Wheat stem rust (Puccinia graminis f. sp. tritici) is historically the most damaging disease of wheat. Under suitable conditions, yield losses of 70% or more are possible. In 1999, a new virulent race of stem rust was identified from wheat fields in Uganda – popularly known as Ug99 after the year and country of discovery. The unique virulence associated with Ug99, or variants, has rendered a large proportion of global wheat cultivars susceptible.
Regularly updated situation reports on cereal rusts are given based on information provided by a global network of rust workers. It is important to note that not all reports of stem rust relate to Ug99 or variants – other local races are also included.
Out of Uganda: An Aggressive Crop Killer That Threatens Global Food
Fungal disease in wheat crops has been a serious but controllable problem, but a newer strain of what’s called “stem rust” has scientists worried.
January 8, 2018 by Kerstin Hoppenhaus & Sibylle Grunze
The video below is the first part in a six-part series examining the scourge of Ug99, a type of fungus that causes disease in wheat crops — one that scientists worry could threaten global food supplies. Visit our series archive for all published episodes.
There was a time when one of the most dangerous crop diseases a wheat farmer could encounter in the field was stem rust. It is caused by a fungus, and its spores look like flecks of rust on metal — first red, later black in color. The fungus spreads along stems and leaves of cereal plants, consuming nutrients and causing the grains to shrivel.
Crops affected by stem rust are often entirely destroyed, and until the 1950s, the fungus was able to wreak havoc on agriculture across the globe — including in the United States. Researchers eventually managed to identify strong resistance genes against the fungus, and successfully bred those genes into new plant varieties beginning in the 1960s, leaving the fungus all but forgotten.
A generation later, however a new strain of wheat stem rust appeared — this time in Uganda in 1998. This new strain, which scientists called Ug99 (Ug for the country where it was first discovered, 99 for the year when it was officially named), was immune to most of the known resistance genes — and it remains a threat today. It is more aggressive than most known stem rusts, and it evolves far more quickly. Indeed, where there was only one strain in 1999, there are now at least 13 new pathotypes of Ug99, and they are spreading fast.
“Why Ug99 is important, first of all is, because it has virulence for may resistance genes,” says Julio Huerta, a wheat breeder and plant pathologist with the International Maize and Wheat Improvement Center. “Second, it’s very aggressive. Extremely aggressive.”
“This is not a race that sleeps,” Huerta added. “That’s why we say rust never sleeps.”
Winds can carry the spores across borders, and scientists have now found Ug99 and its descendants throughout Eastern Africa, from South Africa to Egypt. Reports have also surfaced from Yemen and Iran, and the fungus probably won’t stop there.
Scientists worry that Ug99 will eventually spread further east and reach the wheat and barley breadbasket regions of India and China, and the consequences of this, they say, could be catastrophic — not only for local populations and economies, but for the world.
Coming on Thursday, Part 2: A Precious Crop Under Threat
Kerstin Hoppenhaus and Sibylle Grunze are the founders of Hoppenhaus & Grunze Media, a Berlin-based film production studio specializing in documentary coverage of science.
New, ground-breaking stem rust research is highlighted in the prestigious journal Science published on Dec 22 2017. A perspective entitled “The Quest for Durable Resistance” by Matthew J. Moscou and Peter van Esse provides context and outlines the significance of two research papers that independently identify two Avr effectors from the fungal pathogen Puccinia graminis f.sp. tritici, the causal agent of wheat stem rust. These research papers are considered a milestone in terms of improving our understanding the biology of rust pathogens.
In the first research paper entitled “Variation in the AvrSr35 gene determines Sr35 resistance against wheat stem rust race Ug99”, Andres Salcedo and colleagues identify a fungal gene named AvrSr35 that is required for Sr35 avirulence. The effector protein encoded by this gene binds to the Sr35 resistance protein and as a result activates the plant’s immune response. In the second paper entitled “Loss of AvrSr50 by somatic exchange in stem rust leads to virulence for Sr50 resistance in wheat”, Jiapeng Chen and colleagues identify the gene AvrSr50 that is required for Sr50 avirulence. Similarly this effector protein interacts directly with the Sr50 protein, triggering Sr50-dependant defense responses. In the case of both identified Avr effectors, removal or inactivation renders the plant susceptible to fungal attack.
The results reported in these two papers represent a significant step forward in terms of getting a much better understanding of how plants interact with the rust pathogen.
Related news stories can be found at:
Scientists at the Swedish University of Agricultural Sciences (SLU) are reporting that stem rust has returned to Sweden. In a recently released press release, lead researcher Anna Berlin from the Department of Forest Mycology and Plant Pathology outlines an unusually intense stem rust attack in wheat fields in Almunge, Uppland in the summer of 2017. The last major stem rust outbreak reported from Sweden was in 1951, so this latest outbreak and the initial indications that it is a sexual population emerging from barberry is a major cause for concern.
Full details of the report from the Swedish University of Agricultural Sciences (SLU) can be found at:
A team of scientists from Ecuador, Canada and the USA recently published on the detection of race RRTTF based on samples collected in Ecuador in February 2016. The study led by Dr Charlie Barnes from the Instituto Nacional de Investigaciones Agropecuarias (INIA), Quito, Ecuador has just been published in Plant Disease. Race RRTTF is distinctive by its combined virulence to genes Sr38 and Sr13 and poses a significant threat to wheat production in North and South America as a large proportion of current commercial cultivars are known to be susceptible. The origin of Pgt race RRTTF in Ecuador is unknown, but it is similar to previous isolates of RRTTF from Asia (Pakistan), eastern Africa (Ethiopia), and the Middle East (Yemen). It is unknown whether race RRTTF is a recent long-distance exotic introduction into Ecuador, or a de novo variant of an existing South America lineage that was introduced earlier. Further study is needed to determine how widespread this race is in Ecuador and its potential to migrate to large-scale wheat production areas in South and North America.
New research reveals for the first time the most likely months and routes for the spread of new strains of airborne ‘wheat stem rust’ that may endanger global food security by ravaging wheat production across Africa, the Middle East, Asia and the wider world.
Stem rust (or black rust), named for the blackening pustules that infect plant stems, caused devastating crop epidemics and for centuries before being tamed largely by use of resistance genes.
Since the turn of the century, however, aggressive new strains have emerged – such as ‘Ug99’, first detected in Uganda in 1999 – that infect widely grown varieties of wheat. These diseases threaten to disperse trillions of pathogenic fungal spores on winds across countries and continents.
The fear is that these airborne and highly virulent strains could spread from known sites to some of the world’s most important ‘breadbasket’ regions, such as the Punjab in South Asia, where these strains have not yet been detected.
Now, a team of scientists of the University of Cambridge, the UK Met Office and CIMMYT (International Maize and Wheat Improvement Centre) have adapted modelling systems previously used to forecast, ash dispersal from erupting volcanoes and radiation from nuclear accidents (NAME), to predict when and how Ug99 and other such strains are most likely to spread.
The research, published today in the journal Nature Plants, quantifies for the first time the circumstances – routes, timings and outbreak sizes – under which dangerous strains of stem rust pose a threat from long-distance dispersal out of East Africa to the large wheat-producing areas in India and Pakistan.
The results highlight the role of Yemen as a potential ‘stepping stone’ for the transmission of the disease between continents. The key scenario for disease spread is from Yemen directly to Pakistan or India. In case of a large outbreak in Eastern Yemen results indicate a 30% chance for transmission to occur.
Another important scenario for wheat rust to spread is from Yemen through Middle Eastern countries, in particular Iran, to Central and South Asia. If Iran were to suffer a moderate outbreak of Ug99 – on more than 1000 hectares – then spores would likely spread to Afghanistan, and from there potentially further to the northern plains of Pakistan and India. However, transmission along this route is restricted to a relatively short time-window in March and April, before wheat is typically harvested in South Asia.
“New races of wheat rust are threatening wheat worldwide, and we need to know which areas are at risk,” said senior author Prof Chris Gilligan, from Cambridge’s Department of Plant Sciences.
“From our work, we now believe that if we start to see Ug99 or other new wheat rust strains take hold in Yemen in early Spring then action must be taken immediately to mitigate the risk of further spread.”
Network map of the atmospheric transmission of spores causing wheat stem rust.
(A) Long-distance dispersal network of spores between all major wheat producing countries in Southern/East Africa, the Middle East and Central/South Asia. Nodes represent countries; communities of the same colour indicate regions with high airborne connectivity; the size of nodes indicates node-strength; pie charts show the fraction of out-strength to total node strength (indicating donor and receptor countries).
(B) Spore transmission frequencies along principal migration routes in the Rift Valley zone for the scenario of large outbreaks.
However, the modelling work also offers some encouraging news: the airborne transmission of the disease from East African countries directly to South Asia is highly unlikely, with transmission events possible only on less than one day a year.
The scientific team used field disease surveys from the International Maize and Wheat Improvement Centre (CIMMYT) and weather data from the UK Met Office as key input for the modelling framework.
“This research has allowed us to obtain the first quantitative estimates of long-term airborne spore transmission frequencies for different outbreak scenarios. We compiled risk assessments for pathogen dispersal from key disease locations to important wheat-producing countries. These assessments can effectively inform surveillance and control strategies,” said Cambridge’s Marcel Meyer, the study’s first author.
The team say their work, including 3-D spore dispersal animations and a catalogue of spore dispersal trends (indicating likely directions, frequencies, pathogen loads), provides new ways to raise awareness, communicate risks, and inform agricultural stakeholders.
Their modelling framework can be applied as a tool to analyse risks in case new disease strains should be uncovered in other geographic areas. This has already recently helped in estimating dispersal risks from detection sites of related wheat rust diseases in Europe and Siberia. In ongoing work the team is developing an Early Warning System forecasting disease risk in Ethiopia, East Africa’s largest wheat producing country.
“The combined expertise from plant sciences and atmospheric dispersion sciences has delivered ground breaking tools that highlight the risks, and support the management of the devastating potential of these diseases,” said Dr Matthew Hort, co-author from the UK’s Met Office.
Source: University of Cambridge
Chris Gilligan, University of Cambridge: +44 7964 598 147; firstname.lastname@example.org
Marcel Meyer, University of Cambridge: email@example.com
Matthew Hort, UK Met Office: +44 (0)1392 886242; firstname.lastname@example.org
Dave Hodson, CIMMYT-Ethiopia: email@example.com