Abstract
Sweetpotato (Ipomoea batatas (L.) Lam)
is a valuable root crop widely consumed in Nigeria. However, its postharvest
losses due to storage rots significantly affect its market value and
nutritional quality. This study investigated the host-pathogen interactions and
biological control strategies against fungal pathogens associated with
sweetpotato storage rots. Surveys were conducted in four Local Government Areas
(LGAs) in Kwara State to identify the incidence and severity of sweetpotato
root rot across farm communities and markets. Rotted sweetpotato samples were
collected and subjected to isolation and identification of the causative fungal
pathogens. The most frequently occurring pathogens were Botryodiplodia
theobromae, Rhizopus stolonifer, Aspergillus tamarii, and Aspergillus
ochraceus, with B. theobromae identified as the most virulent. Pathogenicity
tests confirmed their role in postharvest deterioration. Biochemical analysis
revealed significant reductions in protein, fat, carbohydrate, crude fibre, and
mineral content in infected roots, indicating the nutritional degradation
caused by these pathogens. The biocontrol potential of four antagonistic fungi—Trichoderma
harzianum, Penicillium oxalicum, P. chrysogenum, and P.
citrinum—was evaluated both in vitro and in vivo. Culture filtrates of
these bioagents significantly inhibited the radial mycelial growth of the
pathogens on potato dextrose agar and also suppressed rot development in stored
sweetpotato roots. The efficacy of the bioagents was comparable to that of
benomyl, a synthetic fungicide. Among the antagonists, T. harzianum
exhibited the most effective biocontrol activity, followed by P. chrysogenum.
The study also compared different storage methods and found that storage on
racks in ventilated rooms and sawdust heaps was more effective in extending
shelf life and reducing rot than pit storage. The findings underscore the
potential of using eco-friendly biocontrol agents for managing storage rots of
sweetpotato, especially among resource-limited farmers. The use of crude
filtrates from T. harzianum and Penicillium spp. provides an
affordable and sustainable alternative to chemical fungicides. It is
recommended that further chemical profiling of the antagonists’ metabolites be
conducted to explore the development of biopesticide formulations.
Additionally, the adoption of improved storage practices is essential to
minimize postharvest losses, preserve sweetpotato quality, and enhance food
security in Nigeria.
Keywords: Sweetpotato, storage rot,
fungal pathogens, biological control, Trichoderma harzianum
TABLE OF CONTENTS
CHAPTER 1
INTRODUCTION
1.1.
Background to the Study
1.2 Statement of the Problem
1.3 Justification of the Study
1.4 Objectives of the Study
CHAPTER 2
LITERATURE REVIEW
2.1 Origin and Distribution of Sweetpotato
2.2 Botany of the Crop
2.3 Importance of Sweet Potato
2.3.1 Medicinal importance
2.3.2 Economic importance
2.3.3 Food and nutritional values
2.4 Sweetpotato Production
2.5 Soil
and Climatic Requirements
2.6 Harvesting
and Storage of Sweetpotato Root
2.6.1 Harvesting
of sweetpotato
2.6.2 Postharvest
handling of sweetpotato roots
2.6.3 Storage
of sweetpotato roots
2.6.3.1 Types of fresh sweetpotato root
storage
2.7 Constraints
To Economic Production Of Sweet Potato
2.8 Post
Harvest Microbial Deterioration Of Sweetpotato Root
2.8.1 Rot
types and symptoms and predisposing factors of stored sweetpotato root
2.8.2 Sweetpotato
root rots and causal agents
2.9 Characteristic
Features Of Some Fungal Pathogens With Storage Rot Of Sweetpotato
2.9.1 Botrydoplodia theobromae Pat. (1892)
(Synonym: Lasiodiplodia theobromae (Pat.)
Griffin & Maubi 1909).
2.9.2 Rhizopus stolonifer Vuillemin (1902)
(Synonym: Rhizopus nigrican Ehrenberg,
1820)
2.9.3 Aspergillus ochraceus (Wilhelm, 1877) [synomyms: A. alutaceus (Berkeley 1875); Sterigmatocystis
helva (Bainer, 1881)]
2.9.4 Aspergillus tamarii (Kita, 1913).
2.10 Control
Of Rots Of Sweet Potato Tubers In Storage
2.10.1 Cultural
control (Good agronomic practices, field sanitation and store hygiene).
2.10.2 Thermal
and physical control
2.10.3 Use of
resistant varieties in sweet potato root rot control
2.10.4 Chemical
control of rots of sweetpotato roots
2.10.5 Use of
botanicals in the control of root and tuber rots
2.10.6 Biological
control of rots of roots and tubers
2.11 Biological
Control Agents Used
2.11.1 Trichoderma harzianum (Rifia, 1969)
(Syn. T. lignorum var. narcissi Trochinai
and Shimada) Pidopi (1953)
2.11.2 Penicillium oxalicum (Currie JN; Thom.
C, 1915)
2.12 Mechanism
Of Action Of Biological Control Agents
2.12.1 Antagonism
2.12.2 Antibiosis
2.12.3 Competition
2.12.4 Resistance
induction
2.12.5 Parasitism
2.12.6 Metabolite
production
2.12.7 Lysis
CHAPTER 3
MATERIALS AND METHODS
3.1 Experimental
Site
3.2 Survey Of Storage Root Rot Of Sweetpotato In
Some Local Government Areas Of Kwara State
3.3 Source Of Tubers
3.4 Preparation Of Culture Media
3.4.1 Preparation Of Potato Dextrose Agar (Pda)
3.4.2 Preparation Of Potato And Agro-Waste Broths
3.5 Isolation And Identification
Of Fungal Pathogens Associated With Sweetpotato Root Rot In Storage
3.5.1 Isolation Of Pathogenic Organisms
3.5.2 Pathogenicity Test And Identification Of
Pathogens
3.6 Isolation And Identification
Of Bio-Control Agents
3.7 Biochemical Composition Of Healthy And
Infected Sweetpotato Roots
3.7.1 Sample Preparation
3.7.2 Determination Of Moisture Contents
3.7.3 Determination Of Ash Content
3.7.4 Crude Fibre Determination
3.7.5 Fat Content Determination
3.7.6 Protein Determination
3.7.7 Determination Of Carbohydrate Contents
3.7.8 Determination Of Mineral Contents Of
Sweetpotato Roots
3.8 In Vitro Experiments
3.8.1 Effects Of The Antagonists
On Spore Germination Of Pathogenic Organisms.
3.8.2 Effects Of The
Bio-Antagonists On The Radial Growth Of The Test Fungi
3.8.3 Effects Of The Fungal Antagonists On Mycelial
Biomass Of The Pathogenic Organisms.
3.9 Determination Of Inhibitory Factors Of The Antagonists
3.9.1 Determination Of Minimum
Inhibitory Concentration (Mic) Of The Antagonists
3.9.2 Determination Of Minimum Fungicidal
Concentration (Mfc)
3.10 In Vivo Experments
3.10.1 Evaluation
Of Effects Of Biocontrol Agents On Rot Development And Spread In
Sweetpotato Root.
3.11 Evaluation Of Effects Of Different Storage
Methods On Rot Development Of Sweetpotato Root Rot Caused By Pathogens
3.12 Evaluation Of Different Agro-Waste Products As
Sustainable Growth Media For The Bio-Control Fungi
3.13 Histological
Studies Of Infected Sweetpotato Roots
3.14 Data
Analysis
CHAPTER
4
RESULTS
AND DISCUSSION
4.1 Results
4.1.1 On-farm
survey of postharvest root rot of sweetpotato varieties in
Kwara State
4.1.1.1 Sweetpotato varieties in the local government
areas
4.1.1.2 Sweetpotato
root rot incidence and severity after harvest in different locations
4.1.2 Fungi associated with storage rot of
sweetpotato roots
4.1.3 Pathogenic
organisms of stored sweetpotato root
4.1.4 Characteristic features of pathogenic
organisms causing root rot of sweetpotato in storage
4.1.4.1 Botryodiplodia theobromae Pat. (Syn. Lasidiodiplodia theobromae (Pat.) Griffon
and Maubi) (Plates 4.1-4.2)
4.1.4.2 Rhizopus
stolonifer
4.1.4.3 Aspergillus
tamari Kita G. (1913)
4.1.4.4 Aspergillus ochraceus (Wilhem 1877)
4.1.5 Characteristic features of bioagents
4.1.6 Effect of pathogens on the biochemical
compositionof sweetpotato root in storage
4.1.6.1 Effects of pathogen on nutrient content of
sweetpotato roots
4.1.6.2 Effect of pathogens on mineral composition
of stored sweetpotato roots
4.1.7 Effects of the bio-agents on the growth of
the pathogens in vitro
4.1.7.1 Effect of the bio-agents on spore germination
of pathogens in culture
4.1.7.2 Effect of bio-agents on mycelial radial growth
of pathogens in culture
4.1.7.3 Effect of the bioagents on mycelia biomass of
the pathogens in culture
4.1.9 Minimum inhibitory concentration of biogent
filterates (MIC) and minimum fungicidal
concentration (MFC) against pathogenic organisms of sweet potato
4.1.10 Effect
of bioagents on rot development and spread in sweetpotato root in storage (in vivo experiment)
4.1.11 Bioactive
compounds of bioagents
4.1.12 Effects
of the different agro-wastes on growth and sporulation of bio-agents
4.1.13 Effects
of storage sytems on shelf-life of sweet potato roots
4.1.14 Histological
distortion of sweetpotato root tissues by pathogenic organisms during
pathogenesis
4.2 Discussion
CHAPTER
5
CONCLUSION
AND RECOMMENDATIONS
5.1 Conclusion
5.2
Recommendations
References
Appendix 1:Morphological characteristics
of the identified fungal rot pathogens isolated from
the sweet potato tubers
Appendix 2: Inoculated sweetpotato root
specimens in micro-climate condition for pathogenicity test.
APPENDIX 3: Agricultural substrates
inoculated for mass production of antagonistic organisms metabolites.
APPENDIX 4: Effects of storage systems of
rot development on sweet potato
LIST OF TABLES
Table 4.1: Sweetpotato
root rot incidence and severity (2-4) weeks after harvest in different
locations.
Table 4.2: Frequency of occurrence
of fungal isolates of stored sweetpotato root in Ilorin
Table 4.3: Morphological characteristics of the
bioagents
Table 4.4 Effects
of pathogens on nutrient composition of sweet potato root in storage data are
means of three replicates in two
separate experiments.
Table 4.5 Effects of the bio-agents on spore germination
of the rot pathogens of sweetpotato root
Table 4.6 Effects of the bio-agents on radial growth
of the rot pathogens of sweetpotato
in culture
Table 4.7: Minimum inhibitory concentration (mic) and minimum fungicidal
concentration (mfc) (ul/mg)
Table 4.8: Effects of Antagonistic filtrates against
rot-development and spread in
3 months sweetpotato root storage
Table 4.9 Phenolic
and flavonoid compounds of bio-antagonists filterates
LIST OF FIGURES
Figure 4.1: Frequency of occurrence of sweetpotato root varieties in 12
surveyed farms
Figure 4.2: Severity
index of major pathogenic organisms of sweetpotato root in storage
Figure 4.3 Effects of different concentrates of Penicillium oxalicum on biomass accumulation of rot-inducing
pathogens of sweetpotato root
Figure 4.4 Effects of different concentrates of Trichoderma harzianum on
biomass
accumulation of rot-inducing pathogens of sweetpotato root
Figure 4.5 Effects
of different concentrates of Penicillium
chrysogenum on biomass accumulation of rot-inducing pathogens of
sweetpotato root
Figure 4.6 Effects of different concentrates of Penicillium citrinum on biomass accumulation of rot-inducing pathogens
of sweetpotato roots
Figure 4.7: Effects of agro-substrate on growth and sporulation of Penicillium oxalicum
Figure 4.8: Effects of agro-substrate on growth and sporulation of Trichoderma harzianum
Figure 4.9: Effects of agro-substrate on growth and sporulation of Penicillium
chrysogenum
Figure 4.10: Effects of agro-substrate on growth and sporulation of Penicillium citrinum
Figure 4.11: Effects of different storage systems on disease incidence and
severity of sweetpotato root rot
LIST
OF PLATES
Plate 4.1 Sweetpotato root rot due to Botryodiplodia theobromae
Plate 4.2:
Pure Culture of Botryodiplodia theobromae (10-day old) grown on PDA
Plate 4.3:
Photomicrograph of Botryodiplodia theobromae Pat.
Plate 4.4:
Sweetpotato root rot induced by Rhizopus stolonifer during pathogenicity
tests
Plate 4.5: Pure Culture of Rhizopus stolonifer (10 day old) grown on PDA medium
Plate 4.6 Photomicrograph of Rhizopus stolonifer
Plate 4.7:
Sweetpotato root rot incited by Aspergillus tamarii during pathogenicity
tests
Plate 4.8: Pure Culture of Aspergillus tamarii (7-day old) grown on PDA medium
Plate 4.9: Photomicrograph of Aspergillus
tamarii
Plate
4.10: Sweetpotato root rot incited by Aspergillus ochraceus during pathogenicity
tests
Plate 4.11: Pure Culture of Aspergillus ochraceus (10-day old) grown on PDA medium
Plate 4.12: Photomicrograph of Aspergillus
ochraceus
Plate
4.13: Pure Culture of Penicillium chrysogenum grown on PDA
medium
Plate
4.14: Photomicograph of Penicillium chrysogenum
Plate 4.15:
Pure Culture of Penicillium citrinum grown on PDA medium
Plate 4.16:
Photomicrograph of Penicillium citrinum
Plate 4.17:
Pure Culture of Penicillium oxalicum grown on PDA medium
Plate 4.18:
Photomicrograph of Penicillium oxalicum
Plate 4.19:
Pure Culture of Trichoderma harzianum grown on PDA medium
Plate 4.20:
Photomicgraph of Trichoderma harzianum
Plate 4.21: Photomicograph of healthy and B.
theobroma-infected sweetpotato roots after 3 months storage
Plate 4.23: Anatomy
of healthy and A. tamarii infected
sweetpotato root
CHAPTER 1
INTRODUCTION
1.1. BACKGROUND TO THE STUDY
Sweetpotato (Ipomoea batatas (L.) Lam) commonly known as Louisiana yam is a
member of the family Convolvulaceae (morning glory family) which is made up of
45 genera and 1,000 plant species. Ipomoea
batatas is the only member of this family that is of economic importance to
man and livestock (Woolfe, 1992). Louisiana yam is a dicotyledonous, storage
root crop reported to have originated from South America (Yildirim et al., 2011). Today, however, the crop
is grown throughout Africa, Europe and the Americas. Though a perennial plant
with long trailing and slender green or purple vines; sweetpotato is considered
an annual in agronomy adaptable to different agro-ecological conditions (Burt,
2008) including extremely adverse environmental conditions of arid zones (Ahmad
et al., 2014). It has a shorter
growth period than most other root crops (3-5 months) and shows no marked
seasonality (DAFF, 2011).
Sweetpotato produced in 2007 was more than 165
million metric tonnes (MT), among the staple food crops in developing countries
as rice, maize, wheat, maize and cassava, sweet potato ranks fifth in the order
of importance relative to fresh weight (Scott and Maldonado, 1998). Statistics
in 2012 revealed that the total world production of the crop stood at 364
million MT (FAOSTAT, 2016) and China with about 96 million MT per annum is
reported as the world's highest producer and consumer of the crop (FAOSTAT,
2016). Nigeria has the highest yearly yield of 100 million MT of sweetpotato in
Africa followed by Uganda (Bergh et al.,
2012). Yields of 3.1-6.0 metric tonnes per ha have been documented in several
states of Nigeria. However, Kwara State for 3 consecutive years has recorded
sweetpotato root yields of 104,500, 108,910 and 113,750 metric tonnes in 2012,
2013 and 2014 respectively (Kwara State Agricultural Development Project,
2015).
In Nigeria, sweetpotato perceived as one of
the basic food crops particularly in Northern region where they are largely
produced. It is among the six important root and tuber crops grown in Africa.
Within the Sub-Saharan Africa, it is one of the first three (3) root crops
after cassava (Manihot esculenta) and Yam (Discorea spp.) (Ewell and Matuura,
1991; Enyiukwu et al., 2014a,b,d). It
is consumed without much processing in the tropics and presents diverse and
highly profitable industrial uses such as the sweetpotato snacks (Nwanja et al., 2017).
The entire sweetpotato crop is very useful.
The roots are high in calories (energy), fibre and minerals, and are consumed
as food by human while the haulms (vines and leaves) are readily eaten by
cattle, goats, pigs, poultry and fish when fresh or as hay or silage when
dried. Humans also eat the vines as vegetable (DAFF, 2011). Sweetpotato is rich
in sugars, low glycemic index, carbohydrates, vitamins C, B6, beta carotene
(vitamin A equivalent), niacin and folate as well as large profile of minerals
including calcium, iron, magnesium and potassium. Furthermore, it contains
appreciable quantities of dietary fibre. However, they are low in fat and
completely cholesterol-free (Burt, 2008). Its low glycemic index is an
indication of low digestibility of the starch despite its high carbohydrate
content (ILSI, 2008). It has such components as polyphenolics, anthocyanins,
fibre and carotenoids which serve physiological functions such as
anti-oxidation, anti-diabetes, anti-hypertension and anti-ageing attributes
amongst others (Sokoto and Ibrahim, 2007; Yoshimoto, 2010).
In most parts of developing tropical countries
including Nigeria, fresh sweetpotato roots have been reported to have storage
duration of about three weeks only (Rees et
al., 2003; Teye, 2010). However, under controlled atmosphere (Temp. 13 -15oC
and RH of 90 %) the tubers can store for one year (Woolfe, 1992; Rees et al., 2003). Production of sweetpotato
in Nigeria is currently being encouraged for its numerous food security
potential. However, after harvest, the storage of the root is challenged by a
myriad of problems which are often beyond the average farmer's control.
1.2 STATEMENT
OF THE PROBLEM
Besides, immense economic prospects that could
be derived from sweetpotato production and marketing, sweetpotato is highly
perishable. Its perishability arises mainly due to its thin delicate skin which
easily gets damaged during harvesting and post-harvest handling. This is
exacerbated by unfavourable environmental conditions and pest attack in
storage. Under this condition, the roots express deterioration by decay,
shrinkage, weevil infestation and weight loss. It is estimated that in the
tropics each year between 25%-50% of stored agricultural products are lost
because of inadequate farm and village-level storage (Fawole, 2007; Salau and
Shehu, 2015). High water content of its root in addition to damage during
harvesting and post-harvest handling make the storage of the crop difficult and
vulnerable to insects and microbial attacks, resulting in high losses as root
rots and spoilages (Agu et al.,
2015). Other challenges constraining sweetpotato production in Nigeria, include
inadequate government aid, high labour cost, poor access to low interest
credit, lack of new technologies, poor market outlets, poor storage facilities
and high pest and diseases prepronderance are considered principal (Fawole,
2007).
Several postharvest fungal diseases have been
reported to immensely deteriorate nutritional and feed values of sweetpotatao
in storage. They include black rot (Ceratocystics
fimbriata), Scurf (Monilochaetes
infuscans), Soft rot (Rhizopus stolonifer), Java black rot (Diplodia tubericola) and Charcoal rot (Macrophomina phaseoli) (Agu et al.,
2015). In Nigeria mycoflora including Fusarium
oxysporum, Rhizopus stolonifer,
Macrophomina phaseolina, Fusarium solani, Botryodiplodia theobromae are involved in the
postharvest crop spoilage (Clark and Hoy, 1994). Onuegbu (2002) and Oyewale
(2006b) implicated Penicillium sp., Aspergillus flavus Rhizopus stolonifer, Mucor pusillus, Botrytis cinerea and Erysiphe polygoni in the postharvest
storage rot and decay of sweetpotato roots. These rot causing organisms create
local discolouration and disruption of surrounding tissues of infected roots
resulting in changes in appearance, deterioration of texture and organo-leptic
properties of affected roots when cooked (Snowdon, 1991).
Moreover, loss of vital nutrients has been
attributed to these organisms. Depletion of starch granules and loss of protein
and minerals have been reported in potato, sweetpotato and Hausa potato (Amadioha,
1994; Markson et al., 2014; Nwaneri,
2017). According to Jonathan et. al.,
(2017) attack by A. niger, A. tamarii. A. flavus, Fusarium compacticum,
P. chrysogenum and Saccharomyces
spp on sweetpotato root after few months in storage resulted in loss of
carbohydrate (10.00%), fat (0.8%), protein (1.3%), crude fibre (3.8%) and ash
(1.4%). Also, some tuber-borne pathogenic organisms are toxigenic;
contaminating edible roots with toxins that are hazardous to poultry, livestock
and humans. In Egypt, Abdelhamid (1990) detected ochratoxin A (OTA),
aflatoxins, vomiticin, zearalenone and citrinin in feeds of various animals. In
Ekpoma and western Nigeria, contamination of sweetpotato chips, flour, stored
and fermented tubers with aflatoxins B1, B2, G1 and G2 have been documented
(Isibor et al., 2010; Jonathan et al., 2017). These authors argued that
besides microbial deteriorations and reduction in produce quality, these
organisms ultimately lead to reduction in market value of the produce and gross
misfortune to farmers. These pathogens maintain some forms of necrotrophic
lifestyles to invade, colonize and damage sweetpotato root tissues under
storage by the slightest predisposition. Necrotrophic pathogens infect and kill
host tissues with their toxins before extracting nutrients and other growth
factors from the dead host cells (Koeck, 2011; Laluk and Mengiste, 2010).
In order to minimize damage and losses due to
rot incitants, increase sweetpotato root protection in the field, prolong their
shelf-life during storage and transit, several means of plant disease control,
including cultural, immunization, chemical, botanicals and biological measures
that involve antagonistic agents have been invented. Control of rot and storage
deteriorations has been attempted using several cultural approaches like use of
clean vines during planting, crop rotation, field and phyto-sanitation (Wokocha
and Okereke, 2005; Wokocha and Nwaogu, 2008). However, cultural control methods
may not check the disease when epidemics have broken out due to the variability
of the causal agents, which limit the efficacy of use of resistant cultivars.
Chemical control on the other hand has been critical in preventing
losses due to plant diseases, especially with the development of numerous
action-specific fungicides since the 1960s (Wokocha et al., 1986; Wokocha and Uchendu, 2009; Hirooka and Ishii, 2013).
For instance, the fungicides dichloronitroanline protected tubers against
Rhizopus soft rot (Clark and Moyer, 1988). Effective use of other synthetic
fungicides as captan, thiram, mancozeb etc. against the rot diseases (Okigbo,
2004). However, there is obvious fear of mammalian toxicity which results from
chemical residues in pesticides-treated roots and tubers consumed directly by
humans and livestock (Enyiukwu and Awurum, 2013a,b). For instance, thiram and
captan used to protect large volumes of postharvest produce have been banned on
account of mammalian toxicity (Enyiukwu, 2011). In addition, excessive and
inappropriate applications of these chemicals in agriculture have led to the
disruption of ecosystems, several forms of health hazards, pathogens resurgence
and development of resistance to chemo-therapeutants (Amadioha, 2002; 2004).
Besides, resistance of 150 plant pathogens including many rot-inciting fungi to
site-specific fungicides like benomyl, carbendazim, thiophenate-methyl have
been documented (Enyiukwu, 2011a,b). For instance, many of the common
postharvest pathogens of potatoes have been reported to develop resistance to
Mertect (thiabendazole; TBZ), which is the only post-harvest fungicide
presently registered for use on table or processing potatoes in North America
(Platt, 1997; Satyaprasad et al.,1997).
Resistance of pathogenic organisms to agro-chemicals thus far has become a very
serious threat in crop production, being reported to occur every 7-10 years
post-introduction of each synthetic agro-chemical (Enyiukwu et al., 2014a,b). Nevertheless,
post-harvest applications of fungicides for the control of these storage
diseases are grossly expensive.
1.3 JUSTIFICATION
OF THE STUDY
These enormous challenges in the use of
synthetic chemical in plant disease control have stimulated research for
alternative or complements to synthetic fungicides (Asawalam, 2006; Awurum and
Enyiukwu, 2013a). In Nigeria, plant derived-pesticides have been used to
control fungal diseases of several crops and their products including fruits
such as banana in the field and storage (Okigbo and Emoghene, 2004), and tubers
such as yam (Okigbo and Nmeka, 2005), and sweet potato (Amienyo and Ataga,
2007). However, efficacy of pesticides (extracts) of higher plants such as
rotenone, nicoten, pyrethrin and neem extracts are challenged and dwindled by
the influence of high environmental heat and UV- radiation; thus constituting
serious disadvantage to their adoption and use in agriculture (Enyiukwu et al., 2014b)
Some workers have reported the antifungal
potentials of bioagents against pathogens of roots and tubers and agricultural
produce (Okigbo, 2004). These bio-antagonist are eco-compliant and less
phyto-toxic (Amadioha, 2012) and could be used in integrated disease management
(IDM) programmes by low-input farmers without leaving toxic residues on treated
produce when compared to synthetic chemicals (Enyiukwu et al., 2014b). In addition, they are fast growing and produce
multiple bioactive metabolites which are difficult to overcome by rot-inciting
mycoflora making pathogen's resistance to them less likely, suggesting that
they could provide sustainable disease management solutions in organic farming
with zero-synthetic-input tolerance.
Several investigations have shown the inhibitory effects of some
bio-agents on spore germination and growth of pathogenic rot fungi both in the
field and storage (Amadioha, 2004; Okigbo, 2004). A single spray of
soil-derived non-pathogenic strains of Bacillus
subtilis and Trichoderma viride
was reported to potently protect yam tubers in storage for six months against
postharvest rot diseases (Okigbo, 2004). However, reports on the evaluation of
biological antagonists against postharvest fungal rot pathogens are not fully
documented especially against root rot diseases of sweetpotato. Therefore, the
use of these bio-agents including their bioactive ingredients may provide an
ideal and sustainable approach to arresting storage rot diseases of
sweetpotato. These authors also showed that Bacillus
subtilis isolated from cow dung inhibited the growth of pathogenic fungi (B. theobromae and F. oxysporum) isolated from infected yam tuber both in vitro and in
vivo. Biological control is eco-friendly and there is no need for repeated
spray applications as in the case of synthetic chemical or phyto-chemical
interventions against pathogenic organism causing postharvest diseases of root
and tuber crops (Okigbo, 2004). Hence, the evaluation of the effects of some
fungal bio-antagonists and their metabolites against pathogenic organisms
causing rots of sweetpotato root in storage.
1.4 OBJECTIVES
OF THE STUDY
This research was aimed at evaluating the antagonistic activities of
some bioagents (Trichoderma harzianum,
Penicillium oxalicum, Penicillium chrysogenum and Penicillium citrinum) and their metabolites against fungal
pathogens causing rot disease of sweetpotato roots in storage. The specific
objectives of the study were to:
1.
Survey the rot of sweetpotato root in different farm-communities and
major markets in four Local Government Areas (L.G.A.s) in Kwara State.
2.
Isolate and identify the rot causing fungi associated with sweetpotato
roots.
3.
Determine the effects of the rot causing organisms on the biochemical
composition of infected roots of sweetpotato.
4.
Carry out histological studies on the effect of the pathogens on the
tissues of infected sweetpotato root.
5.
Evaluate the biocontrol potentials of Trichoderma harzianum, Penicillium oxalicum, Penicillium chrysogenum and
Penicillium citrinum both in vitro and in vivo.
6.
Compare the fungicidal effects of the bio-agents and benomyl against
the post-harvest rot pathogens of sweetpotato in vitro and in vivo.
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