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).
Justification of the Study
Onion is a highly valued vegetable crop possessing culinary and medicinal
values. Major constraints to sustainable production of the crop include high
incidence of pests and diseases, nutrient deficient soils; inadequate
irrigation facilities, expensive inputs and marketing that gives marginal
returns to the farmers. Kebbi State produces 135,000 mt of onion with an
average yield of 20-35 mt/ha annually, thus making
Kebbi State the second largest onion farming community in Nigeria after Sokoto State (Usman, 2013). Onion production in Nigeria has been declining due to pests infestation (NAERLS and FDAE, 2015) and Kebbi State particularly has had onion yield stagnated at 109 x 103 mt from 2012 to 2014 due to the incidence of a devastating disease locally called „Danzazzalau‟. Total loss especially at seedling stage has been claimed by farmers and all measures to alleviate the menace have proved abortive. When infected stand was uprooted, basal stem plate was rotted and central crown sloughed off. The disease was said to be severe where crop suffers environmental stress and all stages of onion are affected. The need to determine main cause and condition(s) that favour the disease is very important. In view of this, a study to identify the causal organism(s) inducing the disease and proffer management strategies against the disease was carried out.
