Vicia faba L., known as faba bean, horse bean, fava bean or broad bean, is a grain legume consumed both as fresh vegetable and dry grains. It is cultivated across the subtropical to temperate regions worldwide. Faba bean is a plant species of the Old World that was domesticated in the Fertile Crescent of the Near East following the Neolithic era around 9000-10000 BC and subsequently spread around the world (Cole 1961; Maxted et al. 1991; Tanno and Willcox 2006). The primary centre of diversity includes Iran, Iraq, Georgia, Armenia, Azerbaijan, Syria and Turkey (Maxted 1995). Afghanistan and Ethiopia were considered as the secondary centre of diversity. However, some authors reported that the origin of faba bean was Central Asia (Ladizinsky 1975), south-eastern Europe and south-western Asia (Maxted 1995). Cubero (1973) postulated different routes of its spread from the Near East to the other parts of the world. The first route could be across Anatolia to Greece and Mediterranean, the second could be Nile delta to the Maghreb and Iberian lands, third could be River Nile to Ethiopia and the last route could be Mesopotamia to India. Seed morphology distinguished two groups; small-seeded (Eastern group) forms prevalent in south-western Asia and large-seeded (Western group) forms prevalent in Europe (Fouadl et al. 2013). Faba bean was introduced to South America (Andean region) by Portuguese and Spanish travellers during the 15th century that resulted in the evolution of Peruvian and Bolivian landraces (Duc et al. 2010). Zheng et al. (1997) speculated that faba bean was introduced to China during the late 17th century through silk road route through the Middle East.
Faba bean is the fourth important grain legume in top producer countries China, Australia, Ethiopia, France and Egypt. The annual global production is 4.3 million tonnes (FAOSTAT 2016). It was included in Australian agriculture during the late 20th century from Britain and slowly established as a rotation crop with wheat (Adhikari et al. 2016) because of its high potential to fix the significant amount of atmospheric Nitrogen naturally (Jensen et al. 2010). With the access to Middle Eastern markets and release of improved cultivars, Australia became the second largest producer of faba bean after China and its average export value was $168 million during 2010-15 (Pulse-Australia 2016). Like other crops, faba bean is affected by environmental factors, such as diseases, soil architecture, extreme temperature and moisture constraints. Among diseases, pathogenic fungi namely; Uromyces viciae-fabae (Pers.) J. Schrot., (rust), Ascochyta fabae G. J. Jellis & Punith., (blight) and Botrytis fabae Sardiña (chocolate spot) and viruses including faba bean necrotic yellow virus (FBNYV), bean yellow mosaic virus (BYMV), bean leaf roll virus (BLRV) also. The occurrence of fungal diseases is variable in Australia, for example, rust is a major problem in faba bean growing regions spanning from northern New South Wales-southern Queensland, blight in South Australia-Victoria and chocolate spot occurs in both regions depending upon environmental conditions (Adhikari et al. 2016). Several disease control measures that are available including the use of fungicides, agronomic practices, biological crop agents and breeding resistant cultivars (Sillero et al. 2010). The understanding of the local importance, genetic control and availability of sources of resistance to these fungal diseases is crucial to underpin resistance breeding strategies.
Rust is among the devastating foliar diseases, caused by biotrophic pathogen U. viciae-fabae. It is prevalent mostly in North Africa, Mediterranean, China and Australia and can cause the severe reduction in seed size and production, if infection starts in early stages of crop growth (Liang 1986; Rashid and Bernier 1986). U. viciae-fabae is an obligate parasite (carrying both sexual and asexual spore forms), lives an autoecious life (complete its lifecycle on the single host). It can infect vetch (Vicia sativa L.), field pea (Pisum sativum L.) and lentil (Lens culinaris Medik.) (Cummins 1978). Therefore, high diversity among pathogen populations is expected and cross-inoculation studies revealed an existence of intraspecific groups with differential pathogenicity (Rubiales et al. 2013). Early studies in Canada reported variability in virulence among two rust isolates when tested on 17 faba bean genotypes (Conner and Bernier 1982b). Thereafter, 15 physiological races were characterised within 27 Spanish and Portuguese isolates (Rojas-Molina et al. 2006). Herath et al. (2001) identified isozyme variation among rust isolates in Australia, but he failed to relate these variations to virulence due to the unavailability of differentials.
Several sources of rust resistance were summarised by Rashid and Bernier (1984), Bond et al. (1994), Sillero et al. (2010), Adhikari et al. (2016) and Ijaz and Adhikari (2016), but none of them reported the existence of an immune infection type. However, hypersensitive type of rust resistance response, where cell death occurs late during disease progression to reduce individual infection type has also been reported in faba bean (Avila et al. 2003; Sillero et al. 2000). The race specificity of rust resistance was noticed when 64 out of 65 reported resistant lines (Rashid and Bernier 1986) were found susceptible at ICARDA (Bond et al. 1994). Various genetic studies revealed monogenic inheritance of resistance (Adhikari et al. 2016; Conner and Bernier 1982a). In another study, the possible existence of epistatic gene action (Ijaz and Adhikari 2016) was also noticed. Despite the characterisation of single genes in various genotypes, no attempt has been made to assemble a set of differentials for the characterisation of variation in the pathogen population.
The dawn of molecular breeding with the invent of next generation sequencing technologies (NGS) has enabled to pick genetic regions controlling desirable traits to develop molecular markers that allow fast tracking of desirable traits within segregating populations without any field or glasshouse screening (Morozova and Marra 2008). The manipulation of NGS platforms for tracking rust resistance in faba bean remained far behind because of the unavailability of practical molecular markers to breeders. The only reference of developing molecular markers existed in literature was from Avila et al. (2003), who reported Random Amplified Polymorphic DNA (RAPD) markers linked with rust resistance gene Uvf-1 in faba bean line 2N-52 in Spain. The utility of these markers in applied plant breeding has never been reported, presumably their low throughput nature. After more than a decade, using 1536 Illumina ® Infinium assay two major quantitative trait loci (QTLs) were identified by Sudhesh et al. (2016) on chromosome III and V that controlled rust resistance in an old cultivar selection Doza#12034 and an exotic accession Ac1655 in Australia, respectively. Although the information about genomic regions controlling rust resistance is available, it cannot be used in marker-assisted practical breeding unless converted into Polymerase Chain Reaction (PCR) based marker systems such as Simple Sequence Repeats (SSR) or Kompetitive Allele Specific PCR (KASP).
Taking into consideration, the importance of diversity in host resistance, pathogen variation, development of molecular markers linked with rust resistance gene(s) and host range of U. viciae-fabae, the present investigations were planned to:
1. The understanding diversity of resistance in host Vicia faba and virulence in Uromyces viciae-fabae.
2. Molecular mapping and development of markers for rust resistance in faba bean.
3. Microscopic examination of Uromyces viciae-fabae developmental biology in related grain legumes.