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Research Article
 

Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease



S. Vijeth, I. Sreelathakumary, C.S. Aiswarya and Prashant Kaushik
 
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ABSTRACT

Background and Objective: Chilli pepper is an important spice crop and viral diseases hamper the successful of chilli peppers. The present investigation entitled was carried out to identify the sources for Chilli leaf curl virus (ChiLCV) resistance. Screening of genotypes against the ChiLCV is crucial to select the appropriate genotypes to get the successful crop production under the disease pressure. Materials and Methods: A collection of 70 popular chilli genotypes across India were evaluated under the open field conditions and using artificial inoculum for the ChiLCV disease. Thereafter, the virus presence and absence was also determined by using the Polymerase Chain Reaction (PCR) using universal primers (AV494/AC1048). Results: It determined that 23 genotypes were moderately susceptible, 12 each were susceptible and moderately resistant, 10 were symptomless, 6 were resistant, 5 were highly resistant and 2 were highly susceptible. Further, the 10 symptomless and 5 highly resistant genotypes identified under open field conditions were subjected to artificial screening by using whitefly mediated and graft inoculations. Thereafter, the 6 resistant genotypes identified with artificial inoculation by showed the presence of the virus when confirmed with PCR. However, in the whitefly mediated inoculation, four genotypes viz., Sel-3 (T1), Sel-4 (T2), Sel-6 (T3) and CHIVAR-1 (T4) did not show any amplification for the presence of the virus. Conclusion: Overall, this study provides useful information regarding the behaviour of popular chilli cultivars/genotypes against the ChiLCV disease.

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S. Vijeth, I. Sreelathakumary, C.S. Aiswarya and Prashant Kaushik, 2020. Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease. Plant Pathology Journal, 19: 121-131.

DOI: 10.3923/ppj.2020.121.131

URL: https://scialert.net/abstract/?doi=ppj.2020.121.131
 
Copyright: © 2020. This is an open access article distributed under the terms of the creative commons attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.

INTRODUCTION

Chilli is known to be affected by more than 35 viruses. Twenty-four viruses are reported to affect chilli naturally, among them 11 have been reported from India namely Pepper vein bending virus, Pepper veinal mottle virus and Chilli leaf curl virus. Among all these viruses, the Chilli leaf curl virus (ChiLCV) is the most destructive in terms of disease incidence and fruit yield loss. In severe conditions, 100% marketable fruits loss have been reported. Although, the begomoviruses infecting a large quantity of economically essential dicot plants worldwide. The genus Begomovirus belongs to the family Geminiviridae vectored by the whitefly, Bemisia tabaci Gennadius. The Begomovirus members characterized by twin icosahedral particles (18×30 nm size) and the genome consist of one or two circular, ssDNA components (2.5-3.0 kb) known as DNA A and DNA B1. Chilli leaf curl disease on chilli plant has been reported from India. A strain of Chilli leaf curl virus-Pakistan (ChiLCV-PK) was associated with chilli leaf curl disease. The partial DNA-A sequences analysis indicated that this strain was monopartite. Till date genome sequence of four begomoviruses infecting chilli have been characterized from India viz., Chilli leaf curl virus (ChiLCV), Tomato leaf curl New Delhi virus (ToLCNDV), Tomato leaf curl Joydebpur virus (ToLCJV) and recently Chilli leaf curl Palampur virus (ChiLCPV)2.

The symptoms are broadly of three types: (a) Leaf curling, (b) Vein yellowing and (c) Yellow mosaic. The typical symptoms consist of leaf curling, puckering, rolling, shortening of internodes and petioles, blistering of interveinal areas, thickening and swelling of the veins, older leaves turned out to be leathery and brittle, crowding of leaves and stunting of whole plants3. The typical leaf curl symptoms and increase in disease severity in infected plants are due to the presence of cognate beta-satellites associated with the virus. The success of disease resistance breeding depends on the genetic variability and the reliable evaluation tests employed for identification of the resistant sources. It is important to employ most reliable tests of resistance when dealing with destructive diseases like ChiLCV4. Various methods have been employed to screen Capsicum germplasm for resistance to ChiLCV viz., screening under natural epiphytotic conditions and artificial inoculation (grafting inoculation and whitefly mediated inoculation). Breeding for ChiLCV resistance was started in the late sixties in India and natural field screening was mostly used to identify resistance sources based on disease incidence and severity5,6.

Polymerase Chain Reaction (PCR) is now widely followed because of smooth application, rapid, sensitivity, specificity for identification and detection of begomoviruses in epidemiological and disease management studies with minimal sample preparation. In all begomoviruses genomes, a region with high homology is present. Universal degenerate primers are designed to anneal to these regions. These universal primers are identical primers with a base change in one or more places. They act as universal degenerate primers which amplify a DNA base in all begomoviruses7. Therefore, in order to identify the sources for ChiLCV resistance in a collection of germplasm through natural and artificial screening, it screened a collection of 70 popular chilli genotypes across India were evaluated under the open field conditions and using artificial inoculum. Thereafter, the virus presence and absence in the resistant genotypes was determined using the Polymerase Chain Reaction (PCR) by using universal primers (AV494/AC1048).

MATERIALS AND METHODS

The experimental fields were settled at the Department of Vegetable Science, College of Agriculture, Kerala Agricultural University, Vellayani, India, during 2016-2017. The study aimed at the identification of sources of leaf curl virus resistance in the popular chilli cultivars from India. The predominant soil type of the experimental site was red loam to Vellayani series, texturally classified as sandy clay loam. The region appreciates a warm, humid tropical climate.

Plant material and experimental setup: Seventy chilli genotypes had been collected from numerous sources. The list of genotypes and their source of origin is given in Table 1. The randomized complete block design was used with three replications with a spacing of 45×45 cm in a plot size of 3.6×1.8 m. There were 20 plants of each genotype per plot. All other instructions were followed based on the package and practices defined elsewhere8.

Field screening of genotypes for ChiLCV resistance: The field screening was undertaken when the natural ChiLCV pressure was at its peak because of high whitefly population. No plant protection measures were provided. The visual observation on the appearance of ChiLCV symptom was noted at fortnightly periods after transplanting. Chilli genotypes and hybrids were screened for ChiLCV resistance during summer. On each genotype, the severity of symptom was noted on the basis of severity9,10 scale 0-6. The specific disease reaction was assigned for all the genotypes based on the Coefficient of Infection (CI) and Disease Reaction (DR) as suggested by Kumar et al.11.

Table 1: List of 70 genotypes used for the study
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease

Artificial screening for ChiLCV: Ten symptomless and five highly resistant genotypes identified under natural field conditions were subjected to screening under artificial inoculation condition by whitefly mediated inoculation and graft inoculation against leaf curl virus isolate. The genotypes used for artificial screening were presented in Table 2. For the maintenance of ChiLCV inoculum, the susceptible chilli plants affected with ChiLCV were selected and replanted in the clay pot and they were kept in an insect-proof cage. Acquisition of virus from ChiLCV infected plant was prepared by using 2 L plastic bottles. The lower end of the bottles were removed and covered with a muslin cloth and the upper ends were closed with the help of cotton plugs. For the acquisition of virus, ChiLCV infected plant branches were inserted inside the bottles which contain non-viruliferous whiteflies. These flies were allowed to feed on the ChiLCV infected branches for 24 h (Acquisition period).

The virus presence and absence was also confirmed based on Polymerase Chain Reaction (PCR) using the universal primers. The ChiLCV symptomatic samples were collected from whitefly and graft inoculated plants12. From these samples, the genomic DNA was extracted following the CTAB method. The presence/absence of ChiLCV specific PCR band will be observed based on expected size amplicon (~560 bp).

Statistical analysis: The experimental data from all experiments were analyzed by using computer software PBTools (PBTools-1.4, 2014).

RESULTS

Field screening of chilli genotypes for ChiLCV resistance: The field screening was undertaken to evaluate 70 chilli germplasm against chilli leaf curl disease. The genotypes/accessions were assessed based on severity scale 0-6. The symptom severity on an individual plant basis was noted to calculate Disease Severity Index (DSI). The DSI was multiplied by Disease Incidence (DI) and divided by 100 to get the Coefficient of Infection (CI). The responses of 70 chilli genotypes to ChiLCV under natural field conditions are presented in Table 2.

Table 2: Screening of 70 chilli genotypes against ChiLCV disease under field conditions
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease
PDI: Percent disease index, DI: Disease incidence, CI: Coefficient of Infection, SL: Symptom less, HR: Highly resistant, R: Resistant, MR: Moderately resistant, MS: Moderately susceptible, S: Susceptible, HS: Highly susceptible

Field screening against ChiLCV disease: Out of 70 genotypes screened, 10 genotypes were found to be completely free (symptomless) from ChiLCV infection and were, therefore regarded as symptomless genotypes. The genotype which showed a symptomless reaction to ChiLCV included T2, T3, T5, T46, T50, T57, T63, T65, T66 and T67 (Table 2). Out of the remaining 60 genotypes, five genotypes showed highly resistant reaction and they were T51, T60, T61, T68 and T69. The first disease symptom appearance was delayed up to 45 Days After Transplanting (DAT) in genotype T51, whereas, in genotypes T60, T61, T68 and T69 it was delayed up to 60 DAT (Table 3). Out of the remaining 55 genotypes, six genotypes showed resistant reaction with CI ranging from 5 to 10. The genotypes which showed a resistant reaction to ChiLCV included T4, T6, T23, T28, T58 and T64 (Table 2).

Among six genotypes, T6 had early disease appearance (within 15 DAT). Remaining five genotypes expressed delayed symptom development and first symptoms were visible 30 DAT in T23; 45 DAT in T4, T28 and T58 and 60 DAT in T64 (Table 3). Twelve genotypes were moderately resistant with CI ranged from 10 to 20. The genotypes which showed the moderate resistant reaction to ChiLCV included T1, T8, T21, T25, T29, T31, T32, T42, T48, T59, T62 and T70. Four genotypes (T8, T21, T42 and T70) showed disease infection within 15 DAT, T1 and T25 in 30 DAT, T29, T31, T32, T48 and T62 in 45 DAT and T59 in 60 DAT.

Twenty-three genotypes were found to be moderately susceptible with CI ranging from 20 to 40. The genotypes which showed moderate susceptible reaction were T7, T9, T10, T11, T13, T16, T19, T22, T24, T26, T27, T30, T33, T34, T37, T40, T41, T43, T47, T49, T52, T53 and T56 (Table 3). In the genotype, T9 the first disease symptom appeared 30 DAT. Five genotypes (T30, T47, T49, T52 and T53) were free from infection upto 45 DAT (Table 3). Twelve genotypes viz., T12, T14, T15, T17, T18, T20, T36, T39, T44, T45, T54 and T55 showed a susceptible reaction. Two genotypes T35 and T38 showed a highly susceptible response (Table 2). Based on the Coefficient of Infection (CI) and disease reaction under field conditions (Table 2), it was found that greater number of genotypes were moderately susceptible (MS) (23), followed by moderately resistant (MR) (12), susceptible (S) (12), symptomless (SL) (10), resistant (R) (6), highly resistant (HR) (5) and highly susceptible (HS) (2).

Artificial screening for ChiLCV resistance: Selfed progenies of 10 symptomless (SL) genotypes (T2, T3, T5, T46, T50, T57, T63, T65, T66 and T67) and five Highly Resistant (HR) genotypes (T51, T60, T61, T68 and T69) under field conditions were raised under insect-proof cage.

Whitefly mediated inoculation under insect-proof cage: Out of 10 symptomless genotypes, six genotypes viz., T2, T3, T5, T46, T50 and T57 remained symptomless under artificial whitefly mediated conditions (Table 4).

Table 3: Days taken to first ChiLCV symptom expression in 70 genotypes under natural field conditions
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease
SL: Symptom less, HR: Highly resistant, R: Resistant, MR: Moderately resistant, MS: Moderately susceptible, S: Susceptible, HS: Highly susceptible

Table 4: Reaction of symptomless and highly resistant genotypes (under field conditions) against ChiLCV under whitefly mediated inoculation
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease
PDI: Percent disease index, DI: Disease incidence, CI: Coefficient of infection, SL: Symptom less, HR: Highly resistant, R: Resistant, MR: Moderately resistant, MS: Moderately susceptible, S: Susceptible and HS: Highly susceptible, -: Absence, +: Presence of 550 bp viral genome

Two genotypes, namely T63 and T67 were found resistant and the first disease symptoms appeared on 23.67 and 22.33 days after inoculation, respectively. The genotype T65 and T66 were found highly resistant and the first symptom development started 26.67 and 26.33 days after inoculation, respectively. Out of five highly resistant genotypes, T60, T61 and T69 expressed resistant reaction under whitefly mediated inoculation. The symptom development started from 22.00, 21.67 and 22.67 days after inoculation in genotypes T60, T61 and T69, respectively. Two genotypes, namely T51 and T68 showed moderate resistant reaction and the symptom development started from 22.33 and 21.67 days after inoculation, respectively (Table 5).

Graft inoculation under greenhouse conditions: Out of 10 symptomless genotypes under field conditions, none were completely free from ChiLCV infection. Four genotypes showed a highly resistant reaction and six showed a moderately resistant reaction under graft inoculation.

Table 5:
Reaction of symptom-less and highly resistant genotypes (under field conditions) against ChiLCV by graft inoculation under greenhouse conditions
Image for - Screening Of Popular Indian Chili Pepper (Capsicum annuum L.) Genotypes Against the Chili leaf curl virus Disease
SL: Symptom less, HR: Highly resistant, R: Resistant, MR: Moderately resistant, MS: Moderately susceptible, S: Susceptible, HS: Highly susceptible, +: Presence of 550 bp viral genome

The four highly resistant genotypes include T2, T3, T5 and T46 and the first disease symptoms appeared 32.00, 34.33, 33.33 and 34.33 days after graft inoculation, respectively. The genotypes viz., T50, T57, T63, T65, T66 and T67 showed a moderately resistant reaction. In these genotypes, the days to first appearance of disease ranged from 25.67 in genotype T50 to 27.33 in T67 (Table 5). The genotypes which showed highly resistant reaction under field conditions were moderately susceptible under artificial graft inoculation. The genotypes which showed moderately susceptible reaction were T51, T60, T61, T68 and T69 (Table 5). Days to first symptom appearance in these genotypes ranged from 22.00 (T60) to 22.67 (T69).

Molecular detection of ChiLCV by Polymerase Chain Reaction (PCR): In order to confirm the presence of virus from artificially inoculated plants, the DNA from the top young leaves of the artificially inoculated plants were subjected to Polymerase Chain Reaction (PCR) using geminivirus universal primers (AV494/AC1048) for confirmation of ChiLCV. After whitefly inoculation, six genotypes (T2, T3, T5, T46, T50 and T57) were symptomless, two (T65 and T66) were highly resistant and two (T63 and T67) were resistant. Out of six symptomless genotypes, four genotypes, namely T2, T3, T5 and T46 did not show virus-specific amplification, which confirmed the absence of viral genome in the inoculated plants (Table 4). However, two symptomless genotypes (T50 and T57), two highly resistant (T65 and T66) and two resistant genotypes (T63 and T67) showed amplification of 560 bp DNA fragment specific to viral genome indicating the presence of viral genomes in the plants. Under graft inoculation, all tested genotypes (4 highly resistant and 6 moderately resistant) showed the presence of virus (Table 5) by amplification of 560 bp DNA fragment specific to the viral genome.

DISCUSSION

Identification of resistance sources is of utmost importance in any resistant breeding program. Identification of true resistance from large population through artificial challenge inoculation becomes difficult and cumbersome. Keeping this in mind, natural field screening seemed best to eliminate the genotypes, which showed obvious susceptible reaction under natural epiphytotic conditions. In the present experiment, 70 genotypes were screened under natural disease conditions. The phenotypic observations suggested that the chilli plants infected at an early stage remained severely stunted. Their terminal and axillary shoots tend to stay erect and their leaflets were reduced in size and abnormally shaped. A wide range of leaf curl virus symptoms variability was noticed under natural field conditions. Enations on leaves and vein thickening were pronounced in some plants. Upward curling of leaves, leaf bending and cupping were also observed. Severely affected plants showed bushy appearance (stunted growth) due to shortened internodes with numerous small and curly leaves in the upper portion of the plants. These plants were also devoid of flowers and fruits. Senanayake et al.13 observed the most notable field symptoms like curling, mottling, puckering and stunting of plants under field conditions.

The susceptible genotypes T35 (Pusa Jwala) and T38 (Kashi Anmol) showed very severe disease infection (highly susceptible) with 100% disease incidence and the first symptoms of the disease were observed within 15 days after transplanting of the crop. Development of early and severe symptoms on these genotypes suggested that the disease was in epidemic form and screening under natural field conditions was effective. The differential response of genotypes to ChiLCV incidence and symptom expression could be attributed to the fact that the disease incidence and its spread are influenced by the occurrence and population dynamics of the vector whitefly and the weather conditions in the agro-ecosystem14,15. Whiteflies had an affinity for some particular genotypes than others and this resulted in some hybrids being more susceptible to the virus than others under field conditions16. The symptom less reaction of genotypes can either be attributed due to non-preference mechanism or merely due to escape of whiteflies9. Several resistant or tolerant genotypes identified so far are mainly based on field screening.

Under natural conditions, resistance exhibited by some lines cannot be inferred as a true resistance because those lines may manage to escape from whitefly (vector) and hence weren't infected. Sometimes it may also due to feeding of other sucking pests that lead to the slight resemblance of leaf curl symptoms. Annual, seasonal and local variations strongly influence the incidence and severity of virus under natural field conditions17. So, to identify their nature of resistance, the lines that were screened as high resistance and symptomless under field conditions were subjected to artificial whitefly and graft inoculation.

In whitefly mediated screening, the test plants were inoculated by using viruliferous whiteflies under single plant micro cages. The 10 genotypes which showed symptomless reaction under field conditions expressed a varied level of resistance under artificial whitefly mediated inoculation. Genotypes T2, T3, T5, T46, T50 and T57 were remained symptomless under artificial whitefly inoculation. The genotype T65 and T66 showed slight curling and clearing of upper leaves under whitefly mediated inoculation and rated as highly resistant. Genotypes T63 and T67 showed mild curling and swelling of veins, hence rated as resistant. The five highly resistant genotypes (T51, T60, T61, T68 and T69) under field screening were turned out to be resistant (T51 and T68) and moderate resistant (T60, T61 and T69) under whitefly inoculation conditions. The differential response of genotypes under natural and artificial conditions could be attributed to several reasons. Under artificial conditions, high and uniform inoculum pressure is ensured.

Despite efforts to ensure inoculum under the field conditions, some plants still escape infection and are erroneously regarded as symptomless or resistant. One of the reasons for escape under high disease pressure could be due to host non-preference by the vector, whitefly. Symptoms on moderately resistant or tolerant genotypes grown in the field could be inconspicuous, especially if the plant escapes early infection18. Pico et al.19 suggested that artificial cage inoculation is the most efficient, adequate and reliable technique to screen against ToLCV (Tomato leaf curl virus) and screening of tomato for ToLCV resistance under natural infestation conditions could be misleading.

After whitefly inoculation, six genotypes were symptomless, two were highly resistant and two were resistant. Out of these six symptomless genotypes, four genotypes namely T2, T3, T5 and T6 did not show any amplification for the presence of virus whereas, two genotypes (T50 and T57) showed the presence of viral genomes in the plants when subjected to PCR amplification by using degenerate primers. After graft inoculation, all ten genotypes showed symptom development. These genotypes were confirmed for the presence of virus by amplification of 560 bp DNA fragment specific to the viral genome. Though the virus is present in all the graft inoculated plants, the apparent symptoms vary with genotypes, i.e., four genotypes (T2, T3, T5 and T46) were highly resistant and six (T50, T57, T63, T65, T66, T67) were moderately resistant. This suggested that there is a better resistance mechanism working in highly resistant genotypes T2, T3, T5 and T46 and they could be used as testers in the hybridization programme of the present investigation. To confirm the resistance in the symptomless genotypes viz., GKC-29, BS-35 and EC-49 (after graft inoculation). Kumar et al.11 subjected these plant samples to PCR amplification by using degenerate primers and they confirmed the absence of viral genome from these symptomless plants. Overall, the use of genotypes with a high degree of resistance was recommended to obtain better results under disease pressure. Furthermore, the mechanism of resistance must be evaluated in detail by involving the resistance genotypes in the breeding programs and also by applying the omics-based methods.

CONCLUSION

On the basis of Coefficient of Infection (CI) all the genotypes were assigned specific disease reaction. To facilitate the attack of chilli leaf curl disease in the experiment, plant protection measures were not used for proliferation of the vector whitefly. Out of 70 genotypes screened, ten genotypes were found to be completely free from ChiLCV infection and were regarded as symptomless (SL) genotypes. The genotype which showed symptomless reaction to ChiLCV included Sel-3 (T2), Sel-4 (T3), Sel-6 (T5), CHIVAR-1 (T46), CHIVAR-3 (T50), CHIVAR-8 (T57), VS-9 (T63), Sel-40 (T65), Sel-7-1 (T66) and Sel-36-1 (T67). Five genotypes showed Highly Resistant (HR) reaction included CHIVAR-4 (T51), Japani Longi (T60), Perennial (T61), PLS-3-1 (T68) and Sel-20-1 (T69). In order to establish true resistance, the genotypes that were symptomless and highly resistant under field conditions were subjected to artificial screening. In whitefly mediated inoculation single plant inoculation technique was used, where the individual seedling was inoculated at two true leaves stage by 10 viruliferous whiteflies after acquiring virus from ChiLCV infected chilli source. The genotypes, which showed highly resistant reaction under field conditions were moderately susceptible (T51, T60, T61, T68 and T69) under artificial graft inoculation. Moreover, out of six symptomless genotypes after whitefly inoculation, four genotypes namely T2, T3, T5 and T46 did not show any amplification for the presence of the virus, confirming the absence of viral genome in the inoculated plants. Since the virus was present in all the graft inoculated plants, but the apparent symptoms varied with genotypes, there was a better resistance mechanism working in the four highly resistant genotypes.

SIGNIFICANCE STATEMENT

Chilli pepper is an important spice crop from a global perspective and there is a continuous rise in the demand for chilli pepper, but, the ChiLCV is a threat to chilli pepper cultivation. Here, it screened the widespread chilli pepper genotypes from India for their potential against the ChiLCV disease. We hope this research will be useful for the breeders and the farmers to determine the chilli pepper genotype for the successful production of the crop even under the presence of ChiLCV disease.

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