In Vivo Screening of Rhizobacteria Against Phytophthora capsici and Bacillus subtilis Induced Defense Gene Expression in Chili Pepper (Capsicum annuum L.)

Phytophthora capsici (Pc) affects the production of Capsicum annuum L., particularly in susceptible genotypes like ‘California Wonder’ (CW). Beneficial bacteria offer a sustainable strategy for enhancing plant defense. Five bacterial strains, Bacillus subtilis (BMBA), Bacillus amyloliquefaciens (BMBC), Bacillus sp. (BMBH), Pseudomonas putida (BMBI), and Paenibacillus sp. (BMBP), were tested against P. capsici in chili pepper. By 11 days post-inoculation (dpi), BMBH, BMBC, BMBI and BMBP showed a trend toward accelerated symptom development, reaching severity scores of 4–5, compared with control (3.66 ± 0.55), whereas BMBA delayed disease progression (2.5 ± 0.71). At 14 dpi, control plants reached maximum severity (6.0 ± 0.0), whereas BMBA-treated plants maintained lower severity (5.0 ± 0.44). Mortality was 100% in the control, compared with 33.3% in the BMBA+Pc treatment, indicating a partial and transient protective effect. Gene expression analysis in CW roots revealed strong upregulation of PR-1, PR-5, and EAS by P. capsici. In BMBA plants, PR-1 decreased at 24 h post-inoculation (hpi), whereas EAS initially declined at 8 hpi but increase 18.5% at 24 hpi. These results indicate modulation of defense-related gene expression and reduced mortality, supporting the potential of BMBA as a partial biocontrol agent against P. capsici in chili peppers.