Morphological and molecular analyses were carried out to determine the causative agent of black gill disease in narrow-clawed crayfish Pontastacus leptodactylus specimens collected from 6 lakes (Taşkısığı, Beyşehir, Karaidemir, Karataş, Manyas, and Gölhisar) in Turkey. Fungi were isolated from the tissues of crayfish displaying putative black gill disease symptoms. Morphological evaluation was conducted, and the isolates were determined to be similar to Fusarium oxysporum. Subsequent molecular cloning of the ITS region of nrDNA by PCR confirmed species identification; DNA sequences from all 6 isolates were 99% similar to those of F. oxysporum. An experimental infection trial was conducted in triplicate using 1 of the 6 isolates to fulfill Koch's postulates. Three groups of crayfish were used: (1) wounded and contaminated (WC), (2) wounded and not contaminated (WNC), and (3) non-wounded and contaminated (NWC). On Day 3, one individual died in the WC group. Throughout the total 74 d monitoring period, no other mortality was recorded. On Day 14, all crayfish in the WC group showed blackened gills, whereas all crayfish in the NWC displayed partial blackening and all crayfish in the WNC group displayed normal gill coloration. F. oxysporum was re-isolated in pure culture from the WC and NWC groups; thus, the disease was confirmed. Our results indicated that F. oxysporum causes black gill disease in P. leptodactylus crayfish. Moreover, we demonstrated that F. oxysporum can also infect non-wounded crayfish.
Keywords: Black gill disease; Crayfish; Fusarium oxysporum; Pontastacus leptodactylus.