Pea (Pisum sativum L.), being cultivated throughout the world, either for food, fodder, and feed, is considered an important winter season food legume (Rubiales et al., 2019; Parihar et al., 2020). Cotyledons’ color of pea grains varies from yellow, green, and orange that are used in the human diet in different forms such as dal, stew, chhola, vegetables, snacks, soup, chat, and flour, while whole seeds are mainly used as animal feed (Mahajan et al., 2018; Singh et al., 2018). Nutritionally, pea seeds are considered to have about 21–33% protein and 56–74% carbohydrate, with an average iron, selenium, zinc, and molybdenum of about 97, 42, 41, and 12 ppm, respectively (Parihar et al., 2016, 2021). Therefore, it serves as an important ingredient in providing nutritional security for resources poor people in developing countries. Moreover, its consumption minimizes the risk of several chronic diseases such as diabetes (Marinangeli and Jones, 2011), subsides blood cholesterol levels (Ekvall et al., 2006), improves cardiovascular health (Singh et al., 2013), possesses cancer prevention attributes (Kalt, 2001; Steer, 2006), administers body weight, and improves gastrointestinal affairs (Fernando et al., 2010; Lunde et al., 2011).

It is being cultivated widely across many countries in the world (Parihar et al., 2021). Its worldwide cultivated area has increased from 6.58 to 8.09 mha and production from 10.44 to 16.21 mt since 2010. Canada, Russia, China, India, and the United States are the major pea-producing countries (Parihar et al., 2020); however, the United States shares the highest total production of pea (39.33%), followed by Europe (36.98%) and Asia (18.09%). At present, its average productivity is about 2.0 t/ha globally, which recorded an increase of about 36% in a decade (2007–2017), but the potential productivity of this crop is up to 5.0 t/ha in several countries including Netherland, Denmark, Belgium, Germany, and Finland harvests about 3.45–5.01 t/ha (Toker and Mutlu, 2011). However, countries such as India, China, Australia, and Myanmar are recording very low productivity of less than 2.00 t/ha (FAO, 2021). During the past few decades, the gain in yield of pea (15.3 kg/ha/year) is relatively low as compared to other crops, which could be majorly attributed to the least investment in the pea research program (Rubiales et al., 2019). Also, the susceptibility of a pea toward many abiotic/biotic stress is another reason for low productivity which becomes a serious threat to its sustainable productivity especially under changing climatic conditions (Parihar et al., 2020). The most devastating diseases that affect the productivity of pea are powdery mildew (PM), ascochyta blight (AB), rust (PR), wilt (FW), and root rots (Parihar et al., 2013; Mahajan et al., 2018), of which PM caused by Erysiphe pisi (DC.), E. baeumleri (Magnus) (U. Braun & S. Takam.), and E. trifolii (Grev.) has the potential of reducing seed yield by 25–80% (Warkentin et al., 1996; Ghafoor and McPhee, 2012). PR caused by Uromyces viciae-fabae (Pers.) J. Schröt. or U. pisi (Pers.) de Bary is reported to cause yield losses up to 30% (Barilli et al., 2010, 2018; Singh et al., 2015) while, AB, results due to a mixture of fungal species [Ascochyta pisi (Lib.), Peyronellaea pinodes (Berk. & A. Bloxam), Phoma medicaginis var. pinodella (L.K. Jones), P. Koolunga (Davidson), and P. glomerata (Corda) (Wollenw. & Hochapfel)], is one of the most complex and severe diseases worldwide (Bretag et al., 2006; Tran et al., 2014) with a potential of reducing grain yield by about 60% (Liu et al., 2016). Fusarium root rot (FRR) incited by Fusarium solani f. sp. pisi (W.C. Snyder & H.N. Hansen), which may occur in both dry and wet field conditions, reduces yield significantly (Porter, 2010). Similarly, fusarium wilt (FW) caused by F. oxysporum f. sp. pisi (W.C. Snyder & H.N. Hansen) has about 11 different races (Gupta and Gupta, 2019), of which races 1 and 2 are distributed widely affecting the productivity of pea significantly, whereas races 5 and 6 are sporadically distributed (Infantino et al., 2006; Bani et al., 2018). A disease caused by Aphanomyces euteiches (Drechsler) is common root rot (CRR) and is prevalent in the United States, Europe, and Canada causes wilting of the roots (Wicker et al., 2003; Pilet Nayel et al., 2005; Chatterton et al., 2015; Desgroux et al., 2016; Wu et al., 2018). Several insect pests such as pod borer complex [Helicoverpa armigera (Hübner), Etiella zinckenella (Treitschke), and Polyommatus boeticus L.], bruchid (Bruchus pisorum L.) pea leaf weevil (Sitona lineatus L.), leaf miners [Chromatomyia horticola (Goureau)], stem fly [Melanagromyza phaseoli (Vanschuytbroeck)], aphids [Acyrthospihon pisum (Harris)], and cut worms [Agrotis ipsilon (Hufnagel)] seriously reduce the yield of pea by affecting the crop growth (Sharma, 2000; Yadav and Patel, 2015; Yadav et al., 2019). Pod damage of about 40% has been observed in pea due to pod borer complex infestation (Dahiya and Naresh, 1993).

The development of resistant cultivars to the biotic and abiotic stresses is an outstanding tactic to enhance the productivity of any crop including pea. Therefore, knowledge of the genetics of disease and pest resistance is essentially required to breed the resistant/tolerant cultivars. In addition to this, genomic advances especially the accessibility of draft genome sequence of pea (Kreplak et al., 2019) have facilitated the identification of the genes responsible for disease and pest resistance/tolerance and also helped in uncovering the genetics of quantitatively inherited resistance of several major diseases and pests. Moreover, genomics has also facilitated modernizing the conventional breeding for rapid and precise development of resistant cultivars in crop plants including pea. Information on genetics, genomics, and breeding of biotic stress resistance in pea is scattered and only limited attempts were made to review the different aspects of biotic stress resistance (Fondevilla and Rubiales, 2012; Smýkal et al., 2012; Rubiales et al., 2015; Tayeh et al., 2015a). Recently, Mahajan et al. (2018) discussed the genetic improvement in pea in relation to biotic stresses; however, the information provided was largely related to legumes in general and in brief about pea. Thus, an effort is made through this review to make available the comprehensive information pertaining to genetic and genomic advancement at one platform as well as to share a futuristic road map using modern genomic and genetic tools in pea breeding that could aid the crop breeders in developing high-yielding multiple stress resilient pea cultivars.

Genetic improvement in a target crop species requires availability and judicious exploitation of genetic resources. Globally, more than 98,000 pea accessions, comprised of advanced breeding lines (13%), landraces (38%), mutant stocks (5%), wild species (2.6%), and cultivars (34%), are available and conserved in diverse genebanks (Smýkal et al., 2015; Warkentin et al., 2015; Rubiales et al., 2019; Coyne et al., 2020). The National Institute for Agricultural Research (INRA), France, Australian Grains Genebank (AGG), N.I. Vavilov Research Institute of Plant Industry, Russia, US Department of Agriculture (USDA), United States, Leibniz Institute of Plant Genetics and Crop Plant Research, Gatersleben, Germany, and International Center for Agricultural Research in the Dry Areas (ICARDA), Lebanon are the six leading active pea germplasm repositories in the world with about 8,839, 7,432, 6,790, 6,827, 5,343, and 4,596 accessions, respectively (Figure 1). The National Germplasm Repositories of various countries also hold a good number of pea accessions such as 4,558 accessions in Italy, 3,837 in China, 4,484 in India, 3,298 in the United Kingdom, 2,896 in Poland, 2,849 in Sweden, 2,311 in Ukraine, and 2,110 in Aberystwyth University, United Kingdom. Besides, seven other countries hold > 1,000 accessions of Pisum in their national germplasm treasury (Figure 1). Interestingly, the National Genebank of Israel possesses a collection of crop wild relatives (CWRs) such as Pisum fulvum and P. sativum subsp. elatius var. pumilio, which contributes to about 2% of the entire preserved germplasm (Smýkal et al., 2013, 2015; Warkentin et al., 2015). This share of CWR has accessions to P. fulvum (706), P. s. subsp. elatius (624), P. s. subsp. sativum (syn. P. humile/syriacum; 1562), and P. abyssinicum (540) (Smýkal et al., 2013). Besides CWR and cultivated accessions, 575 and 122 accessions of pea mutant stocks are also available at the John Innes Collection, the United Kingdom and the Institute of Plant Genetics Resources Collection, Bulgaria, respectively (Smýkal et al., 2015). A Targeted Induced Local Lesions in Genomes (TILLING) population of 9,000 lines (Coyne et al., 2020) and fast neutron generated deletion mutant resources (around 3,000 lines) are also available, which are being exploited to identify various developmental genes (Smýkal et al., 2015). Internationally, several web-portals have been developed using the database of pea genetic resources such as the European Cooperative Program on Plant Genetic Resources, Cool Season Food Legume Database, Genetic Resources Information Network and System-wide Information Network for Genetic Resources, and KnowPulse for keeping records and disseminating the information related to pea genetic resources.

Crop wild relatives that include Pisum species and subspecies are in general a source of countless fascinating traits including various yield attributing parameters (Mikić et al., 2013). Besides, it is a source of resistance to several biotic stresses, e.g., pea seed weevil (Clement et al., 2002, 2009), PM (Fondevilla et al., 2007b; Esen et al., 2019), PR (Barilli et al., 2010), AB (Jha et al., 2012), and broomrape (Fondevilla et al., 2005). The significance of CWR has been demonstrated by successfully introducing a novel dominant gene (Er3), responsible for resistance to E. pisi from P. fulvum (Sharma and Yadav, 2003; Fondevilla et al., 2008a). Moreover, some P. fulvum accessions were reported to show resistance against bruchid, broomrape, and Mycosphaerella pinodes and are subsequently being utilized in hybridization programs (Fondevilla et al., 2005; Coyne et al., 2020). Similarly, resistance to PR (Barilli et al., 2010, 2018) and AB (Fondevilla et al., 2005; Jha et al., 2012) has been observed in P. fulvum. Diversity for the eIF4E gene and novel alleles for virus resistance has also been identified from CWR (Ashby et al., 2011; Konečná et al., 2014). In a recent report, the relationship between neoplasm and pea weevil (Bruchus pisorum L.) damage was not established in F₁ and F₂ derived from the inter-subspecific crosses of P. sativum subsp. sativum (with neoplasm) and P. sativum subsp. elatius (without neoplasm) in field conditions (Sari et al., 2020).

Interestingly, the germplasm with the least commercial acceptance in terms of colored seed coat and flowers was accredited as a wonderful resistance source for root rot diseases (Grunwald et al., 2003; Weeden and Porter, 2007) and Aphanomyces (Hamon et al., 2011). Most significantly, the resistance to different biotic stresses can also be transferred from Lathyrus species that are harbored in the tertiary pea gene pool (Patto et al., 2007, 2009), preferably through the utilization of contemporary biotechnological techniques. Most recently, super-early progeny derived from an interspecific cross between P. sativum and P. fulvum flowered in 13–17 days and set pod in 18–29 days after emergence. Such progeny could be used as a complementary to “speed breeding,” to generate more than six generations per year in an appropriate climate compartment (Sari et al., 2021). Significant contributions have been made toward the identification of resistant genetic resources for major biotic stresses in pea (Table 1), which might be utilized in breeding programs and further genetic analysis for the identification of new resistance genes.

Knowledge of genes controlling disease resistance is important to accelerate the success of any breeding program (Shashikumar et al., 2010). Understanding gene action/effects operating in a particular breeding population helps to select a suitable parent for hybridization and breeding procedure for making genetic improvements of resistance against that disease (Sharma et al., 2013). Notably, the pea is acknowledged as the original model organism and was utilized in the finding of Mendel’s laws of inheritance, which laid the foundation for modern plant genetics. In the recent years, inheritance has been studied for resistance attributes of disease in pea by several researchers (Lamprecht, 1948; Yarnell, 1962; Blixt, 1974; Gritton, 1980; Kalloo and Bergh, 1993; Kumar et al., 2006; Amin et al., 2010), and genes were identified and mapped using conventional gene mapping approaches. Varieties with inbuilt resistance are the most appropriate, competent, and economic strategies for tackling biotic stresses. Therefore, comprehensive efforts have been made to understand the inheritance of biotic stresses. Inheritance study for PM revealed that it is being operated by two recessive genes (er1 and er2) and one dominant gene (Er3) (Fondevilla et al., 2007a). A recent report illustrated that PM resistance is operated via er1 owing to the non-functioning of gene PsMLO1 (Humphry et al., 2011). The gene er2 is reported to provide complete resistance to PM but is efficient only in location-specific breeding (Tiwari et al., 1997; Fondevilla et al., 2006), while gene Er3 confers resistance in P. fulvum (Fondevilla et al., 2007a,2010).

With regard to PR resistance, it was reported to be operated by a single dominant gene (Ruf) (Tyagi and Srivastava, 1999); however, the polygenic nature of gene action (Singh and Ram, 2001) and partial dominance of a single gene in conjunction with minor and additive genes (2–3) (Singh et al., 2012) have also been found recently. A single dominant gene governs resistance toward races 1 and 2 of F. oxysporum f. pisi, pea enation mosaic virus, F. solani f. sp. pisi, brown root rot, bacterial blight, downy mildew, and other root rot diseases of pea, whereas a recessive gene regulates resistance to pea seed borne mosaic virus (sbm), yellow bean mosaic virus (mo), pea mosaic virus (pmv), and bean virus (Amin et al., 2010; Mohan et al., 2013). However, Davidson et al. (2004) reported downy mildew to be controlled by a single dominant gene and two complementary recessive genes. The nature of inheritance of AB and FRR resistance has been reported to be regulated by many genes (Kraft, 1992; Fondevilla et al., 2007b; Carrillo et al., 2014b; Jha et al., 2017). The pod resistance for pea weevil is quantitatively controlled whereas the seed resistance is operated by three (pwr1, pwr2, and pwr3) major recessive alleles (Byrne et al., 2008). The neoplasm appearance on pods is controlled by a single dominant gene and its expressivity is influenced by one or a combination of environmental factors (Sari et al., 2020).

Numerous biotic stresses including FW, AB, PM, PR, FRR, and CRR are serious threats to pea production (Bohra et al., 2014). These diseases are reported to occur in a severe form in almost all the pea growing countries. Therefore, efforts have been made to exploit the available genetic knowledge of resistance through conventional breeding for these key biotic stresses for developing resistant cultivars (Fondevilla and Rubiales, 2012; Ghafoor and McPhee, 2012). To develop high yielding pea cultivars possessing PM resistance, three genes, namely, er1, er2, and Er3 have been exploited successfully using conventional breeding approaches (Heringa et al., 1969; Fondevilla et al., 2007c). The er1 gene has the highest existence in resistant pea accessions followed by the er2 gene, which is harbored in restricted accessions (Tiwari et al., 1997). Therefore, the er1 gene that provides resistance through the pre-penetration resistance mechanism has been largely exploited in most pea improvement programs worldwide (Fondevilla et al., 2006). PR is another serious disease, scattered across the countries where the pea is being cultivated. Resistance to PR has been reported to be polygenic (Singh et al., 2012) and oligogenic (Vijayalakshmi et al., 2005). AB or black spot disease is one of the most devastating diseases of peas causing yield setbacks of up to 60% (Xue et al., 1996; Liu et al., 2016). Being seed borne, the rate of transmission from seed to sapling for A. pisi and P. pinodes is 40–100% (Maude, 1966; Xue, 2000), with an ability to remain viable on seeds for 5–7 years (Wallen, 1955). To date, the absolute resistant source for AB has not been identified; however, a prominent scale of resistance was found in accession (P651) of P. fulvum, which is being actively utilized in pea improvement (Wroth, 1998; Sindhu et al., 2014). The polygenic inheritance pattern of AB makes the development of resistant cultivars through conventional breeding very difficult. The FRR is considered a serious bottleneck in harnessing the full potential of a cultivar (Bisby, 1918; Jones, 1923). The condensed soil with a temperature of 18–24°C is the ideal thermal regime for the proliferation of FRR (Kraft and Boge, 2001). Unfortunately, complete resistance to this disease is yet to be explored; however, genetic sources carrying partial tolerance to this disease are available in pea (Gretenkort and Helsper, 1993; Porter, 2010). Noteworthy, the majority of the colored flower accessions portrayed a good level of resistance to FRR as compared to white colored flower accessions (Grunwald et al., 2003). Also, the polygenic inheritance of this disease has made the development of resistant varieties more complicated (Muehlbauer and Kraft, 1973; Kraft, 1992). FW is another severe production menace scattered around the world caused by Fusarium oxysporum. f. sp. pisi and causes absolute yield loss under appropriate environmental circumstances (Aslam et al., 2019). The most favorable soil temperature for FW disease development is 23–27°C. In total, 11 different races of fusarium have been discovered considering its virulence (Gupta and Gupta, 2019); of them, races 1 and 2 have become cosmopolitan; on the contrary, races 5 and 6 are prevailing in some areas (Bani et al., 2018). Among these races, race 1 is considered the most devastating and dominating (Kraft and Pfleger, 2001). Being a soil-borne pathogen, it may outlast for a prolonged period below the ground without pea crop (Gupta and Gupta, 2019). McPhee et al. (1999) recognized resistance sources against races 1 and 2 and used them to breed resistant cultivars. Interestingly, one CWR accession (PI 344012) having resistance to races 1 and 2 has been identified. Knowledge of inheritance is vital for incorporating any attribute of interest in the targeted genotype. Therefore, the inheritance pattern of resistance to Fop races 1, 5, and 6 have been studied and confirmed that it is monogenic with dominance in nature, while resistance to race 2 is regulated quantitatively (McPhee et al., 1999, 2012; Rispail and Rubiales, 2014; Bani et al., 2018). The monogenic dominant resistance is successfully introgressed in many pea cultivars (McPhee, 2003). The integration of quantitatively operated resistance in a targeted background is a cumbersome task wherein molecular markers can support significantly to accelerate the introgression process. For such traits, visual selection always remains long-lasting and labor exhaustive. Thus, modern genomic tools and techniques have paved a way for questing, utilizing, and choosing the naturally available sources of resistance against FW in pea (McClendon et al., 2002; Smýkal et al., 2012).

In pea under congruent circumstances particularly under excess moisture in the soil, CRR reduces grain yield significantly by severe damage to the root framework and subsequent wilting of the infected plant (Wu et al., 2018). Unfortunately, the existing old school disease management approaches such as crop rotation and seed treatments are incapable of controlling this disease completely, owing to the prolonged persistence of the pathogen in the form of oospores, which can contaminate crops at any phase. Consequently, resistant cultivar development has been advocated as an ultimate aim in the pea breeding scheme. Few accessions of pea having moderate resistance to CRR have been identified and subsequently used in breeding programs for developing cultivars (Pilet Nayel et al., 2002, 2005; Roux-Duparque et al., 2004; Moussart et al., 2007; Pilet Nayel et al., 2007; Hamon et al., 2011; McGee et al., 2012; Conner et al., 2013; Hamon et al., 2013; Lavaud et al., 2015). However, polygenic inheritance of this disease and its linkage with some objectionable attributes such as lengthy internodes, anthocyanin content, and delayed-flowering made it difficult to breed CRR-tolerant cultivars (Marx et al., 1972; Pilet Nayel et al., 2002).

A close association of markers with a trait of interest is the prerequisite of MAS, which identifies the target traits without assessing their phenotype in the early generation (Tayeh et al., 2015a). Both biparental and association mapping approaches have been utilized in the identification of closely associated markers with genes controlling disease resistance in pea. Such gene-linked markers control resistance to PM (Lakshmana Reddy et al., 2015), pea enation or seed borne mosaic virus (Swisher Grimm and Porter, 2020), FW (Jiang, 2013; Kwon et al., 2013), PR (Singh et al., 2015; Barilli et al., 2018), AB (Carrillo et al., 2014b; Jha et al., 2015, 2017), FRR (Coyne et al., 2019), and CRR (Lavaud et al., 2015; Desgroux et al., 2016) and are available for MAB. The marker-assisted backcrossing (MABC) has been successfully used for the introgression of QTLs for Aphanomyces root rot (ARR) resistance into several recipient genotypes (Hamon et al., 2013; Lavaud et al., 2015). During the recent years, efforts were made to identify markers closely linked with disease resistance genes. However, such markers are not being widely used in the MAB program for developing resistant cultivars due to their poor linkage with target traits. These efforts have proved the utility of MABC and MAS in pea improvement. Accessibility of the reference genome will pave the way toward finding the genes of interest and understanding the genetic background of individuals at the genome level by deploying molecular markers responsive to high-throughput genotyping.

Resistance in the host plant can occur at different stages during compatibility interaction between pathogen and host. Therefore, many mechanisms, metabolic pathways, and proteins are involved in the host plant and pathogen compatibilities. Thus, many genes have to be expressed to control these metabolic pathways or proteins for completing the infectivity of the pathogen with the host plant. Functional knowledge of these genes can help to understand the genetics involved in host plant resistance, which can further be utilized to develop resistant cultivars against a disease. During the recent years, genomic advances have made it possible to know the candidate genes involved in plant resistance by analyzing transcripts of genes expressed during host–pathogen interaction.

Pea is an important and exceptionally high-yielding cool season pulse crop in the world. Numerous biotic stresses are the key constraints in harnessing the full production potential of a pea, of which fungal diseases such as PM, FW, FRR, AB, CRR, and PR causing infection during different growth stages are devastating to the crop. Nevertheless, sincere efforts have been made to elevate the productivity and production of pea, but many more milestones are yet to be achieved for making it a resilient crop to upcoming challenges. Several major and minor genes/QTLs governing important biotic stresses in pea have been dissected and mapped using existing genomic tools, nevertheless, not utilized to a large extent in regular pea breeding programs. The reliable DNA markers flanking the genes/QTLs of interest could accelerate the introgression of resistance from the resistance sources using the genomic-assisted protocol to speed up the pea breeding program accomplishments more efficiently and precisely. Updated research efforts are warranted for the amalgamation of next-generation genomics and phenomics in pea improvement programs. The schematic diagram explains how different genomic approaches can be combined to accelerate the success of a pea breeding program (Figure 2). This figure also explains the combined use of genetic resources, genomic resources, and advanced biotechnological tools in the pea improvement program for the development of biotic stress-resistant cultivars. Underlying resistance mechanisms for AB, PM, and pea aphids have been elucidated using different pathogenic resistance proteins pertinent to the genes and pathways involved in pathogen resistance. However, more concentrated efforts are needed in the future on proteomic and transcriptomic analyses to untangle the disease and pest resistance mechanism in pea at the molecular level and to validate the sequencing results at the functional level for the identification of candidate genes controlling biotic stress resistance. This information will be certainly useful for editing or modification of crop genomes or realtered pathogens to develop resistant cultivars. Genome-wide association and genomic selection, which elucidate specific genetic variations at the genome scale, should be judiciously used for the identification of several gene(s)/QTLs exerting smaller effects on the biotic stress resistance. The transgenic technology should be exploited to let researchers utilize the variability existing outside the crop’s primary/secondary gene pool and also offer an opportunity to conquer crossability constraints. In addition, induced gene-specific DNA methylation and epigenome editing can be exploited to generate new epigenetic alleles for different biotic stresses. Most recently, speed breeding or rapid generation advancement protocols developed for shortening breeding times (4–5 cycles/year) have emerged as a potent technology for accelerating genetic gain in pea. Though, several tools and technologies are in hand judicious use to reap the best of them is challenging, certainly, there is a huge scope to achieve new heights in productivity enhancement by breeding biotic stress-resistant pea cultivars.

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