A Study of Fungal Diseases of Onion (Allium Cepa L.) with Special Reference to Purple Blotch

Abstract

Onion (Allium cepa L.) belonging to the family Alliaceae is one of the most important spice as well as vegetable crops of the world. Its origin is in Iran, Pakistan and the mountaineous region to the north of these countries (Purseglove, 1972). The primary center of origin of onion is central Asia and a secondary center for larger types of onion is Mediterranean regions (Mc Collum, 1976). Its use is neither limited to any climate nor associated with any nationality and it is probably as widely grown as any cultivated crop. The leading onion producing countries, in order of production are the United States of America, Japan, Spain, Egypt, Turkey and Italy (Jones & Mann, 1963). Onion is relatively high in food energy and is rich in calcium and riboflavin. It can be used as medicine. This crop can be grown over a wide range of conditions from sea level up to the elevation of about 1828 metre and from warm summer temperature of plains to cold weather down to a temperature of about l 6°C. However it is not suited to heavy rainfall and the rainfall should not exceed 762 mm during cultivating season (Aiyer, 1980). Onion grows in almost all districts of Bangladesh but widely cultivated in the greater districts of Faridpur, Dhaka, Rajshahi, Comilla. The onion cultivating area of Bangladesh covers 33000 ha. of land, total production 1,31,000 mt and total yield 3948 kg/ha (FAO, 2000). The local cultivars namely, Taherpuri, Jhitka, Salta, Kalasnagari and Faridpur bhati are mainly grown in the country (Rahim and Siddique, 1991). The crop suffers from many diseases (Meah and Khan, 1987; Ahmed and Hossain, 1985; Munoz et. al., 1984). Various kinds of diseases are found in the field as well as in the storage. About 45 different fungal and 5 bacterial species are reported to cause diseases of onion in the field. Five viral entities are also responsible for onion diseases in the field conditions, the percentage of infection and/or the extent ofloss due to diseases may vary from 0-100% (Harrison, 1954 and Vovk, 1959). A variety of disease and disorders affect onions and related crops in Ontario. Most of the diseases are caused by fungi or bacteria whereas disorders may be caused by adverse weather, air pollutions, soil conditions, nutritional imbalances and pest control products, sometimes several diseases and/or disorders can be present at the same time (J. Chaput, 1995). Purple blotch is one of the field disease caused by A/tern.aria porri (Ellis) Cif. is noted as the major disease in many onion growing countries including Bangladesh (CasteIJanos-Linares et. al., 1988; Rahman et. al., 1984; Ashrafuzzaman, 1977). Ahmed and Goyal (1988) mentioned that purple blotch, incited by A. porri is a major problem of onion (A. cepa) in India. The disease is a serious problem for seed production in tropical countries like Bangladesh (Rahman et. al., 1988; Anonymous, 1985). Initially symptoms appear on leaves or seed stalk as small water-soaked lesion that quickly develops white centers. As lesions enlarge, they become zonate and brown to purple, and are surrounded by a yellow zone. The lesion may be covered with brown to dark grey fruiting structures of the fungus. A few large or seed stalk and tissues distal to the lesions will die (Schwartz and Mohan, 1994). Purple blotch of onion causes breaking of floral stalks and thus the seed production is seriously affected. Damage of foliage and breaking of floral stalks due to the disease resulting in failure of seed production are common (Munoz et. al., 1984; Asrafuzzaman and Ahmed, 1976). The infected seed stalks break at the point where the lesion is developed (Singh, 1987). Under favorable environmental conditions for the disease complete failure of the crop may take place if appropriate control measures are not followed (Sharma, 1986).................................................