Wheat stem rust, caused by Puccinia graminis f. sp. tritici, causes severe losses in wheat production under epidemic conditions. The detection of isolate Ug99 in east Africa (Pretorius et al. 2000) has raised global concerns of the vulnerability of wheat to stem rust. Since initial detection, 10 variants of Ug99 have been reported across 13 countries (Patpour et al. 2015). Wheat stem rust infection was widespread in Kenya in 2014. Fifty-two samples from common wheat were collected from the Mount Kenya and North, South, and Central Rift regions and analyzed for race identity in a level-3 biocontainment laboratory in Canada. Of these samples, 41 yielded viable spores for race pathotyping. Each sample was inoculated on 8-day-old seedlings of 20 single-gene differential lines using an inoculator, incubated for 16 h in a dew chamber in the dark, and subsequently moved to a growth cabinet set at 18 ± 1°C and 16-h photoperiod. Infected plants were rated 14 days postinoculation using a 0 to 4 infection type scale. Virulence analysis using the letter-code nomenclature system (Jin et al. 2008) identified two new races in the Ug99 race group from repeated experiments. Race TTHSK was identified from samples collected at Ngorengore (South Rift) and Njoro (field 13), which differs from the original Ug99 isolate (race TTKSK) by avirulence on gene Sr30. Race TTHSK is similar to race TTHST, which was detected previously by Newcomb et al. (manuscript in preparation). Race PTKTK was identified from samples collected at Rotian and Eor-Enkitok in the South Rift region, Cheplasgei and Kaplogoi in the North Rift region, and at the technology farm in Njoro in the Central Rift region. Race PTKTK differs from race PTKSK (first identified in 2007) by additional virulence to gene SrTmp, or alternatively differs from race TTKTK by avirulence to gene Sr21. Races TTKTK and TTKTT in the Ug99 race group with virulence to gene SrTmp were also found in 2014 (Patpour et al. 2015; Patpour et al. 2016). Virulence to SrTmp was detected soon after the deployment of the variety ‘Kenya Robin’ (which has SrTmp) in 2011. This report now brings the total number of variants in the Ug99 race group to 13 and highlights the importance of stem rust surveillance and race pathotyping, particularly in Kenya and surrounding countries in eastern Africa where evolution of new stem rust virulence is frequent.
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The basidiomycetous fungus, Puccinia graminis f. sp. tritici (Pgt) causes stem rust disease as one of the most destructive wheat pathogens, worldwide. TTKSK and other Pgt races under Ug99 race group are considered as major threats to wheat production in east Africa and CWANA region by defeating the stem rust resistance gene Sr31, while its ineffectiveness was reported in Iran in 2007. Race TKTTF of Pgt caused a severe stem rust epidemic in southern Ethiopia in 2013, and was spread to Europe through 2016 Sicily outbreak. This research describes race identification of Iranian isolates collected during the widespread distribution of stem rust in 2014-16. Purified urediniospores of 123 Pgt isolates were separately inoculated on seedlings of 20 North American differential wheat cultivars carrying different Sr resistance gene/s. Infection types were recorded at 14 days post inoculation (dpi) using Stakman et al. 0-4 scale. Based on the letter code nomenclature, we identified the Pgt races TKTTF, TTTTF, TTKSK, TTKTK, PKTTF, TKSTF, PKSTF, PKTTC, PTRTF, PTTTF, PKSTC, TTRTF, TKSTC and PKRTF in Iran. TKTTF and TTTTF were determined as prevalent Iranian Pgt races. This is the first report of race TTKTK, a new variant of Ug99 race group with virulence on Sr31 and SrTmp resistance genes, in Iran. Since TTKTK primarily occurred in south west of Iran, the migration route for this new race seems to be similar to race TTKSK. The high race variation observed in this study could indicate a high genetic diversity among P. graminis f. sp. tritici populations in Iran, as a wheat center of origin.
Most South African winter wheat varieties display all stage resistance (ASR) to stem rust caused by Puccinia graminis f. sp. tritici (Pgt). To study inheritance, four resistant varieties were crossed to a susceptible parent (Line 37) and F2 populations were phenotyped at the seedling stage with stem rust race PTKST (Ug99 lineage). Populations derived from varieties Koonap, Komati, Limpopo and SST 387 segregated in a 3:1 ratio, indicating that a single, dominant gene confers resistance in each population. Assessment of F2 seedlings of four intercrosses between these varieties failed to deliver susceptible segregants therefore suggesting that they carry the same resistance gene. Genotyping of F2 plants with microsatellite markers produced consistent linkage of resistance with markers on chromosome 6DS. Experiments are underway to determine the relationship between resistance in the four winter wheat varieties and resistance genes Sr42, SrCad and SrTmp, all located on 6DS. Current evidence shows that ASR in the South African winter wheat varieties Koonap, Komati, Limpopo and SST 387 is based on a single gene and thus vulnerable to pathogenic adaptation in Pgt.
Puccinia graminis f. sp. tritici (Pgt) is the major wheat production constraint in Ethiopia causing recurrent epidemics that resulted in the withdrawal of widely grown wheat cultivars from production. Among the current Pgt races detected in Ethiopia, TKTTF is the most frequent and has caused a severe epidemic in the south wheat growing regions (Bale and Arsi) after its first detection in 2012. Therefore, to avert the current situation, identifying sources of resistance to race TKTTF in breeding germplasm is a top priority to the National Wheat Breeding Program. Hence, 82 promising bread wheat lines including five check cultivars were evaluated in Debre Zeit in a TKTTF single race nursery for three consecutive seasons, 2014-2016. Ethiopian bread wheat cultivar Digalu was used as a spreader row and was inoculated using a single isolate of race TKTTF at different growth stages. The nursery was bounded by oat to reduce interference with any other stem rust race. The 82 lines were tested in the greenhouse at Cereal Disease Laboratory and were also tested with known diagnostic molecular markers. Twenty-nine lines displayed low levels of terminal stem rust severity in the field and low coefficient of infections. Fourty-one lines were resistant to race TKTTF at the seedling stage. Bread wheat lines resistant to TKTTF are valuable sources of resistance that can be deployed in wheat growing regions of Ethiopia prone to stem rust.