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.
By: Dave Hodson, Mogens Hovmøller, Alexey Morgunov, Elena Salina and Vladimir Shamanin
3 September 2017
During the period 14-18th Aug 2017 a field trip was made to the Omsk, Novosibirsk and Altai Krai regions of Western Siberia, Russian Federation by CIMMYT and the Global Rust Reference Center (GRRC), Aarhus University. Organised by Dr. Alexey Morgunov (CIMMYT-Turkey), the visit aimed to get more information on the recent reports of large scale stem rust outbreaks covering millions of hectares in the region (Shamanin et al., 2016). It provided the opportunity to meet with the leading wheat scientists in the region and visit key wheat research institutes to gain a better understanding of the current stem rust situation. With approximately 7 million ha of short season high latitude spring wheat grown in Western Siberia, along with smaller – but increasing areas of winter wheat and some durum wheat, the region is an extremely important wheat production area.
In the Omsk region, most field plots and commercial fields were 1-2 weeks prior to harvest time (90-100 day growing season) whereas wheat crops were close to maturity or mature in the Novosibirsk and Altai regions (80-90 day growing season). 2017 proved to be a non-epidemic year for stem rust, but even so the disease was universally present at every site visited. Bread wheat, durum wheat and barley were all affected, especially late maturing materials. Several grass species were also affected and barberry, including B. vulgaris, was relatively common in urban areas. Some resistant materials were present, but several susceptible or highly susceptible lines or varieties were observed in the trials visited (Photo 1). Stem rust was not considered economically important until 2015 when a local epidemic occurred in the Omsk region of Russia and neighboring areas of Kazakhstan and affected more than 1 million ha. Stem rust occurred again in 2016 though the spread, severity and losses were less (Shamanin et al., 2016). The weather in 2015 and 2016 was conducive for stem rust – not too hot and dry in June (which is often the case) followed by moist and warm weather in July.
The trip started in Omsk with visits to the research trials of the Omsk Agrarian State University (Photo 2) and the Siberian Research Institute of Agriculture. This region was of special interest, as in 2015 wheat production in this area was affected by a major stem rust epidemic on an estimated 1 million ha. The biggest outbreak in this region in recent history. In 2015, stem rust appeared early with symptoms visible in the field at the end of July (heading). Typically in the past stem rust would not appear until mid August, too late to build up and cause any damage.
The 2017 season proved to be a more typical year with hot and dry conditions and absence of June rains. As a result stem rust had only appeared after the first week of August, too late to cause any damage in commercial fields. Trials in commercial production areas approximately 120 km south-east of Omsk were affected by drought stress and only trace levels of stem rust were present. Trial sites close to Omsk city had significantly higher incidence and severity of stem rust. Some late planted or late maturing susceptible materials were showing reactions up to 40-50S. Stem rust was observed on bread wheat, durum wheat and barley. Grasses, notably bromus sp were also observed to be infected with stem rust. Leaf rust was prevalent throughout the trial sites visited. Trace amounts of yellow rust were detected at one site – an extremely unusual occurrence for this region and so far not reported according to our local hosts. Barberry, including B. vulgaris, was present in urban areas – commonly planted as hedges. But apparently it was rare or absent in natural forested areas. Previous sampling of stem rust in the Omsk area in 2016 has revealed high race diversity for stem rust (14 races from 14 samples – see GRRC report). Samples of stem rust from the 2017 season will be analysed by the GRRC in Denmark.
In research trials of the Institute of Cytology and Genetics, Novosibirsk, stem rust was universally present, but incidence and severity was lower than in Omsk. In this region, it was considered that stem rust was appearing too late to cause any economic damage and no large scale epidemics were reported. A randomly surveyed commercial field on the way to Barnaul, revealed low levels of stem rust on late tillers (and also grass sp.) but the crop was close to maturity and uneffected by stem rust.
Altai Krai Region
Research trials of the Altai Research Institute of Agriculture, Barnaul were visited (Photo 3). As at previous locations, stem rust was universally present throughout the trials. But as with other locations, in 2017 it appeared late and was not a problem in commercial production fields. Late planted durum materials on station were heavily affected by stem rust.
In 2016, the situation reported was entirely different in this region. Rains in June (and July) combined with warm temperatures had resulted in stem rust appearing in mid July. An estimated 2 million ha were considered to have been severely affected, resulting in an estimated 30% total production loss in the region. Increasing areas planted to winter wheat were also considered another potential factor that may be influencing the stem rust cycle in the region, giving the pathogen an opportunity to move from maturing spring wheat to emerging winter wheat at the end of the season. Control efforts in 2016 were compounded by the unusual rainfall, making it difficult to undertake spraying operations. An example was cited of two adjacent fields both growing the same variety; one field was able to be sprayed with a resultant yield of 3 t/ha, the other was unsprayed resulting in a yield of 1.7 t/ha.
The visit highlighted the importance of wheat in this region and the strength of the research programs, but also the vulnerability of grown varieties to stem rust. Significant changes appear to have occurred in recent years, making stem rust an emerging disease of economic concern. Further research is urgently needed, both to understand the pathogen dynamics and also to increase the proportion of resistant varieties. The scale of the reported epidemics if weather conditions are suitable, coupled with the apparent high race diversity may have serious implications for neighboring regions and beyond.
Sincere thanks are given to all the wheat scientists at all the institutes visited for the time they dedicated to show their research activities and for the wonderful hospitality that was offered throughout the trip.
Shamanin, V., Salina, E., Wanyera, R. et al. Euphytica (2016) 212: 287. https://doi.org/10.1007/s10681-016-1769-0