Epidemics of wheat stem rust (Puccinia graminis f. sp. tritici; Pgt) in the UK and Southern Ireland have, to date, been consigned to historical records with the most recent outbreaks dating back to 1955 and 1963 (Hogg et al., 1969). The apparent eradication of wheat stem rust in the UK and Ireland has been widely attributed to the removal of its alternate host (Berberis vulgaris) and introgression of resistance genes into cultivated wheat varieties during the Green Revolution. However, in the past decade wheat stem rust outbreaks have once again started to reoccur within the more temperate climes of Europe, including (but not exclusive to) Germany and Denmark (2013), Sicily (2016), Sweden (2017), Italy and Spain (2019). In the UK we also recorded the first occurrence of wheat stem rust in around 60 years, identified on a single infected plant in Suffolk in 2013 (Lewis et al., 2018). In most cases in western Europe, outbreaks have been largely attributed to the spread of the Digalu wheat stem rust race (TKTTF). Accordingly, the isolate identified in Suffolk in 2013 (UK-01), also pathotyped to the TKTTF race.
Since 2013 we have recorded sporadic outbreaks of wheat stem rust in the UK and just last year (2020) it was identified for the first time in five decades in experimental plots across multiple locations in southern Ireland. We have shown through genetic analysis and pathotyping of stem rust isolates purified from these occurrences that all isolates so far have typed to the TKTTF race. This illustrates that the TKTTF race is also endemic in this region of Europe. Although in each instance only a handful of plants were infected, the concern is that, given the right conditions, epidemics could arise. The German epidemic of 2013 established after wet winter conditions and an early onset of summer temperatures favoured wheat stem rust (Olivera Firpo et al., 2017), a climatic trend that is becoming more commonplace year on year.
In the UK and Southern Ireland, the asexual Pgt urediniospores are unable to survive during the cold winter months. Thereby, infections can be started each year either by (i) asexual spores blowing in from regions with warmer climates or (ii) from its overwintering sexual stage completed on Berberis. To understand the role of Berberis in acting as a source of stem rust inoculum in the UK we also conducted a physiological survey to identify and genotype rust aecia found on Berberis across the UK. Interestingly, we recovered a wheat stem rust isolate found on B. vulgaris in Scotland, that in pathology tests also typed to the TKTTF race. This illustrates that the TKTTF race is completing sexual reproduction and overwintering in the UK. When combined with our analysis showing that climatic conditions are becoming more conducive to stem rust growth and infection and that more than 80% of commonly cultivated UK wheat varieties are highly susceptible to the TKTTF race (Lewis et al., 2018), the potential threat of stem rust re-establishment in western Europe is of critical concern and emphasises the need for continuous monitoring in this region.
