Comparative Efficacy of Trichoderma harzianum and Pseudomonas putida in Mitigating Verticillium Wilt of Brassica napus

Verticillium dahliae is a devastating pathogen that causes Verticillium wilt of Brassica napus. This study investigated the application of Pseudomonas putida and Trichoderma harzianum as biocontrol agents against V. dahliae, and their impact on the growth of B. napus was studied. In vitro, dual-culture assays revealed significant mycelial growth inhibition of V. dahliae by both T. harzianum (96%) and P. putida (83%). For in vivo studies, the fungus (T. harzianum) was directly introduced into V. dahliae-infested soil, and B. napus seeds were sown (T + VD). For the inoculation with bacteria, seeds of B. napus were primed with P. putida and sown in V. dahliae-infested soil (P + VD). Both treatments significantly improved physiological parameters (seed germination, root length, shoot length, relative water content and chlorophyll contents) and decreased relative electrolyte leakage and oxidative burst (malondialdehyde, H2O2). These treatments also increased the concentrations of osmolytes (proline and sugar content) and enhanced enzymatic activities (catalase, peroxidase and superoxide dismutase). After 21 days of germination, control and treated plants (inoculated only with T. harzianum or P. putida) displayed no wilting symptoms. V. dahliae-inoculated plants displayed severe symptoms of wilting, and the lower leaves turned yellow and showed curling. Treatment T + VD revealed almost no disease symptoms, while treatment P + VD exhibited negligible disease symptoms. Histological analysis revealed decreased mycelial colonisation of V. dahliae in the vascular system of B. napus in both T + VD and P + VD treatments. These findings successfully demonstrated the significance of both biocontrol agents for controlling the Verticillium wilt of B. napus.