Bacillus Amyloliquefaciens IKMM and Zinc Nanoparticles as Biocontrol Candidate Induce the Systemic Resistance by Producing Antioxidants in Tomato Plants Challenged with Early Blight Pathogen

Early blight disease of tomato is a devastating disease causing severe yield losses. The chemical control of this disease has become a challenge due to emerging fungicide resistance in Alternaria solani. The present study investigates the biocontrol potential of Bacillus amyloliquefaciens IKMM strain and zinc nanoparticles (Zn-NPs) against A. solani. The in vitro results demonstrated that B. amyloliquefaciens IKMM strain displayed strong mycelial growth inhibition (88.76%) of A. solani followed by the control. Various concentrations (0.1 ppm, 1 ppm 10 ppm, 50 ppm, 100 ppm) of Zn-NPs and conventional fungicide mancozeb showed considerable in vitro mycelial growth reduction (82.98 and 76.55% respectively) at higher concentration (100 ppm) with EC₅₀ (4.56; R² 0.945) for Zn-NPs and fungicide (8.8937; R² 0.966351). In greenhouse experiments, the plants treated with B. amyloliquefaciens IKMM strain, Zn-NPs and fungicide showed lower disease severity (13.33%, 16.35% and 25.58% respectively) followed by the infected control (69.24%) and ultimately increased the biomass accumulation. The flavonoid contents in B. amyloliquefaciens treated leaves were higher after 72 h as compared to Zn-NPs and fungicide treated plants whereas, after 48 h of inoculation, higher total phenol contents were recorded. The antioxidants enzymes viz., peroxidase (PO), phenlylalanine ammonialyase (PAL) and polyphenol oxidase (PPO) demonstrated significant increase after 72 h in B. amyloliquefaciens treated plants followed by control. The inoculation of B. amyloliquefaciens IKMM strain and Zn-NPs induced the accumulation of reactive oxygen species (ROS) and down-regulated the defense related genes (PO, PPO and PAL) in tomato plants by activating the defense mechanism after 48 h of inoculation. Under field experiment, the plants treated with B. amyloliquefaciens IKMM strain, fungicide and Zn-NPs caused the reduction in disease severity in both seasons under natural infection conditions. The fruit yield in B. amyloliquefaciens IKMM strain treated plants was relatively higher in both seasons than fungicide and Zn-NPs treated plants. The results herein presented indicate that indigenous B. amyloliquefaciens IKMM strain along with Zn-NPs could be considered as putative biocontrol approach to combat this challenging pathogen and will assist to develop biofungicides against A. solani. Future research should be focused on the use of B. amyloliquefaciens and Zn-NPs as a combination to develop batter management strategies for this pathogen in sustainable agricultural production system.