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Most South African winter wheat varieties display all stage resistance (ASR) to stem rust caused by Puccinia graminis f. sp. tritici (Pgt). To study inheritance, four resistant varieties were crossed to a susceptible parent (Line 37) and F2 populations were phenotyped at the seedling stage with stem rust race PTKST (Ug99 lineage). Populations derived from varieties Koonap, Komati, Limpopo and SST 387 segregated in a 3:1 ratio, indicating that a single, dominant gene confers resistance in each population. Assessment of F2 seedlings of four intercrosses between these varieties failed to deliver susceptible segregants therefore suggesting that they carry the same resistance gene. Genotyping of F2 plants with microsatellite markers produced consistent linkage of resistance with markers on chromosome 6DS. Experiments are underway to determine the relationship between resistance in the four winter wheat varieties and resistance genes Sr42, SrCad and SrTmp, all located on 6DS. Current evidence shows that ASR in the South African winter wheat varieties Koonap, Komati, Limpopo and SST 387 is based on a single gene and thus vulnerable to pathogenic adaptation in Pgt.
Stripe (yellow) rust, caused by the fungus Puccinia striiformis f. sp. tritici (PST), is a major global wheat disease. New PST strains that show higher infection rates and rapid adaptation to less favourable environmental conditions have been observed over the last 15 years. It has also continued to spread to areas where it was not previously recorded. In South Africa, stripe rust was first detected in 1996. In subsequent years three more PST races were observed, with what seemed to be a step-wise virulence gain. A better understanding of the South African PST pathotypes and how they fit in the global context is needed. We aimed to address this by sequencing the genomes of four historical PST isolates displaying the four distinct virulence profiles. This allowed us to characterise the genetic diversity between these stripe rust races and develop diagnostic markers to easily genotype current detections. We also placed the South African PST isolates in context with global PST isolates where sequence data was available. This analysis illustrates that the South African PST races are more closely related to PST from other African countries when compared to isolates from Africa, Europe and Asia. Through pairwise comparison of isolates, we identified 27 candidate effector genes showing specific polymorphisms between the four isolates that could be related to their distinct virulence profiles. We are currently undertaking gene expression profiling of these candidates to determine if these effectors are specifically upregulated during infection–a key characteristic of effector genes. This study has shed new light on the potential origin and adaptation of stripe rust in South Africa and provides tools for rapid genotypic classification of infections in the field.