As we have seen so often in recent years when it comes to human pathogens, eradicating a disease from a major population, country, or continent does not mean those results will be permanent forever. Any sort of laxity in preventative treatment and strict efforts against global spread can result in the re-emergence of a deadly illness. The same is true of agriculture and wheat farmers have had to deal with this issue first hand over the past few years.
A Rusty History
Wheat stem rust has been a devastating condition for wheat and barley farmers to combat for arguably thousands of years, ever since the first agricultural cultivation of the crops. The fungus Puccinia graminis has preyed on the wild forms of the plants for long before that and can wipe out whole fields. Luckily, the spread of the disease has never been too severe, even with the specialized variety of the fungus with the extra additional species classification of tritici for the stem rust it causes, with other niches of the same fungus targeting other body parts.
Tackling it has been a huge priority over the past century and Green Revolution advocates held stem rust as a necessity to wipe out if food production was to be sustainably increased. Thanks to their efforts, countries in western Europe were successful in completely removing it from the farms and from the wild. They did this through a combination of strict quarantine and control of wheat and barley growth and, arguably more importantly, removing the secondary host the fungus was using to stick around longer than it was wanted.
This host is the barberry shrub, a genus of small evergreens found throughout the world. Since they can also be infected by P. graminis and will then spread that infection to any nearby susceptible plants, primary removal of any barberry near agricultural lands was a strong concern for control of the fungus. Reducing its reproductive possibilities also slows its rate of diversification and resistance to control methods. These tactics allowed farmers in the United States to lower the number of P. graminis races from 17 to 8 over time, though they were never able to completely destroy it like some European nations.
A Rust Revival
In total, these actions improved conditions in Europe and especially the UK over a period of decades, with the last known outbreak of the fungus in the latter country happening in 1955. That would have been the end of the story in most cases, but we’ve unfortunately got a case of a zombie disease on our hands.
In 2013, a major outbreak happened in Germany, even though they hadn’t seen any cases of P. graminis for years. That same year, a single infected wheat plant was found in the UK, stoking fears of an additional revival across the rest of Europe as well. 2016 saw the most massive outbreak to date in the region, erupting on the island of Sicily off the coast of Italy.
The following year in 2017, a new strain of the fungus was discovered in Sweden, where it has apparently been multiplying in barberry bushes. All of this coincided with the rise of a new race group of the species in Africa labeled Ug99 that was far more virulent and deadly than any other seen before it. Would this strain also travel north to Europe and destroy the wheat harvests for years to come?
The Spread of Rust
Researchers at the John Innes Centre shared concern for this outcome and decided to look into the one infected plant that appeared in the UK, along with going over the wheat cultivars currently in use in the country to see how many are at risk for infection if exposed to the fungus. After obtaining an isolate of the fungus on the infected plant, they ran a population genetic analysis to compare it to the 42 other isolates from sixteen countries. This included a full genomic and transcriptomic comparison. The isolate dubbed UK-01 was found to be most closely related to outbreaks in Ethiopia in 2014 and with isolates obtained in Sweden and Denmark in 2013, implying that the wheat stem rust outbreaks there were from the same source and they do appear to have originated in Africa.
The fungal race in question appeared to be particularly fond of infecting a bread wheat cultivar named Digalu and it seems to have originated in Turkey, before spreading to the Middle East, parts of Africa and, as can be seen from their results, apparently Europe where it also caused that original German infection.
The following step was to find out how affected the UK wheat cultivars would be if a broader plague of the isolate (or even worse ones) happened beyond just a single plant. UK-01 was tested against the 43 varieties on the UK’s Recommended list of wheat breeds, along with 14 other more ancient breeds still in use today. Of those 57 in total, 39 had a high infection rate during the seedling stage, 9 had an intermediate response, and 11 had some amount of resistance. Therefore, only about 20% would have any possibility of putting up a fight against the fungus.
Secondary Hosts of Rust
In the years since the presumed erasure of wheat stem rust in the UK, there has been a social incentive to plant more barberry bushes, as they are the home for the endangered Barberry Carpet Moth. The act of planting more, however, has given new possibility of them acting as a spreading force for any new form of P. graminis that comes to the UK. Therefore, in June of 2017 in the course of making this study, the researchers went through barberry bushes throughout England, especially those in close proximity to wheat or barley fields.
They were able to successfully identify a hedgerow infected with the fungus and that was growing within a meter of a barley farmer’s crop. When genomically tested, it was found to be a different race than the one studied before, but still highly capable at infecting barley. Their suggestion is to destroy any barberry near agricultural areas and to push for social conservation efforts in planting them in wilderness regions that will help the carpet moth, but not have a meaningful likelihood of causing spread of the fungus.
Climate and Rust
The final test they made was to create a probabilistic model in order to see if the impact of climate change would accelerate the growth and spread of P. graminis. Since warm temperatures and strong sunlight are requirements for its growth, they focused their model on tracking the summer months. One benefit is that wet conditions are also needed along with the warm temperatures, so an increased dryness would not heavily improve the fungus’ chances. Overall, they saw some increase in fungal viability from 1996 to 2016, but models based on future changes through 2050 did not seem to indicate an increase in wet periods during the summer. Thus, in this instance, climate change does not appear to be a large influencer on what happens with P. graminis’ spread.
There is a caveat to note though, as the new strain in Africa, Ug99, appears to be much more aggressive in general and toward cooler temperatures than other known races. So it may not be deterred and could spread during other months as well. To combat this, the scientists heavily suggest a return to resistance breeding methods against the fungus and to expand the amount of planted wheat and barley that are of cultivars with some resistance. Crossbreeding and gene editing transfer of the resistance genes to other cultivars should also be considered so as not to concentrate the types of grown cultivars too much.
Protecting Against Rust
Even with those protection efforts being taken now, wheat stem rust remains a threat to food production in Europe and the emerging Ug99 strain is a concern for the entire world. Hopefully it can be contained in time and prevent the possible famines that would result if it broke free onto the world stage. We can only do our best to stop that from happening and then see what comes of it.
Photo CCs: Stem rust close up from Wikimedia Commons