Ug99 is a lineage of wheat
stem rust (Puccinia graminis f. sp. tritici), which is present in
wheat fields in several countries in
Africa and the
Middle East and is predicted to spread rapidly through these regions and possibly further afield, potentially causing a wheat production disaster that would affect
food security worldwide.[1] In 2005 the noted
green revolution pioneer
Norman Borlaug brought great attention to the problem, and most subsequent efforts can be traced to his advocacy.[2] It can cause up to 100% crop losses and is
virulent against many
resistance genes which have previously protected wheat against stem rust.
Although Ug99-
resistant varieties of wheat do exist,[2] a screen of 200,000 wheat varieties used in 22 African and Asian countries found that only 5-10% of the area of wheat grown in these countries consisted of varieties with adequate resistance.[1]
The original
race of Ug99, which is designated as 'TTKSK' under the North American nomenclature system, was first detected in
Uganda in 1998[3] and first characterised in 1999[3] (hence the name Ug99) and has since been detected in
Kenya,
Ethiopia,
Eritrea,
Sudan,
Yemen,
Iran,
Tanzania,
Mozambique,
Zimbabwe,
South Africa,[4] and
Egypt. There are now 15 known
races of Ug99.[5] They are all closely related and are believed to have
evolved from a common ancestor, but differ in their
virulence/avirulence profiles and the countries in which they have been detected.[1]
Genetics
Ug99 is the product of a type of somatic nuclear exchange event which has not been observed in other stem rust races.[6] During this event and thereafter the nuclei have not experienced
recombination.[6]
Gene resistance
Ug99 and its variants differ from other strains of the Black Stem Rust (BSR) pathogen due to their ability to overcome
resistance genes in wheat that have been durable against the BSR pathogen for decades.[7] These resistant Sr genes, of which 50 are known, give wheat different resistances to stem rust.[3] The virulence in Uganda was virulent against Sr31 and is specific to Ug99.[3] The massive losses of wheat that have occurred have been devastating, but in recent years the wheat rust epidemic has been effectively controlled through selection and breeding for additional Sr genes.[3] (In the decades since, however, Sr31-virulence has evolved in other strains in other locations.[8] Patpour et al., 2022 finds it in
Spain and
Siberia.)[8]
United States Department of Agriculture (
USDA) researchers are testing genes to determine their Ug99 resistance, which will ultimately aid in the development of wheat varieties that will be able to fight off the rust. Resistance has been identified in a small number of spring wheat land races from
North America - 23 out of 250 races with
adult plant resistance, 27 out of 23,976 SNPs conveying APR, and only 9 races having
seedling resistance.[9] This resistance was present without the Ug99 pathogen challenge being present in NA to drive its
selection.[9] USDA has studied winter wheat land races where resistance is more probable.[10]
In addition to the research being conducted by the
USDA, The United Kingdom’s
Department for International Development (DFID) along with
Bill & Melinda Gates Foundation, announced in February 2011 that they will be granting $40 million to a global project led by
Cornell University to combat virulent strains of Ug99.[11] The five-year grant to the Durable Rust Resistance in Wheat (DRRW) project supported attempts to identify new resistance genes as well as reproduce and distribute rust resistant wheat seeds to farmers.[11]
There has been a continuous process of development of new resistant cultivars and failure of those cultivars.[12] This demonstrates the need for continuous improvement.[12]
Sr35 confers resistance to all other severe Pgt races and the original Ug99.[14] Salcedo et al., 2017 finds its Avr target, AvrSr35.[14] Races virulent on Sr35 benefit from
nonfunctionalization of AvrSr35 by insertion of a
mobile element.[14]
Races
There are 15
races of Ug99, which (under the North American nomenclature system) have the designations TTKSK, TTKSF, TTKST, TTTSK, TTKSP, PTKSK, PTKST, TTKSF+,[4] TTKTT, TTKTK, TTHSK, PTKTK, TTHST, TTKTT+, and TTHTT.[5] They are all closely related and are believed to have
evolved from a common ancestor.[1]
Discovered in
Kenya in
2006[16] was the first Ug99 race found to be virulent against Sr gene Sr24.[1][16] TTKST is now the predominant stem rust race in Kenya.[1]Virulent on Sr31.[5]
First detected in Kenya in
2014.[5] Also detected in
Iraq in 2019, the first such detection in the country.[5] Found in Nepal in 2023.[22]Virulent on Sr31, Sr24, and SrTmp.[5]
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Because stem rust (as with many fungi) spreads its
spores across long distances with the help of natural
air currents, containment is difficult.[28] Advances in
fluid mechanics which are commonly used for
meteorology have also aided Ug99 dispersal prediction.[28] This is especially important for inter-continental, intermittent spread, such as from Eastern
South Africa to
Western Australia.[28]
China
Although Ug99 has not yet reached
China,[29] other stem rust races already have,[29] and an effort is under way to marry resistance against present races with future needs for resistance against Ug99 whenever it arrives.[29]
Lebanon
Although Sr5, Sr21, Sr9e, Sr7b, Sr11, Sr6, Sr8a, Sr9g, Sr9b, Sr30, Sr17, Sr9a, Sr9d, Sr10, SrTmp, Sr38, and SrMcN are no longer effective in
Lebanon, Sr11, Sr24, and Sr31 still are which is diagnostic for the absence of Ug99 from Lebanon.[30]
Low-levels of TTKTT were detected in Nepal in 2023, but surveillance has not revealed any propagation in the region.[31] As of 2013[update] it was the US
Director of National Intelligence's assessment that Ug99 would arrive in
South Asia soon, in the following few years. This was expected to cause worldwide supply disruptions because, although productivity was growing in
Eastern Europe and could theoretically fill that gap, governments worldwide had shown a readiness to forbid exports.[32]
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abcdefghHodson, D. P.; Grønbech-Hansen, J.; Lassen, P.; Alemayehu, Y.; Arista, J.; Sonder, K.; Kosina, P.; Moncada, P.; Nazari, K.; Park, R. F.; Pretorius, Z. A.; Szabo, L. J.; Fetch, T.; Jin, Y.
"Tracking the Wheat Rust Pathogens"(PDF). 2012 Borlaug Global Rust Initiative Technical Workshop Proceedings.
Borlaug Global Rust Initiative.
Archived(PDF) from the original on October 5, 2019. Retrieved 28 November 2012.
• Singh, Ravi P.; Hodson, David P.; Jin, Yue; Lagudah, Evans S.; Ayliffe, Michael A.; Bhavani, Sridhar; Rouse, Matthew N.; Pretorius, Zacharias A.; Szabo, Les J.; Huerta-Espino, Julio; Basnet, Bhoja R.; Lan, Caixia; Hovmøller, Mogens S. (2015). "Emergence and Spread of New Races of Wheat Stem Rust Fungus: Continued Threat to Food Security and Prospects of Genetic Control". Phytopathology. 105 (7).
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^
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