Dose-dependent mortality of the noble crayfish (Astacus astacus) to different strains of the crayfish plague (Aphanomyces astaci).

Several reports of the European crayfish species carrying a latent infection of the crayfish plague (Aphanomyces astaci) have emerged and the discussion has focused especially on the lowered virulence of As-genotypes behind decreased mortality. The aim of this study was to compare the killing rate of different A. astaci strains in controlled infection experiments. Two separate infection experiments with three A. astaci strains (UEFT2B (As), Evira6462/06 (As) and UEF8866-2 (PsI)) were made to compare the noble crayfish populations from the Lake Viitajärvi, Tervo, (Expt I) and the Lake Mikitänjärvi, Hyrynsalmi (Expt II). In the Expt III, the Lake Koivujärvi population noble crayfish were infected with A. astaci strains UEF8866-2 (PsI) and Evira6462/06 (As) using different dosages (1, 10, 100 and 1000sporesml(-1)) of A. astaci zoospores. The results confirmed that PsI-genotype strain is highly virulent and kills all the crayfish within a few days. The tested two As-genotype strains caused the mortalities more slowly, and part of the challenged crayfish survived until the end of the follow-up period. Our results also confirmed the variance of virulence among A. astaci strains within the As-genotype and demonstrated that the mortality is dependent on the number of zoospores used in the infections. It also appeared, that some noble crayfish populations show increased resistance towards the crayfish plague, especially against the As-genotype of A. astaci.