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Journal of Food and Agriculture 2020, 13(II): 1 - 19
DOI: http://doi.org/10.4038/jfa.v13i2.5229
1
Assessment of Sweet Potato Virus Disease Under the Presence of a
Mulch-material and an Organic Insecticide on Sweet Potato and the
Viral Distribution in Kwara State of Nigeria
T. H. Aliyu 1*, J. Popoola 2, L. K. Olatinwo 3, E. K. Eifediyi 4, O. Arogundade 5 and B. H.
Taiwo6
ABSTRACT
Assessments of 30 sweet potato farms in
three districts of Kwara State of Nigeria
were carried out to evaluate the incidence
of sweet potato virus disease (SPVD). A
serological study by Antigen-coated plate
enzyme-linked immunosorbent assay
(ACP-ELISA) was set up to assay six
viruses on sweet potato samples. A field
experiment to appraise the influence of
variety, mulch and organic insecticide on
virus incidence was performed on two
______________________________________
1Department of Crop Protection, Faculty of Agriculture,
University of Ilorin-Nigeria.
2Department of Statistics,, Faculty of Physical Science,
University of Ilorin-Nigeria.
3Department of Agricultural Extension and Economics,
Faculty of Agricultural Sciences, Kwara State University,
Malete-Nigeria.
4Department of Agronomy, Faculty of Agriculture,
University of Ilorin – Nigeria.
5National Horticultural Research Institute, Ibadan –
Nigeria.
6School of Agriculture and Policy University of Reading
RG6 6AR, United Kingdom.
*aliyutaiyehussein@yahoo.com
http://orcid.org/0000-0002-8638-1742
This article is published
under the terms of
the Creative Commons
Attribution 4.0 International License which permits
unrestricted use, distribution and reproduction in
any medium provided the original author and source
are credited.
genetically improved sweet potato
varieties. The surveys revealed the
occurrence of SPVD in all locations with
incidence from 2.6% in Oyun (Kwara
Central district) to 39.1% in Agbamu
(Kwara South district). The serology test
confirmed the predominance of Sweet
potato feathery mottle virus (SPFMV) and
Sweet potato chlorotic stunt virus
(SPCSV) correspondingly as 100% and
30% respectively in the study. The field
experiment indicated lower virus disease
incidence (15.3%) and increased numbers
in storage roots (7.5/plant) and weight of
storage roots (32.4 kg/plant) in Orange
Fleshed Sweet Potato variety, mulched
with organic insecticidal treatment. This
is the first authentication of the
occurrence and prevalence of sweet potato
virus disease in Kwara State, Nigeria. The
study recommends Orange Fleshed Sweet
Potato variety, mulching with guinea corn
stalk and organic insecticide application
for sustainable management of SPVD,
particularly for resource-poor farmers.
Keywords: ACP-ELISA, Food security,
Guinea corn husk, Sustainable disease
management, Wood ash.
INTRODUCTION
Sweet potato (Ipomoea batatas L) is
among the most important food crops
T. H. Aliyu et al.
2
in the world and an extremely
important food crop for subsistence
farmers in Sub-Saharan Africa (SSA). It
is a vegetatively propagated vegetable
and ranks as the world’s seventh most
important crop, with an estimated
annual production of approximately
122 million metric tons (Wau and
Komolong, 2019). In developing
countries, sweet potato is especially
valued because it is highly adaptable,
and tolerates high temperatures, low
fertility soil and drought (Yamakawa
and Yoshimoto, 2002).
Practically, most production and
consumption of sweet potatoes occur in
developing countries. In Africa, sweet
potato cultivars commonly have white,
cream or yellow flesh; while few are
orange-fleshed cultivars. Vitamin A
deficiency is especially prevalent in
SSA because most available foods
contain negligible amounts of ß–
carotene, a precursor of vitamin A
(Yessoufou et al., 2016) whereas white
to yellow-fleshed sweet potatoes have
been reported to contain little or no
Vitamin A; the orange-fleshed sweet
potato cultivars contain good quantities
of Vitamin A, predominantly as β
carotene (Karuppanagounder, 2008).
China is the world’s leading
producer of sweet potato, accounting
for about 80% of the total production
worldwide. Nigeria is the most
abundant sweet potato producer in
Africa and second in world production
with an annual output of 3.92 million
tons (FAO, 2018). However, production
is 44% below the potential capacity of 7
million tons especially in the North
Central part of the country due mainly
to low-yielding varieties planted by
farmers (Nedunchezhiyan et al., 2012).
The low-yielding varieties usually
cultivated include Kukunduku,
Katsina, Dunku and Ex-igbariam
(Mwanja et al., 2017). This brings to fore,
the need to fast track the evaluation of
advanced breeding lines through
participatory variety selection and
release of superior clones (Onunka et
al., 2012). One major limitation in sweet
potato production is cultivar decline,
mostly due to the cumulative effect of
virus infection on this vegetatively
propagated crop.
Viral diseases are considered a
major limiting factor in sweet potato
production worldwide, and
particularly in SSA. The other identified
problems that have hindered high yield
are the use of poor agronomic practices
and poor control of pests and
Assessment of Sweet Potato Virus Disease on Sweet Potato
3
diseasesPests and diseases are the
greatest limitations that affect
production and reduce yields (Motsa et
al., 2015). Viral diseases are the greatest
threat to sweet potato production,
causing yield losses of up to 80%
(Tesfaye et al., 2011; Gibson and Kreuze,
2015). Since early reports of suspected
viral diseases of sweet potato in the
United State of America (Moyer and
Salzer, 1989) and Eastern Africa
(Hansford, 1994), at least 30 viruses
have been recognized as pathogens of
sweet potato worldwide (Mukasa et al.,
2003).
The control of plant virus
infections is a challenge due to the lack
of therapeutic measures. Therefore,
disease management has to focus on
indirect measures such as reduction of
insect vector populations through
chemical or cultural methods and
induction of systemic resistance in host
plants by exogenous application of
biotic/abiotic inducers such as
mulching (Prasannath et al., 2014). The
understanding of pathogen dynamics
and implementing pathogen
management strategies require
consistent and reliable detection of
microorganisms in hosts, reservoirs,
and dispersal agents (Haydon et al.,
2002).
Mulching is a common practice in
rainfed ecosystems in smallholder
farming (Ray and Ravi, 2005). The use
of mulching is a simple cultural method
that requires no specialized equipment
(Doring et al., 2006). The occasional use
of abrasive mineral dust, natural
desiccants like wood ash and various
plant materials with repellant or
insecticidal properties is well-
documented (Golob and Webley, 1980).
Sand kaolin, paddy husk ash, wood ash
and clays constitute a group of
materials that are used commonly by
small-scale farmers in the developing
world as protectants (Golob et al., 1982).
The awareness of the consequences of
environmental pollution, the increasing
cost of storage insecticides and growing
problem of insect resistance has led to
pest management specialists
reappraising inert dusts. Unlike
conventional contact insecticides, inert
dusts function through their physical
properties and, are generally slower
acting (Maciljski and Korunic, 1972).
Wood ash is a product of incomplete
combustion of wood containing organic
and inorganic compounds. It contains a
mixture of oxides, hydroxides,
carbonates and silicates but usually has
very little nitrogen as it gets volatilized
during the combustion process
(Ozolincius et al., 2005). Food security
T. H. Aliyu et al.
4
remains a huge challenge for millions of
Africans who depend on agriculture for
their subsistence. Emerging and
reemerging pathogens, including many
viruses, continue to cause devastating
losses of food production. However,
there is a widespread lack of basic
information and understanding of virus
populations throughout Africa, even
though such basic information is
required to manage the spread and
impact of these viral diseases.
Orange-fleshed sweet potato
(OFSP) is a special type of biofortified
sweet potato that contains high levels of
beta-carotene. Beta-Carotene is an
organic, red-orange pigment abundant
in plants and fruits. Beta-carotene what
gives OFSP an orange color and is
converted to Vitamin A in the body
after consumption to provide
additional nutritional benefits.
Biofortification enhances the nutritional
value of staple food crops by increasing
the density of vitamins and minerals in
a crop through either conventional
plant breeding, agronomic practices or
biotechnology (Bechoff et al., 2011). The
intensity of orange-coloured flesh in
sweet potatoes root indicates the level
of beta carotene (Nwankwo et al., 2012).
Therefore, these OFSP varieties could
be useful to combat the widespread
Vitamin A deficiency that results in
blindness and death of 250,000-500,000
African children yearly (Wariboko and
Ogidi, 2014). Hence, the evaluation of
improved sweet potato varieties is
timely, needed.
The objectives of the study,
therefore, were (i) to assess the
occurrence and distribution of sweet
potato virus disease in Kwara State,
Nigeria and (ii) to evaluate the effect of
mulching and wood ash extract
application on incidence of sweet
potato virus disease on improved
Orange-Fleshed Sweet Potato (OFSP)
TIS 2498 and local White-Fleshed Sweet
Potato (WFSP) varieties. The results
from the study could reveal important
information on the status of sweet
potato virus diseases in Kwara State,
Nigeria and proffer sustainable
management initiatives for farmers
who have poor resources in a
developing country.
MATERIALS AND METHODS
Study Location and Estimation of
Virus Incidence
Kwara State is located on
longitude 2°6’E and 5°2’E and latitude
7°30’N and 9°40’N. It is situated within
Assessment of Sweet Potato Virus Disease on Sweet Potato
5
the North Central geopolitical zone of
Nigeria. The climate of Kwara State
exhibits both wet (rainy) and dry
seasons in response to the South West
Monsoon wind and the North East
continental wind which are the major
prevailing winds that blow across the
state. The wet or rainy season begins
towards the end of April and lasts till
October. The dry season begins in
November and ends in April. The
temperature of the state ranges from
33°C to 35°C from November to
January and from 34°C to 37°C from
February to April. The total annual
rainfall ranges from 990.3 mm to 1318
mm. The rainfall exhibits a double
maximal pattern. Relative humidity
ranges from 75% to 88% from May to
October and 35% to 80% during the dry
season (Ajadi and Adeniyi, 2017).
A virus-disease-survey of 30
sweet potato farms across three districts
of Kwara State was carried out at the
vegetative growth stage of the plants
grown between 2018 and 2019. The
surveyed locations and the Global
Positioning System (GPS) coordinates
are shown in Table 1. The farms
selected had an area not less than 10,000
m2. The incidence of viruses was
estimated based on 100 sweet potato
plants per field and by visual inspection
of virus disease symptoms on the plants
such as mosaic, chlorosis, mottling,
stunting, mottling, necrosis, leaf
deformation and bunching (James,
1974). The plants were identified by
walking in a “W” shaped pattern
method of sampling with 10 plants per
side spaced at an equal distance of 0.5
m from each other (Olawale et al., 2015).
The percentage incidence of virus
disease on the field was calculated
based on the formula:
=
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑓𝑒𝑐𝑡𝑒𝑑 𝑝𝑙𝑎𝑛𝑡𝑠
(𝑠𝑦𝑚𝑝𝑡𝑜𝑚𝑎𝑡𝑖𝑐 𝑝𝑙𝑎𝑛𝑡𝑠)
100
(𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑝𝑙𝑎𝑛𝑡𝑠
𝑖𝑛𝑠𝑝𝑒𝑐𝑡𝑒𝑑)
𝑥 100
A total of 20 sweet potato vine cuttings
and leaves (sourced from WFSP and
OFSP varieties) from each farm were
randomly collected, tagged, put in
plastic containers and stored in iced
packs prior to virus testing using
antigen-coated plate enzyme-linked
immunosorbent assay.
Virus Detection by Enzyme-Linked
Immunosorbent Assay (ELISA)
Antigen-coated plate enzyme-
linked immunosorbent assay (ACP-
ELISA) was the protocol used in
confirming the presence of 6 viruses
using monoclonal antibodies. The
antibodies were specific to Sweet potato
feathery mottle virus (SPFMV), Potato
T. H. Aliyu et al.
6
virus X (PVX), Potato virus Y (PVY),
Sweet potato chlorotic stunt virus
(SPCSV), Soybean mosaic virus (SBMV)
and Cowpea mild mottle virus
(CPMMV). An initial antibody dilution
of 1:100 was made followed by double-
fold serial dilution to 1:51,200.
Antibody dilutions were made in
phosphate-buffered saline containing
0.05% v/v Tween20 (PBS-T).
Table 1: Survey-locations and percentage incidence of viruses on sweet potato in Kwara
State
S/N
Location
District
Latitude
Longitude
Virus Incidence
1
Suku
Kwara North
8.72817
5.75762
16.7
2
Kusegi
Kwara North
8.71667
5.93333
13.2
3
Koro
Kwara North
8.55001
5.95002
11.6
4
Yagbagi
Kwara North
8.72851
5.75561
17.4
5
Gbagufa
Kwara North
9.10816
3.11222
19.2
6
Gwanara
Kwara North
9.55073
3.22805
21.6
7
Tsoho
Kwara North
9.56083
3.228049
16.9
8
Lafiagi
Kwara North
7.21041
5.0028
23.4
9
Teshe
Kwara North
8.76736
5.0028
27.4
10
Kuhiaro
Kwara North
8.61662
4.45211
13.2
11
Afon
Kwara Central
8.45111
4.3332
11.1
12
Fufu
Kwara Central
8.44811
4.72111
9.6
13
Egbejila
Kwara Central
8.53201
4.54621
6.3
14
Ile-Apa
Kwara Central
8.6166
4.3833
11.1
15
Badi
Kwara Central
8.47821
4.70611
14.3
16
Oke-oyi
Kwara Central
8.58333
4.71666
7.6
17
Woru
Kwara Central
8.71663
4.90002
3.8
18
Lajiki
Kwara Central
8.50002
4.71544
4.1
19
Oyun
Kwara Central
8.19921
4.63163
2.6
20
Afeyin
Kwara Central
8.43211
4.70311
4.9
21
Ilemona
Kwara South
8.11664
4.66665
36.3
22
Ira
Kwara South
8.08342
4.60621
28.8
23
Ilofa
Kwara South
8.11112
5.15463
33.3
24
Etan
Kwara South
8.03333
5.21667
29.1
25
Oke-opin
Kwara South
8.06667
5.23333
26.4
26
Agbamu
Kwara South
8.11876
4.87012
39.1
27
Offa
Kwara South
8.14913
4.72076
28.6
28
Kajola
Kwara South
8.34663
5.16843
21.4
29
Ibobo
Kwara South
8.36664
4.40001
26.1
30
Idofian
Kwara South
8.38332
4.71665
31.3
Assessment of Sweet Potato Virus Disease on Sweet Potato
7
The sap (200 µl) obtained by
grinding infected leaf tissues and
healthy yam leaves in grinding buffer
(PBS-T containing 0.5 mM polyvinyl
pyrrolidone (PVP)-40 and 79.4 mM
Na2SO3) were loaded in duplicate wells
for each antibody dilution. The initial
antibody dilution of 1:500 was followed
by a two-fold serial dilution to
1:256,000. Antibody dilutions were
made in conjugate buffer (half strength
phosphate-buffered saline-PBS
containing 0.05% (v/v) Tween-20, 0.02%
(w/v) egg albumin, 0.005 mM PVP-40).
Wells of microtitre plates were coated
with 100 µl of sap obtained by grinding
infected leaf tissues and healthy leaves
in 0.05 M sodium carbonate buffer (pH
9.6) containing 0.5 mM PVP-40 and 79.4
mM Na2SO3. After a skimmed milk
blocking step, 100 µl of each antibody
dilution was loaded into duplicate
wells pre-coated with diseased and
healthy sap. Goat anti-rabbit alkaline
phosphatase (Sigma, UK) diluted with
1:40,000 in conjugate buffer were added
to each well, followed by addition of
200 µl p-nitrophenylphosphate
substrate (pNPP) (1 mg ml-1 in 10%
diethanolamine, pH 9.8) into each of the
wells to detect the antigen-antibody
reactions. The optical density (OD)
values were measured after 1 hour at
absorbance of A405 using a Bio Tek
ELx800, Universal Micro plate Reader.
An optical density value greater than
three times the mean of the negative
controls i.e. virus – free plants, was
considered as positive (Resende et al.,
2000).
Field Experiment
The experiment was laid out in a
Randomized Complete Block Design
(RCBD) with three replicates. The total
plot size was 33 m x 10 m with each
replication having a plot size of 11 m x
2 m. The intra and inter-row spacing
was 30 cm and 90 cm, respectively.
Rows of each variety were separated by
a 1 m boundary and replications were
separated by 2 m boundaries. The vines
of each variety were planted in
triplicate rows containing 6 plants each
with a total of 18 plants per block. The
sweet potato cuttings measuring at
least 30 cm in length with at least 4
nodes were planted on top of the ridges
with cuttings facing the right-side-up
(Esan and Omilani, 2018). The Sweet
potato varieties used for the field trial
were the improved variety Orange
Fleshed Sweet Potato (OFSP) TIS 2498
and local White Fleshed Sweet Potato
(WFSP) variety Iwo. The two varieties
are more preferred and highly
promoted in Nigeria to fight with
T. H. Aliyu et al.
8
hunger and malnutrition because of
high levels of beta-carotene content.
The treatments were mulching
with guinea corn straw (7.5 cm
thickness) applied on the ridge at 1
week after planting and wood ash
extract applied as a foliar spray at
weekly intervals until 12 WAP at the
rate of 3 L/ 25 m2. The materials are cost-
effective as conventional control and
can be easily sourced locally and
applied. The extract was prepared by
dissolving 5 kg of sieved wood ash in 40
L of water, stirred properly every day
for 7 days and filtered using a clean
white cloth (Moyin-jesu, 2010). The
combination treatments were: (i) OFSP
x mulched (ii) OFSP x insecticide (iii)
OFSP x mulched x insecticide (iv) OFSP
control (no mulching, no insecticide) (v)
WFSP x mulched (vi) WFSP x
insecticide (vii) WFSP x mulched x
insecticide (viii) WFSP control (no
mulching, no insecticide).
Data Collection and Statistical
Analysis
Data were collected from 8th to 16th
week after planting (WAP) for
parameters such as plant height,
number of leaves, and number of leaves
with virus disease symptoms. The
symptoms were visually identified
(Figure 1) based on typical virus
symptoms manifested by the plants and
with the aid of disease diagnosis
manuals (Chen et al., 2010; Samarakoon
et al., 2012). The virus disease incidence
was measured by the number of
infected plants relative to the total
number of plants inspected (100) and
expressed as a percentage. At harvest,
the number of storage roots per plant
and the weights were measured. The
data collected were subjected to
analysis of variance (ANOVA) using
the Statistical Package for the Social
Sciences SPSS version 15.0. The
significantly different means were
separated using New Duncan Multiple
Range Test at 5% level of probability.
RESULTS
Survey on the Incidence of Viruses on
Sweet Potato
The result of the survey on the
incidence of viruses on sweet potato in
3 districts of Kwara state (Table 1)
showed that the highest virus incidence
was in Kwara South district. The
locations were Agbamu (39.1%),
Ilemona (36.3%), Ilofa (33.3%), Idofian
(31.3%), Etan (29.1%), Ira (28.8%) and
Offa (28.6%). The lowest virus
Assessment of Sweet Potato Virus Disease on Sweet Potato
9
incidences were however in Oyun
(2.6%), Woru (3.8%), Lajiki (4.1%),
Egbejila (6.3%), Fufu (9.6%), Ile Apa
(11.1%) and Afon (11.1%) in Kwara
Central district. The survey revealed
100% presence of virus disease on sweet
potato in all the districts of Kwara State.
However, the incidence varied among
survey areas with a range of 2.6% in
Oyun (Kwara Central district) to 39.1%
in Agbamu (Kwara South district). The
average virus incidence was between
7.5% in Kwara Central district and 30%
in Kwara South district.
ELISA Identification
The ACP-ELISA detected an infection
of SPFMV and SPCSV in 100% and 30%
of the surveyed locations, respectively.
These were either a single viral
infection or in mixtures of the two
viruses (Table 2). Further scrutiny of the
results revealed the presence of mixed
sweet potato virus disease on samples
from Kusegi, Gwanara, (Kwara North
district); Fufu, Ile-Apa, (Kwara Central
district); Ilemona, Ira, Offa, Kajola and
Idofian (Kwara South district).
.
Figure 1. Symptoms observed during the field survey: (a) vein clearing with intense mosaic
patterns, (b) reddening leaves with a leathery feel, (c) leaf purpling and narrowing along the
leaf veins and (d) leaf deformation, yellowing and bunching of leaves.
c
a
b
d
T. H. Aliyu et al.
10
Table 2. Virus detection on sweet potato plants optical density with ACP-ELISA
S/
N
Location
District
Viruses (OD values in ELISA units)
SPFMV
PVY
PVX
SPCSV
SBMV
CPMMV
1
Suku
North
++0.734
--
--
--
--
--
2
Kusegi
North
++0.925
--
--
++0.686
--
--
3
Koro
North
++1.998
--
--
--
--
--
4
Yagbagi
North
++2.111
_
--
--
--
--
5
Gbagufa
North
++1.874
--
--
--
--
--
6
Gwanara
North
++1.624
--
--
++0.695
--
--
7
Tsoho
North
++1.887
_
--
--
--
--
8
Lafiagi
North
++0.767
--
--
--
--
--
9
Teshe
North
++2.222
_
--
--
--
--
10
Kuhiaro
North
++1.775
--
--
--
--
--
11
Afon
Central
++0.896
--
--
--
--
--
12
Fufu
Central
++1.232
--
--
++0.708
--
--
13
Egbejila
Central
++0.791
--
--
--
--
--
14
Ile-Apa
Central
++1.861
--
--
++0.852
--
--
15
Badi
Central
++1.127
--
--
--
--
--
16
Oke-oyi
Central
++1.654
--
--
--
--
--
17
Woru
Central
++1.773
--
--
--
--
--
18
Lajiki
Central
++1.225
--
--
--
--
--
19
Oyun
Central
++1.49
_
--
--
--
--
20
Afeyin
Central
++1.776
--
--
--
--
--
21
Ilemona
South
++1.434
--
--
++0.642
--
--
22
Ira
South
++1.696
--
--
++0.854
--
--
23
Ilofa
South
++0.896
_
--
--
--
--
24
Etan
South
++0.941
_
--
--
--
--
25
Oke-opin
South
++1.116
_
--
--
--
--
26
Agbamu
South
++0.987
_
--
--
--
--
27
Offa
South
++0.816
_
--
++0.569
--
--
28
Kajola
South
++1.187
_
--
++0.896
--
--
29
Ibobo
South
++1.996
_
--
--
--
--
30
Idofian
South
++1.736
_
--
++0.702
--
--
Positive
2.1222
4.042
Negative
0.133
0.321
Buffer
0.122
0.313
Key: SPFMV = Sweet potato feathery mottle virus, PVY = Potato virus Y, PVX = Potato virus X, SPCSV
= Sweet potato chlorotic stunt virus, SBMV = Soybean mosaic virus, CPMMV = Cowpea mild mottle
virus. ++ denotes presence of virus, -- denotes absence of virus. Figures in parentheses are OD values
of positive samples.
Assessment of Sweet Potato Virus Disease on Sweet Potato
11
The result showed the
predominance of sweet potato virus
disease in Kwara South district. The
other viruses namely: PVX, PVY, SBMV
and CPMMV were not found infecting
any samples and therefore deemed not
occurring on sweet potato in the study
area
Effects of Mulching and Wood Ash
Extract Application on Virus Incidence
The effects of mulching with
guinea corn husk and wood ash extract
as an insecticide on the incidence of
virus infection on sweet potato are
shown in Table 3. The result indicated
that irrespective of the sweet potato
variety, mulching with guinea corn
husk and wood ash extract as
insecticide triggered a significant
reduction in virus incidence. The result
indicated the significantly lowest virus
infection at 8 WAP (12.4%), 12 WAP
(14.8%) and 16 WAP (15.3%) in the
mulched and wood ash treatment.
Contrariwise, the control plants
without mulching and wood ash had
the significantly highest virus incidence
at 8 WAP (50.2%), 12 WAP (53.7%) and
16 WAP (56.4%).
Effect of Mulching and Insecticidal
Application on the Yield of Sweet
Potato
The effect of treatments on yield
parameters such as the number of
storage roots and weight of storage
roots indicated that there were
significant differences in the values
obtained (Table 4). There is an
indication that the action of combining
mulching with wood ash extract
application was more prolific
compared to the other treatments. The
highest average number of storage
roots per plant (7.5) and weight of
storage roots per plant (32.4 Kg) were
obtained in the improved OFSP variety
mulched with guinea corn hush and
wood treatment.
DISCUSSION
The symptoms observed on the
field were consistent with those
reported by Ndunguru and Kapinga
(2007). The survey study detected the
distribution and occurrence of 2 viruses
namely SPFMV and SPCSV in the study
area. The climate influences the
incidence as well as temporal and
spatial distribution of plant virus
diseases (Fabre et al., 2011). Climate
affects all life stages of the pathogen
T. H. Aliyu et al.
12
and host and clearly poses a challenge
to many pathosystems.
This study is the first report of
viruses infecting sweet potato varieties
Table 3. Effect of treatments on percentage virus disease incidence
Treatment
Virus disease incidence (%)
8 WAP
12 WAP
16 WAP
OFSP x mulched
25.2b
26.6b
27.1d
OFSP x insecticide
18.0d
19.1d
19.9f
OFSP x mulched x insecticide
12.4f
14.8f
15.3g
OFSP x control
49.5a
52.9a
55.2b
WFSP x mulched
25.5b
28.3b
30.1c
WFSP x insecticide
23.7c
24.8c
25.3e
WFSP x mulched x insecticide
14.1e
16.3e
16.9g
WFSP x control
50.2a
53.7a
56.4a
Standard error of mean (S.E.M)
1.249
1.525
1.547
Means within a column followed by the same letter(s) are not significantly different using
New Duncan Multiple Range Test at (p < 0.05).
Table 4. Effect of treatments on yield parameters
Treatment
Number of storage
roots/plant
Weight of storage
roots/plant (kg)
OFSP x mulched
3.8e
25.6c
OFSP x insecticide
5.2c
28.5b
OFSP x mulched x insecticide
7.5a
32.4a
OFSP x control
1.1f
12.4d
WFSP x mulched
3.7e
25.5c
WFSP x insecticide
4.2d
28.2b
WFSP x mulched x insecticide
6.4b
31.8a
WFSP x control
0.9f
10.3e
Standard error of mean
(S.E.M)
0.384
1.249
Means within a column followed by the same letter(s) are not significantly different
using New Duncan Multiple Range Test at (p < 0.05).
Assessment of Sweet Potato Virus Disease on Sweet Potato
13
in the Kwara State of Nigeria with the
incidence between 2.6% to 39.1%. This
report validates Olabiyi et al. (2017),
who assessed the climatic dynamics on
the incidence and severity of plant
diseases for climate prediction
purposes in Nigeria and revealed virus
disease range of 16.0% to 97.0%. This
indicates that favorable climate
conditions enhanced disease
development, resulting in higher
incidence and severity.
In many parts of the world,
climate variability is already having
serious impacts especially on the
environment and agriculture (Onu and
Ikehi, 2016; Olubanjo and Adebolu,
2018). Interestingly, studies are
reported viruses having varying, but
significant impacts on yields (Paprotka
et al., 2010). However, until only
recently there had been no reports on
their impact on African sweet potato
varieties (Mulabisana et al., 2019). The
two viruses detected in the study are
considered the most damaging viruses
of sweet potato due to their ability to
induce synergistic viral diseases with
several other viruses (Opiyo et al., 2010).
In a synergistic interaction, the virus
titer of both, one or neither virus may be
enhanced and, as a consequence, the
rate of disease spread may be affected.
It is the opinion of Zhang et al. (2001)
that many virus diseases of plants are
caused by a synergistic interaction
between viruses within the host plant.
The mixed infection of SPFMV
and SPCSV has been reported to
constitute Sweet potato virus disease
(SPVD) with higher yield reductions
compared to SPFMV and SPCSV
(Gutierrez et al., 2003). This is an
indication that the viruses can be a
limiting factor to achieving sustainable
and high-yielding sweet potato crop
varieties and this is in agreement with
Mulabisana et al. (2019). In the present
study, SPFMV was observed in all
samples while SPCSV in 30 % of
locations. The disparity in the incidence
of viruses across the 3 districts of the
State has been studied by Gibson and
Kreuze (2015) and was thought to be a
result of the specific virus strains and/or
varieties planted by the farmers. It
could also be the result of limitations in
sampling as symptoms may not always
be an adequate indicator of virus
presence as symptoms disappear over
time, making such plants difficult to
identify. Symptoms may also be caused
by other pathogens, environment, pest
and nutrition that could be mistaken for
virus infection (Mukasa et al., 2003).
T. H. Aliyu et al.
14
The non-detection of PVX, PVY,
SBMV and CPMMV on sweet potato in
the study area does not however
preclude the absence of these viruses in
the study area. In general, most viruses
have a limited host range and the
inability of interaction between such
host factor and virus leads to resistance
on the part of the host. The non-
detection of the viruses could be the
result of the existence of incompatible
interactions restricting virus replication
(Jaubert et al., 2011). It is yet possible
that the viruses can be confirmed using
more sensitive virus diagnostic
techniques such as Polymerase Chain
Reaction (PCR) and sequencing of
genomes (Wau and Komolong, 2019).
Although in developing countries, this
might prove quite cumbersome and
expensive for routine implementation.
The field experiment revealed the
capability of the treatments to lessen
virus incidence with improved crop
growth and yield. There have been
reports on mulching reducing the
incidence of virus diseases in
vegetables (Vani et al., 1989). The
general trend is that straw mulches
have positive effects on the crop and
soil moisture (Saeed and Ahmad, 2009).
It is assumed that changes in the
microclimate within the growing plants
induced increased growth and
production (Awal and Sultana, 2006). In
addition, it has also been agreed by
Alcantara et al. (2007) that mulches
reduce aphid-borne viruses and
whiteflies with resultant higher overall
yields.
As revealed in this study, straw
mulching, which is less common in
sweet potato cultivation can thus be of
encouraging effect in improving plant
health and suppressing virus infection.
The improvement is prompted by the
rapid decomposition of the mulch,
enhanced soil microorganisms status,
aeration and water conservation.
Mineral nutrients are essential for the
growth and development of plants and
microorganisms. They are important
factors in plant-disease interactions and
also affect the formation of mechanical
barriers in plant tissue and play a role
in disease resistance (Amtmann et al.,
2008).
The doses of wood ash applied as
insecticide may also have increased the
amount of chlorophyll and total
carotenoids, since changes in pigment
concentration are linked with more
photosynthetic activity (Parekh et al.,
1990; Adekayode and Olojugba, 2011).
It is also feasible that wood ash extract
Assessment of Sweet Potato Virus Disease on Sweet Potato
15
contributed immensely in the reduction
of activities of insects associated with
virus epidemiology. The increase in the
population of insect vectors is
positively correlated with the spread of
virus diseases. Hence, lower virus
incidence observed in the mulched and
insecticidal plots is connected to a lower
abundance of vectors (Awodoyin and
Ogunyemi, 2005; Prasannat, 2014).
Similar conclusions were made by
Ayoade (1977) on the effectiveness of
wood ash in the control of pests and
insect vectors of viruses.
CONCLUSIONS
The research showed the diverse
occurrence and prevalence of sweet
potato virus disease in all districts of
Kwara State, Nigeria. There is a need
therefore to put in place adequate
management to curtail the spread of the
disease for increased production and
food security. The field
experimentation endorses the use of the
two sweet potato varieties, mulched
with guinea corn stalk and wood ash
extract applied as an organic insecticide
in the management of SPVD especially
in sub-Saharan Africa with subsistence
agricultural practice.
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