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.
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.