Which bacterium causes the fish disease columnaris, recognizable by pale lesions and cotton-like growth?

Flavobacterium columnare causes the fish disease columnaris, recognizable by pale lesions and cotton-like growth.

Flavobacterium columnare is a Gram-negative bacterium that affects freshwater fish, including farmed species and aquarium fish. Columnaris disease can produce pale areas on the skin, damaged fins, mouth lesions, and masses that resemble fungus. The visible cotton-like material is often bacterial growth and damaged tissue, although true fungal infections can look similar.

Outbreaks are influenced by warm water, crowding, handling stress, and poor water quality. The name columnaris refers to characteristic column-like bacterial arrangements seen under a microscope. Aeromonas hydrophila and other aquatic bacteria can also cause fish disease, so appearance alone does not establish the diagnosis; laboratory identification is needed for certainty.