Evaluation of the Efficacy of Eucalyptus camaldulensis Leaf Extract as a Biocontrol Agent for Managing Bacterial Wilt Disease in Solanaceous Crops

Abstract: Background: Ralstonia solanacearum is a soil-borne bacterium that causes bacterial wilt, a devastating disease in affecting vegetable crops world, and necessitating sustainable biological alternatives to conventional chemical pesticides. Objective: This study amid to critically assess both in vitro and in planta efficacy of Eucalyptus camaldulensis (E. camaldulensis) leaf extract against R. solanacearum. Methodology: Ethanolic extracts were made through a process of maceration and profiled in terms of phenolic and flavonoid content. Antibacterial activity was determined by MIC, MBC, synergy by checkerboard, and biofilm inhibition assays and in planta trials were done on tomato seedlings. Results: The extraction yield was 18.5% (w/w), total phenolic (125.3 mg GAE/g) and flavonoid (78.9 mg QE/g) content which mainly comprised gallic acid and quercetin. In vitro tests revealed have shown that the MIC was 0.625 mg/mL and MBC was 1.25 mg/mL. A synergistic effect was observed (FICi=0.5) in combination with streptomycin. Additionally, the extract reduced biofilm formation by up to 90.1% at the MBC. In planta, the Disease Severity Index was significantly reduced in the treated group (85.6% of the infected) to 19.5% at MBC and growth parameters (height and weight) were restored to normal, with no phytotoxicity evident. The extract was also able to disrupt cell membranes of bacteria, as well as inhibit key enzymes, which helped in its bactericidal effect. Conclusion: E. camaldulensis leaf extract is an effective, environmentally-friendly biopesticide with a high potential to manage bacterial wilt and activate crop growth to be used as an integrated pest control approach.