ABSTRACT
Screenhouse and field experiments were conducted between 2008 and 2010 to quantify yield loss
due to Rice yellow mottle virus (RYMV) in some selected rice cultivars; determine the
epidemiology of RYMV in relation to weather factors on the incidence of RYMV and insect vector
population composition and identification studies; identify its alternative hosts; determine serotypes
of the pathogen; and molecular characterization of the strains and their distribution in the northern
states of Nigeria. The screenhouse experiment was arranged in a strip plot design with three
replicates. Significant interaction between the effects of inoculation regimes and rice cultivars were
found between the disease, growth and yield parameters. The critical periods of RYMV infection
for most of the rice cultivars used was within 4 – 8 Weeks after sowing (WAS). Paddy yield losses
of 6.59 – 79.11 % and 3.65 – 81.67 % were recorded on the test cultivars in rainy and dry seasons,
respectively. Keeping the plants RYMV- free for up to 10 WAS resulted in grain yield almost
similar to control. The epidemiology experiment was laid out in a randomized complete block
design (RCBD) with three replicates. Significant positive and negative correlations were recorded
between RYMV incidence and severity index, some insect vectors and weather factors. Insect
species collected in and around the experimental field, included Oxya sp., Locris rubra Fabriculus,,
Chnootriba similis Thunberg, Chaetocnema sp., Cheilomenes sp., Zonocerus variegatus L.,
Paratettix sp., Conocephalus sp., Nezara viridula L. and Dactylispa sp. they transmitted the virus
to different degree. Serological studies showed that some weed species with or without symptoms
collected, across RYMV infected areas in Northern Nigeria tested positive to RYMV. These
included Imperata cylindrica L., Erograstic tenella L., E. Ciliaris L., Digitaria horizontalis Wild.,
Eleusine indica (L) Gaertner., E. corocana L., Cynodon dactylon (L) Pers., Panicum sp.,
Dactyloctenium aegyptium L., Brachiaria lata (Schumach) C.E. Hubbard ex Robyns., Axonopus
compressus (Sw.) P. Beauv., Cyperus rotundus L., C. difformis L., Oryza barthii A. Chev., O.
longistaminata A. Chev.& Roehr. and Pennisetum pedicellatum Trin. Six serological profiles
(Sep.) or serotypes were identified (Sep A to F) in this study. Sep A was found in Gombe, Kaduna,
Kano, Niger, Sokoto and Zamfara states, Sep B was found in Borno, Gombe, Kaduna, and Kano,
states, Sep C was found in Gombe state, Sep D was found in Borno, Sokoto and Zamfara states,
Sep E was found in Borno and Zamfara states and Sep F was found in Kano, Niger and Sokoto
states. The molecular characterization of RYMV isolates revealed that more than two strains of the
virus were present in northern Nigeria. Strain S1 is widespread in Borno, Gombe, Kaduna, Kano,
Niger, Sokoto and Zamfara states. By contrast, strain S2 was only detected in Gombe, Kaduna and
Niger states. Strains SX occurred in swampy sites in Borno, Gombe, Kaduna, Kano and Zamfara
states. The information provided on the vulnerable phase of the plant growth on yield loss to the
pathogen can be useful in integrated pest and production management for rice in RYMV endemic
areas. The relationship established between RYMV incidences, insect vector population and
variation in weather factors data could be used in RYMV disease magnitude prediction. In addition,
Knowledge of weed hosts, potential insect vectors in and around RYMV infected fields, and
distinction between isolates/strains is critical and would provide useful information in
pathogenicity and breeding studies of RYMV in Northern Nigeria.
TABLE OF CONTENTS
Title page . . . . . . . . . . i
Declaration . . . . . . . . . . ii
Certification . . . . . . . . . . iii
Dedication . . . . . . . . . . iv
Acknowledgement . .. .. .. .. .. .. .. v
Abstract . .. .. .. .. .. .. .. vii
Table of Content .. .. .. .. .. .. .. .. .. viii
List of Figures .. .. .. .. .. .. .. .. .. xv
List of Tables .. .. .. .. .. .. .. .. .. .. xvi
List of Plates .. .. .. .. .. .. .. .. .. .. xx
List of Appendices .. .. .. .. .. .. .. .. .. xxi
Symbols .. .. .. .. .. .. .. .. .. .. xxxii
Abbreviations .. .. .. .. .. .. .. .. .. .. xxxiii
CHAPTER ONE
1.0 INTRODUCTION .. .. .. .. .. .. .. .. 1
1.1 JUSTIFICATION .. .. .. .. .. .. .. .. 4
1.2 OBJECTIVES OF THE STUDY . .. .. .. .. .. 6
CHAPTER TWO
2.0 LITERATURE REVIEW … .. .. .. .. .. .. .7
2.1 RICE … .. .. .. .. .. .. .. .. .. 7
2.2 IMPORTANCE OF RICE YELLOW MOTTLE VIRUS … .. .. 7
2.3 DISTRIBUTION OF RICE YELLOW MOTTLE VIRUS … .. .. 8
2.4 SYMPTOMS OF RICE YELLOW MOTTLE VIRUS .. .. .. 10
2.5 MOLECULAR CHARACTERISTICS AND SEROTYPES OF RICE
ix
YELLOW MOTTLE VIRUS . .. .. .. .. .. 13
2.6 TRANSMISSION OF RICE YELLOW MOTTLE VIRUS .. .. .. 15
2.7 HOST RANGE OF RICE YELLOW MOTTLE VIRUS. .. .. .. 16
2.8 EPIDEMIOLOGY OF RICE YELLOW MOTTLE VIRUS. .. .. .. 18
2.9 VARIETAL REACTION AND YIELD LOSS ASSESSMENT . .. 21
2.10 MANAGEMENT STRATEGIES OF RICE YELLOW MOTTLE VIRUS .. 22
CHAPTER THREE
3.0 MATERIALS AND METHODS .. .. .. .. .. .. 26
3.1 EXPERIMENT I: EFFECT OF RYMV ON PLANT GROWTH AND
YIELD LOSS ASSESSMENT OF RICE CULTIVARS .. .. .. 26
3.1.1. Data Collection and Analyses .. .. .. .. .. .. 26
3.1.1.1. RYMV Disease assessment .. .. .. .. .. .. 26
3.2. EXPERIMENT 1I: EPIDEMIOLOGY OF RYMV .. .. .. .. 30
3.2.1. Sampling and identification of insect vector of RYMV associated
with rice plant .. .. .. .. .. .. .. .. 31
3.2.2. Transmission test of insect associated with rice plants in the Fields . .. 31
3.2.3 Relative abundance of insect vectors associated with rice plant in
relation to weather factors and RYMV incidence . .. .. .. 32
3.3. EXPERIMENT III: IDENTIFICATION OF WEED HOSTS OF RYMV . 33
3.3.1. Field Survey and Sampling .. .. .. .. .. .. .. 33
3.3.2. Double Antibody Sandwich Enzyme – Linked Immuno Sorbent Assay . 34
3.4. EXPERIMENT IV: DETERMINATION OF SEROTYPE(S) OF RYMV .. 35
3.4.1. Serological Characterization of RYMV Isolates .. .. .. .. 36
3.4.1.1. TAS-ELISA . .. .. .. .. .. .. .. 36
3.5. EXPERIMENT V: MOLECULAR CHARACTERIZATION OF RYMV
ISOLATES .. .. .. .. .. .. .. .. .. 37
3.5.1. RNA Extraction Protocol .. .. .. .. .. .. .. 37
x
3.5.2. Reverse Transcriptase- Polymerase Chain Reaction (RT-PCR) .. .. 38
3.5.2.1. First Strand- cDNA Synthesis .. .. .. .. .. .. 38
3.5.3. Polymerase Chain Reaction (PCR) .. .. .. .. .. .. 40
3.5.3.1. PCR Cocktail .. .. .. .. .. .. .. .. .. 40
3.5.4. Agarose gel Electrophoresis of PCR Products . .. .. .. 40
CHAPTER FOUR
4.0 RESULTS .. .. .. .. .. .. .. .. .. 42
4.1. SCREENHOUSE EXPERIMENTS .. .. .. .. .. .. 42
4.1.1. Effect of inoculation regime of RYMV on plant height of rice cultivars
in the Screenhouse during 2008, 2009 and 2010 rainy and 2008/ 2009
and 2009/ 2010 dry seasons. .. .. .. .. .. .. .. 42
4.1.2. Interaction of rice cultivars and inoculation regime of RYMV on
plant height in the Screenhouse (Combine analysis) .. .. .. .. 42
4.1.3. Effect of inoculation regime of RYMV on number of tillers per plant
of rice cultivars in the Screenhouse during 2008, 2009 and
2010 rainy and 2008/ 2009 and 2009/ 2010 dry seasons. .. .. .. 45
4.1.4. Interaction of rice cultivars and inoculation regime on number
of tillers per plant in the Screenhouse (Combine analysis) .. .. .. 48
4.1.5. Effect of inoculation regime of RYMV on days to 50 % flowering
of rice cultivars in the Screenhouse during 2008, 2009 and 2010 rainy
and 2008/ 2009 and 2009/ 2010 dry seasons. .. .. .. .. .. 49
4.1.6. Interaction of rice cultivars and inoculation regime of RYMV on
the days to 50 % flowering in the Screenhouse (Combine analysis) .. .. 50
4.1.7. Effect of inoculation regime of RYMV on the number of grains
per panicle of rice cultivars in the Screenhouse during 2008, 2009
and 2010 rainy and 2008/ 2009 and 2009/ 2010 dry seasons. .. .. 53
4.1.8. Interaction of rice cultivars and inoculation regime of RYMV
on the number of grains per panicle in the Screenhouse
(Combine analysis) . .. .. .. .. .. .. .. 54
4.1.9. Effect of inoculation regime of RYMV on paddy yield of rice
cultivars in the Screenhouse during 2008, 2009 and 2010 rainy
and 2008/ 2009and 2009/ 2010 dry seasons… .. .. .. .. 57
4.1.10. Interaction of rice cultivars and inoculation regime of RYMV
xi
on paddy yield in the Screenhouse (Combine analysis).. .. .. 58
4.1.11. Effect of inoculation regime of RYMV on the paddy yield loss
of rice cultivars in the Screenhouse during 2008, 2009 and 2010
rainy and 2008/ 2009 and 2009/ 2010 dry seasons. .. .. .. .. 61
4.1.12. Interaction of rice cultivars and inoculation regime of RYMV
on paddy yield loss in the Screenhouse (Combine analysis ) .. .. 62
4.1.13. Effect of inoculation regime of RYMV on the threshing percentage
of rice cultivars in the Screenhouse during 2008, 2009 and 2010 rainy
and 2008/ 2009 and 2009/ 2010 dry seasons. .. .. .. .. 65
4.1.14. Interaction of rice cultivars and inoculation regime of RYMV on
threshing percentage in the Screenhouse (Combine analysis) .. .. 66
4.1.15. Effect of inoculation regime of RYMV on 1000- grain weight
of rice cultivars in the Screenhouse during 2008, 2009 and 2010 rainy
and 2008/ 2009 and 2009/ 2010 dry seasons. .. .. .. .. 69
4.1.16. Interaction of rice cultivars and inoculation regime of RYMV on
1000-grain weight in the Screenhouse (Combine analysis) .. .. 70
4.1.17. Effect of inoculation regime and rice cultivars on the incidence of RYMV
in the Screenhouse during 2008, 2009 and 2010 rainy and 2008/ 2009
and 2009/ 2010 dry seasons. .. .. .. .. .. .. 73
4.1.18. Interaction of inoculation regime and rice cultivar on the
incidence of RYMV in the Screenhouse (Combine analysis) .. .. 74
4.1.19. Effect of inoculation regime and rice cultivar on the severity index of
RYMV in the Screenhouse during 2008, 2009 and 2010 rainy and
2008/ 2009 and 2009/ 2010 dry seasons. .. .. .. .. .. 77
4. 1.20. Interaction of inoculation regime and rice cultivar on the
severity index of RYMV in the Screenhouse (Combine analysis) .. 79
4.1.21. Correlation coefficient for Screenhouse experiments in 2008, 2009
and 2010 rainy, 2008/ 2009 and 2009/ 2010 dry seasons .. .. .. 82
4.2. FIELD EXPERIMENTS .. .. .. .. .. .. .. 87
4.2.1. Incidence of RYMV insect vectors during the 2008, 2009 and 2010
rainy seasons at Bomo .. .. .. .. .. .. .. 87
4.2.2 Incidence of RYMV insect vectors during the 2008, 2009 and 2010
rainy seasons at Sayen Gobirawa. .. .. .. .. .. .. 90
4.2.3. Incidence of RYMV insect vectors during the 2008/ 2009 and
2009/2010 dry seasons at Nasarawan Buhari . .. .. .. 92
xii
4.2.4. Effect of weather factors on insect vector population, RYMV incidence
and severity index at Bomo from 2008-2010 rainy seasons .. .. 94
4.2.5. Effect of weather factors on insect vector population, RYMV incidence
and severity index at Sayen Gobirawa from 2008-2010 rainy seasons .. 96
4.2.6. Effect of weather factors on insect vector population, RYMV incidence
and severity index at Nasarawan Buhari in 2008/ 2009 and 2009/ 2010 dry
seasons .. .. .. .. .. .. .. .. .. 98
4.2.7. Effect of RYMV incidence and severity index on paddy yield during the
2008 rainy seasons at Bomo .. .. .. .. .. .. 100
4.2.8. Effect of RYMV incidence and severity index on paddy yield during the
2009 rainy seasons at Bomo .. .. .. .. .. .. 102
4.2.9. Effect of RYMV incidence and severity index on paddy yield during the
2010 rainy seasons at Bomo .. .. .. .. .. .. 106
4.2.10. Effect of RYMV incidence and severity index on paddy yield during the
2008 rainy seasons at Sayen Gobirawa .. .. .. .. .. 108
4.2.11. Effect of RYMV incidence and severity index on paddy yield during the
2009 rainy seasons at Sayen Gobirawa .. .. .. .. .. 110
4.2.12. Effect of RYMV incidence and severity index on paddy yield during
the 2010 rainy seasons at Sayen Gobirawa .. .. .. .. 113
4.2.13. Effect of RYMV incidence and severity index on paddy yield during
the 2008/ 2009 dry season at Nasarawan Buhari .. .. .. .. 117
4.2.14. Effect of RYMV incidence and severity index on paddy yield during
the 2009/ 2010 dry season at Nasarawan Buhari. .. .. .. .. 118
4.2.15. Transmission test of some insect found in and around the experimental
fields .. .. .. .. .. .. .. .. .. 122
4.2.16. Correlation coefficient for the field experiments in 2008, 2009 and
2010 wet and 2008/ 2009 and 2009/ 2010 dry seasons .. .. .. 127
4.3. THE WEED HOSTS OF RYMV ACROSS THE SURVEYED STATES .. 140
4.4. SEROLOGICAL PROFILE OF RICE YELLOW MOTTLE VIRUS
ISOLATES .. .. .. .. .. .. .. .. .. 142
4.4.1. Hierarchical Cluster Dendrogram analysis of RYMV serological profile .. 146
xiii
4.5. MOLECULAR CHARACTERISATION AND DISTRIBUTION OF RYMV
ISOLATES . .. .. .. .. .. .. .. .. 148
4.5.1. Percentage Distribution of RYMV Strains across the Surveyed States .. 151
CHAPTER FIVE
5.0 DISCUSSION . .. .. .. .. .. .. .. 156
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION . .. 176
6.1 SUMMARY AND CONCLUSION .. .. .. .. .. .. 176
6.2. RECOMMENDATION .. .. .. .. .. .. .. 179
6.3. RESEARCH FINDINGS .. .. .. .. .. .. .. 181
REFFERENCES .. .. .. .. .. .. .. .. 182
APPENDICES .. .. .. .. .. .. .. .. 198
xiv
CHAPTER ONE
1.0 INTRODUCTION
Rice (Oryza sativa L.) is a staple food for about one half of people in the world and is mainly
harvested and consumed in Asia and Africa (Anonymous, 2005; Sun et al., 2006). In many regions
it is eaten with every meal and provides more calories than any other single food (Hynes, 2007). It
is not only consumed by humans and fed to livestock as it is also useful industrially for processing
into acetic acid, glucose and starch. Rice straw and stems are used by local farmers as bedding for
animals and for weaving roofs, hats, baskets and sandals. The inedible rice hull is used as fuel,
fertilizer and insulation (Schalbroeck, 2001; Gao et al., 2004; Hynes, 2007).
Rice is produced in about 10% of all cropland (144 million hactares) in over 110 countries (Luth,
1980). It has the ability to adapt to diverse environmental conditions and is therefore, grown in
varied climatic conditions ranging from Sahel to the Rainforest ecological zones (Luth, 1980) in
every country in West Africa (WARDA, 1995).The main production ecologies are rainfed lowland,
rainfed upland, irrigated lowland, deep water/flooding and mangrove swamp (Onyishi et al., 2010).
Nigeria cultivated 2.191 million hectares of land with rice paddy production of 3.28 million metric
tonnes in 1999 while land area under rice production increased by 0.4 % with 3.30 metric tonnes in
2000 (Anonymous, 2001). The potential land area for rice production in Nigeria is between 4.6 and
4.9 million hectares (Nwilene et al., 2009a). Despite the continuous increase in the land area that is
cultivated with rice, Nigeria still imports rice annually (FAO, 2010). It was reported in order to
meet local demand, Nigeria imported 1.7 and 1.8, million tonnes of rice in 2008/ 2009 and 2009/
2010, respectively (FAO, 2010). Post-forecast Nigeria’s rice imports in 2010/ 2011 is to grow to
1.9 million tonnes (FAO, 2010).
Rice is the primary and secondary host of many viruses (Koklu and Yilmaz, 2004) but the most
important is the rice yellow mottle virus (RYMV) genus Sobemovirus (Awoderu, 1991a; Fargette
2
et al., 2002). It is endemic in Africa and became important after the introduction of new high
yielding exotic varieties that are susceptible to the virus (Rossel and Thottappilly, 1988),
availability of water through irrigation, which allows for sequential planting and maintenance of
higher crop intensity without dry season gaps (IITA, 1981, 1982; Rossel et al., 1982a; Thresh,
1989). The pathogen was first noticed in 1966 in Kenya (Bakker, 1970). However, it has since
spread to other parts of Africa (Koklu and Yilmaz, 2004). The virus is characterized by high
genetic variability due to its intrinsic high rates of mutation which is evident from the detection of
several strains of the pathogen (N’Guessan et al., 2001; Pinel- Galzi and Fargette, 2006). Six
strains of the virus have been reported so far: S1, S2 and S3 in West and Central Africa, S4, S5 and
S6 in East Africa (Pinel et al., 2000; Fargette et al., 2002; Banwo et al., 2004a). Both virulent and
avirulent isolates were found among the strains, indicating that phytopathological traits of an
isolate are not hereditary or confined to a specific strain or a particular region, but occurs naturally
in different lineages in different parts of Africa (Sorha et al., 2005). The emergence of several
strains of the virus is due to its high replication, as well as intensification of rice cultivation in most
countries of Africa (Awoderu, 1991a; Traore et al., 2005).
Generally, RYMV infection is characterized by mottle and yellowing symptoms of varying
intensities, depending on rice genotype and age at infection (Bakker, 1970; Thottappilly and
Rossel, 1993a), as well as the virulence of the invading virus strain. Other symptoms include
wilting, streaking of the leaves, stunting of the plant, grain discolouration, malformation and partial
emergence of the panicles, non – synchronous flowering, brown to dark brown discolouration of
panicles and grain, sterility and death of infected plants (Bakker 1971; 1974; Fauquet and
Thouvenel, 1977; Thottappilly and Rossel, 1993b). Symptoms occur at any stage from
transplanting to booting (Abo et al., 1998).
3
RYMV is environmentally stable and is transmitted mechanically from rice to rice (Bakker, 1971;
Fauquet and Thouvenel, 1977). The virus can be spread mechanically through contact between
healthy and infected plants, contaminated hands, cow dung and soil containing undecomposed
infected plant materials, debris, gestation fluid, irrigation water and harvesting tool such as sickle
(Bakker, 1974; Reckhaus and Andriamasintseheno, 1995; Tsuboi et al., 1995; Abo, 1998; Sarra et
al., 2004; Traore et al., 2006; Traore et al., 2008). It is also transmitted by insect vectors belonging
to the families Chrysomelidae (Chaetocnema spp., Dactylispa spp., Hispa unsambarica Weise,
Sesselia pusilla Gartucker, Trichispa sericea Guerin); Tettigonidae (Conocephalus longipennis de
Haan, C. merumontamus Sjostedt)) and Coccinelidae (Chnootriba similis Thunberg) (Bakker,
1971; Reckhaus and Adamou, 1986 and Abo, 1998).
RYMV disease has become a major limiting factor to rice production for lowland and irrigation
ecosystems in Africa. It is now found in virtually all rice growing countries in Sub-Saharan Africa
(Fomba, 1990; Aworedu et al., 1991a) and on the islands of Zanzibar and Madagascar (Kouassi, et
al., 2005). According to Traore et al. (2005), RYMV occurs in diverse cultivated rice system
(rainfed, irrigated lowland and mangrove), in all kinds of ecosystems (forest, woodland Savanna,
grass Savanna up to the Sahelian region as well as upland Savanna), in countries where rice is
either a main or a minor crop and also where epidemics are widespread or localized.
Diversification and spread of RYMV has been concommitant with an extension of rice cultivation
in Africa since the 19th century. This major agro- ecological change increased the encounter
between primary hosts of RYMV and the cultivated rice; it also modified the landscape ecology in
ways that facilitated virus spread (Traore et al., 2009). It is known to be naturally very destructive
and therefore threatening the rice production in the African continent on a large scale (Banwo et
al., 2004b) rapidly spreading among and within the major rice growing areas (Ali, 1999) and cause
severe yield losses ranging from 25 to 100% depending on the date and time of infection as well as
4
type of rice variety (Rossel et al., 1982b; Taylor et al., 1990; Sy, 1994; Konate et al., 1997). Crop
abandonment due to the infection of rice field by RYMV was also reported in Zamfara state of
Nigeria (Alegbejo et al., 2006).
1.1 JUSTIFICATION
RYMV remains one of the major constraints to rice production in rainfed lowland and irrigated
ecosystems in Africa. In Nigeria management of RYMV has been largely concentrated on the
identification and development of resistant materials without success to develop cultivar with
durable resistance to the disease, rice production is undermined and yield is always made uncertain
by the occurrence of RYMV. Although, some work has been carried out on yield loss due to this
virus, yet none has related the effect of the virus to the growth stages or established the stage of the
plant growth at which virus infection is most detrimental to growth and yield attributes Therefore,
more research is needed to assess the crop loss at different stages of the plant growth due to RYMV
incidence and severity, hence the need to identify the critical stage of the plant growth when the
disease has high effect on the yield components or suggest previously unrecognized associations
between risk time factor and the disease.
Just like there is no information on responses of rice cultivars to RYMV infection stages in the
country, hence there is need to concentrate on filling in the gaps in existing knowledge of the virus
epidemiology with a view to gathering more information on the virus epidemics under the
prevailing weather conditions in the country. Despite the strong influence that the local
environmental condition may have on the spread of the disease, individual climatic factors with
respect to epidemiological knowledge has still received little attention from researchers. It is
probably the reason why research is yet to come up with efficient methods to manage the disesase.
Therefore, the impact of specific climatic factors on the incidence and severity of the virus,
5
population density of insect vector and responses of the host plant was initiated so that progress in
designing intervention strategies that are sustainable are recommended for disease management.
There is scanty information on the alternative hosts where the virus population survive during off
season as only a few weed species have been reported in Nigeria, resulting in their role in RYMV
epidemiology being poorly understood. This is a major research challenge which would broaden
the existing knowledge of the disease epidemiology with a view to developing management
strategies of RYMV in the country.
Although RYMV is probably present every where rice is grown in Nigeria and poses serious threats
to rice production. There is limited information on the different serotypes of RYMV in Nigeria.
The existence of different RYMV serotypes in the field that cause different kinds of diseases is a
matter of considerable practical importance. Therefore, more research is needed for distinguishing
and identifying those serotypes. Knowledge of the serological relationship between RYMV isolates
is valuable in diagnostic work and may prove to be important in epidemiological research and in
breeding for RYMV resistant programmes.
Considering the several strains of RYMV and its rapid spread within and between states in Nigeria,
information on the nature of variability and assessing the molecular characteristics of the virus, is
indispensable in understanding RYMV biology and inherent variability as well as shifts in
pathogen population. Obtaining such information of RYMV will be greatly useful for
understanding how the virus strains changed and evolved from a common ancestor. Therefore,
more study is needed to understand the molecular characteristics of the virus, to provide more
accurate and rapid identification of the pathogen, and may contribute to the development of
methods of management of the virus in the study area.
6
1.2 OBJECTIVES
The objectives of the study are to:
i. Quantify yield losses due to RYMV in selected rice cultivars
ii. Determine the influence of weather factors on the vectors populations and RYMV
incidence and severity
iii. Identify the alternative hosts of the virus
iv. Identify the serotypes of RYMV and their distribution in northern Nigeria, and
v. Molecular characterization of strains of the virus.
7
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