Screening of antibacterial activity of Goniothalamus calvicarpa extracts against Xanthomonas axonopodis pv. citri in vitro

Citrus canker caused by Xanthomonas axonopodis pv. citri (Xac) is a serious disease in many parts of the world and is difficult to control. The use of copper-based pesticides is becoming a concern due to the accumulation of heavy metals in orchard soils and more benign treatment methods are needed. Eighteen Thai native plant extracts were screened for antibacterial activity against a Thai isolate of Xac and it was found that ethanolic extracts of Goniothalamus calvicarpa leaves showed the strongest antibacterial activity against Xac in vitro. The G. calvicarpa extracts were then sequentially dissolved with hexane, ethyl acetate and methanol and retested. A 70% aqueous ethanol extract and a methanol soluble extract produced strong inhibition zones against Xac. Although thin layer chromatographic profiles revealed the likely presence of flavonoids in the biologically active extracts of G. calvicarpa, the active compounds have yet to be identified. Work is proceeding to determine whether specific extracts of G. calvicarpa have biological activity against citrus canker in the field