Azotobacter chroococcum Enhances Phytoalexin-Mediated Defense and Growth in Cotton (Gossypium hirsutum L.) in a Genotype-Dependent Manner

Abstract Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum (FOV) severely limits cotton production in Central Asia. Re-sowing of infected fields, reduced fungicide effectiveness, and rising input costs create substantial economic losses for farmers. Therefore, identifying biological alternatives that enhance plant immunity and productivity is essential. The objective of this study was to evaluate the potential of the diazotrophic strain Azotobacter chroococcum K2020 to promote growth, improve yield traits, and stimulate phytoalexin synthesis in several cotton varieties under F. oxysporum stress. Inoculation with A. chroococcum significantly increased chlorophyll a, chlorophyll b, and carotenoid levels (up to +115%, +339%, and +52%), accompanied by improved plant height, leaf area, and branching. Boll weight increased by 5-8%, fiber weight by 9-12%, and fiber length by 1–2 mm, with higher seed number and lint percentage. Under fungal stress, co-inoculation with A. chroococcum + F. oxysporum strongly enhanced phytoalexin production. In ‘Afsona’, gossypol doubled (6759.8 vs. 3149.7 mg/g) and resveratrol increased significantly, indicating activation of induced systemic resistance. Azotobacter chroococcum K2020 effectively strengthened cotton growth, yield, and phytoalexin-mediated defense against FOV, demonstrating its potential as a dual biofertilizer and bioprotective agent for sustainable cotton cultivation.