Assessing the phytosanitary risk posed by an intraspecific invasion of Cryphonectria parasitica in Europe.

Intraspecific cryptic invasions may occur when new strains of an invasive species are introduced into an area where this species had already been introduced previously. In plant pathogens, such invasions are not well studied, even if potentially they can have severe consequences. Here, we investigated the effects of a potential intraspecific invasion in Europe of Cryphonectria parasitica, the causal agent of chestnut blight. Specifically, we tested the hypotheses that (1) non-European strains are more virulent on Castanea sativa than those already present in Europe because they have never encountered this new host, and (2) the variation in virulence among strains is higher within native than within introduced populations. In a greenhouse, two-year old C. sativa seedlings were inoculated with C. parasitica strains from South Korea, the USA and Switzerland and lesion development and seedling mortality weekly recorded. Additionally, growth and sporulation of the strains were measured in vitro on agar medium at 15 and 24 °C. While lesion growth was similar for all strains, seedlings inoculated with strains from South Korea and Switzerland died faster than seedlings inoculated with strains from the USA. Moreover, in vitro strains from South Korea grew faster and produced more spores at both temperatures than the strains from the other two countries. In conclusion, our results did not support the two hypotheses. All strains, regardless of their origin, were found to be highly virulent on the inoculated chestnut seedlings. Nevertheless, current phytosanitary measures to avoid the introduction of new genotypes of C. parasitica into Europe should be further implemented.