Wheat stem rust recorded for the first time in decades in Ireland.

Wheat stem rust, caused by the fungus Puccinia graminis f. sp. tritici (Pgt), occurs in most wheat-growing areas worldwide, and, in western Europe since 2013, has started to re-emerge after many decades of absence. Following this trend across western Europe, in 2020, we also detected and recorded wheat stem rust for the first time in five decades in experimental plots across five locations in Ireland. To examine the potential origin of the Irish Pgt infection in 2020, we carried out transcriptome sequencing on 12 Pgt-infected wheat samples collected across Ireland and compared these to 76 global P . graminis isolates. This analysis identified a close genetic relationship between the Irish Pgt isolates and those from Ethiopia collected in 2015 after a severe stem rust epidemic caused by the TKTTF Pgt race, and with the UK-01 Pgt isolate that was previously assigned to the TKTTF race. Subsequent pathology-based race profiling designated two Irish isolates and recent UK and French Pgt isolates to the TKTTF Pgt race group. This suggests that the Irish Pgt occurrence most probably originated from recent long-distance windborne dispersal of Pgt urediniospores from neighbouring countries in Europe where we confirmed the Pgt TKTTF race continues to be prevalent. The identification of wheat stem rust in Ireland at multiple locations in 2020 illustrates that the disease can occur in Ireland and emphasizes the need to re-initiate local monitoring for this re-emergent threat to wheat production across western Europe.