1.1 Background of the Study
Yams (Dioscorea spp.) are among the most important staple foods in the world, especially some parts of the tropics and subtropics (Okigbo and Ogbonnaya, 2006). The role played by yam in the food economy in most West African countries cannot be over- emphasized. It is one of the most important dietary sources of energy produced within the tropics. Significantly, yam contributes to food security in Ghana and its availability on the market for a considerable part of the year helps prevent food shortages, particularly in the urban communities because it stores relatively longer than other root crops. The income generated by rural poor farmers who are engaged in yam production improves their living standards. Stored yam represents stored wealth which can be sold all-year-round by the farmer or marketer.
Cultivated yams belong to the family Dioscoreceae and to the genus Dioscorea (Coursey, 1967) the most cultivated species in Nigeria are the Dioscorea rotundata (water yam) Dioscorea alata (white yam). There are also species of wild yam growing in Nigeria whose tubers are collected for eating in times of food shortage. Yams are a valuable source of carbohydrate to the people of the Tropical and Subtropical Africa, Central and South America, parts of Asia, the Caribbean and Pacific Islands (Coursey, 1967; Adelusi and Lawanson, 1987). Dioscorea plants are large twiners and are easily propagated by means of the bulbils or portions of the tubers. Yams after peeling the tuber can be cooked in various ways by boiling and mashing, but roasting and frying are also widely used, yam in Nigeria is also processed into various staple intermediate and end product forms (Okaka and Anajekwu, 1990), which are used for direct consumption by animals, used as the basic ingredient for snacks or made into flour used for making instant puree (Coursey, 1967; Okaka and Anajekwu, 1990).
In 2005, FAO reported that Nigeria produces about 66.6% (26.6 million metric tonnes) of total world’s yam production, with Ghana producing 9.8% (3.9 million metric tonnes) every year. Ghana produces yams for local consumption and the export market. The country is a leading exporter of yam in the world (export of about 12,000 tonnes annually). In Ghana, yam is produced mostly in the Guinea-Savanna and Forest-Savanna transition zones. However, reasonable production occurs in almost all regions. White yam (D. rotundata) is much preferred to the other yam varieties and it constitutes about 80% of total yam produced in Ghana (Tetteh and Saakwa, 1994). Water yams (D. alata) are consumed when white yam becomes scarce or expensive.
There are several constraints to the yam industry in the country. Of these constraints, diseases contribute greatly to high yield losses before and after harvest. Yam plants are prone to infection by fungi, bacteria, and viruses at all stages of growth and also during storage of tubers.
Rot is a major factor limiting the post-harvest life of yams and losses can be very high. Losses due to post-harvest rot significantly affect farmers’ and traders’ income, food security and seed yams stored for planting. The quality of yam tubers are affected by rots, which makes them unappealing to consumers. In Nigeria, over 60% of white yam varieties get rotten when stored for less than six months (Adesiyan and Odihirin, 1975).
Statement of the problem
Out of the world production of over 30 million tonne per annum, Nigeria alone produces 26.6 million tons (FAO, 2005). Despite this, the demand for yam tubers in Nigeria has always exceeded it’s supply. However, it has been estimated that an average of over 25% of the yield is lost annually to diseases and pests (Arene, 1987; FAO, 1998). Okafor (1966) and Okigbo (2005) also reported that over 50% of the yam tubers produced and harvested in Nigeria are lost in storage. The disease causing agents not only reduce the quantity of yam produced, but also reduce the quality by making them unappealing to the consumers. There are many microorganisms associated with yam in Nigeria,which may include Fusarium solani, Rhizopus, stolonifera, Botrodiploidia theobrome, Geotriclum candidum (Okafor, 1966; Coursey, 1967; Adeniji, 1970) some such as Trichoderma viride and Bacillus subtilis are also effective in the control or reducing storage rot in yam (Okigbo, 2005). The use of synthetic chemicals such as sodium orthiophenylphenate and borax has been found to reduce storage rot in yam (Booth, 1974). But biological control is generally favoured as a method of plant disease management (Okigbo and Ikediugwu, 2000; Okigbo, 2002; 2005). Plant extracts have been used to control yam diseases (Okigbo and Ogbonnaya, 2006). It is important to search for a method of control of yam rot that will be affordable, durable and free of chemicals that can pollute environment. Therefore, in this report the antifungal properties of Azadiracta indica and Tridax procumbers.
Significance of the Study
Though figures are not available, observations from farmers’ fields and discussions reveal that some farmers lose as high as 70% of their stored yam to rot organisms. Because of their poor storability, farmers sell produce just after harvest to avoid losses, and this result in low income or reduced profits. This practice also affects farmers’ food security particularly in the lean season. Most rots of yam tubers are caused by pathogenic fungi such as Aspergillus flavus, Aspergillus niger, Botryodiplodia theobromae, Fusarium oxysporum, Fusarium solani, Penicillium chrysogenum, Rhizoctonia spp., Penicillium oxalicum, Trichoderma viride and Rhizopus nodosus (Okigbo and Ikediugwu, 2002; Okigbo, 2004; Aidoo, 2007). The storage methods for yam in most farming communities in Nigeria are very poor. Yams are normally stored either in dug pits, in barns or by gradual harvesting. These storage systems are prone to infections by soil-borne microorganisms (especially pathogenic fungi) that are likely to cause rot infections when storage is prolonged. Hence, this work was done with the primary aim of improving farmers’ indigenous storage practices that are cost effective and could easily be practiced to increase on-farm storage life of some white yam varieties. In addition, the primary organisms that are associated with rots of white yam tubers and their pathogenicity were studied.
Aim and Objectives of the Study
The study aims at examining the antifungal activities of leaf extracts of Azadirachta indica and Tridax procumberis on diseased yam tubers. In order to achieve the stated aim, the specific objectives of the study included;
Investigating the causal agent of fungal diseases of stored yam tubers
Isolating and identifying the causal agents of fungal diseases of Yam
To study the effectiveness of some botanical extracts in controlling yam tuber rot fungi using
Dioscrea alata (white yam) is an important staple crop. Most farmers depend on it for income however in the field as well as in the store. The storage of Dioscrea in Mkpat Enin LG.A is impeded by parasites and fungal disease attack. There is therefore, need to investigate these diseases and develop some control measures.