Wheat stem rust: the return of an old foe
The reappearance of stem rust on wheat in Ireland after decades of no reports led researchers to investigate the cause behind recent outbreaks of the disease.
In early July 2020, unusual rust pustules were observed in control plots of winter wheat across a number of locations in Ireland. Unlike the typical yellow rust or brown rust pustules that are regularly observed, these were darker and larger in appearance, and were observed both on leaves and – more importantly – on the stems of wheat.
A further detailed look into the characterisation of these pustules confirmed the worst – these were wheat stem rust pustules. Whilst stem rust itself can be found on numerous grasses in Ireland from mid-summer onwards, the last reports of stem rust on wheat are from the early 1960s.
Its identification in 2020 came as quite a surprise, leading researchers at Teagasc and the John Innes Centre in the UK to investigate further.
The most serious of wheat diseases
Wheat stem rust is caused by a fungal pathogen, and is regarded globally as the most socio and economically destructive wheat disease throughout the majority of wheat-growing regions. The disease is recognisable by its oval shaped pustules that appear on the stems and leaves of both wheat and barley, extruding dark orange-red spores.
By bursting through the leaf or stem, these pustules severely restrict the flow of nutrients to the developing grains. They have the potential to destroy susceptible varieties and inflict significant yield losses.
The pathogen survives the cool winter months in climates such as Western Europe by developing hardy teliospores (the thick-walled resting spore of rust fungi). The development of these teliospores in the latter stages of the disease cycle often give a distinctly black appearance to infected stems, hence the disease is often commonly referred to as black stem rust.
However, for the teliospores to successfully lead to further infections, they require an intermediary host – most commonly, barberry plants. The identification of barberry as the key intermediary host of wheat stem rust allowed control strategies to be developed, limiting the development of the disease by restricting its overwintering capacity. For centuries, this has been the cornerstone of wheat stem rust control.
The removal of barberry combined with changes in arable (land used or suitable for growing crops) practices associated with more intensive production, meant the once endemic disease was no longer seen in Irish crops. That is, until two years ago.
Investigating the 2020 outbreaks
The detection of wheat stem rust at numerous locations in the southern half of Ireland in 2020 was both unexpected and worrying in equal measure. As most current European varieties of winter wheat are deemed susceptible to the disease, it was important to determine if the strains causing this infection were novel and posed a significant threat to production.
To answer these questions, detailed characterisations of its genetic material and causes of the disease were conducted. These included conducting virulence (ability to infect) screens on stem rust isolates obtained from these outbreaks, using an internationally recognised panel of varieties known to carry differing resistance genes. In parallel, novel pathogenomic techniques (high-throughput sequencing technology and bioinformatics) were applied to determine the specific genetic make-up of the strains.
Pathologically, the isolates of stem rust obtained from the Irish outbreaks were assigned to the TKTTF race. This race has also been detected in recent European outbreaks, including those in the UK and Germany. The assignment to this race was further supported by analysis conducted following RNA-sequencing as part of the pathogenomic approach used. It placed the Irish outbreaks amongst those previously reported in the UK and Germany, but also significant outbreaks that occurred in Ethiopia in 2015.
An ongoing threat
Combined with the fact that no barberry was observed in the local vicinities where recent infections occurred, the findings of this analysis suggests that the appearance of stem rust in Ireland in 2020 resulted from long-distance dispersal of wheat stem rust spores. It also highlights that although wheat stem rust has been absent for well over half a century, it can occur at any time in modern wheat production systems in Ireland.
Given the limited resistance available in commercially available varieties, care is needed to ensure it doesn’t become a significant threat in the future.
Did you know?
Because of barberry’s susceptibility to being a host of wheat stem rust, it was included in the Irish Noxious Weed Act of 1958, making it an offence to not remove barberry identified on your land.
The last reports of wheat stem rust, before it’s recent re-emergence, are from around 60 years ago.
This project is primarily funded by the UK Biological Sciences Research Council (BBSRC; BB/S003975/1) and the European Union’s Horizon 2020 research and innovation programme under grant agreement number 773311 (RustWatch).
Additional funding was provided by the European Research Council (number 715638), BBSRC Institute Strategic Programmes BB/P012574/1 and BB/P016855/1, the John Innes Foundation and Teagasc (project 0154).
We’d like to thank Kerstin Flath (Institute for Plant Protection in Field Crops and Grassland, Germany) for vital contributions to the study, all those who assisted with sample collection and processing and Fiona Hutton for lesion photography.
Ayako Tsushima, Postdoctoral Researcher, John Innes Centre, Norwich Research Park, Norwich.
Clare Lewis, Postdoctoral Researcher, John Innes Centre, Norwich Research Park, Norwich.
Diane Saunders, Group Leader, John Innes Centre, Norwich Research Park, Norwich.
Steven Kildea, Senior Research Officer, CELUP, Oak Park. email@example.com
This article featured in the TResearch Spring 2022 Magazine. TResearch is an official science publication of Teagasc. It aims to disseminate the results of the organisation’s research to a broad audience. Find out more about Food in Teagasc here