The Insidious Threat: Assessing the Dangers and Spread of Tomato Leaf Curl New Delhi Virus
Article information
Abstract
Begomoviruses have significantly threatened many important crops worldwide, causing substantial issues for years. New viruses from this genus are frequently identified, displaying increasingly harmful traits. Among the numerous species within this group, Tomato leaf curl New Delhi virus (ToLCNDV) stands out as a particularly dangerous member due to its notable pathogenic characteristics. This virus poses a serious threat to crops, leading to considerable economic losses. Although ToLCNDV has not been detected in Korea, there is no definitive assurance that it will not appear in the future. Thus, understanding the features and mechanisms of this virus, alongside extensive research on ToLCNDV characteristics and the development of effective preventive strategies, is essential. This review underscores key aspects of ToLCNDV, stressing the risks this virus poses to agriculture. Furthermore, recent advances in breeding natural resistance in key crops are discussed, offering a foundation for improved control methods and preparedness in regions currently unaffected, such as Korea, to mitigate potential agricultural impacts should the virus emerge.
Begomoviruses, the largest genus in the Geminiviridae family, are a serious threat to many economically important crops due to their rising incidence and the severity of the diseases (Leke et al., 2015; Seal et al., 2006). Reports increasingly document their pathogenicity on new hosts and their spread to broader geographical areas. Many important crops, such as tomato and cucurbit species, have been notably affected by begomoviruses, particularly in the Indian subcontinent and Mediterranean regions, with the Tomato leaf curl New Delhi virus (ToLCNDV) being one of the most significant.
ToLCNDV has caused severe agricultural losses across regions, leading to complete yield failures in elite tomato cultivars and up to 100% losses in sponge gourd due to yellow mosaic disease in the Indian subcontinent (Hanssen et al., 2010; Kumar et al., 2015; Sohrab et al., 2003). In the Mediterranean, the virus has had significant impacts, with crop losses of over 20% in zucchini and melons in Spain and up to 80% in pumpkins in Italy (Crespo et al., 2020; Panno et al., 2019; Sáez et al., 2017). Most recently, in autumn 2022, ToLCNDV was detected in various cucurbit crops across Jiangsu Province, Zhejiang Province, and Shanghai, China, affecting approximately 650 hectares and resulting in an estimated economic loss of $15 million (Cai et al., 2023; Zeng et al., 2023). The recent emergence of ToLCNDV in China suggests that the pathogenic rate of this virus and its characteristics may be intensifying, underscoring the need for thorough study and the development of preventive measures through scientific and technological advancements. This review summarizes the critical features of ToLCNDV and highlights global efforts in developing resistance against this virus, providing Korean plant pathologists with the foundational knowledge to devise effective control strategies should ToLCNDV appear in Korea.
Global Occurrence of ToLCNDV
The distribution of ToLCNDV has expanded significantly over the past few decades, affecting an increasing number of regions worldwide (Bragard et al., 2020; Moriones et al., 2017). Originally identified in the Indian subcontinent, ToLCNDV has spread across Asia, Europe, the Middle East and parts of Africa, likely facilitated by global trade and the movement of infected plant materials (Fig. 1). A total of 23 countries have officially reported the presence of ToLCNDV. Asia, including the Indian subcontinent and Southeast Asia, and Europe represent the largest shares, with 11 and 8 countries affected, respectively. In the Middle East, Turkey and Iran have documented occurrences, while Africa has seen ToLCNDV reports in four countries: Tunisia, Algeria, Morocco, and Seychelles. The timeline and outbreak patterns of ToLCNDV vary significantly across regions. In 1995, ToLCNDV was first identified in tomatoes in India, later spreading to Pakistan. Thailand became the first Southeast Asian country to report the virus (Ito et al., 2008; Mansoor et al., 1997; Padidam et al., 1995; Zaidi et al., 2017a). From that time until 2012, ToLCNDV was primarily detected in various plants in Iran and Indonesia (Mizutani et al., 2011; Yazdani-Khameneh et al., 2013, 2016). In 2012, Spain—a geographically distant location—reported its first case of ToLCNDV, marking the beginning of a significant outbreak in Europe (Juárez et al., 2014; Ruiz et al., 2017). Between 2013 and 2020, the virus spread rapidly, affecting additional European countries and parts of North Africa, as well as infecting new hosts in previously affected countries. Since 2021, fewer countries have reported new occurrences of ToLCNDV (Fidan et al., 2023; Gu et al., 2023; Just et al., 2022; Mnari-Hattab et al., 2015; Orfanidou et al., 2019; Panno et al., 2016; Radouane et al., 2018; Scussel et al., 2018). However, the recent identification of distinct ToLCNDV isolates in China indicates a potential for further outbreaks in East Asia and neighboring regions. This development highlights the virus’s ability to re-emerge and poses significant risks for agriculture in these areas.

Worldwide distribution of ToLCNDV over time. Countries are color-coded based on the year of the first reported case of ToLCNDV, as shown in the color index on the right. Each cluster is highlighted to indicate the geographical region from which the isolates originate. Full sequence analysis reveals that ToLCNDV isolates fall into three distinct groups: ToLCNDV-ES, ToLCNDV-In, and Southeast Asia.
Analysis of ToLCNDV sequences from previous studies reveals that the genetic relationships among these isolates are influenced more by region than by host species (Moriones et al., 2017; Zaidi et al., 2017a). Southeast Asian isolates are part of a single lineage within a larger, genetically varied group from the Indian subcontinent. The genetic structure of ToLCNDV populations in the Mediterranean basin is notably uniform, with isolates classified under the ToLCNDV-ES genotype. These Mediterranean isolates show considerable genetic differentiation from those originating in the Indian subcontinent. Due to their high nucleotide similarity, it is likely that the Mediterranean ToLCNDV population stems from a single introduction, suggesting a shared origin for these viral strains (Fortes et al., 2016; Juárez et al., 2019). This distinct strain exhibits different pathogenicity across several host plants, as observed in field surveys and laboratory tests. In open fields in Italy, tomato plants infected with the Indian strain of ToLCNDV (ToLCNDV-In) show severe symptoms, whereas the Spanish strain (ToLCNDV-ES) infects tomato plants only with difficulty, often without causing symptoms. In laboratory conditions, infectious clones constructed from isolates in Pakistan and Italy produced similar infection patterns across three tomato cultivars—Moneymaker, Italian San Pedro, and Korean Seogwang (Vo et al., 2023b). Additionally, experimental cucumbers showed differences in symptom development when inoculated with two ToLCNDV strains. The data revealed that the Asian strain caused severe leaf curling and mosaic yellowing in infected cucumber leaves, while cucumbers infected with ToLCNDV-ES exhibited a normal phenotype even though viral DNA can be detected in all inoculated plants (Vo et al., 2023a). Pathogenic differences between ToLCNDV isolates from the Mediterranean and Southeast Asia have been observed, suggesting regional adaptations of the virus. For instance, research using infectious clones derived from Spanish and Indonesian isolates demonstrated varied levels of infectivity and symptom expression in tomato plants. Tomatoes inoculated with the ToLCNDV [ES-Alm-Cuc-16] strain from Spain largely showed no symptoms, with only a few plants exhibiting mild vein-yellowing. In contrast, infection with ToLCNDV-[BACu-20] isolated from Indonesia induced typical begomoviral symptoms of leaf yellowing and curling in the same tomato cultivar (Yamamoto et al., 2021). These variabilities indicate that isolates may evolve unique pathogenic traits depending on their geographic origin, environmental conditions, and host interactions, complicating control efforts and emphasizing the need for region-specific strategies in managing ToLCNDV outbreaks.
The Genetic Framework and Symptomatic Impact of ToLCNDV
Genome organization
ToLCNDV is a bipartite begomovirus with two genomic components referred to as DNA A and DNA B of around 2.7 and 2.6 kb in size (Fig. 2A). The DNA-A segment contains six open reading frames (ORFs): AC1, AC2, AC3, AC4, AV1, and AV2. These ORFs play crucial roles in virus replication, pathogenesis, and encapsidation. The DNA-B segment facilitates virus movement through two proteins encoded by the BC1 and BV1 ORFs (Hanley-Bowdoin et al., 2013; Padidam et al., 1995). The product of the AC1 ORF is the replication initiator protein (Rep), which plays a crucial role in initiating rolling circle replication through its nicking and ligation activity. The AC4 ORF, embedded within the AC1 gene, is characterized as a pathogenicity determinant and acts as an RNA-silencing suppressor (RSS). The AC2 ORF encodes a 15-kDa protein that functions as both a pathogenicity factor and an RSS in begomoviruses. Meanwhile, AC3 produces a 15.2-kDa replication enhancer protein. The AV1 ORF codes for the coat protein, and AV2, which overlaps AV1, encodes a pre-coat protein with an additional likely role as an RSS. The DNA-B segment includes BV1 and BC1, which encode a nuclear shuttle protein (NSP) and a movement protein (MP). NSP is unique to the begomovirus bipartite members and functions both as a symptom determinant and an avirulence determinant. MP encodes a protein required for cell-to-cell movement. Like other bipartite begomoviruses, the NSP and MP are crucial for facilitating systemic infection.

Genome structure and disease phenotype of ToLCNDV. (A) Diagram of ToLCNDV DNA-A and DNA-B segments, with arrows indicating their respective genes. The DNA-A segment contains six genes encoding: a replication-associated protein (Rep) via AC1, a replication enhancer protein (REn) via AC2, a transcriptional activator protein (TrAP) via AC3, a coat protein (CP) via AV1, an AV2 protein, and an AC4 protein. The DNA-B segment contains two genes, BC1 and BV1, encoding a movement protein (MP) and a nuclear shuttle protein (NSP), respectively. (B) Disease symptoms in ToLCNDV-infected plants: tomato (a), melon (b), pumpkin (c), and zucchini (d).
Symptoms of ToLCNDV infection
ToLCNDV causes distinct symptoms across various host plants (Fig. 2B). In tomatoes, the virus leads to leaf curling, yellowing, mosaic patterns, and stunted growth, often resulting in poor fruit setting (Chakraborty et al., 2008). In cucurbits such as zucchini, melons, and pumpkins, typical symptoms include yellow mosaic patterns, vein clearing, leaf blistering, and misshapen or deformed fruits. Sponge gourd plants exhibit severe yellow mosaic and mottling on leaves while watermelon and cucumber leaves show yellowing and upward leaf curling (Bragard et al., 2020; Juárez et al., 2019; Venkataravanappa et al., 2020). These characteristic symptoms reflect ToLCNDV widespread impact on plant health, varying in intensity but consistently leading to reduced crop quality and yield potential across hosts.
Critical Characteristics of ToLCNDV Contributing to Crop Damage
ToLCNDV may stand out from other begomoviruses due to its broad host range and multiple transmission methods. By infecting a variety of crops and non-crop plants, ToLCNDV can persist in the environment, even when target crops are not in season. Its multiple transmission modes make control challenging, allowing the virus to spread rapidly and unpredictably across regions.
Wide host range
ToLCNDV is considered one of the most important and dangerous members of the begomovirus genus because of its pathogenicity. This virus has a wide host range, with around 58 plant species identified as natural hosts for ToLCNDV (Bragard et al., 2020). Initially identified in major crops like tomato, potato, eggplant, and pepper in the Indian subcontinent as well as some Southeast Asia countries, and later spread to cucurbit species such as pumpkin, zucchini, cucumber and melon in the Mediterranean region (Bragard et al., 2020; Charoenvilaisiri et al., 2020; Juárez et al., 2014; Khan et al., 2006; Luigi et al., 2019; Mizutani et al., 2011; Moriones et al., 2017; Padidam et al., 1995; Parrella et al., 2020; Phaneendra et al., 2012; Pratap et al., 2011; Ruiz et al., 2015; Usharani et al., 2004; Zaidi et al., 2017a). Several other cucurbit species such as gourd (wax gourd, sponge gourd, bottle gourd, ridge gourd, spine gourd), luffa, watermelon and chayote have also been identified as hosts for ToLCNDV, particularly in Asia (Anwar et al., 2020; Espino de Paz et al., 2019; Kumar et al., 2019; Kumari et al., 2021; Nagendran et al., 2017; Rajeshwari and Reddy, 2014; Tahir and Haider, 2005; Troiano and Parrella, 2023; Venkataravanappa et al., 2018b, 2019, 2020; Wilisiani et al., 2019). In addition to the main families (Solanaceae and Cucurbitaceae), crops in the Apiaceae (carrot), Caricaceae (papaya), Fabaceae (soybean, mungbean), and Malvaceae (kenaf, okra, cotton) families have also shown susceptibility to ToLCNDV infection, though official records do not yet specify losses in these crops (Jamil et al., 2017; Raj et al., 2007, 2008; Sivalingam et al., 2011; Venkataravanappa et al., 2018a; Zaidi et al., 2016).
The host range orientation of ToLCNDV varies across different geographical regions (Table 1). In the Indian subcontinent, ToLCNDV shows severe pathogenicity in a wide range of important crops, whereas in the Mediterranean region, it primarily targets cucurbit plants. This pattern suggests possible genomic differences between Mediterranean isolates and the original ToLCNDV strains from the Indian subcontinent.
Beyond major crops, non-crop plants are also susceptible hosts for ToLCNDV. The ornamental species belong Asteraceae (Chrysanthemum indicum, Dahlia pinnata, Tagetes erecta), Acanthaceae (Crossandra infundibuliformis), Apocynaceae (Catharanthus roseus, Calotropis procera), Euphorbiaceae (Jatropha spp.), Papaveraceae (Papaver somniferum), Phyllanthaceae (Sauropus androgynus), and Solanaceae (Cestrum nocturnum) were reported to infect with ToLCNDV (Ashwathappa et al., 2020; Pant et al., 2018; Shih et al., 2013; Srivastava et al., 2016; Sundararaj et al., 2020; Vo et al., 2022; Zaidi et al., 2017b). High-market value species such as Chrysanthemum and Dahlia pinnata are vulnerable to this virus, potentially impacting their trade and market value within horticulture. Since these ornamental plants are often grown near agricultural areas, they increase the risk of virus transmission, which can spread from ornamental plants to crops. This proximity not only amplifies the spread of infection but also leads to economic repercussions across both the ornamental and agricultural sectors.
Previously, monocots—particularly weeds—rarely harbored begomoviruses. However, over time, many viruses have adapted to infect different types of plants. Notably, ToLCNDV has been increasingly detected in weeds from nine different families, comprising 24.1% (14 of 58 total species) of its host range compared to major crop hosts (Ansar et al., 2021; Bragard et al., 2020; Haider et al., 2006; Juárez et al., 2019; Lager et al., 2022; Mall et al., 2014; Pant et al., 2018; Zaidi et al., 2017a). The detection of ToLCNDV in a growing number of weeds (Table 2) highlights the adaptability of this virus, and signals a potential for more severe outbreaks, especially given the common presence of weeds in field and greenhouse environments. Weeds can act as intermediate hosts for viruses capable of infecting crops and provide habitats for insect vectors. Their unique traits—high survival rates across seasons—enhance the threat of the virus. Further study of the ToLCNDV-weed interaction is essential to developing effective control strategies for ToLCNDV, which could be applied in open-field environments.
Diversity of transmission mode
A significant factor amplifying the threat of ToLCNDV is its diverse transmission methods, allowing it to infect healthy plants through multiple avenues: insect vectors, seeds, mechanical transmission, and pollen transmission recently (Fig. 3). This flexibility in transmission not only accelerates the virus’s spread but also enhances its adaptability across various environments, making it a formidable pathogen in managing crop health.

Transmission pathways of ToLCNDV, illustrating the virus four primary modes of spread: natural transmission through whiteflies, seed-borne transmission, pollen-mediated transmission, and mechanical transmission.
Like other begomoviruses, ToLCNDV is transmitted naturally by the whitefly Bemisia tabaci (Padidam et al., 1995). Additionally, Trialeurodes vaporariorum has been reported as a ToLCNDV vector in India, although these findings have not been widely replicated (Sangeetha et al., 2018). However, research by a Spanish group indicates that the ToLCNDV Spain strain is not transmitted by T. vaporariorum as none of the inoculated zucchini were infected by the virus in the presence of this insect (Farina et al., 2023). Biological differences among whitefly species can influence virus emergence and epidemiology, affecting host range and transmission efficiency. The specificity of virus transmission is also illustrated by various begomoviruses, which are transmitted differentially by species within the B. tabaci complex.
Seed transmission of ToLCNDV has been documented more recently when the entry and spread of the geminivirus in the seed were thought to be almost impossible. The first evidence of ToCNDV seed transmissibility was recorded in chayote (Sechium edule) in India, where ToLCNDV infected 25% of seedlings in 2015–2016 (Sangeetha et al., 2018). In 2017, ToLCNDV was detected in young seedlings that naturally germinated from fallen fruits on an Italian farm. Harvested seeds from two ToLCNDV-infected zucchini squash fields showed infection rates above 60%, suggesting seeds might play a role in vertical transmission (Kil et al., 2020). However, a research group from Spain contested ToLCNDV seed transmissibility, finding that bleach treatment could remove the virus from the surface of melon seeds. The researchers concluded that ToLCNDV is seed-borne but does not transmit through seeds in melon (Fortes et al., 2023). Another study from a Taiwanese research group examined ToLCNDV seed transmissibility in cucumber (Chang et al., 2023). They reported that ToLCNDV-CB (from cucumber) and ToLCNDV-OM (from oriental melon) were detected in cucumber seed coats and seedlings at infection rates above 79%. Additionally, in 2023, a major experiment was conducted in India to test for seed transmission of ToLCNDV in bitter gourd. Two seed sources were analyzed: one set of seeds was procured from the seed market, and the other set consisted of seeds from the same cultivars collected from infected field plants (Gomathi Devi et al., 2023). Results showed that ToLCNDV was detectable in both seed sources, with a higher detection rate in market seeds (62.96%) compared to field-collected seeds (33.33%). Evidence supporting seed transmission was provided by grow-out experiments conducted in insect-proof glasshouses, where both symptomatic and asymptomatic plants grown from these seeds showed high infection rates with ToLCNDV, at 43.2% and 34.8%, respectively. Although the mechanisms influencing seed transmission remain unclear, multiple studies indicate that seed transmission of ToLCNDV is possible. This transmission route is critical to ToLCNDV danger, as it establishes an important source of primary inoculum. Further, the spread of the virus increased from 43% (from seed transmission alone) to 72% after whiteflies were introduced in a greenhouse, suggesting that whiteflies acquired ToLCNDV from infected seeds and spread the virus to other plants. The discovery of seed-borne infection in begomoviruses offers a new perspective, sparking significant interest in better understanding its validity and potential impact on begomovirus disease epidemics.
A new type of ToLCNDV infection has been identified, revealing that the virus can be transmitted through pollen (Chang et al., 2023). When ToLCNDV was detected in the pollen of infected plants, tests involving the application of infected pollen onto healthy plants resulted in ToLCNDV-infected fruits, with an infection rate exceeding 70%. This indicates that pollen from virus-infected plants can act as a natural source of infection, transmitting the virus to other plants. When virus-carrying pollen lands on the stigma of female plants, it can germinate and enable the virus to infect the plant ovules. These virus-infected pollen grains may be dispersed by humans, wind, or insects, facilitating both vertical and horizontal transmission of the virus. This case represents the first known instance of pollen-mediated transmission of begomovirus.
Another transmission route for ToLCNDV is mechanical sap transmission, with reports for Asian and Mediterranean strains. This feature underscores the danger of ToLCNDV, as begomoviruses are rarely transmitted mechanically through rub or sap inoculation. In India, a ToLCNDV isolate from potato was successfully sap-transmitted to Nicotiana benthamiana and potato plants, an observation reported initially and again in 2020 when ridge gourd was also found to support sap transmission of ToLCNDV (Kaur et al., 2020; Usharani et al., 2004). In Taiwan, scientists demonstrated that cucurbit species, including oriental melons, bottle gourd, cucumber, zucchini, and sponge gourd, could be infected by mechanical transmission of a ToLCNDV isolate from oriental melon (Chang et al., 2010). Similarly, the Spanish ToLCNDV isolate was reported to be transmitted mechanically to the two major Cucumis crops, melon and cucumber (López et al., 2015). However, not all ToLCNDV strains are capable of being mechanically transmitted in host plants. Detailed studies on the molecular factors involved in mechanical transmissibility were conducted with ToLCNDV-OM and ToLCNDV-CB (Lee et al., 2020). Genetic evidence revealed that the DNA-B component in ToLCNDV-OM is associated with mechanical transmissibility, suggesting that understanding the molecular mechanisms behind this transmission could aid in developing more effective strategies to manage begomovirus-caused diseases. With such varied transmission methods, ToLCNDV not only enhances its ability to spread disease but also becomes a more formidable challenge in plant-virus interactions.
Advances in Virus Control Strategies: Emphasizing Host Resistance for Sustainable Management
Various disease control strategies, including chemical, biological, and cultural approaches, have been combined to maximize effectiveness in limiting virus spread. However, no single strategy has proven effective or universally suitable for all viruses, including ToLCNDV. Developing immune plant genotypes is recognized as one of the most effective methods for managing ToLCNDV. Consequently, multiple studies are actively working to identify natural plant resources that exhibit resistance or tolerance to this virus. This review highlights research that employs advanced biology to screen for and incorporate natural resistance genes into breeding programs for Solanaceae and Cucurbitaceae species, which are among ToLCNDV primary hosts.
Solanaceae
Research on wild Solanum species, such as S. habrochaites, S. chilense, S. peruvianum, and S. pimpinellifolium, has highlighted their inherent resistance to begomoviruses in tomatoes, encouraging further screenings. From these species, six resistance genes—Ty1/Ty3, Ty2, Ty3/3a, Ty4, ty5, and Ty6—have been mapped and characterized using molecular markers for their effectiveness against tomato yellow leaf curl disease (Hanson et al., 2006; Hutton et al., 2012; Ji et al., 2007; Prasanna et al., 2015; Rai et al., 2013; Zamir et al., 1994). In Bangladesh, four cultivars (TLB111, TLB130, TLB133, and TLB182), resistant or tolerant to South Indian tomato leaf curl virus, were screened against ToLCNDV, following its emergence in 2003–04 (Maruthi et al., 2005). A study in Pakistan used chip graft inoculation assays to evaluate 170 tomato genotypes from various Solanum species for ToLCNDV resistance (Akhtar et al., 2019). The screening revealed resistance in five wild Solanum species and 13 tomato accessions carrying Ty genes. Recently, in 2021, Indian scientists identified the ToLCNDV resistance gene SlSw5a in the tomato cultivar H-88-78-1, which lacks any known Ty genes (Sharma et al., 2021). This resistance mechanism is regulated by the transcription factor SlMyb33, modulated by sly-miR159 microRNA. The study demonstrates that the miR159-Myb33 module affects Sw5a expression, triggering a hypersensitive response that enables resistance against geminiviruses in tomatoes.
Potato cultivars with tolerance or resistance traits against ToLCNDV are available, with the cultivar Kufri Bahar showing the lowest seed degeneration even under high vector pressure. Microarray analysis comparing this cultivar with a susceptible one has identified differentially regulated genes in response to ToLCNDV-[potato] infection, providing a foundation for developing new disease management strategies (Jeevalatha et al., 2017).
Cucurbitaceae
In 2015, Cucurbita pepo, Cucurbita moschata, and Cucurbita maxima and wild species were researched for resistance to ToLCNDV in Europe, marking a significant step in understanding and managing the virus’s impact on crops in the region (Sáez et al., 2016). Through symptom severity scoring and virus quantification, C. moschata accessions with notable resistance showed promise for breeding programs aimed at reducing the impact of ToLCNDV. Subsequent studies showed that a major quantitative trait locus (QTL) was identified in chromosome 8 controlling resistance to ToLCNDV by genotyping with a single nucleotide polymorphism (SNP) collection evenly distributed along the C. moschata genome (Sáez et al., 2020). Additionally, molecular markers tightly associated with the resistance loci have been developed and were able to correctly predict resistance and susceptibility with an accuracy of 94.34% for ToLCNDV in F2 and back cross populations derived from C. moschata (Duchesne ex Poir.) the breeding line AVPU1426 (Schafleitner et al., 2024).
Screening efforts have also been focused on melon by the Spanish group, with the first resistance sources identified in Cucumis melo subsp. agrestis var. momordica and wild agrestis accessions from India demonstrated tolerance to ToLCNDV (López et al., 2015; Sáez et al., 2017). Their findings revealed that resistance in melon is primarily controlled by a major QTL on chromosome 11 and additional regions on chromosomes 2 and 12, with the wild agrestis genotype (WM-7) showing the highest resistance. Furthermore, French researchers identified five resistant cultivars requiring two recessive and one dominant gene for ToLCNDV resistance (Romay et al., 2019). Additionally, a field screening of 60 melon germplasm, including commercial (vars. reticulatus and inodorus) and wild types (vars. momordica, conomon, and callosus), in India revealed three genotypes with high resistance to ToLCNDV (Padmanabha et al., 2024). To date, a total of 11 accessions (DSM 132, DSM 19, DSM-11-7, PI 124112, PI 414723, WM9, WM7, AM 87, IC-274014, PI 282448 and PI 179901) have been reported with high resistance to ToLCNDV in melon.
Cucumber (Cucumis sativus), an important food source, has been studied for resistance against ToLCNDV. A screening of 40 accessions from various Spanish provinces was conducted, and none exhibited high resistance. However, the Indian accessions CGN22297 and CGN22986 displayed variable responses in symptom severity and viral load, indicating non-fixed resistance. In contrast, accessions CGN23089, CGN23423, and CGN23633 were uniformly resistant, showing no symptoms and significantly lower ToLCNDV accumulation compared to susceptible controls (Sáez et al., 2021). Genotyping with SNP markers across the cucumber genome identified a QTL on chromosome 2 associated with ToLCNDV resistance. This finding enhances the understanding of genetic factors underlying resistance and could inform breeding programs in developing more resilient cucumber varieties.
ToLCNDV poses a significant biotic threat to luffa species, particularly sponge gourd (Luffa cylindrica) and ridge gourd (Luffa acutangula), with potential crop losses reaching 100%. Research by Islam et al. (2010) identified a single dominant gene for resistance to ToLCNDV in advanced breeding lines DSG-6 and DSG-7. Additionally, other resistant lines including IIHR-137, IIHR-138, and IIHR-Sel-1 also has been reported in this species (Kaur et al., 2021). Evaluations at the World Vegetable Center in Thailand found 13 and 59 of ridge gourd and sponge gourd lines resistant to both ToLCNDV and downy mildew respectively, showcasing variability in horticultural traits (Dhillon et al., 2020). In 2024, a study of 50 diverse genotypes over three years under natural conditions revealed eight genotypes exhibiting stable resistance to ToLCNDV, alongside desirable traits such as early maturation, higher fruit numbers, and increased yield (Singh et al., 2024).
As research on plant varieties and resistance genes capable of combating ToLCNDV infection continues to expand (Table 3), preventing outbreaks and managing this virus is expected to become increasingly effective and straightforward.
Conclusion
ToLCNDV represents a serious risk to diverse plant species, leading to considerable economic losses worldwide. Having expanded from Asia to the Middle East, Europe and Africa, the virus recently re-emerged in East Asia with new isolates in tomato and cucurbit reported in China. This resurgence, following a period without reports in the region, underscores the capacity of ToLCNDV to appear in new countries, possibly through unknown pathways. Although there have been no reported cases of ToLCNDV-related disease in Korea, several factors may facilitate its emergence in the future and climate change is expected to play a significant role. Rising temperatures and changing weather patterns can greatly influence the populations and behavior of insect vectors, such as whiteflies- the primary natural vector of ToLCNDV. Warmer temperatures could accelerate the whitefly life cycle, enabling faster reproduction and leading to population surges. Additionally, these conditions expand the habitable range of whiteflies to higher altitudes and previously cooler regions, potentially bringing them into contact with susceptible crops in new areas. Moreover, extreme weather events, such as heatwaves and drought, can further exacerbate the risk by weakening crops, making them more vulnerable to feeding damage and virus transmission by vectors. In Korea, whitefly populations have been documented across various regions, particularly in agricultural areas where vegetable and ornamental crops are extensively cultivated. If ToLCNDV is introduced through imported plant material or seeds, the whitefly populations could act as efficient vectors, facilitating the rapid spread of the virus in susceptible crops. Furthermore, the virus’s adaptability to various geographical regions indicates potential evolutionary changes that may influence its pathogenic traits. This adaptability is evident in the differing infectivity observed between Southeast Asian, Indian subcontinent, and Mediterranean isolates in tomatoes. Such variations underscore the need for ongoing monitoring and research to understand the evolution and impact of the virus on plant health.
The wide host range of ToLCNDV increases its potential for spread, as it infects not only main food crops but also various ornamental plants, weeds, and other wild species. This broad adaptability suggests that non-crop hosts that often grow near or within crop fields can act as reservoirs and facilitate the virus’s transmission across landscapes. The presence of ToLCNDV in these plants raises concerns about its persistence and spread, emphasizing the importance of comprehensive virus management strategies that account for both agricultural and non-agricultural hosts.
Developing effective control strategies for ToLCNDV and other plant viruses remains a top priority. This review focuses on advances in screening for natural resistance, especially in tomatoes and cucurbits, providing insights to guide future research and applications in crop protection.
With a solid understanding of ToLCNDV, effective preparations can be made in uninfected areas like Korea to combat the spread of this virus and prevent infections, especially as the virus evolves. This proactive approach is essential to safeguard both plants and agricultural communities.
Notes
Conflicts of Interest
No potential conflict of interest relevant to this article was reported.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2023-00241106).