Biogeographical patterns and determinants of invasion by forest pathogens in Europe.

A large database of invasive forest pathogens (IFPs) was developed to investigate the patterns and determinants of invasion in Europe. Detailed taxonomic and biological information on the invasive species was combined with country-specific data on land use, climate, and the time since invasion to identify the determinants of invasiveness, and to differentiate the class of environments which share territorial and climate features associated with a susceptibility to invasion. IFPs increased exponentially in the last four decades. Until 1919, IFPs already present moved across Europe. Then, new IFPs were introduced mainly from North America, and recently from Asia. Hybrid pathogens also appeared. Countries with a wider range of environments, higher human impact or international trade hosted more IFPs. Rainfall influenced the diffusion rates. Environmental conditions of the new and original ranges and systematic and ecological attributes affected invasiveness. Further spread of established IFPs is expected in countries that have experienced commercial isolation in the recent past. Densely populated countries with high environmental diversity may be the weakest links in attempts to prevent new arrivals. Tight coordination of actions against new arrivals is needed. Eradication seems impossible, and prevention seems the only reliable measure, although this will be difficult in the face of global mobility.

Are plant pathogen populations adapted for encounter with their host? A case study of phenological synchrony between oak and an obligate fungal parasite along an altitudinal gradient.

Biotrophic fungal pathogens are expected to have adapted to their host plants for phenological synchrony, to optimize the possibility of contacts leading to infections. We investigated the patterns and causes of variation in phenological synchrony in the oak-powdery mildew pathosystem, a major disease in natural ecosystems. The study was carried out along an altitudinal gradient, representing a wide temperature range, in mature oak stands. Both sporulation (pathogen infective stage) and oak flushing (host susceptible stage) were delayed with increasing elevation, but with a significantly different sensitivity for the two species. This resulted in a variable host-pathogen synchrony along the gradient. A common garden experiment did not give evidence of among-population genetic differentiation (past adaptation) for fungal phenology. This could be explained by the high phenotypic variation in phenology within host populations, precluding selection on fungal phenology at the population scale, but possibly favouring adaptation at the within-population scale. Phenotypic plasticity was the major cause of the observed variation in the phenology of the fungal populations.