CHAPTER ONE

 INTRODUCTION

One of the most pressing problems facing the developing nations is food scarcity. Salami and Popoola, (2007); Kana et al., (2012), reported that nearly one billion people are challenged by severe hunger in these nations of which 10% are reported dead from hunger-related complications. In Nigeria, the root crops that are majorly important include cassava, sweet potatoes, sweet potatoes, and Irish potatoes. After harvest, these tubers suffer losses that result from physical, physiological or pathological factors or the combination of the three factors (Opara and Agugo, 2014).

Food security and sustainability is one of the ways of addressing this problem of food scarcity and shortage due to activities of microorganisms after harvesting. In a FAO/WHO report of (2012), food security was defined as a situation in which all people at all times have both physical and economic access to adequate and nutritious food for an active and healthy life; the manner in which the food is produced, preserved and distributed are in consideration of the natural processes of the earth and thus sustainable thus reducing spoilage, scarcity, malnutrition and poverty.

Sweet potato has an enormous potential to be an effective and economic source of food energy (Oyeyipo, 2012). It is an important source of antioxidants and anthocyanidins (Oladoye et al., 2013). It can be incorporated with sweet potatoes to make Amala and pounded sweet potatoes. The production of sweet potato, especially vegetable potato, is seriously affected by rots. According to the survey carried out in Iran, 10% pre-harvest and 20% post-harvest rots occurred in sweet potato (Bidarigh et al., 2012). These rots constitute major impediments to the drives for food security in Nigeria.

Sweet potatoes have been described as having thin, delicate skin that is easily damaged by cuts and abrasion during harvesting, transportation or distribution. Striking the roots with harvesting equipment or dropping them into containers injures their skin. Bruises and abrasions must be kept at a minimum degree to avoid microbial attack. The sweet potato may be cut or bruised if they are placed in containers having sharp edges or roughly hauled or handled and this may give rise to microbial infestation (Rupsa et al., 2017).

It has been reported by some workers that the microorganisms that are responsible for the spoilage of sweet potato produce extra-cellular enzymes such as amylases, celluloses, polygalactunases, xylanases, and pectin-methyl esterases and these enzymes degrade the cell wall components of produce that are susceptible leading in some cases to emission of offensive odour and water (Salami and Popoola, 2007; Amadioha, 2012; Oladoye et al., 2013).

Several rots that affect sweet potato after harvesting have been substantially reported (Onifade et al., 2004; Oyeyipo, 2012) substantially reported several rots that affect sweet potato after harvesting. These rots are linked to a number of factors that are physiological, physical, and microbiological. During harvesting, storage or transportation, mechanical damage occurs and this damage has been known to be predisposing tuber to spoilage and storage rots (Oyeyipo, 2012). Contamination through natural openings or wounds by the pathogenic microorganism is considered the most critical factor in tuber decay (Udo et al., 2000).

According to Arya (2010). Post-harvest pathogens can be divided into those that penetrate the produce on-farm, but develop in their tissues only after harvest, during storage or marketing on one hand; and those that initiate penetration and colonization during or after harvest on the other. Enormous postharvest losses have been attributed to fungal deteriorations (Okigbo, 2002, 2003; Shukla et al., 2012; Khatoon et al., 2012; 2016).

         1.2 STATEMENT OF PROBLEM

Fungal pathogens cause spoilage and post-harvest spoilage of sweet potato by producing various types of mycotoxins. Mycotoxins are low molecular weight toxic secondary metabolites from fungal species. These mycotoxins are dangerous in small quantities and present extreme toxicity due to their heat resistivity (Okigbo, 2004; Shukla et al., 2012). Fumonisins, aflatoxins, ochratoxin A, zearalenone, and deoxynivalenol are mycotoxins of most agricultural importance (Bankole and Adebanjo, 2003).

Several fungi have been implicated in the spoilage of sweet potato. In (2002), Onuegbu reported Penicillium sp. Ceratocystis fimbriata, Aspergillus niger, Diaporthe batatalis, and Aspergillus flavus as fungi responsible for the post-harvest decay of sweet potato.

Oyewale (2006), reported fungi that were associated with post-harvest fungal rot of sweet potato and they include Motierella ramanniana, Rhizopus stolonifer, Mucor pusiluss, Botrytis cinerea, Erysiphe polygoni and Aspergillus flavus. During the post-harvest storage of sweet potato, Aspergillus flavus is the most dominant fungal species followed by Aspergillus niger, Rhizopus stolonifer, Trichoderma viride, Fusarium oxysporum, Penicillium digitatum, Cladosporium herbarum, and Aspergillus ochraceus.

The black rot of sweet potato is caused by Ceratocystis fimbriate (Lewthwaite et al., 2011). The soft rot disease of sweet potato storage roots and post-harvest storage rot is caused by Fusarium solani and Macrophomina phaseolina (Washington, 2013). In some instances, bacteria (Pseudomonas and Erwinia) may play associative roles in rots of vegetables. Only 36% of postharvest rots of vegetables are attributed to bacteria (Agriculture Information Bank, 2013). Staphylococcus acuri and Rabniell sp. were associated with spoilage of vegetable sweet potato based on the DNA sequencing studies in Southwestern Nigeria (Oladoye et al., 2013).

1.3 RESEARCH OBJECTIVES

The specific objectives were to:

1        To document farmers’ knowledge on field and post-harvest diseases of sweet potatoes.

2        To establish the pathogenicity of fungal organisms associated with tuber rots of white sweet potatoes varieties.

3        To study the effectiveness of some chemical fungicides in reducing sweet potatoes rot in storage.

4        To study the effectiveness of some botanical extracts in controlling sweet potat oes rot fungi.

1.4 RESEARCH QUESTIONS

1        What is farmers’ knowledge on field and post-harvest diseases of sweet potatoes.

2        What is the pathogenicity of fungal organisms associated with tuber rots of white sweet potatoes varieties.

3        What is the effectiveness of some chemical fungicides in reducing sweet potatoes rot in storage.

4        What is the effectiveness of some botanical extracts in controlling sweet potat oes rot fungi.

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