Efficacy of Rhizobacteria as Biocontrol Agents Against Verticillium Wilt in Eggplant

Abstract

The study aimed to explore the influence of Bacillus subtilis (Bs1) and Pseudomonas putida (Psp1) on Verticillium dahliae, the causal agent of Verticillium wilt in eggplant, under in vitro and greenhouse conditions. Seventeen purified bacteria exhibiting broad-spectrum antifungal activity were isolated from soil in the North Sinai Governorate, Egypt. The isolates Bacillus subtilis (Bs1) and Pseudomonas putida (Psp1) were identified through 16S rRNA gene analysis. Assays of antibacterial activity showed that Bs1 and Psp1 produced the highest amounts of hydrogen cyanide (HCN), siderophores, and indole-3-acetic acid (IAA). Bs1 and Psp1 isolates recorded the highest levels of soluble phosphate in Pikovskaya's broth medium (6.39 and 6.46 ppm, respectively). Additionally, the highest values of soluble potassium in liquid medium were recorded by Bs1 and Psp1 isolates at 15.3 and 14.4 ppm, respectively. The maximum number of phenolic compounds was observed in Bs1 and Psp1, yielding 142.1 and 136.6 mg GA ml⁻¹. Under in vitro conditions, Bs1 and Psp1 demonstrated a strong ability to inhibit the mycelial growth of V. dahliae. A greenhouse experiment was conducted to evaluate the effects of Bs1 and Psp1 rhizobacteria, either individually or in combination, against Verticillium wilt using the long black hybrid cultivar. The fermentation broth of Bs1 and Psp1 reduced disease development (20%), decreased the disease index (13%), and achieved the highest control efficiency (86.4%). The combined treatment of Bs1 and Psp1 significantly increased both bud length (32.2 mm) and germination rate (97.5%) in eggplant. It also enhanced chlorophyll content and enzyme activity, with peroxidase (POD) and polyphenol oxidase (PPO) levels recorded at 6.2 U min⁻¹ g⁻¹ and 2.3 U min⁻¹ g⁻¹, respectively, in eggplant treated with Bs1 + Psp1 compared to the control. Additionally, the combined treatment significantly reduced disease severity and demonstrated potential as a plant growth-promoting agent.