Evaluating the Potential of Aspergillus niger and Salicylic Acid to Induce Resistance against Fusarium-Associated Wilt in Watermelon

Abstract

This study evaluated the potential of salicylic acid (SA) and Aspergillus niger culture filtrate to induce resistance against Fusarium-associated wilt in two watermelon cultivars, Sugar Baby and Aseel, under greenhouse conditions. Molecular identification based on ITS sequencing identified Fusarium proliferatum, F. solani, and F. oxysporum among the recovered isolates. Disease incidence reached 100% in plants inoculated with F. oxysporum and F. solani, whereas F. proliferatum caused 66.67% incidence in Sugar Baby and 100% in Aseel. Application of A. niger or SA markedly reduced disease incidence, with the lowest value of 23.33% recorded in Sugar Baby treated with F. oxysporum + A. niger or F. oxysporum + SA, and in Aseel treated with F. proliferatum + A. niger. A. niger alone produced the highest growth values in Sugar Baby, including 0.17 mm stem diameter, 17.20 cm root length, 25.00 cm plant height, 16.22 SPAD chlorophyll content, and 2.52 cm² leaf area. In Aseel, the corresponding maximum values were 12.90 cm root length, 21.47 cm plant height, 10.27 SPAD, and 1.04 cm² leaf area. Peroxidase activity was enhanced in combined treatments, reaching 0.82 µmol tetra-guaiacol min⁻¹ g⁻¹ FW in Sugar Baby treated with F. oxysporum + SA and 0.79 µmol tetra-guaiacol min⁻¹ g⁻¹ FW in Aseel treated with F. proliferatum + A. niger. Overall, SA and A. niger reduced disease development and improved several growth and defense-related traits, although their effectiveness varied according to the watermelon cultivar and Fusarium species.