What organism is primarily associated with true fungal infections in fish?

The organism primarily associated with true fungal infections in fish is Exophiala, which is primarily responsible for opportunistic infections in aquatic environments. Saprolegnia, while often cited in discussions about fungal infections in fish, is actually classified as a water mold and does not belong to the true fungi. It is more commonly associated with saprophytic infections and can be triggered by environmental stressors or wounds rather than being categorized as a true fungal pathogen.

On the other hand, Pseudomonas is a genus of bacteria, not fungi, and may cause bacterial infections in fish. Fusarium, although a true fungal genus, is more frequently associated with terrestrial plant infections rather than aquaculture environments.

The distinction lies in the classification of the organisms; true fungal infections are specifically linked to pathogens that belong to the Ascomycota or Basidiomycota phyla, like Exophiala, which can cause systemic mycoses in stressed fish populations.