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Title: First report of a variant within wheat stem rust (Puccinia graminis f. sp. tritici) race TTKSF in South Africa and Zimbabwe

item PRETORIUS, Z - University Of The Free State
item Szabo, Les
item BOSHOFF, W.H.P. - Pannar
item HERSELMAN, L - University Of The Free State
item VISSAR, B - University Of The Free State

Submitted to: Disease Note
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2012
Publication Date: 4/1/2012
Citation: Pretorius, Z.A., Szabo, L.J., Boshoff, W., Herselman, L., Vissar, B. 2012. First report of a variant within wheat stem rust (Puccinia graminis f. sp. tritici) race TTKSF in South Africa and Zimbabwe. Disease Note. 96(4):590.

Interpretive Summary: Ug99 (race TTKSK), a highly virulent race of the wheat stem rust fungus, was first observed in east Africa. Since this discovery six additional variants of this race have been identified. A new race (TTKSF+) of the wheat stem rust fungus with virulence to the winter wheat cultivar Matlabas, was identified in South Africa and Zimbabwe. Pure strains of this pathogen were characterized using 45 wheat stem rust differentials and showed identical avirulence/virulence phenotype to race TTKSF. The only difference in the phenotype between TTKSF and TTKSF+ is this new strain is able to overcome an unknown resistance gene in the wheat cultivar Matlabas. Molecular analysis indicated that this new strain belongs to the Ug99 race group and is genetically distinct from the strain of TTKSF previously characterized from South Africa. This discovery increases the number of races in the Ug99 family to eight, further emphasizing the variability within this internationally important race group. This research provides additional information and biological resources for scientists studying the evolution of races in the wheat stem rust pathogen. In addition, this work provides important information to agricultural professionals and farmers on resistance of wheat cultivars to the Ug99 race group.

Technical Abstract: Seven races have been described in the Ug99 lineage of Puccinia graminis f. sp. tritici (1). Variants of Ug99 previously recorded in South Africa are TTKSF, TTKSP and PTKST (2). In December 2010, severe stem rust infection was observed on the winter wheat cultivar Matlabas at Afrikaskop in the eastern Free State, South Africa. Race analysis using the 20 North American (NA) stem rust differential lines and letter code system classified the race as TTKSF. In comparative infection studies in a greenhouse, Matlabas seedlings were susceptible (infection type [IT] 4) to isolate UVPgt61/1 (TTKSF+) collected from Afrikaskop, whereas the cultivar was resistant (IT 12) to stem rust isolates 2013 (TTKSF), UVPgt55 (TTKSF), UVPgt59 (TTKSP) and UVPgt60 (PTKST). Isolate 2013 represents the original collection of race TTKSF in South Africa (3). In addition to the NA differentials, no differences in the seedling response of lines with Sr7a, 8b, 12, 13, 14, 16, 18, 19, 22, 25, 26, 27, 28, 29, 32, 33, 34, 35, 39, 41, 42, 43, 44, Em, R, Tt2, and Satu were observed between UVPgt61/1 and UVPgt55. With the exception of Matlabas, a collection of 106 South African cultivars responded similarly in their seedling ITs to UVPgt61 and UVPgt55. All seedling IT studies were conducted at least twice. Microsatellite analysis using unpublished primers (2) showed that all single pustule isolates established from the original Matlabas isolate formed part of the Ug99 lineage. When characterized with selected single nucleotide polymorphisms (SNPs), all single pustule isolates shared an identical genotype that differed from UVPgt55 (TTKSF), a foreign introduction into South Africa (3; 4). On phenotypic level TTKSF+ thus represents a single step variant from TTKSF with the acquisition of virulence to the unknown Sr gene in Matlabas. However, SNP genotype analysis suggests that UVPgt61 is genetically dissimilar to UVPt55, as is Zim1009, another TTKSF+ isolate that was recently collected from Birchenough in Zimbabwe. Studies are currently underway to determine the identity of the Sr gene in Matlabas and the cultivar has been added to the South African differential set. TTKSF+ is the eighth variant detected within the Ug99 family, emphasizing the variability within this internationally important race group.