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
|
Tables
|
Title
|
Page
|
|
1
|
Cultural and Microscopic Features of the Fungal Isolates
|
29
|
|
2
|
Percentage Occurrence of Molds Isolated from the Diseased
Corms of Cocoyam
|
30
|
|
3
|
Percentage Severity of Rots Obtained from Test Corms
During Pathogenicity Test.
|
31
|
|
4
|
Diameter of Rot induced by the Isolates after days of
Storage
|
32
|
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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