Bacillus velezensis YL2021, a plant growth-promoting rhizobacterium originally isolated from rice field rhizosphere, can produce catechol-type siderophores under low-iron conditions. Previous studies have shown its broad-spectrum inhibitory activities against phytopathogenic bacteria and fungi. However, its specific anti-oomycete potential and field application performance remain uncharacterized. In this study, we describe the anti-oomycete activity of strain YL2021 and its derived siderophores through plate bioassays and replicated greenhouse trials. Using the classic Chrome Azurol S (CAS) detection assay, we found that strain YL2021 can produce a considerable amount of siderophores as early as 10 h post-incubation in Plich's medium, while the tested oomycetes could only produce detectable siderophores after 2-5 days of cultivation in the same medium. Treatment with 600 μM siderophores produced by strain YL2021 significantly suppressed sporangium germination and mycelial growth of the tested oomycetes. The 1,200 μM siderophore treatment showed no significant difference (P>0.05) in pepper Phytophthora blight control compared with the 200 μg/ml azoxystrobin treatment, reaching a control efficacy of 71.46% ± 3.17% on climate-chamber-cultivated pepper fruits and 75.59% ± 0.55% on greenhouse-grown pepper seedlings. Moreover, the control efficacy was positively correlated with increasing siderophore concentrations. In the repeated field trials, the control efficacy of the 1,200 μM siderophore treatment on cucumber downy mildew achieved 70.29% ± 0.44%, while the 600 μM siderophore treatment only reached 35.20% ± 1.67%. Collectively, these findings demonstrate that siderophores produced by B. velezensis YL2021 are promising biocontrol agents for the management of oomycete diseases.
Liu et al. (Wed,) studied this question.