For 150 years the crayfish plague disease agentAphanomyces astacihas been the cause of mass mortalities among native European crayfish populations. However, recently several studies have highlighted the great variability ofA. astacivirulence and crayfish resistance toward the disease. The main aim of this study was to compare the response of two crayfish species, the European native noble crayfish (Astacus astacus) and the invasive alien marbled crayfish (Procambarus virginalis), to anA. astacichallenge with a highly virulent strain from haplogroup B and a lowly virulent strain from haplogroup A. In a controlled infection experiment we showed a high resistance of marbled crayfish against anA. astaciinfection, with zoospores from the highly virulent haplogroup B strain being able to infect the crayfish, but unable to cause signs of disease. Furthermore, we demonstrated a reduced virulence in theA. astacistrain belonging to haplogroup A, as shown by the light symptoms and the lack of mortality in the generally susceptible noble crayfish. Interestingly, in both marbled crayfish and noble crayfish challenged with this strain, we observed a significant decrease of the detected amount of pathogen’s DNA during the experiment, suggesting that thisA. astacihaplogroup A strain has a decreased ability of penetrating into the cuticle of the crayfish. Our results provide additional evidence of how drastically strains belonging toA. astacihaplogroup B and haplogroup A differ in their virulence. This study confirmed the adaptation of one specificA. astacihaplogroup A strain to their novel European hosts, supposedly due to reduced virulence. This feature might be the consequence ofA. astaci’s reduced ability to penetrate into the crayfish. Finally, we experimentally showed that marbled crayfish are remarkably resistant against the crayfish plague disease and could potentially be latently infected, acting as carriers of highly virulentA. astacistrains.
