An international team of researchers found that wheat stem rust, a largely eradicated disease, is becoming an increasing threat to European wheat and barley production.
The study, published on Thursday in the journal Communications Biology, examined the potential for re-emergence of wheat stem rust in the United Kingdom. Wheat stem rust is a devastating disease that affects wheat and barley and is caused by a fungal pathogen. The disease was largely eradicated in Western Europe during the mid-to-late 1900s, but isolated outbreaks have occurred in recent years.
In 2013, Germany experienced its first major outbreak of stem rust in decades after high early summer temperatures followed an unusually cold spring. Three years later, bread and durum wheat were devastated by stem rust in Sicily, marking the largest European outbreak for many years. Researchers investigated possible causes of the resurgence, including lack of resistance in modern European varieties of wheat and barley, changes in climatic conditions, and increased presence of common barberry, a shrub that also hosts the disease.
Researchers found a single wheat plant in southern England affected with stem rust in 2013. It was the first wheat stem rust occurrence in the UK in nearly 60 years. An analysis of the stem rust isolate (UK-01), along with samples from Denmark, Sweden, and Ethiopia, showed that the strain likely spread from south to north across Europe and originated from a common source. Further analysis revealed that only 20% of UK wheat varieties are resistant to the UK-01 strain, leaving a vast majority susceptible to disease.
Scientists also examined how the replanting of barberry affects the risk of stem rust. As an alternate host for stem rust, barberry populations can provide a “seasonal bridge” in temperate zones and boost the pathogen’s genetic diversity. According to the report, dormant stem rust spores can overwinter and germinate in the spring to infect barberry, and then re-infect primary grass and cereal hosts.
During the late nineteenth and early twentieth century, barberry eradication was a huge success in the UK and drove wheat stem rust to almost complete extinction. Barberry planting is currently increasing, however, largely due to a habitat conservation programme for the endangered barberry carpet moth.
Researchers raised concerns that stem rust pathogens will use the increasing number of barberry plants as a host for sexual reproduction. In Sweden, the oat stem rust fungus, for which common barberry is an alternate host, substantially increased in genetic diversity after the repeal of the barberry eradication law in 1994. Sweden also recently reported the first occurrence in decades of a sexual population of stem rust occurring on barberry, “signifying a worrying turn for wheat stem rust in Europe”, according to the report.
The study also found that climate changes over the past 25 years suggest increasingly conducive conditions for fungal pathogen growth and subsequent infection of crops. The pathogen has the highest rate of infection when there are high temperatures as well as high moisture and light levels. Researchers found a trend of increasing risk up to 2006, with risk levelling off in the past few years, except in the very wet year of 2012.
Although climate change projections for 2050 predict slight warming of the central part of the wheat-growing area in the UK, wet conditions required for infection are unlikely to become more common. However, scientists are concerned that the pathogen’s ability to adapt through sexual reproduction on barberry plants could facilitate the emergence of strains of fungus that are able to infect crops in drier conditions.
To protect European cereals from a large-scale re-emergence of wheat stem rust, scientists recommended reinitiating breeding resistance into wheat and barley varieties. They also suggested implementing rules to prohibit the replanting of barberry near arable land, which would limit the pathogen’s ability to rapidly overcome any introduced resistance and climatic constraints.
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