveness of the invasive Heterobasidion compared to the native on Italian hosts. However, increased transmission in dry environments may enhance the invasion of the exotic Heterobasidion.Using molecular population-genetic methods, I found that the recently described P. nemorosa and P. pseudosyringae are both likely to be introduced to western North America. This finding contrasts with the hypothesis, based on their relatively large ranges and low virulence, that both are indigenous. In order to evaluate potential effects of coinfection of the sympatric P. nemorosa, P. pseudosyringae and P. ramorum on a shared, disease-transmitting host, I conducted simultaneous inoculations with one, two and three of the pathogens. Phytophthora nemorosa abundance was suppressed in all coinfection treatments, while P. ramorum and P. pseudosyringae had no detectable effects on each other.Taken together, these results show the utility of molecular methods to identify "invisible invasions" of morphologically cryptic or relatively mild pathogens. Moreover, understanding the factors facilitating invasion, the role of genetic introgression and effects of multiple coinfection of one host can have many applications in these and other forest ecosystems, including disease control, preventing future pathogen introductions, and understanding the impact of invasive pathogens on microbial community dynamics. |
