Two Bacterial Bioagents Boost Onion Response to Stromatinia cepivora and Promote Growth and Yield via Enhancing the Antioxidant Defense System and Auxin Production

White rot, caused by Stromatinia cepivora (Anamorph: Sclerotium cepivorum Berk), is a serious soil-borne disease of the onion that restricts its cultivation and production worldwide. Herein, we isolated and characterized a plant growth-promoting rhizobacterium Stenotrophomonas maltophilia from healthy onion roots and an endophytic bacterium Serratia liquefaciens from healthy bean leaves. Both isolates showed strong fungistatic activity against S. cepivora using the dual culture and culture filtrate methods. This effect might be due to the presence of several volatile compounds, especially menthol in both culture filtrates as shown with a GC-MS analysis. Additionally, the root drench application of cell-free culture filtrates of S. maltophilia and S. liquefaciens significantly reduced the incidence and severity of white rot disease on treated onion plants, which was associated with the activation of both enzymatic (POX and PPO) and non-enzymatic (phenolics and flavonoids) antioxidant defense machineries of S. cepivora-infected onion plants. Moreover, the culture filtrates of both bacterial bioagents remarkably enhanced the growth (as expressed by root length, plant height, and number of leaves) and yield parameters (as indicated by bulb circumference, fresh weight of the bulb, and bulb yield per plot) of treated onion plants under field conditions during two successive seasons (2020/2021 and 2021/2022). This might be because of a reduced disease severity and/or the accumulation of the main auxin, indole-3-acetic acid (IAA), and its precursor, the amino acid tryptophan. Our findings suggest that both bioagents might be utilized as eco-friendly alternative control measures to reduce the utilization of chemical fungicides entirely or partially for the safer production of onion in S. cepivora-infested soils.