STUDY OF ONION SULFUR-CONTAINING COMPOUNDS AS BIOFUMIGANTS AGAINST SOIL BORNE PLANT PATHOGENS

Soil-borne plant pathogens are a major constraint to sustainable agriculture due to their persistence, adaptability, and resistance to conventional control strategies. Besides fungal pathogens, Gram- negative bacteria such as Xanthomonas campestris and Pseudomonas aeruginosa significantly contribute to plant disease complexes, largely due to biofilm formation and antimicrobial resistance. The present study evaluated the biofumigant potential of natural sulfur-containing compounds derived from Allium cepa (onion) against these soil-borne bacterial pathogens. Sulfur-rich compounds were extracted from fresh onion bulbs through organic solvent extraction following enzymatic activation of S-alk(en)yl-L-cysteine sulfoxides. Antioxidant activity assessed by the phosphomolybdenum assay produced a linear ascorbic acid calibration curve (y = 0.0009x). The extract exhibited substantial total antioxidant capacity, with values of 578 µg/mL and 678 µg/mL expressed as ascorbic acid equivalents (AAE), confirming the presence of redox-active sulfur metabolites. Antimicrobial activity evaluated by agar well diffusion showed inhibition zones of 12.6 mm against P. aeruginosa and 14.3 mm against X. campestris, comparable to streptomycin (14–15 mm). The contact time assay demonstrated strong time-dependent bactericidal activity, with 96.55% reduction in P. aeruginosa and 99.54% reduction in X. campestris after 24 hours. In soil-based pot experiments, onion extract significantly reduced pathogen populations compared to infected controls and showed suppression trends comparable to synthetic fumigant treatment. Although the synthetic fumigant produced slightly stronger immediate reduction, onion-derived sulfur compounds provided sustained suppression under soil conditions.