Crayfish plague epidemics (caused by Aphanomyces astaci) have been causing population collapses among native European crayfish stocks since the late 1800s. Recent indirect and direct evidence has shown that its virulence has been variable, with native European crayfish even acting as carriers. We tested the differences in A. astaci virulence under experimental conditions using both PsI- and As-genotypes with 3 Finnish noble crayfish Astacus astacus populations. We infected crayfish with adjusted quantities of A. astaci zoospores and monitored the symptoms and mortality of the crayfish. The PsI-genotype isolate caused rapid and total mortality among the tested populations, while the As-genotype isolates expressed more variable virulence. In some cases, mortality among the As-genotype-infected crayfish did not exceed the mortality level of the control group. All of the tested noble crayfish stocks showed lower mortality towards the As-genotype of A. astaci isolated from the River Kemijoki epidemic. We conclude that there are clear differences in virulence between different A. astaci genotypes and also differences in virulence within As-genotypes. Furthermore, we observed clear signs of increased resistance in different populations of noble crayfish towards some of the tested strains belonging to the As-genotype of A. astaci.
