Ochroconis humicola causing muscular black spot disease of Atlantic salmon (Salmo salar)

Abstract

A dematiaceous hyphomycete was isolated from black discoloured areas of the somatic musculature of a specimen of Atlantic salmon. The fungus caused an endogenous mycosis and obviously must be classified as a facultative or opportunistic pathogen of marine fish. The optimum temperature for the strain was in the range between 20 and 25 °C, and the temperature tolerance for growth ranged from 10 to almost 37 °C. The morphological and physiological investigation revealed that the pathogen belongs to the genus Ochroconis de Hoog et von Arx, which is synonymous with Scolecobasidium Abbott. However, the definite species identification raised some difficulties, because the characteristics of our pathogenic isolate H 14 670 variably matched with at least four of the known species within the Ochroconis – Scolecobasidium — Dactylaria complex, i.e., O. humicola, O. constrictum, O. gallopavum, and O. simplex. Because of this multispecies affinity and taking into account the actual confused taxonomic state within this group of hyphomycetes, especially with regard to the pathogenic strains, the identification of the present isolate as O. humicola remains with some reservation. Key words: Atlantic salmon (Salmo salar L.), Dactylaria, fish disease, marine fungi, Ochroconis humicola, opportunistic fungi, Scolecobasidium.