Aqueous methanol (water:methanol 20:80) extract of leaves (AMEL) of Indian curry leaf plant was found to be highly efficient in the rapid and controlled synthesis of stable and fluorescent monometallic (Ag and Au) and also bimetallic (Ag/Au alloy) nanoparticles with wide spectrum of task specific morphologies under sonochemical condition. The nanoparticles synthesized by the present economically viable and environment-friendly protocol showed characteristic fluorescence activity. This was exploited in the fluorometric sensing of the dithiocarbamate pesticide, Mancozeb in aqueous medium. The surface chemistry of these nanoparticles was extensively studied to understand their sensing activity. The naturally occurring flurophoric/chromophoric compounds (carbazole alkaloids and polyhydroxy flavonoid) present in AMEL instilled (in situ) strong and characteristic fluorescent behavior to the synthesized nanoparticles which opened up their utility as the fluorometric sensors and detectors for pesticides in aqueous medium.