Detection of wheat stem rust race RRTTF in Ecuador in 2016

Wheat stem rust is a devastating disease that has incited numerous severe epidemics resulting in extreme yield losses over the past century. Stem rust infection in plots of wheat line UC11075, known to carry the Sr38 resistance gene, was severe in February 2016 in a nursery at the Instituto Nacional de Investigaciones Agropecuarias Austro station near Cuenca, Ecuador. Stems with heavy infection of Puccinia graminis f. sp. tritici (Pgt) were sent for genotyping in labs at Ottawa, ON and St. Paul, MN, and for phenotyping at Morden, MB. None of the samples received directly from the field had viable spores, but genotyping was done in Ottawa and St. Paul using DNA extracted from spores or infected plant materials killed in ethanol. DNA was extracted using the OmniPrep™ kit (G-Biosciences, St. Louis, MO USA). Results from the Ottawa genotyping lab using 11 of 20 SSR markers (Stoxen, 2012) indicated that all February 2016 samples from the Austro station were genotypically identical, and nine of 11 markers matched to Pgt isolate 10-Eth-1-2 from Ethiopia (collected in 2010) that previously was pathotyped to race TRTTF. Results from the St. Paul genotyping lab using 17 selected SNP markers also found that all samples were genotypically identical, and grouped into Pgt Clade III (Olivera et al, 2015) along with isolates 14ETH136-2 from Ethiopia (2014), 86PAK1030a from Pakistan (1986), and 06YEM34-1 from Yemen (2006) that previously pathotyped to race RRTTF or TRTTF. Original samples collected in February were increased in August of 2016 in Ecuador. These, along with other samples collected in December 2016 were sent to Morden for phenotyping in February of 2017. Of 18 samples that were sent, five were viable. Each sample was first increased on susceptible McNair in separate Plexiglas cubicles, and then inoculated on 20 single-gene differential lines. Infected seedlings were rated for infection type at 14 days post-inoculation, and the letter-code nomenclature (Jin et al. 2008) was used to identify the race. Four isolates had mixed infection, but isolate 16-ECU-22 was clearly pathotyped to race RRTTF and matched to results from both genotyping labs. Isolate 16-ECU-22 is a urediniospore increase from the original field collection at the Austro station in February 2016 that was sent for genotyping, and was increased in a greenhouse in Ecuador in August. Race RRTTF is distinctive by its combined virulence to genes Sr38 and Sr13 and poses a significant threat to wheat production in North and South America. Previous seedling assessment found that 55% of Canadian hard red spring wheat varieties were susceptible to Pgt race RRTTF (Fetch et al. 2012). The origin of Pgt race RRTTF in Ecuador is unknown, but was clearly similar to isolates of RRTTF from Asia, eastern Africa, and the Middle East. It is unknown whether race RRTTF is a recent long-distance exotic introduction into Ecuador, or a de novo variant of an existing lineage in South America that was a result of an earlier introduction. Further study is needed to determine how widespread this race is in Ecuador and its potential to migrate to large-scale wheat production areas of South and North America.