Potassium Silicate Promotes the Biocontrol Potential of the Rhizobacterial Strain  Bacillus velezensis Bv-22 Against Fusarium Root and Stem Rot of Cucumber

Fusarium root and stem rot (FRSR), caused by Fusarium oxysporum f. sp. radicis-cucumerinum (Forc), is an economically devastating disease affecting cucumber greenhouses. To reduce the use of fungicides, farmers need more environmentally friendly alternatives for disease control, and combining biocontrol microbes with plant nutrition becomes a promising option. This study investigated the potential of combining potassium silicate (Si), an eco-friendly agrochemical, with the native rhizobacterial strain Bacillus velezensis Bv-22 for the biocontrol of FRSR. First, strain Bv-22 and Si (3 mM) significantly inhibited the growth of Forc. Second, Si did not inhibit the growth of Bv-22 in vitro; rather, it enhanced the antagonistic potential of Bv-22 by upregulating transcript levels of antagonism-related genes. Third, greenhouse trials proved that applying Bv-22 and Si together exhibited excellent control efficacy against FRSR compared to separate treatments. Also, the decrease in cucumber plants’ growth due to the Forc-induced stress was synergistically reduced. Quantitative PCR indicated that Si, when incorporated into soils, positively influenced the rhizospheric colonization of Bv-22. Furthermore, there is a negative correlation between Bv-22 and Forc abundances in the rhizosphere soil. The results presented here suggest that Bv-22 + Si-controlled FRSR is achieved by lowering pathogen colonization in plant tissues and the rhizosphere, along with increased tissue Si content, decreased Forc-induced oxidative stress, and enhanced enzymatic and non-enzymatic defense responses in cucumber plants. Collectively, in organic cucumber-cultivated soil, a strategy depending on the enhancement of the antagonistic capability of biocontrol microbes by Si can be proposed for the integrated management of FWS.