Introduction
Tomato leaf
curl virus disease (TLCVD) is a notable biotic stress for the production of tomato
worldwide (Chakraborty 2008). This disease is of economic importance (Valizadeh
et al. 2011) as the yield of infected
plants is reduced both in qualitative and quantitative (Fang et al.
2013). TLCVD is differentiated by stunting, chlorosis, upward curling of
leaves, crinkling, puckering, and yellowing with reduced flower and fruit set.
Infected plants have a bushy appearance due to the shortening of internodal
length with more lateral branches (Kumar et
al. 2012). TLCV is a species of the genus Begomovirus in the family
Geminiviridae and is exclusively transmitted by the whitefly Bemisia tabaci
in a persistent and circulative manner (Ghanim et al. 2001; Haq et al.
2018). The whitefly is described as ‘superbug’ because of its effect on
agricultural production (Dalton 2006; Liu et
al. 2007; Barro 2008).
Whitefly B. tabaci (Gennadious)
belongs to the order Hemiptera and family Aleyrodidae (Boykin et al. 2007). Whitefly B. tabaci
(Genn.) is the most damaging pest of tomato crop in the tropical and
subtropical areas which cause heavy losses by direct feeding and transmitting
the geminiviruses (Inbar and Gerling 2008; Haider et al. 2017). It has become a global threat for many greenhouse
crops and could be able to infect plants at any stage of growth (Martin et
al. 2000). B. tabaci induces
phytotoxic disorders to crops by phloem-feeding, excretion of honeydew, and
transmission of plant viruses. It infests more than 600 plant species and
transmits Begomoviruses (Oliveira et al.
2001).
Chemical control methods remained a major approach for
the management of insect infestations, but this approach has become less
effective because the insect populations develop resistance against
insecticides (Siebert et al. 2012).
Due to the increasing trend of resistance development in insects against
commonly used insecticides and environmental hazards; insect control programs
have relied upon the use of new chemistry insecticides (Jeschke and Nauen
2008). New chemistry insecticides are environmentally safer and specific (Cloyd
and Bethke 2011). The chloronicotinyls or neonicotinoids (imidacloprid,
acetamiprid, nitenpyram, and thiamethoxam) have shown good efficacy in
controlling insects (Bacci et al.
2007; Ishaaya et al.
2007). Imidacloprid and acetamiprid have a systemic mode of action and these
have negligible impacts on the environment (Tomizawa and Casida 2005). The
botanicals obtained from plant extracts also act as effective insecticides in
reducing the problems such as insecticide resistance and environmental hazards
caused by synthetic compounds (Abou-Yousef et
al. 2010). The aqueous extracts of plants are efficient in repelling the
whiteflies because of the elevated amount of hydrocarbons that they contain
(Patel 2011). The plant health plays an important role in pest management
(Altieri and Nicholls 2003). Nutrient management improves plant health which
enables the plant to tolerate the incidence and herbivory of sucking as well as
of chewing insect-pests. In a study, the nutrients (Zn and B) significantly
reduced the population of whitefly in treated plots as compared to control
(Gogi et al. 2012). Zinc has an
essential role in the plant defense against insects and pathogenic attacks
(Machado et al. 2018). It expresses
the defense-related genes and enhances the function of the concerned proteins
(Li et al. 2016). Zn affects the
plant-microbe interaction by the activation of metalloenzymes which helps to
overcome the stress (Deepak et al.
2006). Boron plays a vital role in the activation of dehydrogenase enzymes,
sugar translocation, strengthening cell wall structure, and fruit setting
(El-Sheikh et al. 2007). It regulates
the carbohydrate and sugar contents in phloem which have been impaired by the
insect and pathogen attack (Jonathan 2012). Plant defense activators provide
effective control against sucking insects (Boughton et al. 2006). Salicylic acid (SA) is one of the prominent defense
activators that activate resistance in plants (Kamel et al. 2016). The use of SA reduces the infestation of sucking
insects in tomato plantations (Goggin 2005). SA reduces the harmful effects on
plants caused by extensive insect attack (Catinot et al. 2008). Tomato plants treated with SA produce more quantity
of terpenes that results in the repulsion of whiteflies (War et al. 2011).
Extensive studies have been conducted on the management
of whitefly and TLCVD by using conventional insecticides that revealed less
control in horticultural production systems due to repeated use (Gravalos et al. 2015). Whitefly develops
resistance against synthetic insecticides (Palumbo et al. 2001). The studies about the effects of neonicotinoids, micronutrients,
and defense activators against whitefly and TLCVD in tomato crops are lacking.
Moreover, the effect of the above said chemicals on rice growth and yield was
also evaluated. It is hypothesized that the application of treatments with
varied modes of action may decrease the whitefly infestation in tomato crops
and subsequently TLCV disease incidence. The present study was planned to evaluate the relative
effectiveness of neonicotinoids (imidacloprid and acetamiprid), plant extracts
(Azadirachta indica, Eucalyptus globulus), micronutrients (Zn & B
solution), and salicylic acid against whitefly infestation and TLCV disease
incidence in different tomato genotypes.
Materials and Methods
Experimental layout
The
experiments were conducted at the research area of the Department of Plant
Pathology, University of Agriculture, Faisalabad,
Pakistan during two consecutive crop growing seasons of the years 2014 and
2015. In both the seasons, five tomato varieties (Carmen, Roker,
Uovo Roseo, Po-02, and Lyp#1) were sown in the
rows of 3m length with 70cm row to row and 30 cm plant to plant distance. These
genotypes were obtained from Vegetables Research Institute, Faisalabad,
Pakistan. The recommended agronomic practices (irrigation, fertilizers,
weeding) were opted to keep the crop in good condition. The field was plowed
and leveled thoroughly. Farmyard manure (FYM) and NPK (1:2:2) was added as
basal. Seedlings were transplanted on 25 cm high ridges by maintaining a 40cm
plant to plant distance. Irrigation was applied weekly basis that was reduced
after flowering. Weeding was done routinely just after transplanting with a
garden hoe. The experiments were laid down in a randomized complete block
design (RCBD) with three replications. The plot size was 1.3 Kanal with
dimensions (60 × 100 ft2). All the treatments were randomly applied
in the sub-plots with fifteen rows each.
Preparation of plant extracts
For the
preparation of aqueous extracts, fresh leaves from the healthy plants of A. indica, and E.
globules were collected and macerated with sterilized water at the
dose of 1 kg/L and then thoroughly homogenized. Neem and Eucalyptus were
selected for extract preparation because of their active ingredients (A. indica and
Eucalyptol) which act as insect growth regulators. These plant extracts have
been reported effective in the management of plant virus diseases (Kumar and
Singh 2012). Many studies revealed that these two plant extracts are more
effective than other plant extracts in terms of insect repellency, plant
growth, and disease management (Ali et al.
2011). The composition neem extract (A. indica,
tannins, alkaloids, oxalate, hydrogen cyanide, phenols, flavonoids, saponins,
and steroids (Shah et al. 2017).
Eucalyptus extract contains Eucalyptol (1-8, cineol), globulol,
transpinocarveol, terpineol. The macerated extracts
were passed through two folds of muslin cloth and diluted up to ten times and
stored at 4°C until use. To prepare the required concentration, 5 mL of each
plant extract was measured and dissolved in 100 mL of water. A knapsack sprayer
was used to apply these solutions. The spray was done until drip off occurred
and control was not sprayed with any insecticide/chemical (Ashfaq et al. 2006).
Application of treatments
For the management of B. tabaci and TLCVD, plant extracts (A. indica and E. globulus), insecticides (Imidacloprid and
Acetamiprid), micronutrients consisting of 6% Zn and 4% B solution and salicylic acid (0.35%) were applied
randomly to each row of experimental plot. Micronutrients were applied by
following the direction of use as provided with the product. Salicylic acid was
used at a very low dose to avoid toxicity. The above-mentioned treatments were
applied as Neem (A. indica) extract (5 mL/L); Eucalyptus (E. globulus) extract (5 mL/L);
Acetamiprid (2 mL/L); Imidacloprid (3mL/L); Classic™
comprising 6% Zn (Zinc) and 4% B (Boron) solution (6 mL/L);
Salicylic acid (3.5 g/L) and Control (Water)
Whitefly (B. tabaci) identification and data
recording
The effect
of treatments on whitefly infestation was calculated by selecting three plants
randomly from each row and recording the whitefly population data from upper,
middle, and lower leaves and the average was calculated. For the identification
of B. tabaci, pseudo pupae were observed under a microscope, and pairs
of setae and transverse molting suture were examined (Bellows et al. 1994).
TLCV disease incidence recording
Disease incidence of TLCV infected plants on each
variety was recorded weekly basis from the ratio of infected plants to the
total number of plants and was expressed in percentage.
Data recording
for growth and yield parameters
Fresh
weight was calculated by selecting ten plants randomly from each variety
applied with the same treatment at the time of harvesting. The harvested plants
were separated into leaves, stems, and roots, all the parts were weighed and
the average of ten plants was calculated. The plant parts were dried in an open-air
draught oven at 80°C for 72 h, and then their dry weights were estimated. Plant
height was taken with measuring tape.
Fruit weight, fruit yield/plant, and number of
fruits/plant was recorded by selecting ten plants
randomly from all varieties having the same treatment and average was
calculated.
Statistical analysis
Data for the evaluation of the above-mentioned
treatments on B. tabaci population and TLCV disease incidence was
recorded before and after the application of treatments and analyzed through
Statistix 8.1 software. All possible interactions and comparisons of treatments
were determined through analysis of variance. All the treatments were compared
with one another and with control by the least significant difference (LSD)
test at P= 0.05 (Steel et al.
1997).
Results
The individual
effect of year, spray, variety, and treatment was significant against B.
tabaci population (Table 1). The two-way interactions of spray with year,
variety with year, treatment with year, and variety with spray were also
significant, whereas the interaction of variety with treatment and spray with
treatment were non-significant. The three-way interaction between variety,
spray and the year was significant, whereas the interaction of variety with
spray and treatment, variety with treatment and year, spray with treatment and
year were non-significant. The four-way interaction of variety with spray,
treatment, and year was also not significant.
All the treatments
were significantly effective in reducing B. tabaci population compared
to untreated control during 2014 and 2015 (Table 2). Imidacloprid was the most
effective in reducing the B. tabaci population (68.21%) as compared to
control followed by Acetamiprid (68.19%), neem extract (56.21%), salicylic acid
(55.63%), classic™ (Zn and B solution, 28.39%) and eucalyptus extract (15.01%),
respectively.
All the treatments
were effective in reducing B. tabaci population compared to untreated
control during the years 2014 and 2015 (Table 2). In 2014, all the treatments showed significantly different results in
reducing B. tabaci population, while in 2015 the salicylic acid and neem extract were not
significantly different from each other in reducing the B. tabaci population as compared to control. In 2014, three treatments i.e., imidacloprid, classic (Zn and B
solution), and eucalyptus extract showed significantly different results as
compared to their respective treatments in 2015. In 2014, three treatments i.e., acetamiprid, salicylic acid, and
neem extract were not significantly different from their respective treatments
in the year 2015. During both years (2014 and 2015), imidacloprid was the most
effective in reducing B. tabaci populations compared to other treatments
and control.
Table 1: Analysis of variance for B. tabaci
population and TLCVD during two seasons
|
Source of variation |
DF |
MS for B. tabaci |
MS for TLCVD |
|
Year |
1 |
0.14* |
230.91* |
|
Spray |
2 |
124.47* |
170.46* |
|
Variety |
4 |
21.12* |
16489.73* |
|
Treatment |
6 |
18.03* |
1370.32* |
|
Spray × Year |
2 |
0.62* |
1.37* |
|
Variety × Year |
4 |
0.69* |
107.83* |
|
Treatment × Year |
6 |
0.44* |
9.54* |
|
Variety × Spray |
8 |
0.05* |
3.28* |
|
Variety × Treatment |
24 |
0.09
NS |
11.93
NS |
|
Spray × Treatment |
12 |
2.71
NS |
1.12
NS |
|
Variety × Spray × Year |
8 |
0.86* |
0.53* |
|
Variety × Spray × Treatment |
48 |
0.001
NS |
0.54
NS |
|
Variety × Treatment × Year |
24 |
0.35
NS |
4.82
NS |
|
Spray × Treatment × Year |
12 |
0.32
NS |
0.63
NS |
|
Variety × Spray × Treatment × Year |
48 |
0.04
NS |
0.24
NS |
|
Error |
418 |
0.002 |
0.37 |
|
Total |
629 |
|
|
SOV source
of variation, DF degree of freedom, MS mean sum of square
*Significant
at P < 0.05 NS=Non-significant
Table 2: Comparisons
of different treatments against whitefly population during two seasons
|
Treatment |
Mean B. tabaci
Population |
Mean B. tabaci Population |
% Inhibition |
||
|
2014 |
2015 |
Before Spray |
After Spray |
||
|
Imidacloprid |
1.28 i |
1.07 j |
3.27 g |
1.04 g |
68.21 |
|
Acetamiprid |
1.78 h |
1.79 h |
3.68f |
1.17f |
68.19 |
|
Classic (Zn and Boron) |
4.87 d |
4.14 e |
4.72 c |
3.38 c |
28.39 |
|
Salicylic acid |
3.18 f |
3.16 f |
5.95 d |
2.64 d |
55.63 |
|
Neem extract |
3.06 g |
3.13 g |
4.59 e |
2.01 e |
56.21 |
|
Eucalyptus Extract |
6.91 b |
5.89 c |
5.86 b |
4.98 b |
15.01 |
|
Control |
10.71 a |
10.69 a |
9.75 a* |
9.69 a* |
|
*Means with similar letters in a
column are not significantly different at P = 0.05, LSD = 0.018
Table 3: Whitefly
B. tabaci population on all the varieties
after different sprays during two seasons
|
Mean B. tabaci
Population |
Varieties |
||||||
|
Sprays |
2014 |
2015 |
Carmen |
PO-02 |
Roker |
Uovo Roseo |
Lyp#1 |
|
1st Spray |
3.78 a |
3.75 b |
2.94 e |
3.95 a |
2.97 e |
3.48 b |
3.22 d |
|
2nd Spray |
2.73 d |
2.84 c |
2.32 m |
3.39 c |
2.52 l |
2.86 f |
2.51 l |
|
3rd Spray |
1.25 f |
1.41e |
1.62 q |
2.72 k |
1.77 p |
2.18 n |
1.87 o |
*Means with similar letters in a
row and column are not significantly different at P = 0.05, LSD = 0.013
Three sprays were applied for the management of B. tabaci during
two years (2014 and 2015). There was a significant difference in B. tabaci
population after each spray during 2014 and 2015 (Table 3).
The mean B.
tabaci population was significantly reduced in all the tested genotypes i.e., Carmen, Po-02, Roker, Uovo Roseo,
and Lyp#1in first, second and third sprays (Table 3). In the first spray, three
genotypes i.e., Po-02, Uovo Roseo, and Lyp#1 had a significant difference in B.
tabaci population while Carmen and Roker showed non-significant difference.
All the genotypes showed a significant difference in B. tabaci
population after the second spray except Roker and Lyp#1 which showed a
non-significant difference with each other. In the third spray, all genotypes
(Carmen, PO-02, Roker, Uovo Roseo, and Lyp#1) genotype showed a significant
difference in reducing B. tabaci population.
The mean B. tabaci population significantly reduced in all
genotypes i.e., Carmen, Po-02, Roker,
Uovo Roseo, and Lyp#1in first, second and third sprays during two years i.e.
2014 and 2015 (Fig. 1). All genotypes had a significant difference in mean B.
tabaci population after the third spray with respect to the first and
second sprays during 2014 and 2015. In the first spray all genotypes showed a
significant difference in mean B. tabaci population during 2014 and
2015. In second spray all the genotypes i.e.,
Carmen, Po-02, Roker, Uovo Roseo, and Lyp#1 showed a significant difference in B.
tabaci population during the years 2014 and 2015. All the genotypes showed
a significant difference in mean B. tabaci population third spray during
the year 2015 but Carmen and Lyp#1 showed a non-significant difference in mean B.
tabaci population during 2014.
Comparisons of
different treatments against TLCVD incidence
Table 4: Comparisons
of different treatments against TLCVD incidence during two seasons
|
Treatments |
Disease incidence (%) |
Disease incidence (%) |
% Efficacy |
||
|
2014 |
2015 |
Before Spray |
After Spray |
||
|
Imidacloprid |
13.83
h |
11.85
i |
34.95 g |
11.34 g |
67.56 |
|
Acetamiprid |
16.34
g |
16.02
g |
36.25 f |
16.47 f |
54.57 |
|
Classic (Zn and Boron) |
21.24
d |
20.97
d |
47.42 |
26.71 c |
43.67 |
|
Salicylic acid |
19.42
e |
18.26
f |
44.91 |
28.18 d |
37.25 |
|
Neem extract |
18.12
f |
17.94
f |
42.15 e |
20.16 e |
52.17 |
|
Eucalyptus Extract |
24.23c |
23.71
c |
37.68 b |
23.52 b |
37.25 |
|
Control |
49.09
b |
54.21
a |
56.15 a* |
57.04 a* |
|
*Means with similar letters in a column are
not significantly different at P = 0.05, LSD = 0.16
%201157-1166_files/image002.png)
Fig.
1: Comparisons of whitefly population with
variety, spray and year
The individual effect
of year, spray, variety and treatment was significant for disease incidence
(Table 1). The two- way interactions of spray with year, variety with year,
treatment with year and variety with spray were significant; whereas the two
way interactions of variety with treatment and spray with treatment were not
significant. The three-way interaction between variety, spray and year was
significant. Three-way interactions between variety, spray and treatment;
variety, treatment and year; spray, treatment and year were not significant.
The four-way interaction of variety with spray, treatment and year was also
non-significant.
All the treatments
were significantly effective in reducing TLCVD incidence compared to untreated
control. The comparative efficacy of all treatments was significantly different
from each other. Imidacloprid was the most effective in reducing TLCVD
incidence as compared to control followed by acetamiprid, neem extract,
salicylic acid, classic (Zn and B solution), and eucalyptus extract (Table 4).
All the treatments
were effective in reducing TLCVD incidence compared to untreated control during
the years 2014 and 2015 (Table 4). In 2014 all the treatments
showed significantly different results in reducing TLCVD incidence while in
2015 salicylic acid and neem extract were not significantly different from each
other in reducing the TLCVD incidence. In 2014, the efficacy of imidacloprid and
salicylic acid against TLCVD incidence was significantly different from their
respective treatments in 2015. In 2014, three treatments i.e., acetamiprid, classic (Zn and B
solution) and neem extract were not significantly different from their
respective treatments in the year 2015. During both years (2014 and 2015)
imidacloprid was the most effective in reducing TLCVD incidence as compared to
other treatments and control.
Three sprays were applied for the management of TLCVD during two years
(2014 and 2015). There was a significant difference in TLCVD incidence after
each spray during 2014 and 2015 (Table 5). After the first spray, 38.65%
disease incidence was recorded which reduced to 17.25% after the third spray
during 2014, while disease incidence reduced from 36.03–17.41% after first and
third spray, respectively during 2015.
The mean TLCVD
incidence significantly reduced in all genotypes i.e., Carmen, Po-02, Roker, Uovo Roseo, and Lyp#1in first, second
and third sprays (Table 5). In the first spray, three genotypes i.e., Po-02, Uovo Roseo, and Lyp#1 had
significant differences with respect to disease incidence while Carmen and
Roker showed a non-significant difference. All the genotypes showed a
significant difference in TLCVD incidence in second spray. In the third spray,
only Carmen showed significant difference as compared to all other
varieties/lines, while the disease incidence was non-significant in Po-02 and
Uovo Roseo; Roker and Lyp#1.
The TLCVD incidence significantly reduced in all genotypes i.e., Carmen, Po-02, Roker, Uovo Roseo,
and Lyp#1in first, second and third sprays during two years 2014 and 2015 (Fig.
2). All genotypes had a significant difference in disease incidence in the third
spray with respect to first and second sprays during 2014 and 2015. In the
first spray all genotypes showed a significant difference in disease incidence
during 2014 and 2015. In second and third sprays, three genotypes i.e., Po-02, Uovo Roseo, and Lyp#1
showed significant difference with respect to disease incidence while two
genotypes Carmen and Roker showed non-significant difference with each other
during the year 2014. All the genotypes showed a significant difference in
disease incidence in second and third sprays during the year 2015.
Table 5: TLCVD
incidence on all the varieties after different sprays during two seasons
|
Disease incidence (%) |
Varieties |
||||||
|
Sprays |
2014 |
2015 |
Carmen |
PO-02 |
Roker |
Uovo Roseo |
Lyp#1 |
|
1st Spray |
38.65 a |
36.03 a |
22.13 e |
53.67 a |
22.19 e |
47.55 b |
27.94 de |
|
2nd Spray |
24.73 b |
27.21 b |
13.72 hij |
28.15 de |
15.36 h |
31.83 c |
18.19 g |
|
3rd Spray |
17.25 c |
17.41 c |
5.46 lm |
14.79 hi |
9.33 kl |
18.19 g |
9.46 k |
*Means with similar letters in a
row and column are not significantly different at P = 0.05, LSD = 0.26
%201157-1166_files/image004.png)
Fig.
2: Comparisons of TLCVD incidence with variety,
spray and year
The growth and
yield of treated plants were significantly higher than the untreated tomato
plants in all genotypes during both years (Table 6). The plants treated with
imidacloprid showed significantly higher values of growth and yield parameters
as compared to other treatments and control. The maximum plant height (39.01
cm) was recorded in imidacloprid treated plants in 2015 which was significantly
higher than control. There was a non-significant difference between plant height in imidacloprid treated plants during both years.
Among treated plants, the minimum plant height (31.19 cm) and (30.95 cm) was
recorded in 2014 and 2015, respectively. A similar trend for other growth and
yield parameters (fresh weight, dry weight, no. of fruits/plant, fruit weight
and fruit yield/plant) was recorded in case of all the treatment during two
seasons. The maximum growth and yield were recorded in imidacloprid treated
plants followed by acetamiprid, neem extract, salicylic acid, Zn & B
solution, and Eucalyptus extract.
Discussion
Tomato leaf
curl virus disease (TLCVD) causes severe damage to tomato crops worldwide every
year (Kumar et al. 2012). TLCV transmission is accomplished by the
phloem-feeding of whitefly (Boykin et al.
2007). Different insecticides are used against whitefly to
minimize the virus transmission (Aktar et al. 2008). The repeated use of
conventional insecticides results in the development of resistance (Nauen et al. 2015).
The present study describes that there are diversified
ways to minimize the losses caused by whitefly such as transmission of TLCV.
Genetic resistance of the host plant can play a significant role to avoid yield
losses. The cultivation of
resistant varieties is the most economical method to manage the plant diseases
(Bosch et al. 2006) but when the
disease appears suddenly and at a very rapid rate in the field, the farmers
have no option except to spray the crop with some effective chemicals (Pal and
Gardener 2006). Whitefly infestation was recorded in all the varieties and none
was found resistant or immune. As none of the tested varieties showed
resistance, different insecticides, plant extracts, and nutrients were
applied for the management of insect vector of TLCV; the whitefly B. tabaci.
All the treatments reduced B. tabaci population significantly compared
to untreated control. Among the insecticides, imidacloprid was the most
effective to manage the B. tabaci population followed by acetamiprid in
that order. The imidacloprid and acetamiprid being the member of neonicotinoids, bind to the acetylcholine receptors (AChRs)
in the Central Nervous System (CNS) of insects (Zhang et al. 2000). Neonicotinoids mimic acetylcholine and induce
abnormal excitement in the insect by disturbing the systematic synaptic
transmission. Subsequently, the insect undergoes excitation and paralysis,
followed by death. The neonicotinoids are effective on contact and through
stomach action (Lind et al. 1999).
New chemistry insecticides caused maximum reduction in whitefly infestation
resulting in minimum TLCV transmission (Abbas et al. 2012).
In the
present study, the plants treated with neonicotinoids (imidacloprid and
acetamiprid) exhibited a minimum disease incidence than other treatments. TLCV
infection is delayed in early growth stages of tomato plants if treated with
imidacloprid because it protects the plant by following a systemic pathway
(Karim et al. 2008). Neonicotinoids
stimulate plant defense by expressing the salicylic acid (SA) pathway (Ford et al. 2010). These insecticides
stimulate the SA pathway by expressing pathogenesis-related (PR) proteins
(Karthikeyan et al. 2009).
Imidacloprid is absorbed by the plants systemically and translocated thus
controlling the sucking insects Table 6: Effect
of different treatments on growth and yield parameters of tomato during two
seasons
|
Treatments |
|
Parameters |
|||||
|
Years |
Plant
height (cm) |
Fresh
weight (kg) |
Dry
weight (kg) |
No.
of fruits/plant |
Fruit
weight (g) |
Fruit
yield/plant (kg) |
|
|
Imidacloprid |
2014 |
38.34
g |
3.06
g |
1.03
g |
84.6
g |
177.2
g |
8.92
g |
|
2015 |
39.01
g |
3.12
g |
1.05
g |
84.3
g |
177.3
g |
8.77
g |
|
|
Acetamiprid |
2014 |
36.23
f |
3.42
f |
0.97
f |
73.2
f |
147.5
f |
8.52
f |
|
2015 |
36.65
f |
3.45
f |
0.98
f |
73.5
f |
146.9
f |
8.33
f |
|
|
Neem
extract |
2014 |
35.67
e |
2.74
e |
0.84
e |
65.8
e |
144.7
e |
7.42
e |
|
2015 |
35.99
e |
2.66
e |
0.81
e |
65.7
e |
143.4
e |
7.53
e |
|
|
SA |
2014 |
34.82
d |
2.46
d |
0.77
d |
62.4
d |
142.6
d |
6.72
d |
|
2015 |
34.78
d |
2.49
d |
0.75
d |
62.9
d |
142.3
d |
6.57
d |
|
|
Zn
& B |
2014 |
33.75
c |
2.35
c |
0.68
c |
58.3
c |
124.5
c |
5.26
c |
|
2015 |
33.54
c |
2.34
c |
0.63
c |
58.5
c |
125.6
c |
5.41
c |
|
|
Eucalyptus
extract |
2014 |
31.19
b |
2.14
b |
0.59
b |
55.5
b |
116.3
b |
4.74
b |
|
2015 |
30.95
b |
2.18
b |
0.55
b |
55.4
b |
116.8
b |
4.83
b |
|
|
Control |
2014 |
23.25
a |
1.82
a |
0.47
a |
41.9
a |
103.7
a |
3.07
a |
|
2015 |
23.17
a |
1.85
a |
0.43
a |
41.7
a |
103.2
a |
3.07
a |
|
*Means with similar letters in a
row and column are not significantly different at P = 0.05, LSD = 1.2
(Kagabu
2003). The use of imidacloprid increases resistance against pathogens
and is regarded as induction of stress shield (Thielert 2006) because the
resultant PR proteins suppresses the viral replication and movement (Ahmed et al. 2001). Due to slow virus
movement, minimum TLCD incidence was recorded in neonicotinoids treated tomato
seedlings (Dempsey et al. 2017).
After absorption into plants, imidacloprid is converted into metabolites like
2-chlorothiazolyl-5-carboxylic acid (CTA) that enhances the plant growth and
vigor apart from insect control (Gonias et
al. 2008). Reduced diseased severity and improvement in plant growth and
yield parameters are attributed to the imidacloprid driven SA pathway that
helped in resuming NAC transcription factors of tomato from the replication enhancer
protein of TLCV (Riley and Srinavasan 2019). Neonicotinoids trigger soluble
protein content in plants that increases their ability to fix more CO2
and photosynthesis resulting in enhanced yield (Li et al. 2020).
Although chemical control is easy, direct and
rapid action to solve pest and disease problems but continuous dependence on
pesticides has contributed to environmental pollution and degradation (Palumbo et al.
2001) and has become less effective due to the development of resistance
against insecticide in insects (Siebert et
al. 2012). Bio-pesticides
can solve the problems of insecticidal resistance and
environmental hazards (Abou-Yousef et al.
2010). In the current
experiment, the extract of A. indica (neem) was very effective against
the B. tabaci population and TLCVD incidence after the synthetic
insecticides (imidacloprid and acetamiprid) followed by the extract of E.
globulus (Eucalyptus). The insecticidal
activity of neem extract is due to the components that are capable of
influencing the physiology and behavior of a wide range of insects (Schaaf et al. 2000). A. indica interacts with the corpus cardiacum, thus blocking the activity of the molting
hormone and acts as an insect growth regulator, suppresses fecundity, molting,
pupation and adult emergence (Ascher 1993). A.
indica produces antifeedant
effects by stimulating specific deterrent chemoreceptors and blocking the sugar
receptors in the mouthparts of whitefly (Butler et al. 1991). The anti-feeding and deterrent effects of neem had
forced the insects to leave the locality or chronic effect of the neem
compounds (Khattak et al.
2006). Eucalyptol present in the aqueous extract of E. globules causes toxicity and repellent effects against the
insects (Lee et al. 2002). It has
serious neurotoxic, cytotoxic, and phytotoxic effects on the sucking insects
(Bakkali et al. 2008). The neurotoxic
effect is attributed to the inhibition of acetylcholine esterase (AChE)
activities after the exposure of whitefly to the eucalyptus extract-treated
plants (Lionetto et al. 2013).
Volatile secondary metabolites present in eucalyptus extract are released into
the air that disrupts the olfactory orientation of whitefly (Deletre et al. 2015).
The efficient control of
whitefly led to a considerable reduction in TLCVD incidence. In another study,
the phyto-pesticides significantly reduced the TLCVD incidence and severity
(Bhyan et al. 2007). Eucalyptus
extract manages the disturbed balance of production and scavenging of active
oxygen species under stress situations (Wan et al. 2012) by producing
catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) (Apel and Hirt
2004). Eucalyptus extract contains eucalyptol (1-8, cineol), and many types of
terpenes that initiate the systemic defense in plants by following the JA
pathway (Hong et al. 2012). TLCV
infection results in a decrease of enzymes and photosynthetic pigments
(Montasser et al. 2012) the
deficiency of which is compensated by the application of A. indica (Sujanya
et al. 2008). Apart from A. indica, neem extract also contains tannins, nimbin, nimbidine, and
terpenoids (Mondali et al. 2009), all of these stimulates plant defense mechanisms, hormones
and proteins production that is disturbed due to virus infection (Kumar 2019). A. indica
increases the phenylalanine ammonia-lyase (PAL) activity which is suppressed by
the viral attack. The suppressed PAL activity results in reduced plant growth,
curling of leaves, and thinner cell walls of phloem in virus-infected plants
(Paul and Sharma 2002). It also boosts the production of tyrosine ammonia-lyase
(TAL) which helps in resuming the halted metabolic activities by the viral
infection (Maeda 2016).
Pathogenic attack destroys the
physiology of the plants such as nutrient uptake, assimilation, translocation
from the root to shoot and utilization (Marschner 1995). In the present study,
Classic (Zn and Boron) solution significantly reduced the whitefly population
as compared to control. Nutrients improve the plant health by regulating
metabolic and cellular functions which enable the plant to tolerate the attack
of sucking and chewing insects. The nutrients such as N, P, K, Zn, and B
significantly reduced the whitefly population in cotton (Gogi et al. 2012). The nutrients status of
the plant determines its ability to defend against pests and pathogens (Walters
and Binghum 2007). Several nutrient elements act as catalytically active
cofactors in enzymes while others stabilize the proteins structurally (Hansch
and Mendel 2009). Zn affects the plant defense by the activation of
metalloenzymes after insect attack (Fones and Preston 2012). Zn application
helps in the production of secondary metabolites that are reduced due to the
whitefly attack in tomato plants (Lehman et
al. 2015). High leaf concentration of Zn contributes to increased
structural defense of the plant and defense-related signals (Martos et al. 2016). Boron may affect the
physiology and biochemistry of the plants by strengthening the cell wall and
membrane through binding of apoplastic proteins to cis-hydroxyl groups and by interfering with enzymatic reactions
(Blevins and Leukaszewski 1998). Viruses alter the physiology of plants by
affecting the growth and development and interrupting with defense mechanism.
The concentration of reactive oxygen species (ROS) and free radicals increase
up to two-fold due to the viral attack in Zn deficient cells causing
significant damage to the plants. Zinc improves the defense system of plant
cells against ROS by interfering with membrane-bound NADPH oxidase that
produces ROS and protects membrane lipids, proteins, chlorophyll, enzymes, and
DNA of the cell from oxidation (Cakmak 2000). Boron reduces the severity of many diseases as well as
the susceptibility of plants because it affects the structure of the cell wall,
plant membrane, and metabolism of phenolics or
lignin (Brown et al. 2002).
In current experiments,
salicylic acid (SA) was found the most effective after neonicotinoids and neem
extract for whitefly management. Plant defense responses are regulated by a
complex network of signal molecules and growth regulators. Resistance genes
identify the pathogen and start defense responses. Salicylic acid (SA),
jasmonic acid (JA), naphthalene acetic acid (NAA), and ethylene (ET) mediates
both specific as well as basal defense responses (Jalali et al. 2006). SA at 3% concentration was found superior in reducing
the egg hatchability, adult emergence, adult whitefly population, and CLCuVD
severity both in soil drenching and foliar sprays (Khan et al. 2003). According to Doorn et al. (2015) SA stimulates the plant
defense responses to fight against the whitefly attack. Thaler et al. (2010) found a minimum
infestation of sucking insects on the SA treated plants. SA activates defense
cascades in plants to repel the phloem-feeding insects (Walling 2009). The
exogenously applied SA reduces the fecundity and longevity of whiteflies (Shi et al. 2013). The disease incidence was
also reduced in SA treated plants because it inhibits the systemic movement of
the virus from cell to cell and induces a signal transduction pathway (Mayers et al. 2005).
Conclusion
New chemistry insecticide (Imidacloprid) effectively
controlled the whitefly infestation in all tomato genotypes. The active
ingredients of plant extracts (Azadiracht
indica and Eucalyptol) resulted in significant whitefly mortality by
disturbing the hormonal activities of whitefly. Micronutrients and salicylic
acid stimulated the defense signals of the plants thus decreasing the whitefly
infestation.
Acknowledgments
The authors extend their sincere gratitude to the Higher
Education Commission Pakistan for providing a research grant to conclude this
study.
Author
Contributions
Muhammad Ahmad Zeshan
conceived the idea and conducted research. Muhammad Aslam Khan supervised the
experiment and reviewed the manuscript. Safdar Ali helped in planning of
experiment, analytical work and data collection. Muhammad Arshad helped in
identification of Bemisia tabaci.
Ghulam Mustafa Sahi proofread
for technical details in the manuscript. Muhammad Sagheer
provided technical assistance in whitefly data recording. Nadeem Ahmed did the
statistical analysis. Rana Binyamin assisted in data recording on all the
aspects, sample collection and yield estimations. Muhammad Usman Ghani assisted
in making formulations, plant extract formation, spray applications and other
field activities.
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