Management of downy mildew ( Peronospora destructor) of onion ( Allium cepa)

Abstract

Studies were conducted during winter seasons (rabi) of 1990–1991, 1991–1992, 1992–1993, 1993–1994 to evaluate the effect of heat treatment of bulbs and fungicide sprays on the management of downy mildew ( Peronospora destructor [Berk.] Casp.) of onion ( Allium cepa L.). Heat treatment of bulbs at 35, 40 and 45 °C for 4, 8, 12, 16 and 24 h reduced the disease significantly. Heat treatment of bulbs at 40 and 45 °C for more than 12 h although resulting in a significant suppressive effect on disease severity, also retarded plant growth drastically. Amongst the fungicides evaluated through bulb dip, metalaxyl (Apron 35 SD and Ridomil MZ 72 WP) effectively reduced the initial inoculum and delayed the appearance of disease. In vitro evaluation of thirteen fungicides through inhibition of sporangial germination revealed that Peronospora destructor showed more sensitivity to metalaxyl (Ridomil MZ) and least to fosetyl-A1 with ED 50 values of 2.8 μg ml −1 and 58.6 μg ml −1, respectively. Amongst the co-ordinated products of acylalanines tested, Peronospora destructor was more sensitive to metalaxyl + mancozeb followed by oxadixyl + copper oxychloride and least to benalaxyl + mancozeb with ED 50 values of 2.6, 4.5 and 8.3 μg ml −1, respectively. Similarly, out of nine fungicides evaluated in the field as foliar sprays, three 14-day sprays of metalaxyl + mancozeb (0.3%) effectively controlled the disease. Integration of heat treatment (35 °C for 12 h), bulb dip in metalaxyl (3.5 g kg −1 bulbs for 1 h) and three 14-day sprays of metalaxyl + mancozeb (0.2%) were most effective and resulted in more than 85% control of onion downy mildew.