ISOLATION OF MICROORGANISMS ASSOCIATED WITH DETERIORATION OF SWEET POTATO (IPOMOEA BATATAS(L.) LAM) DURING STORAGE

ABSTRACT

The microorganisms responsible for spoilage in white sweet potato roots (TIS/87/0087 variety) were isolated, characterized and identified. The isolated pathogenic fungi were Aspergillus sp, Botryodiplodia sp Rhizopus sp, Fusarium sp. Of these pathogens, Rhizopus sp has the highest percentage occurrence of (27.3%) followed by Aspergillus niger (24.24%) and Aspergillus flavus (12.12%) has the least frequency percentage. The pathogencity test revealed that Rhizopus sp and Botryodiplodia sp is the most virulent and Aspergillus flavus is the least virulent fungus. The only isolated bacteria pathogen Erwinia sp (3.0mm) had rod shape and was Gram negative, proper storage facilities, essential control measures is therefore important in reducing the loss of sweet potato tubers during storage.

TABLE OF CONTENTS

Title Page i

Certification ii

Dedication iii

Acknowledgements iv

Table of Contents v

List of Tables ix

Abstract x

CHAPTER ONE

1.0 Introduction 1

1.1 Aims and Objective 3

CHAPTER TWO

2.0 Literature Review 4

2.1 The Sweet Potato (Ipomoea Batatas) 4

2.1.1 The Origin 5

2.1.2 Cultivation 5

2.1.3 Nutrient Content 7

2.2 Microbial Spoilage of Potatoes 9

2.2.1 Rhizopus sp 9

2.2.2 Sclerotinia sclerotiorum 10

2.2.3 Botrytis cinerea  11

2.2.4 Fusarium oxysporum  12

2.2.5 Alternaria solani  13

2.2.6 Ralstonia solanacerum  14

2.3 Disease and Pests 15

2.3.1 Fungal Diseases 15

2.3.2 Nematodes 16

2.3.3 Insects 17

2.4 Harvesting, Curing and Storage 18

2.4.1 Harvesting 18

CHAPTER THREE

3.0 Materials and Methods 20

3.1 Materials 20

3.2 Methods 20

3.2.1 Sample Collection 20

3.2.2 Preparation of Media 20

3.2.3 Isolation of Bacteria from Post Harvest

Deterioration of Sweet Potato 20

3.2.4 Isolation of Fungi from Post harvest Deterioration of Sweet Potato 21

3.2.5 Subculturing/Purification of Bacteria Isolates 21

3.2.5.1 Incubation of Culture Media 21

3.2.6 Subculturing/purification of fungal isolates 21

3.2.6.1 Incubation of culture media 21

3.2.7 Characterization and identification of bacterial isolates 22

3.2.7.1 Gram staining method 22

3.2.7.2 Biochemical Test 22

3.2.7.3 Coagulase test 22

3.2.7.4 Oxidase test 22

3.2.7.5 Indole test 22

3.2.7.6 Urease test 23

3.2.7.7 Methyl red test (MR) 23

3.2.7.8 Vogesproskauer test 23

3.2.7.9 Citrate utilization 23

3.2.7.10 Sugar fermentation test 23

3.2.7.11 Catalase production test 23

3.2.7.12 Hydrogen sulfide () production test 24

3.2.7.13 Motility test 24

3.2.7.14 Nitrate test 24

3.3 Characterization and Identification of Fungal Isolates 24

3.3.1 Pathogenicity Test 24

3.3.2 Confirmation of Isolates used for Pathogenicity Test 25

CHAPTER FOUR

4.0 Results 26

CHAPTER FIVE

5.0 Discussion, Conclusion and Recommendation 33

5.1 Discussion 33

5.2 Conclusion 34

5.3 Recommendations 34

REFERENCE 35

APPENDIXES       39

LIST OF TABLES

Table Title      Page

2.1 Nutritional composition of Sweet Potatoes 8  

4.1: Fungi isolated from the spoilt sweet potato 26

4.2: Percentage occurrence of Fungi isolated from

 the spoilt sweet potato 27

4.3: The Fungi pathogenicity test on spoilt sweet potato. 28

4.4: Macroscopic and microscopic characterization

of isolated microorganism. 29

4.5: The bacteria pathogenicity test on sweet potato. 30

4.6: Biochemical and microscopic characteristic of bacteria isolates. 31

CHAPTER ONE

1.0 INTRODUCTION

Important root crop and of much nourishment to South East region of Nigeria includes cassava, yam, sweet potatoes and Irish potatoes. These tubers suffer from post, harvest losses resulting from physical, physiological or pathological factors or the combination of all the three. The late blight diseases of potatoes in Ireland in 1845 caused by Phythophtorainfestans, was responsible for the famous Irish famine of 1845-1846; resulted in the death and migration of millions of people from the country to other countries such as United State of America (Onuegbu, 2002).

The sweet potato (Ipomoea batatas (L.) Lam) is a dicotyledonous plant that belongs to the family Convolvulaceae. It is large, starchy, sweet tasting tuberous root. The young leaves and shoots are sometimes eaten on greens. There are approximately 50 genera and more than 1000 species.  recognized two species in the section batatas, which includes the sweet potatoes and subsequently, three species, have been added and one removed. The closest with relative of the sweet potato appears to be Ipomoea trifida and Ipomoea tabascan. Sweet potatoes are a creeping plant and the only economical species of the family Convolvulcae (Onuegbu, 2002).

Despite its nutritional importance, the tubers have short storage life, generally, less than four weeks in the tropics. Their skin is easily damaged during harvest and post-harvest handling leaving the crops highly perishable in microbial spoilage (FAO; 2008). Potato is an important source of dietary fiber and in Great Britain; it contributes 15% of the intake of food. The starch tuberous roots are the major source of food for millions of persons; the leaves are also useful source of vegetable in some countries.

The plant originated in tropical America and they have spread throughout the world. Sweet potato is ranked seventh in the world’s production after wheat, maize, rice, Irish potatoes, barely and cassava (FAO, 2008). Sweet potatoes are of particular importance as food crops throughout subtropics and tropical regions. The crop is one of the important carbohydrate sources for millions of people, particularly those in developing nations. Sweet potatoes have nutritional value than Irish potato, especially in vitamins A, B and C in calcium. China and Japan are the major consumers of sweet potatoes (Ameinyo and Ataga, 2006). Post-harvest losses of all perishable tropical produce have been conservatively estimated at 25% of production. Attack by fungi, bacteria and viruses are probably the most serious causes of post-harvest 0.losses of between 25% and 60% of the initial weight depending on variety when stored for six and a half month in a semi subterranean. Ameinyo and Ataga (2006) reported Rhizopus oryzae and Aspergillus niger as being responsible for sweet potato rot.  Reported Fusarium oxysporium, Fusarium trichothecoides and Fusarium radicicola to cause potato tuber rot under different condition of temperature and humidity. The three species were susceptible to rot with higher relative humidity and higher temperature. The use of dry as well as cool storage is therefore important in reducing the loss from Fusarium rots of potatoes. However, the most common post-harvest storage disease of potato as reported by Ameinyo and Ataga (2006) include Rhizopus soft rot (Rhizopus stolonifer), bacteria soft rot (Erwinia chrysentheli), Fusarium surface rot (Fusarium oxysporum) and black rot (Ceratocystis fimbriata). Different diseases arise after potatoes are harvested, this is because the storage organs are essentially dormant structure and their cells are physiologically unlike those of the growing plants. The numerous diseases which occur in transit and storage result mainly from the activity of fungi and bacteria. Investigations carried out by various workers clearly indicate that the real cause of the spoilage of tubers in transit or storage is the high temperature and injuries sustained by the tubers during the process of marketing (Ameinyo and Ataga, 2006). Such disease considerably reduces the commercial value of the produce. Harvested tubers are vulnerable to attack by microorganisms because of their moisture content and rich nutrient. Due to harvest, packing and transportation, injuries of various kinds are caused which facilitate the entry of certain pathogens. Some of the pathogens produce extracellular enzymes and start degenerative process in advance of the fungal hypae or bacterial cells of the attacking pathogens. As a result of infection, the market and nutritive value of the tubers are reduced, either due to its ugly appearance or the changes in the stored products of the tubers (Oyewale, 2006).

Recently, only whole (i.e. uncut) sweet potato tubers without apparent wounds were retailed maybe as a result of the difficult economic situation in developing nations. Sweet potatoes are now halved (depending on the size of the tuber) and packed in polyethylene bags for retail. However, the potential for storage rot associated with these practices has been given little attention and tubers showing various amounts of damage are now commonly retailed in the open market. Bruised or cut tubers readily become colonized by propagules of pathogens associated with the surface and those from adjacent infected tubers. A wide variety of microorganisms particularly moulds have been implicated in tuber spoilage, relatively few are implicated as primary pathogens.

1.1 AIMS AND OBJECTIVES

i. To Isolate, Identify and Characterize bacteria associated with deterioration of sweet potatoe (Ipomoea batatas).

ii. To Isolate, Identify and Characterize fungi associated with deterioration of sweet potatoe (Ipomoea batatas)

iii. To determine the percentage occurrence of bacteria and fungi associated with deterioration of sweet potatoe spoilage (Ipomoea batatas).

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