Reconstructing Puccinia graminis f. sp. tritici Genetic Diversity in South Africa Using Herbarium Specimens

In the absence of viable Puccinia graminis f. sp. tritici (Pgt) isolates described before 1980, 15 herbarium specimens collected between 1906 and 1945 and 13 isolates representing current South African races were used to investigate the genetic evolution of the local Pgt population. The study was restricted by complications commonly associated with herbarium specimens, namely, severely degraded genomic DNA and subsequent problematic PCR amplification. This resulted in us using data from only seven microsatellite markers for the genetic analysis, instead of the normal 18 to 20 markers. We, however, showed that the data were sufficiently informative to complete the genetic analysis. Despite a 36-year gap for which no isolates were available, four genetic lineages were identified. Based on collection dates, the first lineage spanned 39 years and included specimens collected from mid-African countries. The second lineage consisted of four herbarium specimens, but included the Australian Pgt race 126-5,6,7,11, thus supporting a southern African origin for this pathotype. The final two genetic lineages represented current South African Pgt races. While it is proposed that all four lineages were the result of an initial foreign incursion into the country, additional incursions contributed significantly to the two current genetic lineages. This confirms that exotic introductions remain the main contributor of genetic variation to the local Pgt population and emphasizes the importance of annual rust surveys in South Africa to quickly detect new variants, which could threaten local wheat production.