Genus-level change in aggressiveness with continuous invasions: a phylogenetically-informed Bayesian quantile regression

Phytophthora is a genus of oomycete plant pathogens consisting of numerous invaders of production and natural systems. To date, only few studies assessed how invasions change over time, and none have examined changes in invasive genera consisting of multiple species. With increasing globalisation, pathogens invade and adapt to new environments and hosts. After establishment, the aggressiveness of pathogens could decrease due to biotic and abiotic factors. Conversely, highly pathogenic genotypes should continuously replace less pathogenic ones. Here, we compiled the disease development data of Phytophthora species from published data and reports that span 105 years to assess how aggressiveness has changed over time. For each aggressiveness trial, we recorded the year of pathogen isolation and a measure of pathogenicity, as well as local environmental variables. Phylogenetic multi-level quantile regression was used to analyse the relationships between aggressiveness and time across pathogen taxa under different environmental conditions. We found that aggressiveness decreased over time. This holds true if only the most commonly isolated taxa, and the most recent isolates, were considered. Highly aggressive pathogens from agriculture and natural ecosystems decreased significantly. However, pathogens from nurseries generally became more aggressive over time, particularly among the most common species. Phytophthora diseases overall are highly pathogenic and have potential to cause outbreaks, especially given that the common species were more aggressive under higher temperature variability. Through a multi-level approach, we uncover how the aggressiveness of pathogens changes globally at the genus-level with continuous emergence and invasion.