- Rust Issues: All 3 rusts are potential constraints. Stem rust historically was the major constraint. Yellow rust was first detected in 1996 – the result of a foreign incursion. Chemical control is undertaken.
- Ug99 Status: Four races in the Ug99 lineage have been detected in South Africa – TTKSF (in 2000), TTKSP (in 2007), PTKST (in 2010) and TTKSF+ (in 2010).
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]
Analysis undertaken by the Small Grains Institute, Agricultural Research Council (ARC), South Africa indicated the continued predominance of stem rust race TTKSF in South Africa. Race TTKSF is one of the Ug99 lineage races, although it is avirulent on Sr31. This race was first detected in South Africa in 2000 by Prof Z.A. Pretorius, University of the Free State and was identified in Zimbabwe in 2007. Of the 40 South African stem rust isolates analysed in 2011, 35 were typed to be race TTKSF. Additional races detected in 2011 were BPGSC+Sr27, Kw, Satu (2SA105) [n=4] and BPGSC+Sr27 (2SA102) [n=1].
A stem rust sample collected from near Greytown, KwaZulu-Natal, South Africa in Nov. 2009 has been confirmed to be Ug99 lineage race PTKST. Samples from another site – Cedara, KwaZulu-Natal, collected in Dec. 2009 showed similar virulence profiles. The confirmed findings have just been published in Plant Disease by Pretorius et al (2010). Race PTKST was only previously known from Kenya, where it was detected in 2008. Race PTKST is notable in that it carries combined virulence for resistance genes Sr24 and Sr31. The published avirulence/virulence profile of race PTKST is:
Avirulent: Sr13, 14, 21, 22, 25, 26, 27, 29, 32, 33, 35, 36, 37, 39, 42, 43, 44, Em, Tmp, and Satu
Virulent: Sr5, 6, 7b, 8a, 8b, 9a, 9b, 9d, 9e, 9g, 10, 11, 16, 17, 24, 30, 31, 34, 38, 41, and McN.
In seedling tests, 59 out of 103 South African cultivars tested were susceptible to race PTKST. It is believed that PTKST may be an exotic introduction into South Africa rather than a single –step mutation from local stem rust races.
Additional reports of wheat stem rust were received from several African countries. In August, wheat stem rust was reported to be widespread in Tanzania. Wheat growing areas in Northern provinces and the Southern highlands both reported the presence of wheat stem rust. In South Africa, wheat stem rust was reported to be widespread in the Southern and Eastern Cape regions during Sept-Oct. One 160 ha field reportedly suffered complete yield loss to stem rust. In Zambia, trace amounts of wheat stem rust were found on 3 breeding lines at Golden Valley Research Station in late October. In all countries, the identity of stem rust races present remains unknown.
In Yemen, reports were received indicating higher incidence of wheat stem rust, compared to 2008, in highland areas during October. No race information is currently available.