Survey and characterisation of Fusarium oxysporum f.sp. cepae causing basal rot of onion (Allium cepa L.) in Perambalur district of Tamil Nadu

Basal rot of onion, incited by Fusarium oxysporum f.sp. cepae, is one of the most destructive soil-borne diseases limiting onion productivity worldwide. A systematic survey was conducted across ten major onion-growing areas of Perambalur district, Tamil Nadu, to determine the incidence of basal rot and to isolate the causal pathogen. Disease incidence ranged from 11.26% to 60.15%, with the highest recorded at Settikulam and the lowest at Nakkasalem. Ten isolates of F. oxysporum f.sp. cepae were obtained and evaluated for cultural and morphological variability. Distinct differences were observed in colony colour, mycelial growth, conidial morphology and sporulation intensity among the isolates, with Foc3 identified as the most virulent based on growth and sporulation parameters. Sand–maize medium supported maximum fungal biomass production compared to sorghum and cumbu grains. The results confirm significant pathogenic diversity among F. oxysporum f.sp. cepae isolates in Perambalur district of Tamil Nadu and highlight the necessity for region-specific management strategies.