Abstract

Purple blotch of onion caused by Alternaria porri is an economically important disease. The field trials were conducted during three crop seasons in order to study the effect of surfactant on the efficacy of fungicides because the use of fungicides for the control of purple blotch of onion is less promising due to the waxy and smooth leaf surface, which does not allow the fungicides to spread and adhere properly on the leaves. Chlorothalonil 75 WP (1.5 g/l) and mancozeb 75 (2.0 g/l) when sprayed along with surfactant (0.5 ml/l) showed minimum per cent disease index (5.38 and 7.96%, respectively) and disease incidence (11.80 and 15.47%, respectively) at 15 days after third spray in comparison to chlorothalonil and mancozeb alone where the disease index was 8.93 and 11.52%, respectively, and disease incidence was 15.92 and 20.38%, respectively. The results also indicated that the efficacy of fungicides was significantly increased by adding surfactant from 23.69 to 39.75 and 19.87 to 25.88% in minimizing disease index and disease incidence, respectively, in different treatments when compared with the fungicides without surfactant. Both chlorothalonil and mancozeb along with surfactant showed minimum disease severity (9.72 and 12.36%, respectively) at 15 days after third spray and highest bulb yield (237.70 and 231.96 q/ha, respectively) as compared to both the fungicides sprayed alone. The per cent increase in bulb yield over control ranged from 5.57 to 27.60 in different treatments. The fungicides along with surfactant reduced the per cent disease severity (16.15–24.12) and increased the bulb yield (5.31–9.81%) in different treatments when compared with the fungicides without surfactant. The study clearly demonstrated the potential of surfactant to improve the efficacy of fungicides against purple blotch of onion.