Chromolaena odorata is an invasive weed from the Neotropics, but we have recently found that an aqueous extract of the plant could control important diseases in rice under both controlled and field conditions by application to the seeds before sowing or by spraying on the leaves. Indeed, significant control was obtained of sheath blight (Rhizoctonia solani), brown spot (Bipolaris oryzae), rice blast (Pyricularia oryzae) and bacterial blight (Xanthomonas oryzae pv. oryzae) [Khoa et al. (2011). Phytopathology 101:231-240]. Expression of different defence-related genes was studied in plants inoculated with R. solani and included genes encoding PR-proteins [?-1,3-glucanase (PR-2), chitinase (PR-3), thaumatin-like protein (PR-5), peroxidase (PR-9), PR-1b and PBZ1 (both PR-1)] and genes encoding two enzymes involved in the hydrogen peroxide metabolism (superoxide dismutase and catalase). Application of the extract prior to pathogen inoculation resulted in elevated transcript levels of the defence-related genes compared to control plants pre-treated with water. This resulted in decreased fungal growth and reduced formation of infection cushions of R. solani. The results indicate that the protection exerted by the extract involves induced resistance since defence responses were enhanced in plants treated with the extract followed by inoculation with the pathogen compared to control plants pre-treated with water. To identify the active compound(s) responsible for the disease-reducing effect, the extract has been fractioned using group separation and analysed by capillary electrophoresis. The sub-fractions are currently being subjected to NMR analyses for structure elucidation of the active compound(s).