Ten newly synthesized organophosphorus derivatives containing substituted chalcones and substituted chalcone semicarbazones were tested for their antifungal efficacy against Colletotrichum falcatum, Fusarium oxysporum, Curvularia pallescens (all sugarcane pathogens). The O,O‐diethylphosphate derivatives containing 2‐chlorochalcone and 2‐chlorochalcone semicarbazone exhibited 70‐85% mycelial inhibition against all the test fungi at 1000 ppm. The screening results were correlated with structural features of the tested compounds.