ABSTRACT
The incidence of bacterial soft rot was investigated in three markets in Zaria area namely:
Samaru, Wusasa and Sabon Gari markets. Examination of symptoms on tubers (both
wounded and unwounded), were done on tubers stored under different environmental
conditions in order to determine the factor(s) that might predispose them to attack by the
soft rot pathogen. High populations of Erwinia spp. were isolated from the samples
collected from various markets, which was an indication of high inoculum level from the
various soils of origin or storehouses. Tubers without symptoms were frequently
contaminated, usually at eye, lenticels and heel end, suggesting contamination from
mother tuber to progeny. Aqueous extracts of neem (Azadirachta indica A. Juss),
Eucalyptus leaves (Eucalyptus citrodorus (L)), Lemon grass (Cymbopogon citratus
(stapf)), Garlic bulb (Allium Sativum (Linn)), Ginger (Zingiber officinale) (Roscoe)) and
Aloe vera were evaluated for the management of bacterial soft rot. Two methods were
used, namely in vitro methods and on the tubers both whole and sliced. Lemon grass had
the greatest inhibitory effect and it’s statistically comparable with garlic, Aloe vera, borax
and neem extracts. Eucalyptus had the least inhibitory effect and statistically similar to
ginger extract. Two methods (Tuber slice assay and point titration assay) were used to
screen yam (Dioscorea rotundata), sweet potato (Ipomoea batatas), Irish potato
(Solanum tuberosum), cocoyam (Xanthosoma sagittifolium), cassava (Manihot esculenta)
and carrot (Daucus carota) for susceptibility to soft rot bacteria. Irish potato ranked the
most susceptible followed by carrot, sweet potato, yam, cocoyam and cassava. The two
methods examined were considered to be effective for screening tubers for bacterial soft
rot. However, with the single site titration assay, it is possible to determine not only the
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extent of tissue maceration but also the bacterial population necessary for initiation of
lesions. Tuber Slice assay on the other hand, has the advantage of being faster than the
single site titration assay.
TABLE OF CONTENTS
Title Page i
Declaration ii
Dedication iii
Certification Iv
Acknowledgements v
Table of Contents vi
List of Tables ix
List of Figures x
List of Appendices xi
Abstract xiii
1.0
CHAPTER ONE
INTRODUCTION
1
1.1 General 1
1.2 Purpose of the study 4
1.3 Objectives 4
2.0
CHAPTER TWO
LITERATURE REVIEW
5
2.1 Diseases of Tubers 6
2.2 Economic Importance of the Disease 6
2.3 Symptoms of Soft Rot 7
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2.4 Host Range 9
2.5 Epidemiology 9
2.6 Factors Affecting Infection and Bacterial Multiplication 11
2.7 Control 12
2.7.1 The use of plant extracts and local materials for the control of plant
diseases
14
3.0
CHAPTER THREE
A MARKET SURVEY OF THE INCIDENCE OF BACTERIAL SOFT
ROT OF POTATOES IN THE ZARIA AREA OF KADUNA STATE
18
3.1 Introduction 18
3.2 Materials and Methods 18
3.2.1 Sampling Sites 18
3.2.2 Sampling Procedure 19
3.2.3 Isolation of the Pathogen 19
3.3 Results and Discussion 20
4.0
CHAPTER FOUR
EVALUATION OF SOME PLANT EXTRACTS FOR THE
CONTROL OF BACTERIAL SOFT ROT OF TUBERS
28
4.1 Introduction 28
4.2 Materials and Methods 28
4.2.1 Experimental Materials 28
4.2.2 Preparation of Plant Extracts 29
4.2.3 Preparation of Bacterial Inoculum 29
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4.2.4 Evaluation of Plant Extract 30
4.2.4.1 Single Site Titration Assay 30
4.2.4.2 Tuber Slice Assay 31
4.2.5 Testing sensitivity of the organism to the extract 31
4.3 Results and Discussion 32
5.0
CHAPTER FIVE
DETERMINATION OF THE RELATIVE SUSCEPTIBILITY OF
TUBERS TO SOFT ROT PATHOGEN
38
5.1 Introduction 38
5.2 Materials and Methods 38
5.2.1 Single Site Titration Assay 39
5.2.2 Tuber Slice Assay 39
5.3 Results and Discussion 39
6.0
CHAPTER SIX
SUMMARY AND CONCLUSIONS
45
7.0 REFERENCES 49
8.0 APPENDICES 65
LIST OF TABLES
Table Page
3.1 Erwinia spp. population on potato tubers in Zaria 22
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3.2 Incidence of Erwinia spp. following storage in polythene bags 23
3.3 Incidence of Erwinia spp. Following storage under room temperature 23
4.1 Effect of plant extracts on the population of Erwinia spp. in vitro using
dilution plate count method
33
4.2 Inhibition of rot by plant extracts (mm) in the single site titration assay 35
4.3 Inhibition of rot by plant extracts (mm) in the tuber assay 35
CHAPTER ONE
1.0 INTRODUCTION
1.1 General
In sub-Saharan Africa where population is ever increasing with low technological
growth, drudgery is always associated with most works (Ofori and Hahn, 1991). This of course,
necessitates the consumption of more energy-giving food like roots and tubers. Roots and tubers as the
name implies are plants that develop starchy roots, tubers, stems, bulbs or corms that acts as a food store
for the plant. Nutritionally, they are composed mainly of water and starch with small amount of protein,
fibre, minerals and vitamins (mainly A and C). These crops include yam, cassava, Irish potato, cocoyam
and sweet potatoes. Most of these tubers were in the recent past regarded as prestigious items of food,
meant only for the affluent (Best, 1996).
Following the failure of earlier emphasis on cereals to bridge the food production gap, serious attention is
currently being given to the development and promotion of traditionally starchy stable foods in many
Africa countries including Nigeria (CICRTCR, 2000; Ofori and Hahn, 1991). Yam has a historic role in
the socio-cultural life of many Africans, so much so that the new produce is seen as a source of celebration,
especially in South-eastern and middle belt of Nigeria; and currently, presentation of specified number of
tubers is a condition for the settlement of bride price in most West Africa countries (Dorosh, 1988).
Root and tubers deserve particular attention because many developing world’s poorest and most food
insecure households look to these crops as contributing, if not the principal source of food, nutrition, and
cash income. Among other things, farmhouse-holds see the value of these crops in their ability to produce
more yields per hectare with low doses of fertilizer and chemical inputs than other food commodities and
also their ability to generate yields under condition where other crops may fail (Scott et al., 2000).
Apart from the importance of these crops in food security and food self-sufficiency, they contribute
significantly to the national economies and rural income and provide employment for most rural women in
sub-Saharan Africa (Committee on Inter-centre Root and Tuber Crops Research (CICRTCR, 2000).
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Consultative Group on International Agricultural Research (CGIAR, 2000) reported that farmers in
developing countries harvested 439 million metric tons of the major roots and tubers with an estimated
value of more then US $41 billion, nearly one fourth, the value of the major cereals. Sweet potato will play
a supplementary role to cassava as food security and cash, while both potato and yam can help eliminate
poverty and improve food security in their respective areas of greatest concentration, making these crops
the most world wide demand food crops of the tropics (Anon, 1975; Albracht and Valleys, 1977; Nweke,
1992; Scott et al., 2000).
The population growth rates in many developing countries are still relatively high (1.5% per annum) and
such will account for over 97% of the increase in the world population by 2020 (Pinstrup et al., 1999;
United Nations, 1999). Sub-Saharan Africa has the highest projected growth rates in population averaging
over 2.4%. It is worthy to note therefore, that in the same sub-region roots and tubers play a greater role in
local food system than they do in many parts of the world (Nweke, 1992; Scott, 1994). The linkages
among agricultural production, income generation, and poverty eradication present another dimension to
the potential contribution of these crops to the global food system and important implication for
maximizing their potential in the years ahead (Maldonado et al, 1998).
Root and tubers mean different things to different people in different regions of the world and at different
levels of economic well-being. These crops have myriad and complex parts to play in feeding the world in
the coming decades. These include food security, raw materials for industries, reducing poverty, feed for
animals and source of income to the farmer and government. CGIAR and CICRTCR Vision 2020 project
that roots and tubers will be integrated into emerging markets through the efficient and economically sound
production of a diversified range of high quality, competitive products for food, feed and industry. These
crops’ adaptation to marginal environment, their contribution to household food security and their great
flexibility in mixed farming system make them an important component of a targeted strategy that seeks to
improve the welfare of the rural poor and to link small holder farmers with this growth market.
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Nigeria produces 40% of the total production of roots and tubers of the 80-90% in Africa. Nigeria leads in
the production of cassava and yam. In 1994 Nigeria produced 47,000 mt of yam, cassava, and cocoyam
compared with 19,700 mt of cereal (FAO, CBN, 1994).
The various yields of these crops are yam 10-25 mt/ha. Cassava 40 mt/ha, Potato 25-40 mt/ha, sweet
potato 15-30 mt/ha and coco yam 20-25 mt/ha (CBN, FAO, 1994).
Erwinia spp. induces soft rot of numerous crops both during the vegetative cycle and on stored tubers and
is therefore considered the most threatening bacteria on potato and other tubers world-wide. Control of this
disease with available chemicals has so far not been effective (Goto, 1992; Lowesz, 1992).
1.2 Purpose of the Study
Root and tuber are the target crops in the new millennium that mean many things to very many people of
the world. These crops will mean the difference between subsistence and achieving a leg up on the
economic ladder; in others, they will mean the difference between survival and starvation (CICRTCR,
2000). These great potential crops have alarming rates of post harvest loss due to soft rot induced by
Erwinia spp. and hitherto, there is no effective strategies to control the disease (Carolina et al., 2000). The
aim of this research therefore is to initiate a control strategy using some natural plant products that are safe
to man and environment-friendly to manage bacterial soft rot.
1.3 Objectives
a) To determine the incidence of bacterial soft rot of potato in markets in the Zaria area.
b) To determine the relative susceptibility of tubers of potatoes, sweet potatoes, yam, cassava,
carrot and cocoyam to the soft- rot bacteria.
c) To evaluate some plant extracts for the control of bacterial soft rot.
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