The global spread of wheat disease worries scientists

A wheat plant infected with pestilence (left) is compared to a healthy wheat. Photo credit: Nature and Science/Alamy

Scientists warn that a fungal pathogen threatening wheat production in South America is about to spread globally.

On April 11, researchers reported in PLoBiology that outbreaks of wheat blast in parts of Africa and Asia originated in a family of fungi from South America. Scientists warn that the lineage could spill over into other places or develop worrisome traits such as antimicrobides.

“This is a very serious disease that could spread around the world and threaten wheat cultivation in some of the poorest parts of the world.” Study co-author Nick Talbot, a plant pathologist at the Sainsbury Laboratory in Norwich, UK, said at a recent press conference.

Rice blast bacteria infect wild and cultivated grasses, especially rice and wheat. In the 80s of the 20th century, researchers first detected this pathogen in wheat crops in Brazil. Since then, the fungus has spread in South America, causing periodic outbreaks and limiting wheat cultivation in some areas.

In 2016, Bangladesh recorded the first outbreak of wheat blast in Asia, which researchers determined was caused by a lineage of wheat blast that is circulating in South America. Two years later, rice blast bacteria first reached Africa, striking Zambia’s wheat crop. But it is unclear whether the pathogen came from Bangladesh or South America.

To pinpoint the source of the pathogen, the researchers analyzed 84 genetic markers from more than 500 samples of the rice blast and sequenced the genomes of 71 isolated samples. Epidemics from Zambia in 2018 and Bangladesh in 2016 belong to different clades of the wheat blast lineage endemic in South America. This conclusion echoes the findings of a preprint study published last June that analyzed the genomes of wheat blast isolates in Africa, Asia and South America.

Hernán Burbano, an evolutionary geneticist at University College London, who participated in the study, said this suggests that wheat blast strains from South America independently reached Africa and Asia, “and also that people are spreading the pathogen in some way”.

“It was a big surprise.” Bruce McDonald, a plant pathologist at ETH Zurich in Switzerland, said. Although he was not involved in the study, he also believes that human activity is to blame.

Researchers say imported infected seeds may have been the cause of the outbreak. Bangladesh imported large quantities of wheat seeds from Brazil a year before the wheat blast outbreak, but the exact source of the pathogen is unknown. Related lineages have also been found in Brazil and Bolivia.

Tofazzal Islam, an ecochemist at Bangladesh’s Bangabangdu Sheikh Mujibur Rahman Agricultural University, said at the launch that researchers are currently using genomic information to monitor the spread of wheat blast across Bangladesh. His team is breeding wheat crops that are resistant to the lineage of pathogens behind the epidemic.

But genomic analysis also identified a threat. While the strain that caused the outbreak remains sensitive to certain fungicides, laboratory experiments have shown that spontaneous mutations may develop drug resistance. This strain, which reproduces by cloning itself, can acquire new traits by mating with another rice blast lineage. The researchers found that wheat blast strains can mate with rice blast strains that infect African millet crops.

McDonald said efforts should be made to eradicate wheat blast in Bangladesh and Zambia. But he is skeptical that genomic surveillance can largely slow the spread of fungi. Global funding cuts mean fewer scientists are paying attention to and reporting on emerging plant diseases, he said. “Global surveillance is woefully inadequate to detect new disease outbreaks before they occur.” (Source: Li Huiyu, China Science News)

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