Current status of wheat stem rust race Ug99 in South Africa Stem rust (black rust) is an important disease of bread wheat in South Africa. It is frequently found in the winter rainfall wheat growing regions of the Western Cape, sometimes reaching epide mic levels and causing significant economic loss. Susceptible cultivars could suffer more than 35% yield loss. The main sign of stem rust is the development of dark-red, elliptical pustules (masses of spores) mainly on the stem, and sometimes on the leaf and head of wheat. Dr Tarekegn Terefe (PTKST), 2SA88+ (TTKSF+) and 2SA42 (PTKSK). ARC-SMALL GRAIN, BETHLEHEM Race TTKSF (virulent on resistance genes Sr9e and Sr38) was first detected in SA in 2000 and LTHOUGH RESISTANT CULTIVARS pro- has dominated the stem rust population over Avide effective stem rust control, the fungus the last 20 years. This race is similar to TTKSP, ex- that causes stem rust frequently acquires new cept for its avirulence on Sr24. The widespread virulence to overcome the resistance in exist- occurrence of TTKSF can be mainly attributed to ing cultivars. The appearance, adaptation and its virulence on several resistance genes and the spread of the stem rust race Ug99 (TTKSK) high- cultivation of susceptible wheat cultivars in the lights the importance of stem rust variability in Western Cape, where stem rust regularly oc- global wheat production. curs. In addition to its broader virulence, TTKSF Ug99 is a highly virulent race which was probably possesses other fitness traits which detected in Uganda, East Africa in 1999. Ug99 may have contributed to its predominance over overcame the resistance gene Sr31 which had the past several years. been effective and widely used in resistance Race TTKSP (virulent on Sr24, Sr38) was breeding for more than three decades. Dur- first identified in 2007 in the Western Cape ing the last 20 years, variants of Ug99 have and has not been detected outside the prov- emerged overcoming additional resistance ince. Given its broader virulence than TTKSF, genes such as Sr24, Sr36 and SrTmp. In addi- race TTKSP was expected to predominate. tion, Ug99 and its variants had spread to sev- However, the population of TTKSP gradually eral countries. To date, thirteen Ug99 variants declined and disappeared as this race has not are known and one or more of these races been detected since 2011. The poor establish- have been confirmed in thirteen countries, in- ment of TTKSP compared to races with rela- cluding South Africa (Table 1). tively lower virulence suggests that it probably To mitigate the negative impact of the stem lacked other fitness traits associated with viru- rust pathogen which continually evolves, re- lence. searchers in many wheat producing countries Race PTKST (virulent on Sr24, 31, 38) was conduct rust monitoring (surveillance). Moni- first detected in 2009 in KwaZulu-Natal. It toring of the stem rust population helps to was also found in 2010 in the Free State. timeously detect and control new races, pre- However, it has not been detected in the sub- venting potential outbreak and economic loss. sequent seasons except in 2014 and 2017 New races detected through surveys can when it was found in a few samples collected also be used in screening and identification of in Bethlehem, Free State. The infrequent oc- effective resistance, thereby ensuring continual currence of PTKST could be due to the ab- availability of resistant cultivars to wheat grow- sence of favourable weather for infection and ers. Therefore, the national rust monitoring dissemination, particularly in KwaZulu-Natal Symptom of stem rust programme at ARC-Small Grain (ARC-SG) has and the Free State, where it was previously on the stem of wheat. been conducting annual surveys which made detected. So far, PTKST has not been detected The main sign is the significant contributions to sustainable control in the Western Cape. development of dark- red, elliptical pustules of stem rust in SA. Through such surveys, con- The fourth race, TTKSF+, was detected in (masses of spores) ducted over the past two decades, five of the the Free State in 2010. This race is similar in its mainly on the stem, thirteen Ug99 variants have been confirmed in virulence profile to TTKSF except for its virulence and sometimes on the SA. These include 2SA88 (TTKSF, North Ameri- on the South African winter wheat cultivar leaf and head of wheat. can race notation), 2SA106 (TTKSP), 2SA107 Photo by ZA Pretorius. TO PAGE 16 JANUARY • FEBRUARY 2021 KORINGFOKUS WHEAT FOCUS 15 Current status of wheat stem rust race Ug99 in South Africa FROM PAGE 15 Table 1. Ug99 (TTKSK) and its variants confirmed in South Africa and other countries. Adapted from RustTracker (https://rusttracker.cimmyt.org/). Matlabas (Sr9h). Since its initial detection in 2010, TTKSF+ has been found only in No. Country Virulence on key stem rust Ug99 variants# resistance genes 2013 in a few samples collected in Tyger- 1 Egypt Sr24, Sr31, Sr38, SrTmp TTKST, TTKSK, TTKTK hoek and Moorreesberg in the Western Cape. 2 Eritrea Sr24, Sr31, Sr38, SrTmp TTKSK, TTKST, PTKST, TTKTK More recently, a few isolates which 3 Ethiopia Sr24, Sr31, Sr36, Sr38 TTKSK, TTTSK, PTKST, PTKSK were collected from wheat near Bethle- 4 Kenya Sr24, Sr31, Sr36, Sr38, SrTmp TTKSK, TTKST, TTTSK, PTKSK, hem in the Free State in 2017 were iden- PTKST, TTKTT, TTKTK, TTHSK, PTKTK, TTHST tified as a new Ug99 variant race 2SA42 5 Mozambique Sr24, Sr31, Sr38 PTKST (PTKSK) (virulent on 31 and 38). This brought the number of Ug99 variants de- 6 Rwanda Sr24, Sr31, Sr36, Sr38, SrTmp TTKSK, TTKST, TTTSK, TTKTK tected in SA to five. Except for avirulence on Sr24, PTKSK is similar to the existing 7 South Africa Sr24, Sr31, Sr38, Sr9h TTKSF, TTKSF+, TTKSP, PTKST, race PTKST. In the subsequent years, PTKSK PTKSK was found in samples collected 8 Sudan Sr31, Sr38 TTKSK in a few localities in the Free State, con- 9 Tanzania Sr24, Sr31, Sr36, Sr38 TTKSK, TTTSK, TTKST firming its establishment and potential to 10 Uganda Sr24, Sr31, Sr36, Sr38, SrTmp TTKSK, TTTSK, TTKST, TTKSF, cause infection in the years to come. TTKTK New races commonly arise through 11 Zimbabwe Sr24, Sr31, Sr38, Sr9h TTKSF, TTKSF+, PTKST genetic mutation occurring in locally ex- isting races. They can also be transported 12 Iran Sr31, Sr38 TTKSK over long distances and enter new areas 13 Yemen Sr31, Sr38 TTKSK, PTKSK through their wind-borne spores or by #The 13 Ug99 variants known so far are underlined sticking to clothes and shoes of travel- lers. Genetic studies on the Ug99 vari- cultivated and the weather is conducive Pyramiding of these genes is being sup- ants and existing stem rust races in SA for stem rust development. ported by the use of diagnostic DNA showed low similarity between TTKSF, Furthermore, different resistance markers. These efforts will contribute to PTKST and the existing races. Therefore, genes that are effective against PTKST the development of cultivars with mul- it is unlikely that these Ug99 variants were identified and some of these have tiple resistance and increased durability. developed locally from existing races been incorporated into breeding lines by through a step-wise mutation. These ARC-SG, as described below. The recent- Conclusion two races should rather be considered ly detected race PTKSK is less virulent on From the thirteen Ug99 variants known exotic introductions. SA wheat cultivars compared to PTKST. worldwide so far, five have been con- Similar genetic studies suggested that Hence, it should not pose a significant firmed in SA. Some of these races are TTKSP might not represent an introduc- threat to local wheat production. most likely foreign introductions into tion but rather a local adaptation. Races To mitigate the challenges of rapid this country, emphasising the continued TTKSF, TTKSF+ and PTKST have been resistance breakdown due to the emer- vulnerability of the South African wheat reported in other African countries, in- gence of new races, researchers from industry to new stem rust races that cluding Zimbabwe and Mozambique ARC-SG together with national and could emerge locally or through migra- (Table 1), suggesting that these coun- international collaborators have been tion from other countries. tries, especially those close to South continually screening wheat germplasm In an effort to reduce the negative Africa, could be the sources of some to identify resistance sources which are impacts of Ug99 variants and other new of the races found in SA. Presently, it is effective against Ug99 variants and stem rust races, ARC-SG has been regu- not known if PTKSK arose locally from other stem races known in SA and oth- larly conducting rust surveys to timeously PTKST or represents a new introduction er countries. The identified resistance detect new races and screen available re- into SA. This race has been reported in genes have been and will continue to be sistance sources. This has assisted in the Ethiopia, Kenya and Yemen. incorporated into breeding material to identification and utilisation of several re- From the relatively older Ug99 vari- develop cultivars with a broad spectrum sistance genes which are effective against ants in SA, PTKST was found to be more of resistance. Ug99 variants present in and outside SA. virulent as several South African spring Efforts are in progress at ARC-SG to The ongoing rust surveys should be wheat cultivars and breeding lines were improve durability of stem rust resistance strengthened and continued in order to susceptible to it. However, PTKST has through pyramiding of two or more reduce the negative impacts of Ug99 vari- still not been observed in the Western major genes in combination with minor ants and other virulent stem rust races that Cape, where spring wheat is widely genes conferring adult plant resistance.