ISOLATION, IDENTIFICATION AND CHARACTERIZATION OF COCOYAM FUNGAL PATHOGENS

ABSTRACT

Isolation of fungal pathogens of cocoyam was undertaken at both pre and postharvest levels. At the pre harvest level four species of fungi were isolated, characterized and identified from diseased cocoyam leaves to belong to species of Phytophthora, Phylostica, Fusarium and Phythium. At postharvest level, the level the species isolated from diseased cocoyam corms and Cormels included Aspergillus, Fusarium, Penicillium, Rhizopus, yeast and Botryodiplodia with occurrence levels of 100%,60%, 60%, 80%, and 100% and 40% respectively. The occurrences of isolates in pre harvest leaf disease was 80% (Phytophtora) 40% (Phylostica), 60% (Fusarium) and 40% (Phythium). Pathogenicity test result shows that four of the six postharvest disease isolate demonstrated pathogenicity while two (yeast and Rhizopus) proved nonpathogenic. However, the level of pathogenicity varied significantly with Aspergillus niger being the most pathogenic (66.27%) while Penicillium was the least pathogenic (48.0%) Fusarium and Botryodiplodia recorded 48.15% and 49.28% pathogenicity respectively. It was concluded that these were the causative agents of phytopathogenicity in cocoyam after harvest. Constraints of available technology, weather and timing which did not permit test on pathogenicity of the isolates from diseased leaves at pre harvest level. But it will be worthwhile of the pathogenicity test are carried on the phytopathogens isolated during the preharvest period to determine those that are pathogenic to the cocoyam plants.

TABLE OF CONTENTS

Certification                                                                                                                            i

Dedication                                                                                                                              ii

Acknowledgments                                                                                                                  iii

Table of Contents                                                                                                                   iv

List of Tables                                                                                                                          vi

Abstract                                                                                                                                  vii

CHAPTER ONE

1.0       Introduction                                                                                                                1

1.1              Aim                                                                                                                             4

1.2              Objectives                                                                                                                   4

CHAPTER TWO

2.0       Literature review                                                                                                         5

2.1       Cocoyam as a staple food                                                                                           7

2.2       Propagation and growth conditions of Colocasia esculenta                                      9

2.3       Importance of Colocasia esculenta                                                                             10

2.4       Blight disease and causal agent(s)                                                                              10

2.5       Impact of blight disease on world’s production of Colocasia                                   11

2.6       Sources of inoculum of Curvularia blight and environmental factors influencing

            blight disease                                                                                                              12

2.7       Signs and symptoms of Curvularia leaf blight                                                           13

2.8       Management of leaf blight disease of Colocasia esculenta                                        13

2.8.1    Diseases and pests of C. esculenta                                                                             13

2.8.2    Good agronomic practices, field sanitation and store hygiene                                   15

2.8.3    Cultural control                                                                                                           16

2.8.4    Chemical control                                                                                                         17

2.8.5    Use of resistant varieties                                                                                             18

2.8.6    Biological control                                                                                                        18

2.8.7    Thermal and physical control                                                                                      19

2.8.8    Biological control of rot organisms                                                                             20

2.8.9    Chemical control of rots and their causal agents in postharvest produce                   21

CHAPTER THREE

3.0       Materials and Methods                                                                                               23

3.1       Collection of materials                                                                                                23

3.1       Materials                                                                                                                     23

3.2       Methods                                                                                                                      23

3.2.1    Preparation of samples                                                                                                23

3.2.2    Preparations of medium                                                                                              23

3.3       Isolation of cocoyam rot organisms                                                                            24

3.3.1    Culture by direct placement                                                                                        24

3.3.2    Culture by spread plate technique                                                                              25

3.3.3    Isolation of organism                                                                                                  25

3.4.0    Characterization of isolates                                                                                         26

3.3.4.1 Characterization of fungi isolates                                                                               26

3.4.2.   Characterization of bacteria isolates                                                                           27

3.5.0    Identification of isolates                                                                                             28

3.6.0    Pathogenicity test                                                                                                       29

CHAPTER FOUR

4.0       Results                                                                                                                        30

SECTION FIVE

5.0       Discussion, Recommendation and Conclusion                                                           34

5.1       Discussion                                                                                                                   34

5.2       Conclusion                                                                                                                  36

5.3       Recommendation                                                                                                        36

            References

            Appendices

LIST OF TABLES

Table                                                            Title                                                                  Page

 1                                 Fungal isolates from rot infected corms                                             31

 2                                 Occurrence of cocoyam rot fungi isolates (pre-harvest in leaves)      32

 3                                 Occurrence of cocoyam rot fungi isolates (post-harvest)                   33

 4                                 Pathogenicity of fungi isolates on healthy corms                               34

CHAPTER ONE

1.0       INTRODUCTION

 In Nigeria, cocoyam (Xanthosoma sagittifolium) is an important staple which besides being a food crop, serves as a major source of income for rural households. Yield losses due to cocoyam root rot disease (CRRD) remains a major constraint to increased cocoyam production in Nigeria. Until now, this disease has been attributed to a pathogen complex including Fusarium solani and Rhizoctonia solani. However, symptoms observed in infected fields in Nigeria are similar to those caused by the oomycete (Perneel et al. 2006). The most popular type available in most South-eastern Nigerian bazaar is ede-uli in Igbo (Colocasia esculenta); it grows in marshy areas and its corms are used as soup thickening agents in most South-eastern Nigerian communities. The second type which is less popular is called ede-oku in Igbo (Xanthosoma sagittifolium), whose corms could be boiled and eaten with various soups.

            Cocoyam (Colocasia esculenta) provide substantial portion of the carbohydrate content of the diet in many regions in developing countries and provide edible starchy storage corms or cormels, this may not be the case in Ghana where Xanthosoma sagittifolium is more popular (Opara, 2002). Xanthosoma sagittifolium is a herbaceous corm belonging to the family Araceae with enlarged root stock that acts as a storage organ (Ojinaka et al., 2009). It is about the third most important root and tuber crop after cassava and yam in Nigeria. Again, Cocoyam production and processing in the country does not meet with other major root and tuber crops, owing to its low storability, decreasing yields, and the socio-anthropological perception of the crop as women’s crop (Okigbo and Ikediugwu, 2000). Women do not have possession of land, labour and capital in various parts of Nigeria especially South-eastern Nigeria. Furthermore, the cultivation of cocoyam in South-eastern Nigeria is jeopardised by the devastating disease, cocoyam root rot blight complex (CRRBC) (Spore, 2003). Storage loss of root and tuber crops has saddled farmers because more than 40% of their harvests are lost as a result of decay. Fungal rot is the main cause of root and tuber loss in storage (IITA, 2006).

The foremost species of microorganisms linked with cocoyam rot in Nigeria include Aspergillus flavus, Penicillium digitatum, Botryodiplodia theobromae, Sclerotia rolfsii, Fusarium solani and the bacterium Erwinia carotovora. These fungi were reported to be pathogenic to four cultivars of Colocasia esculenta, causing rot of cocoyam in numerous parts of southern Nigeria (Quash et al., 2000). Other influences like vermin, prolonged linger in on-set and cessation of rainfall, lengthened dry season and moisture stress, play roles in the low cultivation of cocoyam in Nigeria. Cocoyams are best kept in a cool, dry and well aerated environment. Optimal storage temperature of 7⁰C is best for long term storage, while storage at room temperature will preserve the corms for a fairly reasonable period of time. Cocoyams can also be stored under shade, in pits and covered with ash and plantain leaves. The ash is believed to have some fungicidal effects (Onifade, 2002). The dietary significance of root crops has led people to formulate several ways to evaluate the nutrient profile of food commodities. A host of authors have assayed the chemical composition of Xanthosoma sagittifolium and Colocasia esculenta corms (Amusa, 2003). In spite of the fact that cocoyam corms are overlooked crops, their compositional significance is great with an average protein content of 6 % and 390 calories per 100g dry matter (Ray et al., 2000). One major constraint in the utilization of cocoyam is the presence of oxalates which confer acridity or elicit irritations when foods prepared from them are ingested. Consumption of foods containing oxalates have also been discovered to have caustic effect, exert irritations to the intestinal tract and cause absorptive poisoning (Ellis et al., 2013).

            The exact origin of cocoyam (Colocasia spp) is not clear, but it may have come from India or South-East Asia. They have a long history of cultivation. Ede-uli is further divided into edible and ornamental cocoyam. The ornamental cocoyam has three varieties namely; Colocasia black-magic, Colocasia coffee-cup and Colocasia black-ruffle. On the other hand, the ede-oku (Xanthosoma) comprises of species like; X. eggersii, X. sagittifollum, X. weeksi, and X. violaceum (Ague et al., 2014). These are grown for their starchy corms, an important staple of tropical region (Agu et al., 2016). The cocoyam is known as food crop which provides high yield of roots (or corms) and foliage. It is a tropical food crop that can be grown under flooded or upland condition. The cocoyam plant is considered toxic due to the presence of calcium oxalate crystals typically as raphides. The toxin is minimized by cooking especially with a pinch of baking soda or reduced by steeping roots in cold water overnight. Calcium oxalate is highly insoluble and contributes to kidney stones. Cocoyam contributes significant portion of the carbohydrate content of the diet in many regions in developing countries and provides edible starchy storage corms and cormels. Although they are less important than other tropical roots such as yam, cassava and sweet potatoes, they are still a major staple in some parts of the tropics and sub tropics (Babajide Oyewole and Obadina, 2006). It could be used as thickener in soup, flour (for confectionary), or may be cut up and boiled or fried to make crispy chips or flakes. The leaf stalk can also be eaten. Post-harvest spoilage of cocoyam arises from improper handling of the cocoyam either during storage or harvest. The greatest cause of root rot and tuber loss in storage is the highest disease in cocoyam (Osuji, 2003). The post-harvest loss of root and tuber crops has been a very serious problem to farmers, as more than 40% of their harvest maybe list because of decay (Okigbo et al., 2015). It is estimated that in the tropics each

ear between 25% and 40% of stored agricultural products are lost because of inadequate farm and village-level storage (Okigbo et al., 2015).

Also cocoyams may be attacked by pests like mealybug, aphids, catapillers etc. these pests and diseases can be controlled by the use of chemicals, fertilizers rich in potassium. Insecticides from nurseries (for pests) use of well cleaned planting materials, initiation of early planting (April/May) and planting in well-drained soil with no water logging (Onuegbu, 1999).

1.3              AIM

To isolate, characterization and identification of fungal phyto pathogens cocoyam

1.4              OBJECTIVES 

i.                    isolation, characterization and identification of Pre-harvest fungal phyto pathogens of cocoyam

ii.                   isolation, characterization and identification of Post-harvet fungal phyto pathogens of cocoyam

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