Effects of Peanut Rust Disease (Puccinia arachidis Speg.) on Agricultural Production: Current Control Strategies and Progress in Breeding for Resistance

Peanuts play a pivotal role as an economic crop on a global scale, serving as a primary source of both edible oil and protein. Peanut rust (Puccinia arachidis Speg.) disease constitutes a significant global biotic stress, representing a substantial economic threat to the peanut industry by inducing noteworthy reductions in seed yields and compromising oil quality. This comprehensive review delves into the distinctive characteristics and detrimental symptoms associated with peanut rust, scrutinizing its epidemiology and the control strategies that are currently implemented. Notably, host resistance emerges as the most favored strategy due to its potential to surmount the limitations inherent in other approaches. The review further considers the recent advancements in peanut rust resistance breeding, integrating the use of molecular marker technology and the identification of rust resistance genes. Our findings indicate that the ongoing refinement of control strategies, especially through the development and application of immune or highly resistant peanut varieties, will have a profound impact on the global peanut industry.


Introduction
Peanut (Arachis hypogaea L.), an allotetraploid (AABB, 2n = 4x = 40) originating from South America, constitutes an important feed, food, and oilseed crop due to its high nutritional and economic value worldwide.Cultivated across 114 tropical and subtropical countries, this crop spans an extensive area of 32.72 million hectares (M ha), yielding an annual production of 53.93 million tons (MT) and a productivity rate of 1648 kg/hectare [1].The three leading planting and producing countries, India, China, and Nigeria, collectively account for 45.81% and 61.58% of the global area and production, respectively.China, with a production of 18.36 million tons (MT), is the world's largest peanut producer.However, India has the largest cultivation area, covering 5.97 M ha, while China's area is 4.75 M ha [1].Despite growing interest and demand, peanut crops face substantial challenges in terms of quality and yield, primarily due to various biotic and abiotic stresses-notably, fungal diseases.
Peanut rust (Puccinia arachidis Speg.) is a prevalent fungal disease that predominantly targets peanut leaves but can also affect petioles, support leaves, stems, and other plant parts.Several names were given to the pathogen causing peanut rust since the first report from Suriname in 1827 [2].Ultimately recognized as a distinct species by Spegazzini in 1976, Genes 2024, 15, 102 2 of 24 it was officially named P. arachidis [3].This widespread rust disease is prevalent wherever peanuts are cultivated [4], and it thrives in warm and humid conditions, facilitating rapid spread by way of repeated uredospore infection cycles [5].The impact of peanut rust on the peanut industry is severe, significantly hindering its development.Infected peanut plants exhibit various symptoms of damage during the growing season, including early pod maturity, increased pod senescence, reduced seed size, and decreased oil content [6].Research conducted by Mondal and Badigannavar indicates that rust disease can lead to yield reductions of up to 57% in susceptible genotypes such as Robut 33-1, resulting in substantial economic losses [7].
Given the significant nutritional and economic value of peanuts, a range of strategies have been employed to combat peanut rust in peanut production.These encompass cultural, chemical, and biological control methods and host resistance.However, it is crucial to weigh up the drawbacks of the chemical and cultural methods, which not only increase production costs but also pose risks to food safety and the environment.In contrast, host resistance and the development of disease-resistant cultivars stand out as the most sustainable strategies against peanut rust [6].Consequently, breeding peanut varieties that are resistant to rust has become a focal point for private, national, and international research institutions.While substantial strides have been made in breeding rust-resistant varieties and lines, the creation and deployment of immune or highly resistant varieties that are adaptable to diverse production areas remain vital for all peanut-growing nations around the world.
This review aims to showcase the progress that has been made in this field to inspire and galvanize further research and innovation in countering this challenging disease.By highlighting the progress achieved, we hope to instigate a collective effort towards the development of rust-resistant peanut varieties, anticipating the positive and transformative impact of such research on the industry.

Characteristics and Disease Symptoms of Peanut Rust
Peanut rust (P.arachidis Speg.) is taxonomically classified within the phylum Basidiomycota, subphylum Pucciniomycotina, class Pucciniomycetes, family Pucciniaceae, and genus Puccinia [8,9].As a fungal disease positioned within the realm of higher fungi, it has undergone extensive scrutiny to identify its characteristics and detrimental symptoms.Understanding the biology and pathogenicity of peanut rust is of paramount importance, serving as the bedrock for the formulation of effective management and control strategies.Researchers have invested considerable efforts in exploring the disease's life cycle, modes of infection, and factors influencing its spread and severity.The wealth of knowledge derived from these investigations plays a pivotal role in crafting sustainable and efficient approaches to combatting peanut rust, thereby mitigating its impact on peanut cultivation globally.

Life Cycle of Peanut Rust
The sexual life cycle of peanut rust is a complex process involving transitions between haploid and dikaryotic stages, encompassing five distinct spore stages: the spermagonium (haploid), aecium (dikaryotic), uredium (dikaryotic), telium (dikaryotic), and basidium (dikaryotic/diploid) [5] (Figure 1).Notably, the spermagonium and aecium stages occur in an alternate host, whereas the uredium, telium, and basidium stages perpetuate in the primary host.The formation of the dikaryotic mycelium results from plasmogamy, a fusion between two compatible spermatids and receptive hyphae.
The presence of the aecial form in peanut rust remains unclear given the absence of an alternate host.However, the uredial form predominantly sustains the disease in nature [10].Uredospores, characterized by their obovate, ovate-oblong, or broadly clavate shape, infect peanut leaves and give rise to uredinia.These uredinia are hypophyllous, subepidermal, ellipsoidal, or oblong structures, each housing numerous pedicellate uredospores [10].
Upon maturity, the uredinia transform, turning dark cinnamon brown and releasing a multitude of uredospores upon bursting.Under natural conditions, these nascent uredospores initiate several infection cycles in susceptible hosts.The tibial stage, formed from uredospores under low temperatures and nutrient stress, is not commonly observed in peanut rust [5]; however, instances have been reported in wild peanut leaves in Florida, Brazil, and Central America, as well as in cultivated peanut fields in Karnataka, India, and Gadar [3,10].
Telia, which contain numerous teliospores, are part of the life cycle, but their presence has been minimally reported worldwide.Consequently, further research is imperative to elucidate the existence and function of teliospores [3,10,11].Teliospores and basidia contribute to sexual spores in the peanut rust life cycle, fostering genetic sequence variability among different isolates and potentially leading to the evolution of new pathotypes or races [10].
Importantly, due to the absence of the basidium, occurrences of karyogamy and successive meiosis in the basidium have not been reported, and the emergence of new variants or races of peanut rust has rarely been observed to date [10].The intricate and nuanced aspects of the peanut rust life cycle underscore the necessity for ongoing research to deepen our understanding of this pathogen and enhance strategies for its management.The sexual life cycle of peanut rust encompasses five spore stages: spermagonium (haploid), aecium (dikaryotic), uredium (dikaryotic), telium (dikaryotic), and basidium (dikaryotic/diploid).The spermagonium and aecium stages occur in an alternate host, while the uredium, telium, and basidium stages perpetuate in the primary host.However, it remains unclear whether the aecial form and basidial form exist in peanut rust due to the lack of an alternate host.
Upon maturity, the uredinia transform, turning dark cinnamon brown and releasing a multitude of uredospores upon bursting.Under natural conditions, these nascent uredospores initiate several infection cycles in susceptible hosts.The tibial stage, formed from uredospores under low temperatures and nutrient stress, is not commonly observed in peanut rust [5]; however, instances have been reported in wild peanut leaves in Florida, Brazil, and Central America, as well as in cultivated peanut fields in Karnataka, India, and Gadar [3,10].
Telia, which contain numerous teliospores, are part of the life cycle, but their presence has been minimally reported worldwide.Consequently, further research is imperative to elucidate the existence and function of teliospores [3,10,11].Teliospores and basidia contribute to sexual spores in the peanut rust life cycle, fostering genetic sequence variability among different isolates and potentially leading to the evolution of new pathotypes or races [10].
Importantly, due to the absence of the basidium, occurrences of karyogamy and successive meiosis in the basidium have not been reported, and the emergence of new variants or races of peanut rust has rarely been observed to date [10].The intricate and nuanced aspects of the peanut rust life cycle underscore the necessity for ongoing research to deepen our understanding of this pathogen and enhance strategies for its management.

Infection Process
The infection process of peanut rust plays a pivotal role in the progression of the disease.Following the germination of the uredospores, a singular unbranched germ tube emerges from one of the equatorial germ pores in its wall [12].Notably, the germination of uredospores in dense patches and clumps is impeded by a higher concentration of methyl cis-3,4-dimethoxycinnamate [13].The germ tube extends along the intercellular grooves of the peanut leaf's surface until it establishes direct contact with the stomata.Upon contact, the tip of the germ tube undergoes swelling, giving rise to a thin-walled ellipsoidal appressorium of approximately the same size as the emerging spore [14].
The processes of germ tube elongation and appressorium formation are typically completed within 12 h of inoculation in susceptible genotypes.A thin cross wall forms between the appressorium and the germ tube, encapsulating the dense cytoplasm within the appressorium [15].Subsequently, a narrow infection peg emerges from the appressorium, traversing the stomatal apertures and swelling to form a vesicle in the substomatal chamber.Within 24 h, several infection hyphae (dikaryotic) typically develop from the substomatal vesicle, forming simple knob-like haustoria in adjacent mesophyll cells.
During this phase, the pathogen employs hydrolytic enzymes, including cellulases, glucanases, and proteinases, which play a crucial role in dissolving cell walls and plasma membranes, facilitating intercellular infection.These infection foci evolve into chloronemic flecks, eventually transforming into reddish-brown or orange pustules, known as uredinia, on the lower leaf surfaces (Figure 2).
Ultrastructure studies utilizing scanning electron microscopy have revealed differences in spore reactions on the lower leaf surfaces of two peanut genotypes: Arachis stenosperma V10309 (resistant) and A. hypogaea cv.IAC-Tatu (susceptible) [16].In the susceptible genotype, the germ tube elongates sufficiently to successfully enter the stomata, resulting in a successful intercellular infection within 72 h of inoculation [5].In contrast, the resistant genotype exhibits distinct responses during the infection process, highlighting the intricate dynamics involved in the interaction between the peanut plant and the rust pathogen.

Infection Process
The infection process of peanut rust plays a pivotal role in the progression of the disease.Following the germination of the uredospores, a singular unbranched germ tube emerges from one of the equatorial germ pores in its wall [12].Notably, the germination of uredospores in dense patches and clumps is impeded by a higher concentration of methyl cis-3,4-dimethoxycinnamate [13].The germ tube extends along the intercellular grooves of the peanut leaf's surface until it establishes direct contact with the stomata.Upon contact, the tip of the germ tube undergoes swelling, giving rise to a thin-walled ellipsoidal appressorium of approximately the same size as the emerging spore [14].
The processes of germ tube elongation and appressorium formation are typically completed within 12 h of inoculation in susceptible genotypes.A thin cross wall forms between the appressorium and the germ tube, encapsulating the dense cytoplasm within the appressorium [15].Subsequently, a narrow infection peg emerges from the appressorium, traversing the stomatal apertures and swelling to form a vesicle in the substomatal chamber.Within 24 h, several infection hyphae (dikaryotic) typically develop from the substomatal vesicle, forming simple knob-like haustoria in adjacent mesophyll cells.
During this phase, the pathogen employs hydrolytic enzymes, including cellulases, glucanases, and proteinases, which play a crucial role in dissolving cell walls and plasma membranes, facilitating intercellular infection.These infection foci evolve into chloronemic flecks, eventually transforming into reddish-brown or orange pustules, known as uredinia, on the lower leaf surfaces (Figure 2).
Ultrastructure studies utilizing scanning electron microscopy have revealed differences in spore reactions on the lower leaf surfaces of two peanut genotypes: Arachis stenosperma V10309 (resistant) and A. hypogaea cv.IAC-Tatu (susceptible) [16].In the susceptible genotype, the germ tube elongates sufficiently to successfully enter the stomata, resulting in a successful intercellular infection within 72 h of inoculation [5].In contrast, the resistant genotype exhibits distinct responses during the infection process, highlighting the intricate dynamics involved in the interaction between the peanut plant and the rust pathogen.

Disease Symptoms of Peanut Rust
Peanut rust primarily targets leaves, but it can also affect petioles and stems.Identifying the disease is quite straightforward as visible rust spores appear on the peanut plant.Symptoms of peanut rust typically arise around 8-10 days after the infection.It all begins with whitish flecks appearing on the lower leaf surface (abaxial).Within a day, yellowish flecks start showing up on the upper leaf surface (adaxial); meanwhile, on the lower side, orange-red or brownish pustules (known as uredinia) start forming.Approximately two days after these symptoms emerge, raised pustules become visible.They often take on a circular to elliptical shape, measuring 0.3-2.0mm in diameter.These pustules exhibit elliptical or circular uredospores; these are initially dark orange but mature into a cinnamon brown color [17] (Figure 2).
While pustules might occasionally appear on the upper leaf surface, they are not as plentiful as those on the lower side.When necrosis develops around these pustules, it can cause lesions to merge, ultimately resulting in leaf defoliation.Peanut rust typically spreads outwards in a radiating pattern from a single spot in the field.Under warm and humid weather conditions, the affected area can expand rapidly (Figure 2).Recognizing these distinctive symptoms early on is pivotal for promptly implementing management strategies to curtail the disease's impact on peanut crops.

Epidemiology and Control Strategies for Peanut Rust
Epidemiological studies are pivotal in advancing our understanding of the occurrence and distribution of peanut rust.With their meticulous analyses of the factors influencing the spread and severity of the disease, these studies enable us to forecast the epidemic trend.These predictive capabilities form a robust foundation for implementing efficient strategies in controlling and preventing peanut rust.

Epidemiology
Uredospores play a crucial role in the persistence and dissemination of the peanut rust pathogen across seasons [18].Their dispersal involves various mechanisms, such as rainfall, wind, and adherence to plant materials [19].It is noteworthy that uredospores are airborne and not internally seed-borne spores [20,21].
The epidemiology of peanut rust is profoundly shaped by environmental conditions.Studies indicate that the disease thrives under sustained high humidity (>78%) and warm temperatures (20-30 • C) [5,20].Conversely, the disease exhibits sluggish progression when temperatures drop below 10 • C or exceed 35 • C [22].Uredospores can maintain viability for up to 20 days at field temperatures ranging from 25 • C to 28 • C, with an optimal germination temperature of 25 • C [18].The controlled manipulation of humidity and temperature in experimental environments proves instrumental in regulating the rate of inoculum accumulation.
Environmental variables, including humidity, temperature, and wind speed and direction, wield a substantial influence on the distribution, infection, and development of airborne fungi, including peanut rust [23].The prudent management of wind speeds and directions under specific production conditions can effectively curtail the buildup and spread of the inoculum.
The epidemiology of peanut rust is further complicated by the host genotype.Distinct peanut genotypes may exhibit varying levels of susceptibility to the disease under diverse temperature conditions [22].Light rain showers are markedly more conducive to disease dispersal than heavy showers as the latter significantly diminish the pore content in the canopy [24].Early sowing practices can mitigate the severity of peanut rust during the summer, while late sowing may assist in reducing disease incidence during the rainy season [25].Additionally, levels of spore trapping on the plant canopy tend to be lower in the evening than in the morning, and the overlapping of crop seasons contributes to continuous inoculum buildup and the aerial propagation of uredospores [5,25].

Control Strategies for Peanut Rust
Controlling peanut rust is of paramount importance for mitigating the disease's impact and enhancing the efficiency of peanut production.Control strategies can be categorized into four main approaches: (I) Cultural control: quarantine stations play a crucial role in this strategy.Stringent phytosanitary inspections are imposed to regulate the movement of peanut materials across regions, preventing the introduction and spread of peanut rust to new areas.Employing crop rotation with cereals and other non-host species proves effective in limiting the spread of peanut rust in susceptible regions [14].Cultural control also entails the careful consideration of suitable sowing dates, the appropriate planting density, weeds, fertilizers, and water management, along with the implementation of fallow periods [26].The proper removal of diseased residues after the peanut harvest is imperative for preventing further spread in farmers' fields.
(II) Chemical control: in cases of unexpected and severe infection or outbreaks where cultural control measures are insufficient, it becomes necessary to resort to chemical control.Fungicides such as carbendazim and chlorothalonil are commonly employed to reduce the incidence of peanut rust, with chlorothalonil being reported as the most effective fungicide for controlling peanut rust [26,27].However, chemical control has its drawbacks, including high costs, potential environmental and health hazards, and the risk of resistance buildup in pathogenic strains, making it less than ideal for long-term management.
(IV) Host resistance: various national and international crop breeding institutions have successfully developed rust-resistant peanut genotypes, including GPBD4 [31], Zhonghua No. 9 [32], and ICGV86699 [33].Wild Arachis species generally exhibit higher levels of resistance to rust than A. hypogaea [34,35].Efforts have been made to harness resistance from wild species through the development of interspecific hybrids and derivatives [36].However, factors such as associated linkage drag, ploidy barriers, genetic isolation, and incompatibility limit the widespread use of resistance from wild species [37].Host resistance is considered the most desirable strategy against peanut rust due to its ability to address the shortcomings of the other three approaches.

Genetics of Peanut Rust Resistance
Several previous research studies have indicated that rust resistance in peanuts is generally a recessive trait and is controlled by multiple genes [38][39][40][41][42][43].However, the specific number and nature of the genes governing peanut rust resistance require further investigation.According to a report produced by Singh and Moss [44], rust resistance derived from diploid wild species may be governed by a partially dominant gene.Additional studies by Wynne et al. [45] and Kokalis-Burelle et al. [26] suggest that partial resistance to rust involves several minor genes, resulting in reduced infection frequency and an extended incubation period.
Moreover, the expression of rust resistance seems to be predominantly influenced by non-additive, additive × additive, and additive × dominance gene interactions, as reported by Hayman [46].Subsequent work conducted by Ghewande [47] also revealed that rust resistance can be conditioned by additive, additive × additive, and additive × dominance gene effects.Despite these insights into the genetic regulation of rust resistance in peanuts, information regarding gene regulation or transcript upregulation in response to peanut rust is limited and requires further investigation.
Further research in this area will undoubtedly enhance our understanding of the molecular mechanisms underlying rust resistance in peanuts.This, in turn, has the potential to aid in the development of more effective breeding strategies for rust-resistant peanut varieties.Such advancements are crucial for sustainable peanut production, helping farmers combat the impact of peanut rust and ensuring the development of a more resilient and productive peanut crop.

Resources for Peanut Rust Resistance
The precise evaluation and effective utilization of rust-resistant germplasms are fundamental prerequisites for the development of rust-resistant peanut cultivars.Researchers have identified a substantial number of resources demonstrating immunity or high resistance to peanut rust disease; these originate from diverse sources such as wild Arachis species, interspecific derivatives, cultivated varieties, breeding materials, and landraces.These evaluations have been conducted in laboratory, greenhouse, and field trials, and some of these valuable resources have already been successfully integrated into breeding programs.
Rust resistance resources can be broadly classified into two main groups: (I) Rust resistance resources in wild Arachis species (Table 1) [48][49][50][51].Wild Arachis species act as potential sources of novel genes that can be leveraged to enhance rust resistance in cultivated peanuts (A.hypogaea L.).Various studies have documented the immunity or high levels of resistance of different wild species.For example, Subrahmanyam et al. [48] identified 64 accessions of wild species that were immune to peanut rust, spanning various taxonomic sections.Additionally, six accessions, including ICG 8126, ICG 8125, ICG 8137, ICG 8952, ICG 8954, and GKP 9893, exhibited high levels of resistance, while Arachis monticola PI263393 displayed resistance to peanut rust.Pande and Rao [49] further identified sixty-seven accessions from seventy-four wild species, showcasing varying levels of rust resistance, including one immune accession (ICG 8954) and sixty-four highly resistant accessions.Notably, certain accessions, such as ICG 8123, ICG 8138, ICG 8216, ICG 8190, and ICG 8954, were reported to have different rust resistance levels by both Subrahmanyam et al. [48] and Pande and Rao [49].By averaging the identification results, five accessions were classified as highly resistant, while one accession (ICG 8125) was categorized as resistant.
Moreover, under laboratory conditions, additional immune and highly resistant accessions, such as VSPmSv 13774, Pa s/n, VK 12083, VSPmSv 13710, VPoJSv 10506, VSGr 6389, and VPoBi 9230, were identified [50].Leal-Bertioli et al. [51] also reported Arachis magna K 30097 to be a highly rust-resistant accession of wild species.These findings collectively highlight the diverse and promising array of rust-resistant resources available within wild Arachis species, underscoring their potential for contributing novel genes to enhance rust resistance in cultivated peanuts."-": details not available; "a": Nomen nudum; "b": A hybrid between A. correntina and A. villosa.
In conclusion, the identification and effective utilization of rust-resistant germplasms are crucial for developing rust-resistant peanut cultivars.The diverse array of resources from both wild Arachis species and cultivated varieties, showcasing varying levels of rust resistance, hold great promise for breeding programs focused on enhancing peanut rust resistance.
Furthermore, numerous breeding lines exhibiting high levels of resistance or resistance to rust have been identified.ICGV 86699 and ICGV 87157, elite peanut germplasm lines released by ICRISAT, showcase notable resistance to rust.ICGV 86699 emerged from a single-plant selection derived from CS 29, a product of repeated selections from an interspecific population of [A.batizocoi/A.duranensis//A.hypogaea cv.NC 2] [33,61].On the other hand, ICGV 87157 originated from a single-plant selection in an F 3 population of a cross between Argentine (a Spanish cultivar) and PI 259747 (a rust-and late leaf spot (LLS)-resistant genotype) [72].In Peru, 71 breeding lines, including ICG 1697 and ICG 10025, have been identified as having substantial resistance to rust.Additionally, other countries, such as Israel and Honduras, report 15 breeding lines (e.g., ICG 4746, ICG 7885) demonstrating high resistance to rust and 18 rust-resistant breeding lines (e.g., QT0348, ICG(FDRS)30).These findings underscore the global significance of diversified breeding lines in the pursuit of resilient and productive peanut cultivation practices.Note: "-": details not available.

Breeding for Rust Resistance
Breeding rust-resistant peanuts constitutes a pivotal objective in peanut breeding programs to ensure the enduring sustainability and growth of global peanut production.In recent decades, substantial strides have been made in developing numerous rust-resistant cultivars and breeding lines of A. hypogaea L., leveraging both traditional breeding methods and modern molecular breeding technologies.
The development and release of rust-resistant peanut varieties represent significant achievements in peanut breeding programs (Table 3) [31,32,53,54,56,62,63,67,, contributing substantially to enhanced crop productivity and mitigating yield losses attributed to rust disease.In India, three noteworthy peanut varieties-GPBD 4, Mutant 28-2, and ICGV 86590-have attracted considerable attention for their robust rust resistance.GPBD 4, a promising early-maturing variety released in Karnataka, showcases high levels of resistance to rust.This achievement resulted from a cross between KRG 1 (a susceptible cultivar) and ICGV 86855 (a resistant genotype), which is an interspecific derivative of A. hypogaea × A. cardenasii [36].Mutant 28-2, an improved high-yielding Spanish peanut variety released in Karnataka, also boasts high levels of rust resistance.It was developed via the ethyl methane sulfonate (0.5%)-induced mutagenesis of VL 1, itself a mutant derivative of Dharwad Early Runner [83].ICGV 86590, a rust-resistant peanut variety bred at ICRISAT, resulted from a cross between X 14-4-B-19-B (a Spanish breeding line) and PI 259747 (a rustand late leaf spot (LLS)-resistant Valencia germplasm line) [85].
In the USA, several rust-resistant peanut varieties have been released for peanut production, including Southern Runner [84], C-99R [86], Florida MDR 98 [87], Hull [88], and York [89].In China, peanut varieties with high levels of resistance to rust, such as Guihua 23 and yuanza 9102, were bred through distant hybridization.This involved using peanut diploid wild species A.chacoense with high rust resistance as the male parent and Baisha 1016 as the female parent [94,95].Yueyou 114 [100] and Fuhua 6 [103], have been released for peanut production, achieving notable resistance outcomes.Similarly, rustresistant varieties including Yueyou 79 [52], Guihua 21 [117], and Zhonghua No. 12 [32] have also been developed.Moreover, rust-resistant peanut varieties have been released in other countries.In the Philippines, NSIC Pn 12 [119] and in Tanzania Narinut 15 [62] and Narinut [63] have been released to combat rust disease.This comprehensive global effort underscores the widespread commitment to addressing the challenges posed by rust, ensuring a resilient and productive future for peanut cultivation."*": variety has been approved but not reported in a journal.

Application of Molecular Marker Technology and the Mining of Rust Resistance Gene
Traditionally, the process of breeding peanuts for rust resistance has been characterized by the use of expensive, time-consuming, and sometimes inefficient methods, including artificial inoculation and a reliance on natural occurrences when assessing resistance levels.However, a transformative shift has occurred with the advent of molecular peanut breeding, offering a more efficient and precise approach that can substantially shorten the breeding cycle and enhance overall selection efficiency.Molecular marker technology, a cornerstone of molecular breeding, has proven instrumental in peanut breeding by facilitating the tagging of genes and the selection of rust-resistant genotypes.Various types of molecular markers, such as Amplified Fragment Length Polymorphism (AFLP) markers, Random Amplified Polymorphic DNA (RAPD) markers, Simple Sequence Repeat (SSR, microsatellites) markers, and Single Nucleotide Polymorphism (SNP) markers, have been effectively employed in this revolutionary process.For example, in a study involving the cross Yuanza 9102 × ICGV86699, the researchers identified two AFLP markers, M3L3-46 and M8L8-645, which are linked to rust resistance, at genetic distances of 10.9 cM and 7.86 cM, respectively [120].In another investigation using an F 2 mapping population from VG 9514 × TAG 24, two RAPD markers, J71350 and J71300, were identified.J71350 was linked to the rust resistance gene at a distance of 18.5 cM, while J71300 was completely linked to rust resistance [121].Additionally, certain SSR alleles in crosses ICGV99005 × TMV2 and ICGV99003 × TMV2 were found to be associated with rust resistance [122].Other studies identified candidate SSR loci for mapping rust resistance using an analysis of molecular variance and Kruskal-Wallis one-way ANOVA [35,123].In a separate investigation involving a population of 164 recombinant inbred lines (RILs) derived from VG 9514 (a rust-resistant genotype) × TAG 24 (a susceptible genotype), the researchers assessed rust resistance in the field and generated a genetic linkage map with 24 linkage groups (LGs) using 109 SSR markers.Notably, SSR markers gi56931710 and pPGPseq4A05 were identified as flanking the rust resistance gene at map distances of 4.3 cM and 4.7 cM in the linkage group (LG) 2, respectively [53].In conclusion, peanut molecular breeding that leverages molecular markers has demonstrated significant potential in enhancing the efficiency and precision of rust resistance selection; this constitutes a notable advantage over traditional breeding methods.The integration of these technologies is crucial for ensuring sustainable and disease-resistant peanut production on a global scale.
With the rapid evolution of molecular marker technology, numerous quantitative trait loci (QTLs) linked to peanut rust resistance have emerged, driven by high-density linkage maps and detailed phenotypic data (Table 4) [60,[77][78][79][80][81][82][120][121][122][123][124][125][126][127].In one notable study, 12 QTLs governing rust resistance were pinpointed using composite interval mapping (CIM) within a cohort of 268 recombinant inbred lines (RILs) derived from the cross TAG 24 × GPBD 4. These QTLs were meticulously assessed using 67 polymorphic SSR markers, leading to a key discovery: the identification of the candidate SSR marker IPAHM 103, which is firmly linked to a major QTL (QTLrust01) correlated with rust resistance.This marker explained a phenotypic variation ranging from 6.9% to 55.2% [123].Another investigation, employing two distinct RIL populations (TAG 24 × GPBD 4 -RIL-4 and TG 26 × GPBD 4 -RIL-5), revealed a total of 15 QTLs associated with rust resistance.Among these, RIL-4 exhibited three significant QTLs (QTLR4-Rust01, QTLR4-Rust02, and QTLR4-Rust03), while RIL-5 showcased four major QTLs (QTLR5-Rust01, QTLR5-Rust02, QTLR5-Rust03, and QTLR5-Rust04).These QTLs exhibited a noteworthy spectrum of phenotypic variations, spanning from 10.68% to an impressive 82.96%.Additionally, five SSR markers (IPAHM103, GM2009, GM1536, GM2301, and GM2079) exhibited significant associations with the major QTLs, collectively elucidating a substantial phenotypic variation of 82.96% [124].Moreover, within a specific genomic region on LG AhXV, flanked by GM2009 and GM1954 markers, three robust QTLs for rust resistance were localized, demonstrating an explained phenotypic variance (PVE) of up to 82.96% [128].In a separate study, involving an F_6 population derived from a cross between A. ipaënsis K30076 and A. magna K30097, the researchers identified thirteen QTLs governing rust resistance, with two being classified as major.One of these major QTLs (PVE 5.8% to 59.3%) was situated between 35.1 and 42.9 cM on LG B08, with the closest microsatellite marker of Ah-280.The second major QTL (PVE 13.2% to 34.8%) was mapped between microsatellite markers AHGS1350 and AHGS2541, spanning positions 25.4 to 33.1 cM on the same LG B08.Additionally, competitive allele-specific PCR markers were crafted and validated on a tetraploid background for these QTLs [51].These findings showcase the prowess of molecular marker technology in not only identifying but also characterizing QTLs that are bound up with rust resistance in peanuts.Such insights hold a great deal of promise for breeding programs dedicated to cultivating rust-resistant peanut varieties."*": Major QTLs are not named by the researchers but by the authors of this review in order to distinguish them from others.
Numerous candidate genes associated with peanut rust resistance have been meticulously identified, as outlined in Table 5 [60,[127][128][129][130].One pivotal quantitative trait locus (QTL) of rust, referred to as Rust-QTL, which is responsible for reducing the lesion number, size, and sporulation of rust, was discerned in Arachis magna K30097.This locus, situated on LG B8, is flanked by the closest linked marker, Ah280, which maps near an NB-LRR-encoding gene known as Araip.RV63R [129].In a separate study, an enhanced genetic map featuring 20 linkage groups (LGs), incorporating 139 new SSR and transposable element markers, was deployed for QTL mapping for rust resistance in a recombinant inbred line (RIL) population derived from TAG 24 (a susceptible variety) and GPBD 4 (a resistant variety) [125].The results of the analysis revealed five QTLs associated with rust (with the explained phenotypic variance, PVE, ranging from 44.5% to 53.7%) on AhXV (B03 LG of the B genome), flanked by GM2009-IPAHM103.Additionally, a QTL linked to rust (PVE 49.3-52.3%)was identified on AhV (A05 LG of the A genome), flanked by GM1989-AhTE0839 [125].Diligent efforts were undertaken to construct a high-density genetic map of TAG 24 × GPBD 4, encompassing 29 LGs with 453 loci (including 171 SNPs, 89 transposons, and 193 SSRs) covering a span of 1510.1 cM.The subsequent QTL analysis identified nine QTL candidates for rust resistance; they were concentrated in a genomic region of 2.7 Mb (131.9-134.6Mb on A03).Within these regions, six genes with deleterious mutations (Aradu.C88Z1, Aradu.Z87JB, Aradu.1WV86,Aradu.RW91L, Aradu.NG5IQ, and Aradu.YL3ZN) emerged as potential candidates for rust resistance.This determination was made through ddRAD-Seq analysis, linkage map construction, QTL analysis, sequencing, and whole-genome resequencing analysis [127].To further define the rust resistance gene, an RIL population resulting from a cross between VG 9514 and TAG 24 was subjected to phenotyping and subsequent QTL analysis.The results revealed a consensus rust QTL (Rust_QTL) flanked by two SSR markers (FRS72 and SSR_GO340445) within a 1.25 cM map interval on the A03 chromosome in cultivated peanuts.This region encompasses an R-gene (Aradu.Z87JB; TIR-NB-LRR) and four PR-genes (Aradu.RKA6 M, Aradu.T44NR, Aradu.1WV86, and Aradu.VG51Q) associated with rust resistance [60].Additionally, a conserved Tir-NBS-LRR gene (AH13G54010.1),residing in a common genomic region colocalized with resistance to rust and LLS, appeared to translocate from Chr03 to Chr13 after tetraploidization; thus, it represents a potential candidate for resistance to rust and LLS [130].In a subsequent analysis involving 328 F 2 individuals derived from a cross between two Indian varieties (GJG17 × GPBD4), a major QTL for rust resistance (RustQTL; PVE 70.52%) was validated.This QTL was found to be flanked by markers SSR_GO340445 and FRS72, situated on the A03 chromosome.Notably, in addition to the five genes identified by Mondal and Badigannavar [60], a PR gene (Aradu.RW91L) encoding lipase/lipooxygenase was validated in the RustQTL region, being identified as contributing significantly to rust resistance.Furthermore, seven novel EST-SSRs were validated in 177 peanut genotypes [128].These comprehensive findings shed light on the intricate genetic landscape governing peanut rust resistance, offering valuable insights for future breeding programs aimed at developing resilient peanut cultivars."-": details not available.
The isolation of the dominant rust resistance gene, VG 9514-Rgene (GenBank accession number MK791522), was successfully achieved using map-based cloning, employing VG 9514, TAG 24, and 164 recombinant inbred lines (RILs) derived from the cross of VG 9514 × TAG 24 [64].A BLASTn search identified six genes homologous to VG 9514-Rgene, namely Aradu.Z87JB (Aradu.A03; in A. duranensis), Araip.0R3VU(Araip.B03; in A. ipaensis), Arahy.GFGJ54 (Arahy13; in A. hypogaea), Arahy.T6DCA5 (Arahy03; in A. hypogaea), Arahy.R8KUIR (Arahy03; in A. hypogaea), and Arahy.ZZ0VZ9 (Arahy03; in A. hypogaea) [64].The genomic location of VG 9514-Rgene was predicted to be at 142,544,745.0-142,549,184bp on chromosome arahy03, coinciding with the location of Arahy.T6DCA5.Further analysis of VG 9514-Rgene revealed its structural composition, encoding a typical consensus three-dimensional folding of the TIR-NBS-LRR protein.This protein class is well-known for its role as a resistance (R) protein and is integral in plant defenses against pathogens.Using sequence analysis, three non-synonymous mutations (E268Q in the hhGRExE motif, Y309F in the RNBS-A motif, and I579T in the MHD motif of the NB-ARC domain) were identified in the susceptible version of the R-protein.These mutations were mapped and found to be associated with the loss of the rust resistance function in the susceptible version of the gene [53].The elucidation of the molecular structure and mutational landscape of VG 9514-Rgene provides crucial insights into the mechanisms underlying rust resistance in peanuts.This information holds significant potential for the development of rust-resistant cultivars through both traditional breeding methods and advanced biotechnological approaches.By understanding the genetic basis of resistance, breeders and biotechnologists can make informed decisions to enhance the efficiency and precision of crop improvement programs, ultimately contributing to sustainable peanut cultivation practices.

Outlook
Sustainable peanut production is imperative for global food security given their high nutritional value and abundant seed oil content.However, peanut productivity faces significant challenges, notably peanut rust and abiotic stresses that impede production worldwide.The various proposed control strategies include cultural, chemical, and biological approaches, but each has inherent limitations.Large-scale biological control methods encounter challenges including constraints related to biocontrol fungi, complex operations, unstable control effects, and high costs.Consequently, peanut research has shifted its focus toward breeding immune or highly resistant varieties as the most environmentally friendly, economical, and effective strategy for rust control.
Like other crops, peanuts can achieve enduring resistance if rust immunity or resistance genes are introduced from elite sources into genotypes adapted to diverse production regions.Conventional breeding methods have successfully integrated resistance traits, although it is crucial to note that only specific wild Arachis species in Arachis areas possess immunity or high levels of rust resistance.Interspecific hybridization using these wild relatives can transfer rust resistance to cultivated peanut varieties.By harnessing the genetic diversity in wild relatives, peanut breeders can develop rust-resistant varieties suited to specific agro-climatic conditions, contributing to sustainable and enhanced global peanut production.The ongoing efforts to develop rust-resistant varieties play a crucial role in ensuring food security and improving the livelihoods of peanut farmers worldwide.
Advancements in molecular breeding techniques, including molecular markers, transgene technology, and molecular design, are leading peanut breeding to undergo a transition from traditional "experience breeding" to efficient, precise, and targeted "molecular design breeding."The rapid progress in sequencing technology has facilitated genome sequences for various Arachis species.The whole-genome sequencing of diploid peanut ancestors (A.duranensis and A. ipaensis) and allotetraploid peanuts (A.hypogaea L.) has provided valuable genomic information.This has accelerated the identification and isolation of rust-resistant genes, enhancing the discovery of more rust-resistant genes.
Precision-designed breeding enables the direct selection and efficient stacking of rust-resistant genes, representing the future of breeding peanuts for rust resistance.This approach offers advantages such as a shortened breeding period, the effective resolution of issues related to a narrow genetic basis and limited adaptability, and improved efficiency in achieving rust-resistant varieties.With the integration of molecular design techniques, peanut breeding programs are positioned to achieve enhanced rust resistance more rapidly and effectively, thereby contributing to sustainable peanut production and global food security.

Conclusions
Understanding and managing peanut rust disease is crucial for securing a prosperous future for peanut production and meeting the escalating demand for this nutritious and versatile crop.This review offers an extensive elucidation of peanut rust disease and the breeding of rust-resistant cultivars, describing the disease's characteristics and detrimental symptoms, its epidemiology, and diverse control strategies.Additionally, the review emphasizes the utilization of molecular marker technology and the identification of rust-resistant genes.This comprehensive compilation of information serves as a valuable resource for breeders and biotechnologists, empowering them to collaboratively develop immune or resistant peanut varieties, ensuring the sustainability and vitality of the peanut industry.
Author Contributions: Y.Y., J.L., Z.H., M.K., C.W., Z.Z. and J.M. collected the manuscript data.Y.Y. and J.L. wrote the original draft and contributed equally to the production of the manuscript.Y.X.approved the final manuscript.All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the Sichuan Science and Technology Program (grant number 2022YFH0033 to J.L.), the National Peanut Industry Technology System and the "14th Five-Year" Crops Breeding Program of Sichuan Province (grant numbers CARS-13 and 2021NYZ0018 to Y.X.), and the Sichuan Characteristic Cash Crop Innovation Team of National Modern Agricultural Industry Technology System (none to J.M.).

Institutional Review Board Statement: Not applicable.
Data Availability Statement: Not applicable.

Figure 2 .
Figure 2. Field phenotype of plants infected with peanut rust disease.(A) The harm caused to peanut field production by peanut rust disease; (B) typical symptoms of rust disease on peanut leaves.Figure 2. Field phenotype of plants infected with peanut rust disease.(A) The harm caused to peanut field production by peanut rust disease; (B) typical symptoms of rust disease on peanut leaves.

Figure 2 .
Figure 2. Field phenotype of plants infected with peanut rust disease.(A) The harm caused to peanut field production by peanut rust disease; (B) typical symptoms of rust disease on peanut leaves.Figure 2. Field phenotype of plants infected with peanut rust disease.(A) The harm caused to peanut field production by peanut rust disease; (B) typical symptoms of rust disease on peanut leaves.

Table 1 .
Rust-resistant resources in wild Arachis species reported globally.

Table 2 .
Rust-resistant germplasm resources in cultivated species and breeding lines (A. hypogaea L.) reported globally.

Table 5 .
Candidate genes for rust resistance.