PsI-genotype isolates from different Finnish signal crayfish stocks show variation in their virulence but still kill fast

We detected significant virulence differences among five tested (PsI-Puujärvi, PsI-Pyhäjärvi, PsI-Kukkia, PsI-Saimaa I and PsI-Saimaa II) PsI-genotype crayfish plague (Aphanomyces astaci) isolates against lake Mikitänjärvi noble crayfish population. The crayfish were inoculated with a dose of 300 m·L-1A. astaci spores in ambient water under experimental conditions. Mortalities started from four to seven days after inoculation, depending on the PsI-genotype isolate. In all the experimentally infected groups it took no more than three days for all the crayfish to die after the first mortality. The PsI-Puujärvi isolate proved to be the most virulent strain, while PsI-Kukkia was the least virulent. The average day of death for these experimental groups was fifth and ninth day, respectively. We did not discover correlation between the average day of death and gender or level of additional Psorospermium haeckeli infestation. Our results show that there are, from the practical point of view, minor virulence differences among PsI-genotype A. astaci isolates, and that all the tested five isolates are highly virulent. The present results emphasize the necessity to prevent all further spread of highly virulent strains of A. astaci to aid and shelter successful conservations attempts of the native European crayfishes.