Columnaris‐causing bacteria (CCB) represent a group of four Flavobacterium species, previously classified under Flavobacterium columnaris , causing a threatening condition in salmonid farming characterised by cutaneous and gill lesions, commonly referred to as ‘saddleback disease’. A peracute skin disease outbreak with high mortality in rainbow trout ( Oncorhynchus mykiss ) farms in Northern Italy was investigated. The disease...