Placement of South African stripe rust in a global context and development of diagnostic tools for genotyping field samples

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.