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Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021 79
EFFECT OF FERTILIZER TYPE ON THE LEAF SPOT DISEASE OF SWEET
POTATO (Ipomoea batatas (L.) Lam) IN MAKURDI AND OTOBI, BENUE STATE,
NIGERIA
*Ekhuemelo, C., Okoh, L.E. and Ochoechi, F.P.
Department of Crop and Environmental Protection, Joseph Sarwuan Tarka University, P.M.B.
2373 Makurdi, Benue State, Nigeria.
*Corresponding author Email chiekhuemelo@gmail.com +2347031332667
SUMMARY
A multi locational study was conducted at the Teaching and Research Farm of the JS Tarka University Makurdi and
the National Root Crops Research Institute Otobi Sub-station, Otukpo Benue State, Nigeria to determine the effect of
organo - mineral fertilizers on naturally occurring leaf spot, growth and yield of sweet potato during the 2018
cropping season. The experiment consisted of three rates of mineral fertilizer NPK 15:15:15 applied at 100, 150 and
200 kgha-1, three rates of organo-mineral fertilizer (Fertiplus® ®) at 1, 2 and 3 tons ha-1 and three rates of poultry
manure applied at 5, 10 and 15 tons ha-1 laid out in a randomized complete block design with three replicates and an
untreated control. The data collected were subjected to analysis of variance. Results of the study showed that the
application of organo-mineral fertilizer had no significant (P ≥ 0.05) effect on the incidence of leaf spot disease of
sweet potato at 4 and 6 weeks after planting and leaf spot severity at 10 and 12WAP in both locations. In Makurdi
plots treated with Fertiplus® ® 3t/ha and poultry manure at 15t/ha produced significantly greater number of leaves
(33.53 and 32.67). The application of poultry manure at 15t/ha recorded the highest yield of 13.39 t/ha and 7.50t/ha
at Otobi and Makurdi respectively. Leaf spot incidence was positively correlated with leaf spot severity (0.620**).
There was negative significant relationship between leaf spot severity and number of marketable tubers (-0.515**) at
Otobi. The use of poultry manure at 15tons/ha and NPK 15:15:15 150kg/ha reduced the leaf spot incidence and
severity on sweet potato in the study area.
Keywords: Organo-mineral fertilizer, Sweet potato, leaf spot, poultry manure
SWEET POTATO (Ipomoea batatas (L.) Lam) is a popular root crop in Nigeria. Nigeria is
ranked the third largest producer of sweet potato after China and Uganda and cultivated 2.5% of
the sweet potatoes produced in 2010 with per capita annual consumption of 22.3 Kg (Egeonu and
Akoroda, 2010; Bergh et al., 2012). Sweet potato in Nigeria is ranked after yam, cassava and
cocoyam (Babatunde et al., 2005). Sweet potato can be consumed by boiling and eating with stew,
drying and milling into flour, roasting, frying into chips, pounding and eating with soup, made into
pottage or fermented to make a local drink ‘kunu’ (Egeonu and Akoroda, 2010; Bergh et al., 2012).
The leaves are used for animal feed while it is used industrially for making starch, candy, noodles,
ethanol and biofuel production (Bergh et al., 2012).
Benue State in the Southern Guinea savannah is known for sweet potato production in Nigeria
(Marti and Mills, 2002). Sweet potato production has been impacted by low soil fertility and the
incidence of leaf spot diseases (Ilondu, 2013). The introduction of synthetic inorganic fertilizers
may increase soil fertility but with a corresponding increase in plant diseases (Hoitink and Boehm,
1999; Bailey and Lazarovits, 2003). The progression of leaf spot infections may cause yield losses
due to the reduction of assimilates deposited at the sink (Ilondu, 2013). The combined application
of organic manure and inorganic fertilizers enhances long-term cropping in the tropics (Ipimoroti
et al., 2002). The application of animal manure, green manure, composts and peats improves the
80 Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021
soil fertility and also decreases the incidence of disease caused by soil- borne pathogens (Magid
et al., 2001). The application of organic and inorganic fertilizer may help to reduce the incidence
and severity of leaf spot disease on sweet potato thereby increasing yield.
This study was conducted to determine the effect of varying rates of organic and inorganic
fertilizations on leaf spot severity, growth and yield of sweet potato in Makurdi and Otobi in
Oturkpo Local Government Areas of Benue State.
MATERIALS AND METHODS
The experiment was sited at the Teaching and Research Farm of the Joseph Sarwuan University,
Makurdi, Benue State located between 7o45’01’’N - 8o37’11’’E 98m above sea level and the
National Root Crops Research Institute Otobi Sub-station (7o11’10’’N - 8o10’03’’E) in Otukpo
Local Government Area of Benue State, Nigeria in the Southern Guinea Savannah of Nigeria
between August and October 2018.
The treatments consisted of three levels of mineral fertilizer NPK 15:15:15 applied at 100, 150
and 200 kgha-1, three levels of organo-mineral fertilizer (Fertiplus®) at 1, 2 and 3 tons ha-1, three
levels of poultry manure applied at 5, 10 and 15 tons ha-1 and an untreated control. The experiment
was laid out in a randomized complete block design with three replicates. Soil samples were
collected from 0-30cm depth of the soil and analyzed for nutrient status using standard procedures
(Table 1).
The poultry manure was cured for 7days and incorporated into the soil 2 weeks before planting.
The inorganic fertilizer (NPK 15:15:15) and organo-mineral fertilizers (Fertiplus®) were applied
2 weeks after planting (WAP) using side placement method. The proximate nutrient composition
for the poultry manure was pH 7.03, organic carbon 3.48%, OM 6.02%, N 1.96%,P 1.31 Mgkg-1,
K 0.36 Cmol, Na 0.40% , Mg 0.41%, 0.38% Ca,
Table 1: Chemical properties of the soil amendment materials used at experimental sites
Parameter
Fertiplus®
Poultry manure
NPK(15:15:15) %
pH
6.96
7.03
7.36
Organic carbon (%)
1.91
3.48
-
Organic manure (%)
3.30
6.02
-
Nitrogen (%)
1.56
1.96
13.72
Phosporus (Mgkg-1)
4.80
1.31
10.75
Potassium (Cmol k)
0.27
0.36
9.70
Sodium
0.24
0.40
-
Magnesium
2.80
0.41
-
Calcium
3.10
0.38
-
Manganese (ppm)
69.00
32.6
-
Iron (ppm)
218.00
96.8
-
Copper (ppm)
71.00
2.98
-
Zinc (ppm)
134.00
28.4
-
Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021 81
Planting of sweet potatoes
The experiment was conducted on plots of 0.0012ha consisting of four ridges each 3 meters long.
Planting was done using 25cm vine cuttings of sweet potato variety TIS-8164 sourced from
National Root Crops Research Institute Umudike, Nigeria on the crest of the ridge at a spacing of
100cm by 30cm. Manual weeding was done at 4 WAP.
Data collection
Five plants were randomly selected and tagged from the two middle rows of each plot for data
collection. Leaf spot incidence was determined as the number of infected plants per plot relative
to the total number of plants per plot expressed as a percentage (Agrios, 2005). Disease severity
was assessed on a scale of 0-4 (Onuegbu and Emiri, 2011) where,
0 - leaves without spot.
1 - Leaves with less than 5 spots.
2 - Leaves with 5-10 spots.
3 - Leaves with more than 10 spots.
4 - Dead leaves.
Growth and yield data collected included number of leaves, vine length, plant vigour, girth, leaf
length and leaf width at 4, 6, 8, 10 and 12 WAP. Plant vigour was determined using the scale of
3-7 where 3= low, 5 = average and 7=good.
Data Analysis
The data collected were subjected to Analysis of Variance (ANOVA) using SAS statistical
software (Statistical Analysis System, 2009) while significant means were separated using Duncan
New Multiple Range Test (DNMRT) at 5% probability level. Correlation analysis was done using
Pearson correlation at 0.05 and 0.01 level of significance.
RESULTS
Soil chemical and physical properties of experimental site
Results of the soil analysis from the two locations are presented in Table 2. The physical and
chemical analyses showed that the soil of the experimental site was sandy loam with high
proportion of sand 69.80 %, 10.00 % silt and 20.20 % clay in Otobi while the soil in Makurdi had
73.08 % sand, 11.92 % silt and 15.00 % clay with a neutral soil pH of 6.59 and 6.43, respectively.
The soils from both locations were low in Nitrogen with 0.13% in Otobi and 0.10% in Makurdi.
The phosporous content of the experimental soil in Otobi (3.80 mg kg-1and Makurdi (3.00 mg kg-
1) were below the required amount of 6.80 mg kg-1. The soil at Otobi had higher amount of
exchangeable Ca, K and base saturation. Soil organic C and total N was also higher at Otobi.
82 Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021
Table 2: Physical and chemical properties of the surface soil at the experimental sites before
planting
Soil Properties
Percent composition
Otobi
Makurdi
Particle Size distribution
Clay (%)
20.20
15.00
Silt (%)
10.0
11.92
Sand (%)
69.80
73.08
Soil pH
6.59
6.43
Textural class
Sandy loam
Sandy loam
Exchangeable bases (Cmol kg-1)
Potassium
0.23
0.21
Sodium
0.21
0.18
Magnesium
2.80
2.40
Calcium
3.00
2.70
Exchangeable base
6.24
5.49
Exchangeable Acid
1.10
1.00
CEC
7.34
6.49
Base saturation (%)
85.01
84.59
Chemical properties
Organic carbon (%)
1.08
0.74
Organic manure/matter
1.86
1.28
Nitrogen (%)
0.13
0.10
Phosphorus (mg kg-1)
3.60
3.00
Table 3 shows the meteorological data at the experimental site during the period of the experiment.
The total amount of rainfall recorded at the weather station in the experimental area between June
and October 2018 was 722.2 mm with the highest rainfall recorded in August (245.8mm) while
the lowest rainfall was recorded in July 2018 (44.1 mm). The minimum temperature (June to
October) ranged between 22.0 oC in August to 22.5 oC in June while the maximum temperature
ranged from 32.6 oC in October to 30.8 oC in August. The relative humidity ranged from 74 % in
July to 93 % in September.
Table 3: Meteorological data at the experimental site during the 2018 cropping season.
Month
Rainfall (mm)
Rel. humidity
(%)
Min. temperature
(oC)
Max. temperature
(oC)
June
139.1
80
22.5
32.1
July
44.1
74
22.2
31.3
August
245.8
83
22.0
30.8
September
219.3
93
22.4
31.4
October
73.9
79
22.2
32.6
Source: Tactical Air Command, NAF Base Makurdi
Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021 83
Effect of organo-mineral fertilizer on leaf spot incidence and severity
Table 4 shows the effect of fertilizer type and rates on the incidence of leaf spot disease of sweet
potato at Otobi and Makurdi during the period of the study. The result showed that the application
of organo-mineral fertilizer had no significant (P ≥ 0.05) effect on the incidence of leaf spot disease
of sweet potato at 4 WAP and 6 WAP at both locations. Similarly, the percentage leaf spot
incidence was not significantly different across the soil amendment types and rates of application
in Otobi at 8 and 10 WAP. At 8 WAP, the leaf spot incidence on sweet potato plants at Makurdi
was significantly lower (P≤ 0.05) on plants where poultry manure was applied at 10 tons/ha (35.00
%) compared with the control (53.67%) while at 10 WAP, it was significantly lower (P ≤ 0.05)
when poultry manure was applied at 5 tons/ha (41.25 %) and NPK was applied at 150 kg/ha (46.98
%) compared with the control (62.50 %).
At 12 WAP in Otobi, leaf spot incidence was significantly lower (P ≤ 0.05) on sweet potato plants
treated with 15tons/ha of poultry manure (76.67 %) compared with other soil amendment types
and rates and the control which recorded the highest leaf spot incidence of 98.33%. In Makurdi at
12 WAP, sweet potato plants where Fertiplus® was applied at the rate of 1ton/ha significantly (P≤
0.05) reduced percentage leaf spot incidence by 21.77 % while the application of poultry manure
at 5 tons/ha and 15tons/ha reduced leaf spot incidence by 12.09 %.
Table 4: Effect of fertilizer type and rates on the incidence of leaf spot disease of sweet potato at
Otobi and Makurdi in 2018 cropping season.
Fertilizer
type and
rate
Percentage leaf spot incidence
4WAP
6WAP
8WAP
10WAP
12WAP
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Control
12.50a
13.20a
39.17a
24.40a
55.00a
53.67a
75.83a
62.50ab
98.33a
69.67abc
NPK
100kg/ha
9.17a
13.43a
32.50a
26.50a
58.33a
44.75abcd
73.33a
65.83a
88.33ab
66.00abc
NPK
150kg/ha
10.83a
9.87a
24.17a
23.10a
43.33a
45.25abcd
67.33a
46.98ef
83.33ab
72.50ab
NPK 200
kg/ha
13.33a
11.34a
26.67a
23.60a
49.17a
46.75abc
73.33a
53.25cde
90.00ab
68.50abc
PM 5t/ha)
9.17a
12.67a
30.83a
21.40a
55.00a
37.25cd
73.33a
41.25f
93.33ab
61.25cg
PM 10t/ha)
7.33a
13.37a
30.67a
20.40a
54.17a
35.00d
73.33a
50.50de
92.50ab
76.00a
PM 15t/ha)
11.67a
10.30a
31.67a
23.70a
52.50a
38.75bcd
69.00a
56.75bcd
76.67b
61.25cd
Fplus 1t/ha)
10.00a
9.33a
38.33a
19.20a
58.33a
49.00ab
76.67a
50.83de
83.33ab
54.50d
Fplus(2t/ha)
9.17a
9.74a
27.50a
18.00a
49.83a
42.80abcd
65.00a
59.33abc
94.17ab
73.17ab
Fplus3t/ha)
9.17a
8.87a
32.50a
20.50a
59.17a
39.13bcd
78.33a
49.15e
95.83a
65.50bc
PM= Poultry manure; Fplus= Fertiplus®. Values with the same letter(s) within the same column
are not significantly different using DNMRT at 5% probability.
Table 5 shows the effect of fertilizer type and rates on the severity of leaf spot disease of sweet
potato at Otobi and Makurdi during the 2018 cropping season. Data presented in Table 5 shows a
100 % increase of the severity of leaf spot disease in Makurdi from 1.00 at 4WAP to 2.00 (Leaves
with 5-10 spots ) at 6WAP in all treatments except in sweet potato plants treated with NPK at the
rate of 100 kg/ha and 150kg/ha. In Makurdi at 8WAP, leaf spot disease was more severe in sweet
potato plots treated with poultry dung at 10t/ha with a severity rating of 3.00. Although there were
no significant effect of fertilizer type and rates on leaf spot severity at 10 WAP and 12WAP, the
least leaf spot severity was recorded on sweet potato plants with the application of Fertiplus® at
84 Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021
the rate of 3t/ha 12 WAP with severity rating of 2.00 followed by the application of NPK at the
rate of 100kg/ha and poultry manure at 15t/ha with severity rating of 2.33 in Makurdi. At Otobi,
poultry manure applied at 15t/ha and Fertiplus® applied at 1t/ha recorded the least leaf spot
severity rating of 2.00.
Table 5: Effect of fertilizer type and rates on the severity of leaf spot disease of sweet potato at
Otobi and Makurdi during the 2018 cropping season.
Fertilizer
type and rate
Severity of leaf spot disease
4WAP
6WAP
8WAP
10WAP
12WAP
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Control
1.00a
1.50ab
1.33a
2.67a
2.00a
2.67ab
2.67a
2.67a
2.67a
2.67a
NPK
100kg/ha
1.00a
1.00b
1.00b
1.33b
1.33ab
2.67ab
2.00a
2.00a
2.33a
2.33a
NPK
150kg/ha
1.00a
1.00b
1.00b
1.33b
1.33ab
2.67ab
2.33a
2.33a
2.33a
2.67a
NPK
200kg/ha
1.00a
1.07b
1.00b
1.67b
1.33ab
2.67ab
2.00a
3.00a
2.33a
2.67a
PM 5t/ha
1.00a
1.10b
1.00b
2.00ab
1.33ab
2.67ab
2.33a
2.67a
2.33a
2.67a
PM 10t/ha
1.00a
1.43ab
1.00b
2.00ab
1.33ab
3.00a
2.33a
2.67a
2.33a
2.67a
PM 15t/ha
1.00a
1.50ab
1.00b
2.00ab
1.00b
2.67ab
1.67a
2.33a
2.00a
2.33a
Fplus 1t/ha
1.00a
1.83a
1.00b
2.00ab
1.00b
2.00b
1.67a
2.33a
2.00a
2.67a
Fplus 2t/ha
1.00a
1.33ab
1.00b
2.00ab
1.33ab
3.00a
1.67a
2.33a
2.33a
2.67a
Fplus 3t/ha
1.00a
1.33ab
1.00b
2.00ab
1.33ab
3.00 a
2.33a
2.67a
2.33a
2.00a
PM= Poultry manure; Fplus = Fertiplus®. Values with the same letter(s) within the same column
are not significantly different using DNMRT at 5% probability.
Table 6 shows the effect of fertilizer types and rates on number of leaves of sweet potato at Otobi
and Makurdi in 2018. Data presented in Table 6 shows that at 4 WAP sweet potato plants produced
the highest number of leaves when poultry manure was incorporated at 15tons/ha at both locations
(22.13 and 14.87 at Otobi and Makurdi, respectively). At 6WAP the number of leaves produced
by sweet potato plants treated with 15tons/ha of poultry manure was significantly higher (P≤ 0.05)
with a value of 27.67 in Otobi and 29.73 in Makurdi compared with the number of leaves produced
by plants treated with all other soil amendment types and the control at both locations. At 8WAP
the number of leaves produced by sweet potato plants treated with 15tons/ha and 5tons/ha of
poultry manure was significantly higher (P≤ 0.05) compared with all other soil amendment
materials and the control at both locations. At 10WAP the number of leaves of sweet potato plants
was not significantly different (P≥0.05) across the soil amendment types and rates applied at Otobi.
In Makurdi, sweet potato plants in the plots where poultry manure was applied at 10t/ha (33.40)
and 15t/ha (32.40) produced the highest number of leaves while NPK applied at 150kg/ha
produced the least number of leaves (26.33). Number of leaves was not significantly different at
Otobi 12 WAP. In Makurdi plots treated with Fertiplus® 3t/ha and poultry manure at 15t/ha
produced significantly greater number of leaves (34.53 and 32.67) compared with plots where
NPK was applied.
Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021 85
Table 6: Effect of fertilizer types and rates on number of leaves of sweet potato at Otobi and
Makurdi in 2018.
Fertilizer
type and
rate
Number of Leaves
4WAP
6WAP
8WAP
10WAP
12WAP
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Control
7.53c
9.80c
20.20b
21.73bc
26.73b
24.27c
34.27a
29.00ab
36.33a
31.13abcd
NPK
100kg/ha
9.07c
1.07bc
23.87ab
27.33ab
30.87ab
29.40ab
33.40a
30.13ab
35.27a
30.93abcd
NPK
150kg/ha
9.00c
9.37c
23.40ab
21.80bc
32.27ab
29.40ab
34.80a
26.33b
35.07a
27.87bcd
NPK
200kg/ha
9.73c
10.53bc
22.87ab
25.27abc
30.93ab
30.40ab
34.67a
29.07b
33.67a
26.33d
PM 5t/ha
12.67bc
12.27abc
24.07ab
21.80bc
33.73a
31.13ab
39.20a
30.53ab
38.07a
32.53ab
PM10t/ha
16.47b
12.80ab
24.47ab
27.87a
30.67ab
29.67ab
36.40a
33.40a
36.47a
31.93abc
PM15t/ha
22.13a
14.87a
27.67a
29.73a
35.27a
32.60a
38.07a
32.40a
34.53a
32.67ab
Fplus1t/ha
11.13bc
11.67bc
26.47ab
25.20abc
32.20ab
27.33bc
34.73a
31.47a
32.53a
26.93cd
Fplus2t/ha
8.93c
9.93bc
22.00ab
21.33c
31.00ab
24.27c
35.53a
29.27ab
36.80
a
31.40abcd
Fplus3t/ha
11.73bc
10.33bc
25.40ab
26.20abc
31.20ab
27.73abc
35.07a
31.80a
33.00a
33.53a
PM= Poultry manure; Fplus= Fertiplus®. Values with the same letter(s) within the same column
are not significantly different using DNMRT at 5% probability.
Effect of fertilizer type on plant vigour
Data presented in Table 7 shows the effect of fertilizer type on plant vigour. At 6 WAP the
application of poultry manure at the rate of 15tons / ha produced sweet potatoes plants with
significantly good vigour of 7.00 at Otobi and 6.33 at Makurdi compared with the control with low
vigour of 3.00. Sweet potato plants where Fertiplus® was applied at the rate of 1tons / ha, 3tons /
ha and NPK at the rate of 150 kg/ha recorded a plant vigour of 5.00 classified as average vigour.
At 8WAP all sweet potato plants where the soil was amended in Otobi recorded significantly
higher (P≤ 0.05) plant vigour ranging from 5.67 to 7.00 (average to good) compared with the low
vigour recorded in unamended control plants (3.00). In Makurdi, soil amendment with NPK
fertilizer at the rate of 200kg/ha and poultry manure at the rate of 10tons/ha and 15tons/ha produced
sweet potato plants with significantly higher vigour of 6.33 compared with those treated with the
three rates of Fertiplus® and the control with low vigour (3.00). At 10 WAP in Otobi, the sweet
potato plants which received soil amendment had good vigour (6.33 to 7.00) compared with the
control which had low vigour (4.33). At Makurdi, plant vigour ranged from average vigour in
control plants and plants in soils amended with Fertiplus® to good vigour of 7.00 obtained in
plots amended with poultry manure at 10t/ha, 15t/ha and NPK at 200kg/ha. At 12 WAP plots
amended with Poultry manure and NPK were more vigorous than the unamended control plot at
both locations.
86 Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021
Table 7: Effect of fertilizer type on the Plant vigour of sweet Potato plants at Otobi and Makurdi
Fertilizer
Type and rate
Plant vigour
6WAP
8WAP
10WAP
12WAP
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Control
3.00c
3.00b
3.00b
3.00d
4.33b
5.00b
5.00b
5.00c
NPK 100kg/ha
4.33bc
4.33ab
7.00a
5.67ab
7.00a
5.67ab
7.00a
7.00a
NPK 150kg/ha
5.00abc
5.00ab
6.33a
5.67ab
7.00a
6.33ab
6.33 a
6.33ab
NPK 200kg/ha
4.33bc
5.00ab
7.00a
6.33a
6.33a
7.00a
6.33 a
7.00a
PM 5t/ha
4.33bc
5.67a
5.67a
4.33bcd
6.33a
6.33ab
6.33 a
6.33ab
PM 10t/ha
6.33ab
6.33a
7.00a
6.33a
7.00a
7.00a
7.00 a
7.00a
PM 15t/ha
7.00a
6.33a
7.00a
6.33a
7.00a
7.00a
7.00 a
7.00a
Fplus 1t/ha
5.00abc
5.67a
5.67a
4.33bcd
6.33a
5.00b
6.33 a
6.33ab
Fplus 2t/ha
4.33bc
4.33ab
7.00a
3.67cd
7.00a
5.00b
7.00 a
5.67bc
Fplus3t/ha
5.00abc
4.33ab
7.00a
5.00abc
7.00a
5.67ab
7.00 a
7.00a
PM= Poultry manure; Fplus= Fertiplus®. Values with the same letter(s) within the same column
are not significantly different using DNMRT at 5% probability.
Table 8 shows the effect of fertilizer types and rates on yield parameters of sweet potato at Makurdi
and Otobi in 2018. Although the application of soil amendment did not significantly affect the
yield of sweet potato plants in Otobi, the application of 15t/ha produced the highest yield of
13.39t/ha followed by NPK 150kg/ha (13.11t/ha) and Fertiplus® applied at 3t/ha (13.00t/ha). The
untreated control had the least yield of 4.66t/ha. Sweet potato yield in Makurdi was lower than
yields recorded from Otobi. Sweet potato plots where poultry dung was applied at 15t/ha was
significantly higher producing the highest yield of 7.50t/ha which was followed by the application
of Fertiplus® at 3t/ha having a yield of 7.36t/ha. At Otobi, the application of NPK at the rate of
150kg/ha produced significantly higher (P≤ 0.05) number of marketable sweet potato tubers
(65.33) which was comparable with the number of sweet potato tubers from all other plots except
plots which received poultry manure at the rate of 10t/ha (30.33) and the control (28.00).
In Makurdi, sweet potato plants which received Fertiplus® at the rate of 1t/ha recorded the highest
number of marketable sweet potato tubers (39.00) and this was comparable with the number of
marketable tubers from all other soil amendment types and rates. The untreated control plots had
the least number of marketable tubers of 20.67. The number of unmarketable sweet potato tubers
recorded at Otobi was not significantly different (P ≥ 0.05) among all the soil amendment types
and rates. However, in Makurdi, the application of Fertiplus® at the rate of 1t/ha recorded the least
number of unmarketable tubers (11.33) while the application of poultry manure to sweet potato
plants at the rate of 15t/ha produced the highest number of unmarketable sweet potato tubers
(45.33).
Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021 87
Table 8: Effect of fertilizer types and rates on yield parameters of sweet potato at Makurdi and
Otobi in 2018 cropping season
Fertilizer type and
rates
Yield (t/ha)
Number of marketable
tubers
Number of unmarketable
tubers
Otobi
Makurdi
Otobi
Makurdi
Otobi
Makurdi
Control
4.66a
3.19d
28.00b
20.67b
39.00a
26.33abc
NPK 100kg/ha
10.11a
4.45bcd
56.67a
25.67ab
74.67a
25.00bc
NPK 150kg/ha
13.11a
5.17abcd
65.33a
31.67ab
72.00a
25.67bc
NPK 200kg/ha
10.50a
7.14ab
55.33a
31.33ab
61.33a
23.00bc
PM 5t/ha
9.61a
5.44abcd
43.67ab
35.33ab
58.00a
30.67ab
PM 10t/ha
11.44a
6.67ab
30.33b
36.33ab
48.33a
14.67bc
PM 15t/ha
13.39a
7.50a
58.00a
31.00ab
69.00a
45.33a
Fertiplus® 1t/ha
10.67a
6.41abc
41.33ab
39.00a
49.33a
11.33c
Fertiplus® 2t/ha
9.00a
3.67cd
59.33a
24.33ab
71.33a
25.00bc
Fertiplus® 3t/ha
13.00a
7.36a
64.33a
35.33ab
62.67a
24.00bc
PM= Poultry manure; Fplus= Fertiplus®. Values with the same letter(s) within the same column
are not significantly different using DNMRT at 5% probability.
Pearson correlation coefficient between disease, plant vigour and yield parameters of sweet potato
plants in Otobi is presented in Table 9. Data presented in Table 9 shows there was moderate
positive and significant relationship (P˃0.01) between disease incidence and disease severity
(0.620**), leaf spot severity and plant vigour (0.499**) in Otobi. There was however negative but
significant relationship (P˃0.01) between incidence of leaf spot disease and leaf area in Otobi (-
0.469**). There was also negative significant relationship between leaf spot severity and number
of marketable sweet potato tubers (-0.515**) while leaf area had moderate significant correlation
with number of unmarketable sweet potato tubers (0.532**). Leaf area had moderate significant
positive correlation with yield (0.426*) while leaf spot severity had a strong negative correlation
with yield of sweet potato plants at Otobi (-0.702**).
Table 10 shows the Pearson correlation between disease incidence, disease severity, plant vigour
and yield of Sweet potato in Makurdi. Data presented in Table 10 shows that in Makurdi, leaf spot
severity had weak negative correlation with sweet potato yield (-0.012) while leaf spot incidence
exhibited weak significant but negative correlation on number of marketable sweet potato tubers
(-0.395*). Plant vigour had strong positive and significant correlation with yield (0.645**) and
leaf area (0.667**).
88 Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021
Table 9: Pearson correlation coefficients between disease, plant growth and yield parameters of
sweet potato plants in Otobi at 12 weeks after planting
Number
of leaves
Leaf spot
Incidence
Leaf spot
Severity
Leaf area
Plant
vigour
Yield
Number of
marketable
tubers
Number of
unmarketable
tubers
Number of
leaves
1
-0.098
-0.068
0.080
0.104
0.098
-0.022
0.008
Leaf spot
Incidence
1
0.620**
-0.469**
-0.236
-0.556**
-0.498**
-0.309
Leaf spot
Severity
1
-0.517**
0.499**
-0.702**
-0.515**
-0.186
Leaf area
1
0.687**
0.426*
0.447*
0.532**
Plant vigour
1
0.556**
0.386*
0.331
Yield
1
0.691**
0.023
No of
marketable
Tubers
1
0.448*
No of unmarketable tubers
1
**Correlation is significant at the 0.01 level 2-tailed
*Correlation is significant at the 0.05 level 2-tailed.
Table10: Pearson correlation coefficients between disease, plant growth and yield parameters of
sweet potato plants in Makurdi at 12 weeks after planting
Number
of leaves
Leaf spot
Incidence
Leaf spot
Severity
Leaf
area
Plant
vigour
Yield
Number of
marketable
tubers
Number of
unmarketable
tubers
Number of
leaves
1
0.035
0.108
0.055
0.228
0.077
0.057
0.271
Leaf spot
Incidence
1
-0.040
-
0.023
-0.205
-0.334
-0.395*
-0.114
Leaf spot
Severity
1
-
0.122
-0.211
-0.012
0.137
-0.306
Leaf area
1
0.667**
0.392*
0.162
0.303
Plant vigour
1
0.645**
0.531*
0.202
Yield
1
0.729**
0.025
No. of
marketable
tubers
1
0.010
No. of
unmarketable
tubers
1
**Correlation is significant at the 0.01 level 2-tailed
*Correlation is significant at the 0.05 level 2-tailed.
DISCUSSION
The soils of the experimental sites were classified as sandy loam and has a soil pH of 6.59 which
is reported to be required for sweet potato production. Brandenberger et al. (2014) noted that sweet
potato thrives in a sandy loam soil with rich soils producing high yield with an optimum pH of
between 5.5 and 6.8. The soil analysis in the study indicated an acidic soil with reduced soil
Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021 89
nutrients at Otobi in Otukpo LGA. This result is in line with the report of Abah (2014) who noted
that soils in Otukpo LGA are deeply weathered red and yellowish-brown soils developed
essentially on sedimentary rock and hence lacking nutrients. Similarly, Ojo et al. (2014) reported
that tropical soils were plagued with acidity, soil erosion and low nutrient content. This deficiency
in soil nutrient is corroborated by the report of Aduayi et al. (2002) which reported that Nigerian
soils are deficient in nitrogen, phosphorus and potassium. Marti and Mills (2002) also noted that
Nitrogen availability increases sweet potato leaf area and tuber yield while excess Nitrogen
reduces tuber yield
Although the Phosphorus content of the soil in the experimental site, Fertiplus® and poultry
manure used for soil amendment was lower than the recommended soil phosphorus of 6.80mg/kg,
as recommended by Kareem (2013a), the sweet potato plants in this study were able to produce
moderate yield of sweet potato despite the low nutrient content. This result is in line with the report
of Hartemink (2000) which reported low phosphorus demand in sweet potato plants. Kareem
(2013b) observed that Phosphorus absorption was positively correlated with the number of leaves
produced by sweet potato plants.
Leaf spot disease of sweet potato is an important disease of sweet potato occurring in areas with
high humidity throughout the stages of its production and its occurrence could result in crop failure
(Alabi and Waliyar, 2004; Amienyo and Ataga, 2008; Ilondu, 2013). Ekhuemelo and Nsobundu
(2020) identified Aspergillus flavus, Macrophomina phaseolina, Aspergillus tamarii and
Fusarium verticillioides as fungi inciting leaf spot disease on sweet potato plants in Makurdi.
Waliyar et al. (2000) also noted that leaf spot resulting in leaf defoliation and leaf loss of between
25% and 43% could disrupt the photosynthetic process resulting in lower yield.
The result of this present study revealed that fertilizer types and rates had significant effect on the
leaf spot incidence and severity of leaf spot disease on sweet potato at 12 WAP. This is in line
with the report of Ekhuemelo and Olatunji (2015) which observed that NPK fertilizer and poultry
application significantly increased the incidence and severity of leaf spot on pepper plants in
Makurdi.
Although soil amendment in the form of fertilizer types affected leaf spot incidence of potatoes in
the locations, the yield of sweet potatoes was moderate. This is in contrast with the report of
(Ilondu, 2013) which reported reduced yield in sweet potato plants with leaf spot. Onunwa et al.
(2021) reported nutrients were more readily released by poultry manure hence achieving an
increase in yield compared with other soil amendments.
The increase in sweet potato yield could be attributed to tolerance of the sweet potato variety used
in the study to leaf spot disease. Echezona and Nganwuchu (2006) observed that organic nitrogen
sources may allow greater tolerance of vegetative damage due to the slow release of nitrogen.
Ekhuemelo and Olatunji (2015) also reported increased yield of pepper when poultry manure was
applied. The significant increase in growth and yield of sweet potato plants treated with poultry
manure in the present study could be attributed to the ability of organic fertilizers to be retained in
the soil compared with the chemical fertilizer (Ojo et al., 2014).
90 Nigerian Journal of Plant Protection Vol. 35, No.1, June 2021
In conclusion, the study has shown that sweet potato plants treated with poultry manure at the rate
of 15tons/ha had significantly lower incidence of leaf spots disease compared with other
treatments. The application of poultry manure at 15tons/ha, and Fertiplus® at 1tons/ha to sweet
potato plants recorded least severity of leaf spots disease with higher number of leaves and plant
vigour. Highest yield was also recorded from the plot treated with poultry manure at 15tons/ha
while highest number of marketable tubers was recorded from sweet potato plants treated with
NPK 150kg/ha. The use of poultry manure at 15tons/ha and NPK 15:15:15 150kg/ha is
recommended for the reduction of the leaf spot incidence and severity on sweet potato in the study
area.
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