Biotrophic phytopathogenic fungi are obligate parasites of plants that during evolution developed the ability to penetrate host cells without destruction for obtaining nutrients and energy [1]. Rust fungi of bread wheat (Triticum aestivum L.) cause diseases such as leaf rust (caused by Puccinia recondita Dietel and Holw), yellow rust (Puccinia striiformis var. striiformis Westend), and stem rust (Puccinia graminis Pers.), which may seriously affect wheat yield worldwide [2]. For instance, yield losses could be up to 100% for especially pathogenic races of stem rust [3]. Significant yield losses related to epiphytotics of stem rust were reported in Australia, the USA, Scandinavian countries, Central and South Europe, India, and Asia in the 20th century [4,5]. The situation became even more dramatic in the 21st century starting with stem rust epidemics in Africa. Furthermore, in the last decade there have been significant outbreaks of stem rust in Kenya due to the emergence of new Ug99 races [6], epidemics and devastating yield losses in Ethiopia in 2013 due to the TKTTF race [3], outbreaks in North Kazakhstan and Siberia in 2015–2017 [7,8], epidemics of stem rust in Germany in 2013 [9], the first cases of wheat stem rust infection in the United Kingdom in nearly 60 years [10], an outbreak of stem rust in Southern Italy in durum wheat [11], and further spread of stem rust in Europe [12].

Losses caused by the disease may be explained by the details of the life cycle and pathogenesis of P. graminis. The life cycle of the fungus involves five different spore stages during the asexual reproduction in wheat (the uredinial stage) and sexual reproduction, which starts at the teliospore stage and continues on an alternate host plant (barberry, Mahonia). Ascospores complete the P. graminis life cycle infecting cereals [13]. This process is associated with the formation of urediniospores positioned on the surface of a leaf sheath or a stem and further development of the complex system of penetration of the plant cell, which includes appresoria, a penetration peg, hyphae, haustoria, and a substomatal vesicle to provide nutrients to the parasite. In areas with mild winters and sufficiently wet springs, P. graminis can exist in the uredinial (asexual) state in winter cultivated and wild cereals [4,13]. In the case of significant stem rust infestation of plants, nutrient flow to kernels can be affected causing shriveled grain. Moreover, stems are weakened by the disease resulting in wheat lodging, which causes additional yield losses [13].

Another factor that makes stem rust an especially dangerous disease for wheat is its polymorphism and ability for mutagenesis of the causative agent and the rapid emergence of new P. graminis races, as it is a species with a high-evolutionary potential [14]. Regularly, shortly after the wide implementation of a gene conferring resistance to the disease, a race virulent to that gene emerges causing significant losses to agriculture in some countries [15,16]. Only a few stem rust genes have shown durable effectiveness in breeding history. One of these genes is the stem rust resistance gene on translocation 1BL/1RS from the Petkus rye (Secale cereale L.), which reliably provided stem rust resistance for about 40 years until the emergence of the first race of the Ug99 group, TTKSK, with virulence to Sr31 in Uganda in 1999, which turned out to be virulent to the majority of other widespread resistance genes [15]. Despite preventive measures to localize Ug99, it spread to the southern coast of Africa. In addition, the original race TTKSK has been reported in the Middle East [16]. Moreover, new types (probably mutants) of Ug99 have been detected, which have gained the status of races. In particular, especially virulent not only to Sr31 but also to other genes that, according to initial studies, conferred resistance to Ug99, are the races TTKST, TTTSK, TTKSP, PTKSK, PTKST, TTKSF+, TTKTT, TTKTK, TTHSK, TTHST, PTKTK, TTKTT+, and TTHTT discovered from 2005 to 2020 in Tanzania, Eritrea, Egypt, Rwanda, Kenya, Ethiopia, South Africa, Yemen, Mozambique, Zimbabwe, and Uganda [6,15,16,17,18,19,20,21]. In 2019, the race TTKTT was reported in Iraq [22]. The last decade is characterized by stem rust outbreaks in Europe, Asia, and African regions due to the emergence of new stem rust races with multiple virulences that are distinct from the Ug99 group [3,7,8,9,12,23,24,25,26]. The Digalu race (TKTTF) caused severe epidemics in southern Ethiopia in 2013–2014 when yield losses were up to 100% of the wheat cultivar ‘Digalu’ planted in large areas [3]. Among the currently prevalent European races are TTRTF, TKTTF, and TKKTF. The race TTRTF caused the outbreak of stem rust in Sicily in 2016 [27]. This race was first described in 2014 in Georgia [23] and became widespread in Europe [12]. TTRTF was also detected in 2016 in Eritrea [25] and 2019 in Ethiopia [24] and the south of Iran [25]. This race is avirulent to Sr31 but has virulence to many important genes providing resistance against Ug99 races such as Sr13b, Sr35, Sr37, and Sr50 [12,23]. Moreover, a number of novel stem rust races with virulence to Sr31 and other stem rust genes have been recently described including TKHBK [26] and 22 other races in Spain [12] and the race LTBSK in Western Siberia [12].

2. Own Resistance Genes in Bread Wheat