EPIDEMIOLOGICAL SURVEY ON THE INCIDENCE AND SEVERITY OF PURPLE BLOTCH OF ONION IN BANGLADESH

Abstract

An epidemiological survey on the incidence and severity of purple blotch of onion was conducted during ·onion growing seasons of the year 2007-08, 2008-09 and 2009-10 (Winter- September to March) in the major onion growing areas viz. Vasanchar (Faridpur), Jitka (Manikgonj), Sayedpur (Comilla), Nandina (Jamalpur), Kesobpur (Jessore), BARI (Gazipur), SAU campus (Dhaka), Hathazari (Chittagong), Dhamrai (Dhaka), Taherpur (Rajsbahi) of the country, Data were collected on % Plant infection, % Leaf infection, % LAD ( Leaf Area Diseased ), % SAD (Stalk Area Diseased), Soil pH, Soil temperature, Soil moisture, Air temperature and Air humidity (% RH). The disease incidence and severity of purple blotch of onion varied with locations, and environmental factors. The plant infection and leaf infection ranged from 87 % - 36 % and 62 % - 20 %, respectively while the percent leaf area diseased (% LAD) and percent stalk area diseased (% SAD) ranged from 36.67 % - '9.00 % and 39.00 % - 11.00 %, respectively. The highest disease incidence and severity during the consecutive 3 growing seasons of the year 2007-08,2008-09 and 2009-10 was recorded at Sayedpur (Comilla) where average air temperature, air humidity and pH were 24.08oC, 75.17% and 4.72 respectively. The lowest incidence and severity was recorded at BARI (Gazipur) where average air temperature, air humidity and pH were 23.990C, 85.44% and 5.30 respectively. Soil pH (4.6 -5.17), air temperature (23°C) and air humidity (75 % ± 2) had influential effect on the incidence;3l1d severity of purple blotch of onion.