FUNGI ASSOCIATED WITH THE SPOILAGE OF COCOYAM IN THE FIELD

ABSTRACT

The identification of fungal pathogens responsible for the spoilage of cocoyam corms was demonstrated on Sabouraud dextrose agar media and subsequent microscopic observation. The fungi which were isolated from rot affected tissue identified to be the causative microorganisms responsible for cocoyam spoilage included Aspergillus niger, Sclerotium rolfsii, Fusarium solani, and Penicillium species. All the fungi were found to be responsible for spoilage of healthy cocoyam. Rot was observed by the softness of the corms and oozing out of fluids. Sclerotium rolfsii and Fusarium solani has the highest frequency of occurrence (56.3%) and 37.5% respectively followed by Aspergillus niger (31.2%) and Penicillium species with 18.7% frequency of occurrence. Pathogenicity test showed that the isolated fungal species induced rot in healthy cocoyam corms after days of inoculation. Sclerotium rolfsii and Fusarium solani were the most pathogenic with rapid deterioration of the fresh cocoyam corms in 21days having a rots diameter of 27mm and 22mm, respectively. The least pathogenic fungus was A. niger having a rots diameter of 16mm. This study showed that virulence varied amongst the fungal pathogens. S. rolfsii and F. solani were more virulent whilst Aspergillus niger, and Penicillium species were mildly pathogenic during the 28days storage period.

TABLE OF CONTENTS

Title Page                                                                                                                    i

Certification                                                                                                               ii

Dedication                                                                                                                  iii

Acknowledgements                                                                                                    iv

Table of Contents                                                                                                       v

List of Tables                                                                                                              vi

Abstract                                                                                                                      vii

CHAPTER ONE:      INTRODUCTION

1.0           Introduction                                                                                                              1

1.1       Aim of the Study                                                                                                      8

1.2       Specific Objectives                                                                                                   8

CHAPTER TWO

2.0       Literature Review                                                                                                     9

2.1       Nutritional Composition of Cocoyams                                                                   11

2.2       Sources and Magnitude of Storage Losses in Cocoyam                                        13

2.2.1    Mechanical Injury                                                                                                   14

2.2.2    Physiological Factors                                                                                             14

2.2.3    Pathological Losses                                                                                                15

2.3       Economic Importance of Rot                                                                                 16

2.4       Initiation of Rot                                                                                                      17

2.4.1    Causal Agents of Yam Rot                                                                                     17

2.4.2    Root Rot Disease                                                                                                    18

2.5       Management of Pre-Harvest Pathogens of Cocoyam                                                  19

2.5.1    Biological and chemical control of rot organisms                                                 20

2.5.2    Good Agronomic Practices, Field Sanitation and Store Hygiene                                20

2.5.3    Thermal and Physical Control                                                                                21

2.5.4    Biological Control of Rot Organisms                                                                 21

CHAPTER THREE

3.0       Materials and Methods                                                                                       24

3.1       Materials                                                                                                             24

3.1.1    Source of Cocoyam Species                                                                               24

3.2       Methods                                                                                                              24

3.2.1    Preparation of Culture Media                                                                                         24

3.2.2    Pathogen isolation                                                                                              24

3.2.3    Subculturing and Purification                                                                             25

3.2.4    Characterization                                                                                                 25

3.3       Pathogenicity Test of the Isolated Fungi                                                            26

CHAPTER FOUR

4.0       Results                                                                                                               28

CHAPTER FIVE                                                                               

5.0       Discussion and Conclusion                                                                               33

5.1       Discussion                                                                                                          33

5.2       Conclusion                                                                                                         36

References                                                                                                         37

LIST OF TABLES

CHAPTER ONE

1.0      INTRODUCTION

One of the most pressing problems facing the countries of the third world is food scarcity. The most important losses in agricultural goods production which involve the greatest costs on the farm economy starts from unhealthy pre-harvest processes that often result in spoilage during storage. It is reported that nearly 1 billion people are challenged by severe hunger in the developing nations of which 10% actually die from hunger-related complications. A substantial part of this hunger problem stems from inadequate agricultural storage and produce preservation from microbes-induced spoilages (Salami and Popoola, 2007; Kana et al., 2012). According to Arya (2010), of all losses caused by plant diseases, those that occur after harvest are the most costly often resulting from improper handling during pre-harvesting processes. Unoccasional introduction a cocoyam corm that is infested with a pathogen into a storage barn could likely result in the infection of other tubers stored in the process. Cassava, yam and sweet potato are important sources of food in the tropics. Others are cocoyam, rice, maize, wheat, sorghum, millet and various fruits, legumes and vegetables.

Cocoyam (Araceae; Colocasia esculentus) is one of the important crops in Nigeria.Nigeria leads its production with 3.7 million MT perannum. Nigeria is the largest producer of cocoyam in the world, with 5.49 million MT of corms produced annually which accounts for 37-40% of the world’s total (FAO, 2006). The crop is grown as a staple in African, Oceanic and Asian cultures. Cocoyams are rich in carbohydrates, vitamins (especially vitamin B6), minerals, trace elements and fibre while the leaves contain vitamins A and C. These make it popular amongst diabetics. Its nutritional values compare well with potatoes (Talwana et al., 2009).

Cocoyams are used in much the same way as yams being processed into fufu, chips and different kinds of Nigerian cuisines. Current yield levels of the cocoyam production are low on a worldwide basis. The most mentioned disease symptoms on the crop in wetlands ofEast Africa are tuber rot, smelly tubers and sticky ooze from the tubers (Talwana et al., 2009). In Nigeria, the crop is seriously affected by tuber rot (Sclerotium rolfsii). Attacks from this disease caused a drop in production figures of cocoyam by 11% (Nwachukwu and Osuji, 2008).

Apart from these huge losses in the quantity of produce, rot-losses have a lot of other consequences on produce quality (Nwachukwu and Osuji, 2008). Some workers reported that spoilage organisms produce extra-cellular enzymes such as amylases, celluloses, zylanases, polygalactunases and pectin-methyl esterases which degrade cell wall components of susceptible produce leading in some cases to emission of foul odour and water (Salami and Popoola, 2007; Amadioha, 2012; Oladoye et al., 2013).

Hence rots reduce the market value of affected produce, hamper the addition of value to them and prevent produce to complete their roles in the food chain. In a bid to keep these organisms at bay, controls are employed which increase the cost of production. These costs are passed over to the end users of the commodities.

Many socio-economic factors, and especially pests and disease pressures however; threaten the sustainable production, storage and preservation of these staple foods as well as several leafy vegetables, fruits and spices produced in the country (Awurum et al., 2001; Abate et al., 2011). These threats are projected to increase in the face of climate change challenges (Awurum et al., 2001). Fungi represent the greatest number of pathogens responsible for plant diseases and deteriorations. Pathogenic fungi decimate agricultural crops or products at all stages of growth in the field, transit or storage (Amusa et al., 2003; Enyiukwu et al., 2013). It has been reported that up to 8000– 750,000 crop diseases are caused by fungi with between 50–200 species, races and/or biotypes attacking a single crop (Madden et al., 2008). Infection by fungi interferes with normal physiological function(s) of the host plant such as photosynthesis, biosynthesis, nutrient and water uptake among others, leading to great reductions in crop yield and product quality.

The major important root crop of tropical regions, including Nigeria suffer from pest and post-harvest losses resulting from physical, physiological or pathological factors or the combination of all the three (Okigbo and Nmeka, 2010).Plant pathogens and diseases are a major limitation to crop production worldwide. While plants are naturally immune to the vast majority of potential bacterial and fungal pathogens, the few that manage to establish disease can be extremely problematic and cause economic losses.

Fungi are the most important and have been reported to be responsible for 80% of all storage rots of cocoyam tubers in West Indies and 57 – 77% in Nigeria (Okigbo et al., 2010). The principal species of microorganisms associated with cocoyam rot in Nigeria include: Aspergillus niger, Rhizopus nodosus, Sclerotia rolfsii, Fusarium oxysporum and Botryodiplodia theobromae, these fungi have been reported to be pathogenic to several tuber crops, causing severe loss in several parts of Southern Nigeria (Okigbo et al.,2015).

An appraisal of the major constraints on cocoyam production indicated that it is not due to lack of demand but losses due to on-field deterioration (Nwachukwu and Osuji, 2008). Fungi are the main microbial pathogens that cause storage rot of cocoyam corms (Nwachukwu and Osuji, 2008). Fungi causing storage rot of cocoyams (C. esculenta, C. antiquorum and Xanthosoma sagittifolium) have been reported in USA, India, Egypt, the Pacific Islands and Nigeria (Yusuf and Okusanya, 2008).

Other species of fungi reported as important rot pathogens include Fusarium solani, Fusariumoxysporium, Fusarium moniliforme, Fusarium avenaceum,Botryodiplodia theobromae,Sclerotium rolfsii,Botrytis sp; Pythium sp, Phytophthora colocasiae, Rhizoctonia bunoides (Banito et al., 2010). In Nigeria, B. theobromae, S. rolfsii, F. salani, F. oxysporium, Fusarium sp; and R. stolonifer have consistently been reported as rot pathogens consistently isolated from corms of C. esculenta, C. antiquorum and X. sagittifolim during storage (Ogaraku and Usman, 2008). These fungal organisms have also been reported to be the major cause of storage rots of other root and tuber crops including yam, cassava and sweet potatoes (Okigbo and Nwakamma, 2005; Adegbite, et al., 2005).

Quantitative pathogenic losses of the stored cocoyams result from the rapid and extensive break down of host tissues by microorganisms especially fungi. The pattern of attack is usually an initial infection normally through wounds caused by harvest bruises and points of detachment from mother corms by one of a few specific pathogenic or saprophytic organisms which grow on the dead moribund tissues remaining from the primary infection.

A major problem in cocoyam production in Nigeria has been attributed to the rapid deterioration of the corms soon after harvest. Rotting of stored cocoyam corms reduces the quantity and quality of the crop available for human consumption and for the next planting season. An estimated loss of 40-50% of corms in Nigeria during storage is known to be due primarily to microbial action (Amienyo and Ataga, 2007). These microorganisms include Erwinia carotovora, Fusarium solani (Mart.) Sacc., Sclerotium rolfsii Sacc., Botryodiplodia theobromae Pat.,Rhizopus stolonifer, Trichoderma harmatum (Amienyo and Ataga, 2007).

Quantitative losses of stored cocoyam corms result from the rapid and extensive breakdown of host tissues by microorganisms. The pattern of attack is usually an initial infection through wounds caused by harvest bruises and points of detachment from mother corms by one of a few specific pathogenic or saprobiotic organisms which grow on the dead moribund tissues.

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 (Kana et al., 2012). 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).

One of the most pressing problems facing the countries of the third world is food scarcity. It is reported that nearly 1 billion people are challenged by severe hunger in these nations of which 10% actually die from hunger-related complications. A substantial part of this hunger problem stems from inadequate agricultural storage and produce preservation from microbes-induced spoilages (Salami and Popoola, 2007; Kana et al., 2012). According to Arya (2010), of all losses caused by plant diseases, those that occur after harvest are the most costly. Rots according to Taiga (2011) result in loss of 7 million MT of yams annually. Rot of cocoyam tubers may be soft, wet or dry and could occur pre- or post-harvest. Pre-harvest rots are due to infection of tubers by soil-borne pathogens.

Cocoyam is considered as a food crop; corms, cormels are boiled, roasted or baked, leaves are rich in vitamins and minerals and are prepared as vegetable in West and Central Africa (Sandifola, 2002). It is an important crop in tropical and sub tropical areas because it provides carbohydrates, proteins, fats and vitamins. It is highly nutritious with a considerable amount of carbohydrate (88.04%), ash (3.20%), crude protein (5.19%), crude fibre (2.86) and 0.40% ether extract (Aderoluand Sogbesan, 2010). However, storage conditions have effect on the proximate and nutritive value of the stored root crops because of the growth of some spoilage fungi that strife in such conditions (Abaka and Norman, 2000).

The fungi that invade stored product are generally grouped into two categories namely field fungi which attack developing and matured seeds in the field and storage fungi which are predominantly species of Aspergillus and Penicillium which attack the stored products (Fagbohun et al.,2010).

The conditions of the stored product determine the extent of invasion of the stored product. The environmental factors that aid the development of fungi in stored products include moisture content (Amusa et al., 2002), temperature (Abaka and Norman, 2000), aeration, pH (Aderiye, 2004), relative humidity (Fagbohun and Lawal, 2011). However, the effect of this storage fungi on stored products include deterioration and spoilage of stored products (Abaka and Norman, 2000; Ekundayo and Idzi, 2005), reduction of market value (Fagbohun et al., 2010) and production of chemical substances that are toxic (Richard and Wallace, 2001). The preventive measures that can be employed for the growth of the storage fungi are biological control (Aderiye, 2004), chemical control and physical control (Rice, 2002).

Diseases of Colocasia significantly reduce the number of functional leaves and have led to yield reduction of about 50% worldwide (Arunasri et al., 2011). C. esculenta is affected by a number of infectious diseases caused by fungi, bacteria, nematodes, and viruses as well as noninfectious or abiotic factors. Disease levels in recent years have caused tremendous decline in corm yields of C. esculenta and there is a corresponding loss of revenue to C. esculenta farmers. Rapid decline in Colocasia production is threatening the survival and existence of the crop and extinction is eminent. Recent decline in the production of C. esculenta resulting from fungal disease is a significant threat to food security.

Harvested cocoyam is stored by different methods to extend the shelf life for use in the next planting season and food. Post-harvest loss of root and tuber crops has been a very serious problem to farmers as more than 40% of their harvest may be lost because of decay. Organisms associated with cocoyam rot in Nigeria include Aspergillus flavus, Penicillium digitatum, Botryodiplodia theobromae, Sclerotium rolfsii, Fusarium solani, and Erwinia carotovora (Amienyo and Ataga,2007). In particular, cocoyam cormel rot caused by S. rolfsii is common. S. rolfsiiis distributed in tropical and subtropical regions of the world with high temperatures (Arunasri et al., 2011). Symptoms due to S. rolfsii in host plants include seed rot, seedling blight, collar rot, stem rot, and wilt resulting to economic losses.

There are several constraints to the cocoyam industry in the country. Of these constraints, diseases contribute greatly to high yield losses before and after harvest. Cocoyam 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 cocoyams 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 cocoyam tubers are affected by rots, which makes them unappealing to consumers. In Nigeria, over 60% of cocoyam varieties get rotten when stored for less than six months (Amienyo and Ataga, 2007).

1.1       Main Aim

This research is aimed at isolating and identifying the spoilage molds found in cocoyam corms during storage.

1.2       Specific objectives

1.     To isolate some rot-inciting pathogens of cocoyam.

2.     To establish the pathogenicity of fungal organisms associated with corm rots of cocoyam

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