CHAPTER ONE
INTRODUCTION
Onion (Allium cepa L.) belongs to the family Alliaceae, it is a monocotyledonous, cross-pollinated crop cultivated mainly as a cool season vegetable (Jilani and Ghafoor, 2003). The crop is grown in most regions of the world, with China, India and United States of America being the largest producers (FAOSTAT, 2014). In Nigeria, 1.32 x 106 mt are produced on 42,000 hectares of land which makes it ranks fourth after Egypt, Algeria and Morocco in Africa (FAOSTAT, 2014). Profitable production of onions is favoured by the inclement conditions obtained around the northwestern region of Nigeria. Reports of the National Agricultural Extension and Rural Liaison Services and Federal Department of Agricultural Extension (FDAE) in 2015 showed that the rich alluvial deposits along the River Niger provide suitable soil for its cultivation. The 119.10 x 103 and 109.90 x 103 mt estimates at Sokoto and Kebbi States respectively in 2014 are highest throughout the country.
World Food Programme reported global usage of onions as food and condiment. Its unique flavour and odour makes it indispensible in preparation of many traditional dishes. The numerous ways in which it is prepared from boiling, frying, stewing, baking, pickling, to eating raw make it a versatile food source (WFP, 2014).
Production of onion as a crop is however, constrained by numerous abiotic and biotic factors. In Nigeria, abiotic factors include nutrient deficient soil, shortage of supplementary irrigation facilities, inadequate inputs and poorly defined post-harvest storage and value addition strategies (Ibrahim, 2014). Pests and diseases have been reported to reduce yield and quality of produce (Currah and Felicity, 1990). Major insect pests include onion thrips, cutworms, leaf miners, onion maggot (Hylemya antique Meig.) and several species of mites decimate the crop. Economic diseases of the crop induced by bacteria, nematodes, viruses, parasitic flowering plants and fungi in the northwestern Nigeria agro-ecology have been listed by Emechebe et al. (1980). Some of these diseases include bacterial soft rot, Iris Yellow Spot Virus disease transmitted by thrips (Thrips tabaci Lind); root knot nematode disease caused by Meloidogyne spp. Colletotrichum gleoesporioides causes the onion twister disease which is very prevalent in northwest Nigeria, Alternaria porri causes purple blotch; Peronospora destructor causes downy mildew disease and neck rot disease caused by Botrytis allii are other fungal diseases of this crop. These pathogens cause yield reductions of 50-70 % which discourage production by the farmers.
A relatively new disease of onion was reported by the Kebbi State Agricultural and Rural Development Authority (KARDA) during the 2009 Annual Cropping Scheme Meeting of the Institute for Agricultural Research, Ahmadu Bello University, Zaria. Symptoms ascribed to the disease were rapid protrusion and clock-wise curl of main leaf, die-back and eventual wilting of the whole plant. The plant can be infected at all stages of growth leading to 100 % loss of the crop as reported by the farmers and it discouraged them from continued production. Adopted strategies to manage the disease by farmers have not been successful. The local name „Danzazzalau‟ given to the condition reflects the sudden protrusion and curling of main leaf that the farmers observed (IAR Cropping Scheme Report, 2009).
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