CHAPTER ONE

 INTRODUCTION

One of the most pressing problems facing 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 majorly important root crops include cassava, sweet potatoes, sweet potatoes, and Irish potatoes. After harvest, these tubers suffer losses that result from physical, physiological, or pathological factors or a combination of the three factors (Opara and Agugo, 2014).

Food security and sustainability are one of the ways of addressing this problem of food scarcity and shortage due to the activities of microorganisms after harvesting. In an FAO/WHO report (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; how the food is produced, preserved and distributed are in consideration of the natural processes of the earth and thus sustainable 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 potatoes, especially vegetable potatoes, is seriously affected by rots. According to a survey carried out in Iran, 10% of pre-harvest and 20% of post-harvest rots occurred in sweet potatoes (Bidarigh et al., 2012). These rots constitute major impediments to the drive 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 potatoes produce extra-cellular enzymes such as amylases, celluloses, polygalacturonase, xylanases, and pectin-methyl esterases and these enzymes degrade the cell wall components of produce that are susceptible leading in some cases to the emission of offensive odor 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 several factors that are physiological, physical, and microbiological. During harvesting, storage, or transportation, mechanical damage occurs and this damage has been known to predispose tuber to spoilage and storage rots (Oyeyipo, 2012). Contamination through natural openings or wounds by pathogenic microorganisms 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 deterioration (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 potatoes 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 potatoes. 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 potatoes.

Oyewale (2006), reported fungi that were associated with post-harvest fungal rot of sweet potato and they include Motierella ramanniana, Rhizopus stolonifer, Mucor business, Botrytis cinerea, Erysiphe polygons, 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 herbarium, and Aspergillus ochraceus.

The black rot of sweet potatoes 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 acute and Rabniell sp. were associated with spoilage of vegetable sweet potatoes based on the DNA sequencing studies in Southwestern Nigeria (Oladoye et al., 2013).

1.3 RESEARCH OBJECTIVES

The specific objectives were to:

• To document farmers’ knowledge of field and post-harvest diseases of sweet potatoes.
• To establish the pathogenicity of fungal organisms associated with tuber rots of white sweet potato varieties.
• To study the effectiveness of some chemical fungicides in reducing sweet potato rot in storage.
• To study the effectiveness of some botanical extracts in controlling sweet potato oes rot fungi.

1.4 RESEARCH QUESTIONS

• What is farmers’ knowledge of field and post-harvest diseases of sweet potatoes?
• What is the pathogenicity of fungal organisms associated with tuber rots of white sweet potato varieties?
• What is the effectiveness of some chemical fungicides in reducing sweet potato rot in storage?
• What is the effectiveness of some botanical extracts in controlling sweet potato oes rot fungi?

MICROORGANISMS ASSOCIATED WITH THE SPOILAGE OF POST-HARVEST SWEET POTATOES (IPOMOEA BATATAS). GET MORE SCIENCE LABORATORY TECHNOLOGY PROJECT TOPICS