Emergence of Fusarium oxysporum infection in farmed Labeo rohita: Isolation, growth requirements, antifungal response, and pathogenic potential.

Abstract

Fusarium oxysporum, was isolated from a naturally infected captive-reared Labeo rohita in Tripura, India. The infected fish exhibited clinical signs of fungal growth with haemorrhage on the ventral body surface and erosion of the caudal fin. Morphological identification of fungal colony, including microscopic observation of 5-day-old culture, revealed the presence of multi-septate macroconidia and one-septate microconidia. Growth requirement study revealed that the fungus grew best at 30 °C and pH 6.0. In vitro studies revealed that the isolate produced extracellular enzymes such as caseinase, lipase, starch hydrolase and DNase, which contributed to its pathogenicity. The partial 18 S gene sequence was assigned NCBI accession number PQ013739.1 and confirmed its identity as F. oxysporum. Pathogenicity test by intraperitoneal injection of fungal spore suspension in healthy L. rohita fingerlings with fungal growth at the site of injection; LDvalue obtained by probit analysis was 4.85 × 10spores/ml. Antifungal susceptibility testing indicated that none of amphotericin B, ketoconazole, itraconazole, clotrimazole, and nystatin produced zones of inhibition against the fungal isolate. In contrast, the fungus exhibited sensitivity to sodium chloride and copper sulfate, with reduced radial growth at salinity levels exceeding 0.5% and CuSOconcentrations above 25 ppm. Our findings confirm that the F. oxysporum strain is virulent to L. rohita, and highlights the potential utility of salinity and copper sulfate-based management strategies for control of this fungus in aquaculture.