Functional Screening of Rhizobacterial Isolates of Bacillus subtilis from Cacao Agroecosystems for Plant Growth-Promotion and Antagonism Against Moniliophthora roreri

The application of plant growth-promoting bacteria (PGPB) offers a sustainable alternative for improving cacao (Theobroma cacao L.) health and productivity. This study evaluated the relative growth rate (RGR) of cacao fruits under four field treatments for 30 days: PGPB alone, PGPB plus the pathogen Moniliophthora roreri, the pathogen alone, and an untreated control. Fruits inoculated only with M. roreri exhibited the highest RGR (0.0055 ± 0.002 day−1), significantly higher than the control (0.0030 ± 0.001 day−1; p≤0.05). The combined treatment (PGPB + pathogen) showed intermediate values (0.0050 ± 0.0018 day−1), while PGPB alone presented the lowest RGR (0.0037 ± 0.0014 day−1). These results indicate that pathogen inoculation may lead to transient fruit hypertrophy, while bacterial inoculation alone or in combination moderates this effect. The observed variability among treatments likely reflects the influence of uncontrolled environmental factors (e.g., humidity, temperature, and soil heterogeneity) and natural pathogen presence in the field, which may have masked or modulated the physiological effects of PGPB during the short 30-day observation period. Overall, this work highlights the complexity of plant–microbe–pathogen interactions under field conditions and underscores the need for longer-term, multi-season trials to validate the effectiveness of PGPB-based strategies in cacao agroecosystems.