July 3, 2012: Eighth new Ug99 variant identified – Race TTKSF+ confirmed in South Africa and Zimbabwe

Photo Credit: ZA Pretorius

stem rust samples collected from Afrikaskop, Eastern Free State, South Africa in Dec. 2010 and Birchenough, Eastern Zimbabwe in Sept. 2010 have been confirmed to be a new variant in the Ug99 lineage. The confirmed findings have just been published in Plant Disease by Pretorius et al. (2012). The new variant has been designated as race TTKSF+, this is the eighth race detected in the Ug99 group. The new race was collected from the winter wheat cv. Matlabas in South Africa. This was the first time this cultivar had been observed to be infected with stem rust. Microsatellite analysis showed that all single pustule isolates established from the original Matlabas isolate formed part of the Ug99 group. Race analysis using the 20 North American (NA) stem rust differential lines and letter code system classified the new race as TTKSF. However, cv. Matlabas is resistant to race TTKSF and characterization with selected single nucleotide polymorphisms (SNPs), revealed that all single pustule isolates of the Matlabas race shared an identical genotype that differed from race TTKSF. The Zimbabwe isolate was identical and was also classified as race TTKSF+. Studies are under way to determine the identity of the defeated Sr gene in Matlabas. Since no other cultivars or advanced lines were found to carry the Matlabas gene, it is unlikely that race TTKSF+ will threaten wheat production in South Africa. However, the occurrence of a new Ug99-related race emphasizes the variability within this internationally important group.

[Photo Legend: Left to right: race TTKSF+ on Matlabas; race TTKSF on Matlabas; race TTKSF (original isolate) on Matlabas, Susceptible Control]

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