Compatibility of biocontrol agents Pseudomonas putida PCL1760 and Trichoderma asperellum T302

Biological control is a safe alternative to chemical pesticides; however, the performance of biocontrol agents often varies with environmental and application conditions. Combining multiple biocontrol agents may improve the consistency of their protective effects but requires preliminary assessment of their compatibility. This study aimed to evaluate the compatibility of Pseudomonas putida strain PCL1760 and Trichoderma asperellum strain T302 under in vitro conditions, in a gnotobiotic system, and in the biological control of tomato Fusarium root rot. No antagonistic interactions between the strains were observed in vitro. In the gnotobiotic system, co-inoculation with T. asperellum strain T302 increased the population density of P. putida strain PCL1760 on the apical root region from 1.4 × 10⁶ to 11.5 × 10⁶ cells cm⁻¹, representing an 8.2-fold increase compared to inoculation with PCL1760 alone. The presence of strain PCL1760 did not significantly affect colonization of tomato roots by T. asperellum T302. When applied individually or in combination, both strain PCL1760 and strain T302 maintained their biocontrol efficacy against Fusarium root rot, reducing the disease index from 0.68 in control to 0.13–0.26, with no statistically significant differences among treatments with the biocontrol agents. Combined inoculation of the two strains also effectively suppressed root rot development, decreasing the disease index to 0.23. These findings demonstrate the compatibility of P. putida PCL1760 and T. asperellum T302 and highlight the potential of their combined application for the management of tomato root rot diseases.