Incidence and spatial pattern of viruses in commercial dragon fruit orchards in southern Minas Gerais, Brazil

doi:10.21203/rs.3.rs-3782298/v1

Dragon fruit (Selenicereus spp. syn. Hylocereus spp.) cultivation has grown significantly in recent years both in Brazil and throughout the world. This growth has demanded basic information on diseases caused by pathogens that can affect the crop. Among them, diseases of viral etiology stand out for the lack of curative control methods and for being underreported because the symptoms induced by viruses can be confounded with nutritional disorders. Previously, four viral species of the genus Potexvirus were detected in samples from different regions of Brazil. The objective of this study was to determine the incidence and spatial distribution of potexvirus in 12 commercial dragon fruit orchards in southern Minas Gerais. The number of plants evaluated for incidence determination was calculated based on the size of each orchard, and the spatial distribution was investigated throughout the area. High incidences of potexvirus were detected, ranging from 80–100%. The spatial distribution of diseased plants showed an aggregated pattern in planting rows and a regular pattern in plots, indicating dissemination through cultural operations and infected seedlings as a source of inoculum. This was the first study on the epidemiology of viruses in dragon fruit orchards, and the data obtained indicate that producers are unaware of control measures required to live with the disease. In addition, the results showed the need for control seedling commercialization, aiming at the propagation of healthy material, along with the use of appropriate management practices to decrease the spread of viruses within orchards.

dragon fruit

epidemiology

Potexvirus

CVX

PiVX

ZyVX

SchVX

Fruit growing is one of the main agricultural activities in Brazil, which is the third largest fruit producer in the world, behind only China and India (ABRAFRUTAS, 2022). Due to its continental dimensions and varied soil and climate conditions, the country has the potential to produce a wide variety of tropical, subtropical, temperate and semiarid fruits.

The fruit market has experienced a recent global expansion, especially of exotic fruits, such as pitaya or Dragon fruit, reflecting the changing habits of the world population (SANTOS et al., 2022). Pitaya belongs to the family Cactaceae, genus Selenicereus syn. Hylocereus (KOROTKOVA et al., 2017), and the four main commercial species are Selenicereus undatus, which produces large fruits with red skin and white pulp; Selenicereus costaricensis, which produces medium-sized fruits with red skin and red pulp; Selenicereus megalanthus, which produces medium-sized fruits with yellow skin with thorns and white pulp; and Selenicereus setaceus, which produces small fruits with red skin, thorns and/or white pulp. The species S. undatus and S. megalanthus are the most cultivated species in the world (MERCADO-SILVA, 2018).

Despite the great diversity of native cultivars found in Brazil, most dragon fruit cultivars planted today come from Taiwan and Israel (FALEIRO et al., 2021; SANTOS et al., 2022). The wide temperature range in which dragon fruit can be grown (from 4 to 38°C) allows its cultivation in the regions with different soil and climate conditions. In addition, dragon fruit is highly hardy because it is a CAM plant, i.e., it utilizes crassulacean acid metabolism, a special photosynthesis pathway observed in plants that grow in environments with hot and dry climates. As such, the dragon fruit plant closes its stomata during the day to decrease water loss through transpiration, making it highly efficient in water use.

These characteristics make the dragon fruit a perfect candidate to be cultivated in semiarid regions, covering nine Brazilian states and the northeast and north of the state of Minas Gerais. In 2022, approximately 3 million kg of the fruit were produced in Brazil, generating a revenue of R$ 44 million (CONAB, 2023). This strong market has created a great demand for knowledge about the crop because in addition to knowledge on the phenological cycle and nutritional requirements of dragon fruit, understanding the diseases and pests that affect production is also indispensible. Therefore, investigate their etiology and epidemiology is crucial for sustainable management, and the tools developed in the field of epidemiology are essential to understand how diseases in general occur in a plant population.

Among the diseases that affect cultivated plants, viruses are economically important and, in general, increase proportionally with agricultural intensification, being responsible for approximately 47% of emerging and reemerging diseases (JANSSEN & RUIZ, 2021; JONES, 2021). The unavailability of curative control measures and the ease of dissemination of viruses in the field, whether through vectors or cultural operations, mean that these diseases have great potential to cause considerable damage to cultivated plants.

Technological innovations have allowed considerable advances, especially in the understanding of the epidemiology of viral diseases (JONES, 2021). At the macro scale, examples include the implementation of information systems, modeling capacity, decision support and forecasting of disease incidence, analysis of spatiotemporal propagation patterns and general understanding of the factors responsible for the progress rates of epidemics. In contrast, at the micro level, molecular epidemiological approaches and evolutionary studies, together with the most recent procedures for disease detection, are additional innovations capable of significantly contributing to the understanding of disease dynamics (JONES, 2013).

Recently, four viral species belonging to the genus Potexvirus were detected and identified in commercial dragon fruit orchards in municipalities in southern Minas Gerais State: Cactus virus X (CVX), Pitaya virus X (PiVX), Schlumbergera virus X (SchVX) and Zygocactus virus X (ZyVX). The infected plants presented symptoms such as chlorotic lesions, progressing to mosaic and/or mottled and curled cladodes, which reduced vigor and fruit yield. However, there is a complete lack of knowledge about the different factors related to the etiology and epidemiology of the virus in the field. The objective of this study was to diagnose the infected plants present in these orchards and to determine the incidence and spatial distribution of disease to provide information for the sustainable management of viruses in the dragon fruit crop.

Sampling, sample collection and calculation of viral incidence

Sample collections were performed in 12 dragon fruit orchards in five municipalities in the southern Minas Gerais (Table 1) from July to September 2022. For the spatial distribution analysis, visual inspections were performed in each plant present in these orchards, recording the presence and/or absence of viral symptoms in the plants. The symptoms of mosaic and chlorotic rings were considered references to detect the presence and absence of viruses in the field (Fig. 1).

Table 1 Description, geographic coordinates and the cultivation system of the orchards evaluated in the cities of Boa Esperança, Ingaí, Ribeirão Vermelho, Lavras and Perdões

City	Orchard	Latitude	Longitude	Number of plants	Type of cultivation
Boa Esperança	A	21° 0' 59" S	45° 34' 14" W	722	Organic
Boa Esperança	B	21° 0' 57" S	45° 34' 11" W	765	Organic
Ingaí	C	21° 21' 30" S	44° 58' 3" W	1012	Conventional
	D	21° 21' 28" S	44° 58 '2" W	72
	E	21° 21' 27" S	44° 58' 2" W	117
Ribeirão Vermelho	F	21° 11' 46" S	45° 3' 35" W	617	Conventional
	G	21° 11' 47" S	45° 3' 34" W	211
	H	21° 11' 48" S	45° 3' 35" W	1098
	I	21° 11' 50" S	45° 3' 36" W	351
Lavras^a	J	21° 13' 45" S	44° 58' 53" W	198	Experimental
Perdões	K	21° 5' 7" S	45° 2' 8" W	223	Conventional
Perdões	L	21° 4' 24" S	45° 2' 19" W	305	Conventional

^aFederal University of Lavras, UFLA

Fig. 1 Potexvirus symptoms in dragon fruit, used as a reference in data collection to determine the presence or absence of the virus in the plant. A: healthy; B: mosaic; C, D & E: chlorotic rings; F: cladode coiling

The number of samples collected for determination of viral incidence was calculated based on the total number of plants in each orchard (Table 1). For this, the equation proposed by Cooke (1998) was used n = (Z²pqN)/d²(N-1) + Z²pq, where n = number of samples to be collected; Z = tabulated value referring to the abscissa of the standard normal curve, with a 90% confidence level, that is, 1.65; p = 0.6, an estimate of the probability of finding virus in the sample, according to tests previously performed at the Molecular Virology Laboratory of the Federal University of Lavras; q = 1-p; N = population size or total number of plants and d = sampling error, that is, error attributed to the probability of finding or not virus in the collected sample.

A piece of cladode approximately 20 cm long, with and without symptoms, was used as the sampling unit and collected randomly in different areas within the orchard. These cladode samples were sent for diagnosis of Potexvirus by RT‒PCR. The disease incidence (DI) was determined using the technique of Campbell and Madden (1990), i.e., by multiplying the ratio between the number of infected plants (PI) and the total number of plants sampled (PA). The distribution of normality was calculated using the Shapiro‒Wilk test (P < 0.05), and the significance of differences was tested using the Kruskal‒Wallis test for nonparametric data, which allows for the comparison of three or more groups in independent samples and is used in cases where the ANOVA prerequisites were not met. The p value was adjusted using Duncan's test, using the rstatis, dplyr and ggplot2 packages of the R statistical program (R CORE TEAM, 2022).

RT‒PCR

Considering that previous studies by the authors (data in publication) revealed only the presence of species of Potexvirus in plants that exhibited symptoms suspicious of viruses, in this study, the plants were tested only for diagnosis of the genus and not of the species of potexvirus.

The total RNA of the cladodes was extracted by the lithium chloride method (CHANG et al., 1993). The reverse transcriptase (RT) reaction was performed using M-MLV Reverse Transcriptase (Sigma‒Aldrich Co., Saint Louis, MO, USA) following the manufacturer's instructions. The reaction was prepared to a final volume of 20 µl, initially containing 1 µl of 10 mM, 1 µl of 10 mM dNTPs, 6 µl of ultrapure water and 2 µl of total RNA (500 ng/µl). After heating at 70°C for 10 minutes, the tubes were immediately transferred to a container with ice, and then 2 µl of enzyme buffer (10X), 1 µl of enzyme and 7 µl of ultrapure water were added, and the reaction was incubated again at 37°C for 50 minutes and at 85°C for another 10 minutes.

Recombinant Taq DNA Polymerase (Thermo Fisher Scientific Inc., Carlsbad, California, USA) and degenerate primers for the genus Potexvirus were used for PCR amplification: Potex 1RC 5'TCAGTRTTDGCRTCRAARGT3' and Potex 2RC 5'AGCATRGCNSCRTCYTG3', which generate a fragment with 600 base pairs (VAN DER VLUGT & BERENDSEN, 2002). The amplification conditions were as follows: initial heating at 94°C for 3 minutes followed by 35 cycles of 94°C for 45 seconds, 51.5°C for 30 seconds, 72°C for 1 minute, and a final extension at 72°C for 10 minutes. The PCR product was analyzed on a 0.7% agarose gel counterstained with Red Nucleic Acid Gel Stain (Biotium®).

Spatial pattern of disease

The spatial pattern of the disease was evaluated in the planting rows using the doublet test and the run test (MADDEN et al., 2007; CAMPBELL & MADDEN, 1990; MADDEN et al., 1982). For studies in plots, the dispersion index was considered (UPTON & FINGLETON, 1985). Data analyses were performed using Microsoft Office® Excel.

For the doublet test, the numbers of healthy (-) and diseased (+) plants in the planting rows were used. The expected number of doublets (E(_D)) (MADDEN et al., 1982) was calculated considering E(_D) = m(m-1)/N, where m = number of diseased plants and N = number of plants in the row. The calculation of the doublet variance was performed using the equation variance²(_D) = [m(m-1)[N(N-1)+(2 N(m-2) + N(m-2)(m-3)-(N-1)m(m-1)]/N2(N-1)]. The standardized value (Z(_D)), based on the normal distribution, was obtained using the equation: Z(_D) = [(D + 0.5-E(D)]/σ(D), where D = number of doublets (two adjacent diseased plants) and σ(_D) = standard deviation. The distribution is considered to follow an aggregate pattern when Z(_D) > 1.64 (P = 0.05), and when Z(_D) < 1.64 (P = 0.05), it follows a random pattern. For orchards in which the presence of symptoms was observed in all plants, the variance formula was adjusted to variance²(_D) = [m(m-1)[N(N-1)+(2N(m-2) + N(m-2)(m-3)-(N-1)m(m-1)]/N2(N-1)].

The expected number of runs (E(_R)) (MADDEN et al., 1982) was obtained by the run test E(_R) = 1 + 2 m(Nm)/N and its variance: variance²(_R) = 2m(Nm)[2m(Nm)–N]/[N2(N-1)]. The standardized value (Z(_R)) = [R + 0.5-E(R)]/s(R) was calculated based on the normal distribution, where R: number of runs; m: number of diseased plants; N: number of plants in the row; s(_R): standard deviation. Z(_R) > 1.64 (P = 0.05) indicates a random pattern, and Z(_R) < 1.64 (P = 0.05) indicates an aggregate pattern. For the orchards in which the presence of symptoms was observed in all plants, the variance formula was adjusted to variance²(_R) = 1 + 2 m(Nm)[2m(Nm)–N]/[N2(N-1)].

The dispersion index (D) was obtained from the values of the mean (X), according to the equation (X1 + X2...)/Σt, where: X1 + X2...= the sum of the number of symptomatic plants in the entire area, Σt: total plants in the orchard. The variance (s²), is calculated by (s²)= Σ[(X1-µ)2/n-1], where: X1...= the number of infected plants, µ = arithmetic mean mentioned above, n = total number of plants. Based on the calculated values, the dispersion index (D) is the ratio between s²/X. When D < 1, it indicates a regular spatial pattern, when D = 1, a random pattern, and when D > 1, an aggregate pattern (CAMPBELL & MADDEN, 1990; UPTON & FINGLETON, 1985).

Incidence of potexvirus

The degenerate primers used amplified a 600 bp band in the infected samples (Fig. 2). All samples that showed symptoms were positive, but plants without symptoms were not always negative for the presence of potexvirus, indicating a probable recent infection. The average incidences observed in each orchard are georeferenced in Fig. 3 and detailed in Table 2. All orchards sampled had high incidences of Potexvirus, with the majority between 80% and 100%.

Figure 2 Electrophoresis gel with amplified fragment of 600 bp in dragon fruit cladodes infected with potexvirus. M: marker 10000 bp; 1–7: cladode samples collected in the studied orchards; 8: potexvirus positive control; 9: negative control

Figure 3 Virus incidence map in dragon fruit orchards in the southern region of the State of Minas Gerais, Brazil

Table 2

Incidence of viruses in dragon fruit orchards in the southern region of Minas Gerais
City	Orchard	Samples collected	RT-PCR result		Incidence* (%)
City	Orchard	Samples collected	Positives	Negatives	Incidence* (%)
Boa Esperança	A	10	11	-	100
Boa Esperança	B	11	10	1	90,9
Medium values					95,4
Ingaí	C	20	17	3	85
	D	8	5	3	62,5
	E	8	8	-	100
Medium values					82,5
Ribeirão Vermelho	F	15	12	3	80
	G	10	10	-	100
	H	25	23	2	92
	I	10	8	2	80
Medium values					88
Lavras	J	15	12	3	80
Perdões	K	20	20	-	100
Perdões	L	20	20	-	100
Medium values					100

* P < 0,05

The lowest incidence was observed in crop D (62.5%), and the highest incidence, with all plants infected, was observed in Boa Esperança, in orchard A; in Ingaí, in orchard E; in both Perdões orchards (K and L); and in orchard G of Ribeirão Vermelho. The test for nonparametric data included the assumptions that the incidence data by collection site were independent (Kruskal‒Wallis test P = 0.406) and that the differences were not significant when the collection sites were compared (Table 3).

Table 3

Probability of comparisons between orchards analyzed by municipalities using molecular detection by RT-PCR for potexvirus
City		Valor-p*
Boa Esperança	Ingaí	0,397^ns
Boa Esperança	Ribeirão Vermelho	0,452^ns
Boa Esperança	Perdões	0,562^ns
Boa Esperança	Lavras	0,237^ns
Ingaí	Ribeirão Vermelho	0,874^ns
Ingaí	Perdões	0,139^ns
Ingaí	Lavras	0,558^ns
Ribeirão Vermelho	Perdões	0,155^ns
Ribeirão Vermelho	Lavras	0,476^ns
Perdões	Lavras	0,097^ns
*P < 0,05; ^ns não significativo

Spatial pattern of disease

The maps of the spatial disposition of the disease in the orchards are shown in Fig. 4, where the red squares represent infected plants and the green squares represent healthy plants. They reflect the results that were obtained when random sampling was performed for viral diagnosis in plants. In general, visual observation indicated a slightly lower incidence than that observed in incidence tests based on diagnosis by RT‒PCR. While the diagnostic tests indicated an incidence ranging from 61.5–100% in random collections, visual inspection showed a frequency of infected plants of 69.7–98.5%. Considering that in the diagnostic tests some plants that did not show symptoms were positive for Potexvirus, it is likely that some of the plants evaluated as negative in the visual inspection were infected.

Figure 4 Maps of the spatial position of symptomatic and asymptomatic potexvirus plants in dragon fruit orchards in the south of Minas Gerais. Red: plants with virus symptoms, Green: plants without virus symptoms.

The spatial distribution of viruses between rows, analyzed by the run and doublet tests, showed an aggregate pattern for most of the orchards analyzed (Table 4). The occurrence of the aggregate pattern was observed in all municipalities, in 9 orchards by the run test (75%) and in 8 orchards by the doublet test (66,6%), indicating the occurrence of inoculum dispersion via seedlings and cultural operations. This finding was confirmed in the spatial distribution in plots by the dispersion index, which showed a regular pattern in 11 orchards (91.6%). The exception was one orchard in the municipality of Ingaí (C), wich showed the aggregate pattern.

Table 4

Result of the analyzes of run, doublet and dispersion index spatial patterns with *p < 0.05*
City	Orchard	Run				Doublet				D
City	Orchard	E(_R)	σ² (_R)	Z(_R)	P	E(_D)	σ² (_D)	Z(_D)	P	VM	P
Boa Esperança	A	4.99	0.01	4.55	RD	956,01	0.01	6.25	A	0.002	R
Boa Esperança	B	126.79	20.15	-10.54	A	647,19	5.14	11.16	A	0.26	R
Ingaí	C	365.69	125.24	-11.72	A	642,88	31.49	11.72	A	1.49	A
	D	32.31	13.36	-1.31	A	32,67	3.53	1.51	RD	0.23	R
	E	41.73	13.60	-0.61	A	72,85	3.58	0.87	RD	0.48	R
Ribeirão Vermelho	F	50.95	3.71	-9.06	A	608,06	0.98	9.53	A	0.12	R
	G	1.00	1.00	0.50	A	215,00	21.50	0.11	RD	0.00	R
	H	10.96	0.08	1.94	RD	1123,03	0.03	2.92	A	0.00	R
	I	6.95	0.08	16.39	RD	377,03	0.03	8.37	A	0.02	R
Lavras	J	84.64	35.08	-0.19	A	95,48	8.96	-0.33	RD	0.13	R
Perdões	K	8.91	0.15	1.53	A	363,05	0.05	1.95	A	0.01	R
Perdões	L	23.91	1.91	-7.53	A	293,59	0.54	8.05	A	0.11	R
E(_R): expected value of R; E(_D): expected value for D; σ²(_R): variance of R; σ²(_D): variance of D; Z(_R): standardized value for rums; Z(_D): standardized value for doublets; D: dispersion index; VM: relationship between variance and mean; P: standard; A: aggregate; RD: random; R: regular

This study demonstrated the vulnerability of dragon fruit to viral species belonging to the genus Potexvirus, which is corroborated by the high incidence rates recorded in all sampled fields. In addition to this crop being propagated vegetatively, it is subjected to several pruning and fruit harvesting operations with cutting tools. Therefore, the fact that these viruses do not have a vector in nature does not prevent their spread from easily occurring. Potexvirus are known to be transmitted by contaminated equipment, pollen, seeds and leaves (VERCHOT, 2021; BERCKS, 1949). As dragon fruit is not propagated by seeds in Brazil, transmission must have occurred via cultural operations, without ruling out the possibility of transmission via pollen, which has not yet been studied in this crop.

The introduction of inoculum into the inspected orchards probably occurred via infected seedlings, since no other crop was detected near the visited fields. Once present in the crop, manual operations were responsible for its rapid dissemination to the other plants in the orchard. Several studies have warned of viruses being introduced into dragon fruit crops by cuttings (TAITANO et al., 2021; MERCADO-SILVA, 2018; MIZHARI, 2014, 2015). According to Alonso Barrera et al. (2015) and Masanto et al. (2018), the absence of tool disinfection protocols for during pruning and harvesting is the main route of disease transmission after the arrival of the inoculum in the orchard.

Studies that investigated the occurrence of Potexvirus in dragon fruit in other countries indicated that after the viral particle enters the tissue, the disease develops, leading to systemic invasion of the plant. Thereafter, dissemination occurs rapidly, culminating in high incidences. Liao et al. (2003) reported incidences between 50% and 90% in Taiwan. Similar results were reported in Malaysia, where Masanto et al. (2018) reported incidences of 40–100% and Gazis et al. (2018) found incidences between 55% and 90% in the United States. According to Masanto et al. (2018), when correlating incidence with severity, it was shown that the higher the incidence of infected plants, the greater the severity of the disease in the orchard.

Our findings demonstrate the need for greater care in the production and acquisition of seedlings with high phytosanitary quality, which would have a greater chance of being achieved with the multiplication of healthy matrices in vitro (LEE & CHANG, 2022; EVALLO et al., 2021; CORREIA et al., 2017). In addition to these practices, tissue health must be confirmed by more sensitive diagnostic techniques, such as RT‒PCR with degenerate primers for Potexvirus (VAN DER VLUGT et al., 2002) or species-specific primers for Potexvirus and other viruses that have been reported in cacti (PARK et al., 2018; ZHANG et al., 2016; BAKER & ADKINS, 2015). Simultaneous detection by multiplex PCR and RT‒qPCR are also techniques that have the necessary sensitivity for the detection of the genera and/or species of viruses present (HOFFMEISTEROVÁ et al., 2022; PARK et al., 2021).

In Brazil, dragon fruit cultivation was introduced in the late 1990s (NUNES et al., 2014), so it is a relatively recent phenomenon that still needs to be studied in the context of the edaphic and climatic conditions of the country. Seedling production is usually performed by noncertified nurseries that do not apply any type of diagnostic method to verify seedling health. In addition, there are no large commercial orchards, with most production undertaken by family farmers in low-technology operations, who acquire seedlings from dubious sources, including online, given the lack of options.

The aggregated spatial pattern of the disease observed in the evaluated orchards pointed to a clear viral spread between neighboring plants that certainly occurred through cultural operations. Part of the fields presented a regular spatial distribution pattern, also indicating the possible presence of the inoculum in the seedlings used for crop establishment. In field C of Ingaí, which presented a random distribution pattern, most healthy plants, according to the producers, were replaced by plants that showed high disease severity or that had a drop in vigor, which certainly contributed to the change in the spatial pattern.

Interestingly, in Boa Esperança, field A was established before field B and therefore the plants had longer exposure in the field, which indicates disease spread throughout the crop cycle, allowing an incidence of disease in plants of practically 100%. The same pattern was observed in the Perdões fields, where the K field was established before the L field.

Alonso Barrera et al. (2015) investigated the spatial pattern of viral spread in dragon fruit and observed an aggregate spatial pattern, with an evident direction along the row, suggesting spread through pruning and harvesting operations. Similar results were found by studies on other vegetatively propagated crops: Rice et al. (2019) determined the spatial pattern of spread of the sugarcane mosaic and concluded that spread must have occurred from infected propagative material; Arnold et al. (2017) and Cieniewicz et al. (2017) observed a high degree of spatial aggregation in grapevine, with spread dependent on the existence of inoculum. For infections caused by viruses, the aggregate pattern has been the most common due to the vector mode of transmission or because of spread via mechanical action through wounds caused by tools (JANSSEN & RUIZ, 2021; JEGER, 2020).

For an improvement in dragon fruit production in Brazil, official control agencies must regulate the production and commercialization of seedlings and provide training for farmers to enable them to content with the occurrence of viral diseases through roguing and special care in pruning and fruit harvesting operations.

- All orchards established in municipalities in southern Minas Gerais have a high incidence of potexvirus, indicating a lack of preparation on the part of producers for coexistence with viral diseases.

- The spatial distribution of diseased plants shows an aggregated pattern in most orchards, indicating spread via cultural operations and infected seedlings as an inoculum source.

Acknowledgements

The authors would like to thank the National Council for Scientific and Technological Development (CNPq), the Coordination for the Improvement of Higher Education Personnel (CAPES), Foundation for Research Support of the State of Minas Gerais (FAPEMIG) for their support in this project.

Authors’ contributions

All authors contributed to the study conception, design, data acquisition, data analysis and interpretation, as well as the drafting of the work. All authors read and approved the final manuscript.

Data availability Not applicable.

Code availability Not applicable.

Conflicts of interest/competing interests

The authors declare no competing interests.

Ethics approval/declarations

This article does not contain any studies with human participants or animals.

Consent to participate Not applicable.

Consent for publication Not applicable.

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Incidence and spatial pattern of viruses in commercial dragon fruit orchards in southern Minas Gerais, Brazil

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Abstract

Figures

INTRODUCTION

MATERIALS AND METHODS

RT‒PCR

RESULTS

* P < 0,05

DISCUSSION

CONCLUSION

Declarations

References

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Journal Publication

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