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Article

Evaluation of Pepper (Capsicum spp.) Germplasm Collection for Bacterial Wilt (Ralstonia solanacearum) Resistance

by
Nayoung Ro
1,*,
Mesfin Haile
1,
Hyeonseok Oh
1,
Ho-Cheol Ko
1,
Jungyoon Yi
1,
Young-Wang Na
1 and
Onsook Hur
2,*
1
National Agrobiodiversity Center, National Institute of Agricultural Sciences, Rural Development Administration, Jeonju 54874, Republic of Korea
2
Crop Breeding Research Division, National Institute of Crop Science, Rural Development Administration, Wanju 55365, Republic of Korea
*
Authors to whom correspondence should be addressed.
Agronomy 2024, 14(8), 1753; https://doi.org/10.3390/agronomy14081753
Submission received: 17 July 2024 / Revised: 3 August 2024 / Accepted: 7 August 2024 / Published: 10 August 2024
(This article belongs to the Section Pest and Disease Management)
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Versions Notes

Abstract

This study was conducted to identify resistant pepper accessions against the destructive disease bacterial wilt (BW) caused by Ralstonia solanacearum. A total of 338 pepper (Capsicum species) germplasms collected from different countries and deposited in the National Agrobiodiversity Genebank, Rural Development Administration (RDA), Republic of Korea, were evaluated. The evaluated accessions comprise samples from five distinct species: Capsicum annuum (213), Capsicum baccatum (47), Capsicum chinense (45), Capsicum frutescens (31), and Capsicum chacoense (2). Disease severity scores were recorded over four consecutive weeks and showed an increase in severity from initial inoculation to the end of the evaluation period. A strong correlation was observed between week 1 and 2, as well as between week 3 and 4. Ten resistant pepper accessions were identified. All selected accessions consistently exhibited low disease scores ranging from 0 to 1 throughout the observation period. These accessions belong to C. chinense (2), C. annuum (6), C. chacoense (1), and C. frutescens (1). Accessions such as IT236738 (C. chinense) and IT283498 (C. chinense) were demonstrated to have high resistance, showing no symptoms over all four weeks. Other accessions belonging to C. annuum (IT247232 and IT236340) and C. chacoense (IT158713) maintained a disease score of 0 (no symptoms) for the first three consecutive weeks; however, they developed symptoms with a score of 1 in the fourth week. Other important characteristics of the resistant materials were evaluated, including carotenoids and fruit characteristics. These accessions have important traits beyond resistance to the destructive pepper disease. They will serve as promising resources for breeding resistant pepper varieties against BW to enhance productivity.

1. Introduction

Peppers (Capsicum spp.) are members of the Capsicum genus within the Solanaceae family. This genus encompasses five cultivated species (C. annuum L., C. baccatum L. Ruiz., C. frutescens L., C. chinense Jacq., and C. pubescens Ruiz. & Pavon), along with approximately 35 wild species [1]. According to FAOSTAT (2022), over the decade from 2012 to 2022, green pepper production increased from 31 million to nearly 37 million tons, while dry pepper production rose from 3.37 million to 4.91 million tons. In 2022, China topped the list of fresh pepper producers with 16.57 million tons, followed by Mexico with 2.73 million tons, Turkey with 2.50 million tons, and Indonesia with 2.29 million tons. India led in dry pepper production, contributing 1.74 million tons to the global market [2]. Pepper is known to be a rich source of health-promoting compounds, boasting important nutraceutical and anticancer properties. However, the cultivation of pepper faces challenges from several pests and diseases worldwide, posing a significant limitation to productivity [3,4].
Biotic factors, including bacteria, fungi, and viruses in the fields, cause several diseases, posing threats to pepper production worldwide. Among the destructive diseases caused by phytopathogens, bacterial wilt (BW) caused by the soil-borne bacterial pathogen Ralstonia solanacearum has led to major agricultural losses in crops like potatoes, tomatoes, bananas, and peppers [5]. R. solanacearum is a Gram-negative bacterium that thrives in water and soil [6]. It has the capability to persist in soil for prolonged durations and gains entry into root tissues via root tips or wounds. Subsequently, the pathogen proliferates rapidly to high cell densities, leading to wilting of the leaves and disrupting the aerial parts of the plants [7,8]. BW is a highly destructive disease that has spread extensively in pepper crops across Asia [9,10,11]. The direct yield losses induced by R. solanacearum vary widely, depending on factors such as the host plant, cultivar type, climate conditions, soil composition, cropping methods, and the particular strain of the pathogen [12]. For instance, in tomatoes, losses can range from 0% to as high as 91%, in potatoes from 33% to 90%, in tobacco typically between 10% and 30%, and in bananas, losses can be the most severe, ranging from 80% to 100%, while in groundnuts, they may reach up to 20% [13].
Managing R. solanacearum through integrated methods is challenging, because it can infect crops through various pathways, including soil-borne, water-borne, or seed-/tuber-borne routes [14]. To prevent the spread of R. solanacearum, it is recommended to use seeds that are free from pathogens, ensure that the soil is not contaminated with the pathogen, and verify that the irrigation water is free from R. solanacearum [15]. If the soil becomes contaminated, implementing crop rotation (with intervals of 2–5 years), managing weed hosts, and conducting water surveys for irrigation can help reduce the bacterial load [16]. Chemical control, besides causing potential environmental harm, has not proven to be an effective method for completely eradicating R. solanacearum [17,18].
Breeding for resistance against R. solanacearum in solanaceous crops appears to be location-dependent and affected by climatic conditions [19]. The limited success in developing resistant cultivars against R. solanacearum stems from challenges including the need for durable resistance with desirable traits, adaptation to diverse agro-ecological zones, and prioritizing highly resistant cultivars to prevent further pathogen spread [19]. However, the most widely practiced method usually involves developing improved, resistant varieties through selective breeding as the most effective way to control BW in pepper crops [20]. Several studies have identified multiple pepper accessions that are resistant to bacterial wilt (BW) caused by different strains of R. solanacearum [21,22,23]. The available sources of resistance were observed to be influenced by multiple genes, and quantitative trait loci (QTLs) associated with BW resistance were identified in tomato [24,25,26], tobacco [27], eggplant [28], and pepper [29,30,31].
The evaluation of genetic pepper resources and the identification of BW-resistant germplasms are crucial tasks, offering valuable and promising sources of resistance for breeding programs. In this experiment, we utilized a total of 338 Capsicum accessions sourced from five species (C. annuum, C. baccatum, C. chinense, C. frutescens, and C. chacoense) and diverse origins. These accessions are deposited in the National Agrobiodiversity Genebank, Rural Development Administration (RDA), Republic of Korea.

2. Materials and Methods

2.1. Plant Materials and Cultivation

This study evaluated a total of 338 pepper germplasms from five Capsicum species (C. annuum: 217, C. baccatum: 47, C. chinense: 45, C. frutescens: 27, and C. chacoense: 2) for BW resistance. These germplasms were sourced from various countries and have been deposited at the Genebank of the Rural Development Administration in the Republic of Korea. The research was conducted in controlled plant growth rooms at the National Agrobiodiversity Center (NAC). The experiment consisted of ten plants from each accession, and the experiment was conducted in triplicates. The accession number, accession name, and species information are presented in Supplementary Table S1.

2.2. Pathogen and Inoculum Preparation

The pepper BW pathogen, R. solanacearum (WR-1), was isolated in Suwon, Republic of Korea, in 2012. R. solanacearum (WR-1), preserved in 20% glycerol at −70 °C, and was used. Prior to use, the pathogen isolate was streaked onto NA (nutrient agar 20 g; distilled water 1 L) plates with a loop and cultured at 28 °C for 48 h [32,33]. Then, the BW pathogen was cultured in LB liquid medium (Luria–Bertani broth 25g; distilled water 1 L) at 28 °C for 2 days, followed by suspension in distilled water and adjustment to a concentration of 1 × 108 colony-forming units (cfus)/mL (OD600 = 0.8) for use as inoculum [34].

2.3. Bacterial Wilt Resistance Evaluation

The seedlings were grown in small pots, placed in trays, and then transferred into the controlled growth room for further pathogen inoculation and evaluation. The temperature, light, and relative humidity in the growth room were automatically regulated. It was set at 28 °C with 75% relative humidity and a 12 h photoperiod. The pepper cultivars ‘Daekwonseoneon’ (moderately resistant), ‘Meotjinsanai’ (resistant), and ‘Muhanjilju’ (resistant) were used as resistant controls, whereas ‘Manitta’ was used as the susceptible control [35]. The roots of 10 seedlings per accession at the four fully expanded leaf stages were treated with 10 mL of a 1 × 108 CFU/mL bacterial suspension after being wounded on two sides of the plant by inserting a blade 2 cm from the collar. The progression of symptoms was documented at 7, 14, 21, and 28 days post-inoculation with R. solanacearum. Disease development was assessed using a scale ranging from 0 to 4 [33,36], where a score of 0 indicated no wilted leaves, 1 denoted that less than 25% of leaves were wilted, 2 represented 26 to 50% of leaves being wilted, 3 indicated 51 to 75% of leaves being wilted, and 4 signified 76 to 100% of leaves being wilted. Pepper plants with a mean disease index were categorized as follows: 0 to 1 were classified as resistant (R), 1 to 2 as moderately resistant (MR), 2 to 3 as susceptible (S), and 3 to 4 as highly susceptible (HS). Specifically, a score of 1 was classified as resistant (R), a score of 2 as moderately resistant (MR), and a score of 3 as susceptible (S).

2.4. Fruit-Related Traits and Carotenoid Content Analysis of Resistant Accessions

Following the resistance evaluation, we extracted the fruit-related traits and carotenoid contents of the selected resistant pepper accessions from the RDA genebank genetic management system (GMS). The procedures used for evaluating and analyzing these selected traits are described as follows: The fruit-related traits include fruit length (cm), fruit width (mm), fruit wall thickness (mm), fruit weight (g), and °Brix content. These measurements were taken using standard instruments such as rulers, calipers, digital scales, and refractometers.
Carotenoid analysis was performed on freeze-dried and powdered pepper fruit samples. Following the methodology described by Kim et al. [37], carotenoids were extracted, separated, and measured using High-Performance Liquid Chromatography (HPLC). Initially, carotenoids were extracted from pepper powder (0.05 g) that had been passed through a 0.7 mm sieve. This involved adding 3 mL of ethanol containing 0.1% ascorbic acid (w/v), vortexing for 20 sec, and heating in a water bath at 85 °C for 5 min. Subsequently, the carotenoid extract underwent saponification in an 85 °C water bath for 10 min with potassium hydroxide (120 µL, 80% w/v). Following this, carotenoids were analyzed using HPLC (Agilent 1260/90 Infinity II, Santa Clara, CA, USA) with a C30 YMC column (250 × 4.6 mm, 3 µm; Waters Corporation, Milford, MA, USA). This analysis included assessing individual carotenoids such as α-carotene, antheraxanthin, β-carotene, β-cryptoxanthin, capsanthin, capsorubin, lutein, violaxanthin, and zeaxanthin. Additionally, the total carotenoid content was calculated to provide an overall measure of the carotenoid abundance in the pepper fruits.

2.5. Statistical Analysis

The disease data were summarized using Microsoft Excel 2016, Microsoft Corporation, Redmond, WA, USA. Additionally, correlation analysis and principal component analysis (PCA) were conducted using the R software (version 4.3.2).

3. Results

3.1. Disease Assessment

The evaluation of pepper germplasms for BW resistance was conducted, and the disease severity assessment was recorded for four consecutive weeks. As presented in Table 1, the average disease severity scores from 338 accessions for each week were 0.50, 2.02, 2.78, and 3.06 in weeks 1, 2, 3, and 4, respectively. Detailed descriptive statistics of the experiment are provided in Table 1. The number of accessions categorized by disease score ranges (0–1, 1–2, 2–3, and 3–4) for each week and species is summarized in Table 2. In the first week, the total number of accessions within each disease severity score range were 271 (0–1), 55 (1–2), 11 (2–3), and 1 (3–4). In the second week, the totals were 71 (0–1), 91 (1–2), 89 (2–3), and 87 (3–4). Week three’s records showed 23 (0–1), 45 (1–2), 101 (2–3), and 169 (3–4) accessions across disease severity score ranges. Week four’s distribution included 10 in the range of 0–1, 30 in the range of 1–2, 111 in the range of 2–3, and 187 in the range of 3–4.
The summary of accessions by disease severity score ranges within each species is presented in Table 2. In the initial week, C. annuum had 182 accessions in the 0–1 range, C. baccatum had 25, C. chinense had 39, C. frutescens had 24, and C. chacoense had 1. In the subsequent weeks, the number of accessions within the 0–1 range decreased, while counts in the susceptible (2–3) and highly susceptible (3–4) categories increased. By week 4, C. annuum had 6, C. chinense had 2, C. frutescens had 1, and C. chacoense had 1 accession within the resistant (0–1) range. Additionally, in week 4, counts within the moderately resistant range (1–2) were observed for three species: C. annuum (18), C. baccatum (2), and C. chinense (7).

3.2. Correlation Analysis

The correlation analysis was conducted using the disease scores of four consecutive weeks and provided valuable insights into the progression of BW severity within the pepper accessions (Figure 1). Significantly strong positive correlations were observed between the disease scores of successive weeks, indicating a progressive trend in disease severity. The correlation coefficients (r) between week 1 and week 2, week 2 and week 3, and week 3 and week 4 are 0.77, 0.81, and 0.93, respectively (Figure 1). However, the correlation between week 1 and subsequent weeks decreased as the disease severity increased from week 1 to week 4. Therefore, the correlations between week 1 and week 2, week 1 and week 3, and week 1 and week 4 were 0.77, 0.55, and 0.53, respectively.

3.3. Principal Component Analysis (PCA)

The PCA results offer valuable insights into the underlying trends within the disease score data across the four consecutive weeks (Figure 2 and Table 3). PC1 exhibits the highest eigenvalue and variance percentage (3.23 and 80.74%, respectively), suggesting that it captures the dominant variation and likely represents the overarching disease progression (Table 3). PC2, PC3, and PC4 contribute 14.09%, 3.52%, and 1.65% of the total variance, respectively. The cumulative variance percentages underscore the cumulative explanatory power of the principal components, reaching 94.83% with PC2, 98.35% with PC3, and 100% with all four components. The interpretation of loadings indicates that PC1 is strongly influenced by later weeks, while PC2 reflects earlier stages. Together, these findings illustrate a condensed representation of disease dynamics, emphasizing the primary role of disease progression over time and potentially informing strategic approaches for managing BW within the germplasm population.

3.4. Selection of Resistant Pepper Accessions against BW

Table 4 presents 10 selected resistant pepper accessions against BW. The selection of disease-resistant accessions was based on their disease scores, consistently ranging between 0 and 1 throughout all four weeks. These accessions represent various species, including C. chinense (2), C. annuum (6), C. chacoense (1), and C. frutescens (1 accession) (Table 4). Remarkably, all selected accessions exhibited consistently low disease scores throughout the observation period. These 10 selected pepper germplasms have shown superior resistance compared to the resistant controls, as indicated in Table 4. Accessions such as IT236738 (C. chinense) and IT283498 (C. chinense) demonstrated highly resistant genetic resources, maintaining a consistent disease score of 0 across all four weeks. Other accessions such as IT247232 (C. annuum) and IT158713 (C. chacoense) maintained a disease score of zero for the first three consecutive weeks but developed symptoms with a score of one in the fourth week.

3.5. Fruit-Related Traits and Bioactive Compounds in Selected Resistant Pepper Accessions

The fruit-related traits and bioactive compounds (carotenoids) of the selected resistant pepper accessions (eight accessions) are summarized in Table 5 and Table 6, respectively. Figure 3 depicts the fruit pictures of the resistant pepper accessions. Fruit-related traits, including fruit length, width, wall thickness, weight, and °Brix, were measured (Table 5). The data revealed variations in fruit-related traits among the resistant pepper accessions. For instance, IT247232 from the C. annuum species had the longest fruits, measuring 14.00 cm, while IT283498 from C. chinense had the widest fruits at 23.90 mm. Additionally, IT236398 of C. annuum exhibited the thickest fruit walls, measuring 0.87 mm. Moreover, IT221919 from C. frutescens had the lightest fruits, weighing only 1.67 g. In terms of sugar concentration, IT236398 of C. annuum had the highest °Brix content at 14.8°Brix, while IT283498 of C. chinense showed the highest sugar content (10.8°Brix) among the selected resistant pepper accessions.
Table 6 presents the individual carotenoids and total carotenoid content of the selected resistant pepper accessions. Among the resistant accessions, IT236398 from the C. annuum species showed the highest amounts of β-carotene, antheraxanthin, α-carotene, β-cryptoxanthin, capsanthin, capsorubin, and zeaxanthin, with a total carotenoid content of 908.57 µg/g. Following closely, IT283498 from the C. chinense species displayed the highest amount of β-carotene and β-cryptoxanthin, totaling 670.90 µg/g of carotenoids. Additionally, IT240642 and IT228634 had total carotenoid contents of 665.01 µg/g and 517.50 µg/g, respectively. Lutein was not detected in any of the resistant accessions.

4. Discussion

BW stands as one of the most devastating diseases affecting pepper crops globally, resulting in significant decreases in yield and overall production [38,39]. Managing BW disease is challenging due to its broad range of hosts, diverse array of BW isolates, and ability to survive for extended periods within pepper plants [18,19]. R. solanacearum, the causative agent of BW, is naturally found in soil and infects the internal stems of plants, making traditional chemical control methods ineffective [21]. Developing resistant cultivars offers an alternative approach to managing BW disease. However, the presence of numerous heterogeneous species complexes within R. solanacearum complicates the breeding of resistant cultivars, posing a significant challenge [7]. Therefore, the evaluation of pepper germplasms and identification of potential resistance source accessions are crucial and should continue to develop broad-spectrum resistance to multiple strains of R. solanace. In this study, after evaluating accessions from diverse Capsicum species originating from various geographical regions, we identified accessions that are resistant to R. solanacearum (WR-1) (IT236738, IT283498, IT240012, IT158713, IT221919, IT240642, IT236398, IT247232, IT236340, and IT228634). Among these, three accessions showed high resistance to R. solanacearum (WR-1), IT236738, IT283498, and IT240012. Based on the data from the fourth week, which represents the peak of disease prevalence, we observed the following numbers of accessions: resistant (10), moderately resistant (23), susceptible (108), and highly susceptible (200). Previous studies conducted by several researchers have screened and identified resistant materials for breeding purposes, as well as for grafting as rootstock. For instance, peppers such as LS2341, PI358812, Kerting, PI322726, PI322727, PI369998, PI377688, PI322728, Jatilaba, MC4, MC5, PBC 066, PBC 437, PBC 631, and PBC 1347 showed strong resistance against broad-spectrum BW pathogens [21,22,23]. Similarly, in another study focusing on the solanaceous crop tomato, researchers screened 40 accessions and found 5 resistant accessions [40]. Among these, PI 645370, PI 647306, PI 600993, PI 355110, and PI 270210 exhibited resistance to R. solanacearum (WR-1), with PI 645370 demonstrating the highest level of resistance [40]. The selected 10 pepper accessions have demonstrated superior resistance compared to the resistant controls (Table 4). In previous studies, these resistant cultivars showed varying responses, ranging from susceptible to moderately resistant and resistant, when tested against different groups of R. solanacearum isolates [35]. Hence, the susceptibility observed in these resistant controls in our study might be attributed to race specificity and variations in resistance mechanisms. This emphasizes the critical importance of identifying and utilizing broad-spectrum resistant materials to combat the serious threat of BW in solanaceous crops for different R. solanacearum isolates.
The selected material from this study can be used as a source of resistance for breeding programs or as rootstocks for grafting. Breeding for resistance in solanaceous crops is influenced by regional and climatic conditions [19]. Long-lasting resistance to R. solanacearum, along with improved agronomic traits, is crucial [41]. Resistant cultivars must perform well across diverse agro-climatic locations and overcome emerging strains of R. solanace. However, challenges such as instability across locations and strain-specific resistance [15,42] make developing durable resistance difficult. Collecting germplasms, screening, and employing backcrossing are essential steps in breeding for BW resistance [41]. In addition, the process of developing varieties or hybrids can be time-consuming, and occasionally, resistance traits may come with a reduction in yield [43], which could impede the acceptance of a specific variety or hybrid. The grafting of vegetables and the development of resistant cultivars are extensively utilized as alternatives to chemical methods for controlling soil-borne diseases [44]. The resistant pepper accessions identified in this study can serve as potential rootstocks to combat BW and enhance yield. In modern farming, grafting is gaining increasing importance as an alternative technique, surpassing slower breeding methods [45,46]. It has been effectively used for the management of nematodes and fungal pathogens such as Phytophthora spp. [46,47,48]. This environmentally friendly, sustainable, and efficient method allows for the utilization of resistant genotypes (as rootstocks) to improve the performance of susceptible commercial cultivars (as scions) that are prone to biotic and abiotic stresses [49,50,51,52]. According to Lee [45], grafted peppers demonstrated superior disease resistance and faster growth rates. His findings pointed to the potential of using disease-tolerant pepper rootstocks as a viable strategy for mitigating soil-borne diseases and boosting overall yield. Subsequent studies have demonstrated increased growth and improved resistance to R. solanacearum and Phytophthora blight in grafted peppers [45,53]. Moreover, it contributes to maximizing growth, yield, and nutrient uptake [54]. Grafting could prove instrumental in disease management strategies and organic vegetable production. Therefore, the potential direct and indirect advantages of the resistant materials selected in our study hold significant promise for enhancing pepper crop resistance and productivity.
Genetic factors contributing to resistance against BW have been extensively examined across various crops. Recent studies have reported the identification of QTLs associated with BW resistance in pepper [31,55,56,57]. Researchers employed specific locus amplified fragment sequencing along with bulked segregant analysis to pinpoint a major QTL (qRRs-10.1) linked to BW resistance on chromosome 10 [31]. Utilizing this QTL, they developed closely associated molecular markers. Additionally, using Genotyping-by-Sequencing (GBS), researchers successfully mapped a BW-resistant QTL (pBWR-1) on chromosome 1 within an F2 population [55]. Using the same technology, five QTLs were identified (Bwr6w-7.2, Bwr6w-8.1, Bwr6w-9.1, Bwr6w-9.2, and Bwr6w-10.1) for less virulent strains and three QTLs (Bwr6w-5.1, Bwr6w-6.1, and Bwr6w-7.1) for highly virulent strains of R. solanacearum [56]. More recently, an integrated approach combining bi-parental QTL mapping and GWAS analysis led to the identification of five BW-resistance loci on pepper chromosomes 4, 5, and 8 [57]. Moreover, through exploration of the regions flanking these resistance loci, 13 candidate genes were discovered [57]. These findings offer significant potential for the development of resistant pepper cultivars that are capable of effectively combating R. solanacearum infection. The selected resistant material in this study may harbor genes associated with BW. Further investigation is required to thoroughly understand into the resistance mechanisms of the 10 selected accessions from this study.
Accessions screened with the primary objective of resistance could possess other important traits. The selected resistant pepper accessions were characterized for additional important characteristics, including fruit-related traits and important bioactive compounds such as carotenoids. For example, accessions such as IT 240012 and IT 247232 have relatively large and erect fruit. Additionally, the total carotenoid contents for IT 283498 and IT 236398 were 670.90 µg/g and 908.57 µg/g, respectively. In a previous study by [38], 166 pepper accessions imported from Vietnam and another 29 from Korea and Nepal were evaluated and identified. The selected accessions were characterized, with most of them bearing pendent fruits, except for KC999, which produced erect fruits. The fruit sizes of lines originating from Vietnam appeared smaller, with the exception of KC980 and KC1021 compared to KC350 and KC351 [38]. Therefore, characterizing resistant materials for other desirable traits is essential for identifying multiple traits or resistance with yield and nutritional traits.

5. Conclusions

In conclusion, breeding remains key in the development of disease-resistant Capsicum varieties. In this study, through the evaluation of 338 pepper accessions, we identified 10 promising genetic resources exhibiting resistance to R. solanacearum (WR-1) from different Capsicum species. These accessions belong to C. chinense, C. annuum, C. chacoense, and C. frutescens, indicating that resistance is not species-specific and can be found in diverse Capsicum species. Beyond resistance, we explored important traits for the selected accessions such as fruit characteristics and carotenoid content. Among the 10 accessions, approximately 3 to 4 also possess moderate yield-related traits and bioactive compounds. Therefore, these accessions are potential sources of resistance for future breeding programs to develop Capsicum varieties that are not only resistant to diseases but also provide increased yield and nutritional benefits, thus contributing to productivity and sustainable agriculture.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/agronomy14081753/s1: Table S1: List of pepper accessions used for evaluating bacterial wilt resistance, with accession numbers, accession names, and species name.

Author Contributions

Conceptualization, N.R. and O.H.; methodology, O.H., H.-C.K., J.Y. and H.O.; data curation, N.R., O.H. and Y.-W.N.; data analysis, M.H.; writing—original draft preparation, M.H.; writing—review and editing, M.H. and N.R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Research Program for Agricultural Science and Technology Development (Project No. PJ016045 and PJ013251) of the National Institute of Agricultural Sciences, Rural Development Administration (Jeonju, Republic of Korea).

Data Availability Statement

Relevant data are incorporated in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Analysis of Spearman correlations for the weekly disease scores of bacterial wilt. The correlation coefficient (r) is shown using a color gradient in the heatmap, ranging from deep blue (0) to deep red (1). Significant correlations are marked with an asterisk (*** p < 0.001).
Figure 1. Analysis of Spearman correlations for the weekly disease scores of bacterial wilt. The correlation coefficient (r) is shown using a color gradient in the heatmap, ranging from deep blue (0) to deep red (1). Significant correlations are marked with an asterisk (*** p < 0.001).
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Figure 2. Principal component analysis of pepper accessions based on bacterial wilt disease index across four consecutive weeks. Species names are represented by different colors, and reaction types (R: resistant, MR: moderately resistant, S: susceptible, HS: highly susceptible) are indicated by different signs. Reaction status is visualized based on the fourth-week disease data.
Figure 2. Principal component analysis of pepper accessions based on bacterial wilt disease index across four consecutive weeks. Species names are represented by different colors, and reaction types (R: resistant, MR: moderately resistant, S: susceptible, HS: highly susceptible) are indicated by different signs. Reaction status is visualized based on the fourth-week disease data.
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Figure 3. Fruits of selected resistant pepper accessions. C. chinense (IT 236738), C. chinense (IT 283498), C. annuum (IT 240012), C. frutescens (IT 221919), C. annuum (IT 240642), C. annuum (IT 236398), C. annuum (IT 247232), and C. annuum (IT 228634).
Figure 3. Fruits of selected resistant pepper accessions. C. chinense (IT 236738), C. chinense (IT 283498), C. annuum (IT 240012), C. frutescens (IT 221919), C. annuum (IT 240642), C. annuum (IT 236398), C. annuum (IT 247232), and C. annuum (IT 228634).
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Table 1. Descriptive statistics for the disease severity index of bacterial wilt in Capsicum species.
Table 1. Descriptive statistics for the disease severity index of bacterial wilt in Capsicum species.
Descriptive StatisticsWeek 1Week 2Week 3Week 4
Mean0.502.022.783.06
Standard Error0.030.060.060.05
Median0.332.002.953.20
Standard Deviation0.581.171.030.84
Sample Variance0.341.381.070.71
Kurtosis1.94−1.07−0.080.76
Skewness1.460.02−0.80−0.94
Minimum0000
Maximum3444
Count338338338338
Table 2. Distribution of accessions by disease severity index.
Table 2. Distribution of accessions by disease severity index.
WeeksSpeciesDisease Severity IndexTotal
0–11–22–33–4
Week-1C. annuum1823230217
C. baccatum25165147
C. chinense3933045
C. frutescens2430027
C. chacoense11002
Total27155111338
Week-2C. annuum39636154217
C. baccatum39152047
C. chinense20116845
C. frutescens886527
C. chacoense10102
Total71918987338
Week-3C. annuum112665115217
C. baccatum12133147
C. chinense81491445
C. frutescens2314827
C. chacoense10012
Total2345101169338
Week-4C. annuum61866127217
C. baccatum03113347
C. chinense29191545
C. frutescens10151127
C. chacoense10012
Total1030111187338
Table 3. The principal component analysis results (PCs, eigenvalue, cumulative variance) from 338 Capsicum spp. germplasms.
Table 3. The principal component analysis results (PCs, eigenvalue, cumulative variance) from 338 Capsicum spp. germplasms.
PC1PC2PC3PC4
Week 142.34−84.4532.22−5.94
Week 252.78−6.17−80.9524.92
Week 352.6634.867.93−77.12
Week 451.4540.1648.4158.26
Eigenvalue3.210.570.150.07
Variance %80.2914.313.771.63
Cumulative variance %80.2994.5998.37100.00
Table 4. Selected resistant pepper accessions to R. solanacearum (WR-1).
Table 4. Selected resistant pepper accessions to R. solanacearum (WR-1).
No.IT No.Species NameAccession NameWeek 1Week 2Week 3Week 4Reaction
1236738C. chinenseC044170.00.00.00.0R
2283498C. chinenseC044330.00.00.00.0R
3240012C. annuumChili bangi #30.00.00.20.2R
4158713C. chacoensePI 2604290.00.00.00.4R
5221919C. frutescensP 820050.10.60.60.6R
6240642C. annuumNPL-GYS-2004-390.00.30.50.8R
7236398C. annuumPX23595 (No.8)0.40.40.80.8R
8247232C. annuumP 088-390.00.00.01.0R
9236340C. annuumNew Mexico0.00.00.01.0R
10228634C. annuum9852-193 AVRDC 2110.00.51.01.0R
11Control-1C. annuumMuhanjilju0.01.113.563.56S
12Control-2C. annuumMeotjinsanai0.101.103.303.40S
13Control-3C. annuumDaekwonseoneon0.502.803.303.50S
14Control-4C. annuumManitta0.603.303.703.90S
Note: ‘Daekwonseoneon’ (moderately resistant), ‘Meotjinsanai’ (resistant), and ‘Muhanjilju’ (resistant) were used as resistant controls based on their reactions in a previous study to different R. solanacearum strains [35]. ‘Manitta’ was used as a susceptible control. However, in our study, these materials were found to be susceptible to R. solanacearum (WR-1).
Table 5. Fruit-related traits for selected resistant pepper accessions.
Table 5. Fruit-related traits for selected resistant pepper accessions.
No.IT NumberSpecies NameFruit Length (cm)Fruit Width (mm)Fruit Wall Thickness (mm)Fruit Weight (g)°Brix
1IT 236738C. chinense2.43 ± 0.2823.90 ± 0.893.07 ± 0.765.20 ± 0.367.2 ± 2.42
2IT 283498C. chinense5.67 ± 0.3821.53 ± 1.763.07 ± 0.3110.30 ± 1.6510.8 ± 1.63
3IT 240012C. annuum9.88 ± 0.5217.77 ± 3.001.70 ± 0.3612.23 ± 2.909.7 ± 1.13
4IT 221919C. frutescens4.05 ± 0.149.93 ± 0.121.13 ± 0.251.67 ± 0.127.8 ± 1.59
5IT 240642C. annuum6.65 ± 0.157.30 ± 1.250.60 ± 0.201.93 ± 0.237.4 ± 0.64
6IT 236398C. annuum9.57 ± 0.398.57 ± 0.700.87 ± 0.063.50 ± 0.4414.8 ± 4.78
7IT 247232C. annuum14.00 ± 0.4623.53 ± 0.862.37 ± 0.3127.73 ± 2.579.6 ± 0.46
8IT 228634C. annuum5.20 ± 0.5710.73 ± 0.320.70 ± 0.172.80 ± 0.266.2 ± 0.70
Table 6. Carotenoid contents (µg/g) of the selected resistant pepper accessions.
Table 6. Carotenoid contents (µg/g) of the selected resistant pepper accessions.
IT NumberSpecies Nameα-CaroteneAntheraxanthinβ-Caroteneβ-CryptoxanthinCapsanthinCapsorubinLuteinViolaxanthinZeaxanthinTotal Carotenoids
IT 236738C. chinense1.940.0010.2413.44107.8030.860.000.0023.30187.57
IT 283498C. chinense2.860.0017.99194.43364.5841.660.000.0049.38670.90
IT 240012C. annuum4.020.0057.3094.19233.6410.520.000.0094.22493.88
IT 221919C. frutescens0.520.009.458.5228.432.750.000.007.0256.69
IT 240642C. annuum6.680.00125.36212.31140.7315.110.000.00164.82665.01
IT 236398C. annuum13.8211.35230.72130.86322.6530.070.003.14165.96908.57
IT 247232C. annuum4.940.0031.5749.38275.5427.800.000.0051.34440.58
IT 228634C. annuum2.500.0060.4780.47285.6334.220.000.6353.59517.50
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Ro, N.; Haile, M.; Oh, H.; Ko, H.-C.; Yi, J.; Na, Y.-W.; Hur, O. Evaluation of Pepper (Capsicum spp.) Germplasm Collection for Bacterial Wilt (Ralstonia solanacearum) Resistance. Agronomy 2024, 14, 1753. https://doi.org/10.3390/agronomy14081753

AMA Style

Ro N, Haile M, Oh H, Ko H-C, Yi J, Na Y-W, Hur O. Evaluation of Pepper (Capsicum spp.) Germplasm Collection for Bacterial Wilt (Ralstonia solanacearum) Resistance. Agronomy. 2024; 14(8):1753. https://doi.org/10.3390/agronomy14081753

Chicago/Turabian Style

Ro, Nayoung, Mesfin Haile, Hyeonseok Oh, Ho-Cheol Ko, Jungyoon Yi, Young-Wang Na, and Onsook Hur. 2024. "Evaluation of Pepper (Capsicum spp.) Germplasm Collection for Bacterial Wilt (Ralstonia solanacearum) Resistance" Agronomy 14, no. 8: 1753. https://doi.org/10.3390/agronomy14081753

APA Style

Ro, N., Haile, M., Oh, H., Ko, H.-C., Yi, J., Na, Y.-W., & Hur, O. (2024). Evaluation of Pepper (Capsicum spp.) Germplasm Collection for Bacterial Wilt (Ralstonia solanacearum) Resistance. Agronomy, 14(8), 1753. https://doi.org/10.3390/agronomy14081753

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