The Digalu race (race TKTTF) is not part of the Ug99 race group; it is genetically unrelated. This race has been known from Turkey since at least 2005 and it is widely distributed in the Middle East region. It is being increasingly detected across a wide geographical range. Currently it has been confirmed in 9 countries (Turkey, Iran, Lebanon, Egypt, Ethiopia, Georgia, Azerbaijan, Eritrea, Yemen – see Map 1) with 2 additional countries (Germany and Denmark) detecting TKTTF-like variants in 2013. The German and Danish isolates differed by having virulence on 4 additional Sr genes: Sr33, Sr7a, SrTt-3, Sr45 (Olivera et al., 2015).
Race TKTTF is virulent on: Sr5, Sr6, Sr7b, Sr8a, Sr9a, Sr9b, Sr9d, Sr9e, Sr9g, Sr10, Sr17, Sr21, Sr30, Sr36, Sr38, SrTmp and SrMcN
Race TKTTF was first detected in Ethiopia in August 2012, but at trace levels from a single sample (GRRC data). Most likely this was a potential incursion from neighboring countries. TKTTF was re-confirmed in Ethiopia in early October 2013. By early November 2013 it had reached epidemic levels in south-eastern Ethiopia on the popular cultivar Digalu (carrying SrTmp) (see report Dec 2013). Repeated localized epidemics on Digalu due to race TKTTF were observed in Ethiopia in 2014/15 (see report Jan 2015).
Phylogenetic studies have revealed that 2 very distinct genotypes (type A and type B) are present within the TKTTF isolates analysed to date (Olivera et al., 2015).
Given the rapid and destructive nature of race TKTTF in Ethiopia, close monitoring of this race is advised – especially in countries which have cultivars carrying the SrTmp resistance gene.
Analysis of dead, single pustule stem rust pathogen samples (D-samples) using molecular diagnostic SNP assay at the USDA-ARS Cereals Disease Lab, Minnesota has detected a race TKTTF (“Digalu” race) genotype in Kenya for the first time. Five samples from 2014 and 7 samples from 2015, all collected by Ruth Wanyera and the pathology team from KALRO, Njoro, tested positive for race TKTTF genotype. These results indicated that a single genotype (clade IV-B) was present in Kenya. Clade IV-B is the predominant genotype in Ethiopia.
The D-sample results indicate that race TKTTF is distributed (probably at low frequency) throughout the major wheat growing regions of Kenya. The 2014 positive samples were collected from North Rift (n=2), Central Rift (n=1) and Mount Kenya (n=2). In 2015, positive samples were collected from South Rift (n=1), Central Rift (n=1) and Mount Kenya (n=5). Most of the TKTTF genotype positive samples were collected from the cultivar ‘Robin’ (n=9), but ‘KS Mwamba’ (n=1), ‘Kwale’ (n=1) and an unknown barley variety (n=1) also produced positive results for TKTTF genotype.
At present no race analysis studies on live samples collected in Kenya have detected the presence of race TKTTF. Testing of the SNP assay against known isolates of TKTTF and negative controls has proven 100% reliable, but until there is confirmation by race analysis the D-sample results are considered indicative.
Race TKTTF now totally dominates the stem rust pathogen population in all the wheat growing regions of neighbouring Ethiopia, so its presence in Kenya is not unexpected.