So wheat researchers around the world have long been on their guard. And in early 2016, they received a major jolt.

Ghostly white heads appeared across widely scattered wheat fields in Bangladesh, at the edge of the great wheat-growing breadbasket of the north Indian plain. Plump grains that should have been arrayed in neat green rows at the top of each plant were now shriveled and dry. Reports came in from research stations, agricultural extension agents, and farmers across the country’s southwestern region. Often 20 percent of the crop was lost in affected areas, and in some fields, 100 percent. Bangladeshi wheat researcher Paritosh Kumar Malaker and his colleagues soon diagnosed wheat blast as the cause.

The outbreak was doubly frightening. For one thing, wheat blast on that scale had never appeared outside South America, where the disease was first reported in 1985. Wheat blast is thought to have originated in Brazil when agriculture expanded into areas already harboring isolated colonies of Magnaporthe on wild grasses. Unknown at the time, the colonies included a mutation that could infect wheat under the right conditions; those conditions arose when farmers planted a newly developed, high-yield variety of wheat that only decades later was found to lack a gene resistant to blast.

No one in Bangladesh knew how wheat blast had jumped two oceans, but now it had a foothold on the Asian continent. In March, reports came out that wheat blast had surfaced in the Indian region of Bengal. Should the disease spread widely across Bangladesh and India, and into Pakistan, wheat blast could devastate a crop that feeds more than 1.5 billion people. And waiting to the north is China, the world’s largest wheat producer.

“The government of Bangladesh has been very proactive in taking remedial steps in mitigating the threat of this new disease in the region,” said Malaker, chief scientific officer at the Wheat Research Center at the Bangladesh Agricultural Research Institute in Dinajpur. The government advised care in importing wheat grain or seed, and imposed restrictions on seed purchased from the affected districts. To break the chain of infection, it also discouraged sowing wheat there while encouraging farmers to plant alternative crops like peas, beans, lentils, mustard, or maize, Malaker said. Bangladeshi scientists are also cooperating with colleagues from around the world to understand and control the epidemic.

Genomic analyses of Magnaporthe samples from Bangladesh found that they were clones of one another, indicating a common source, and were virtually indistinguishable from certain South American strains, according to Barbara Valent, a professor of plant pathology at Kansas State University and a leader of a team of researchers from the United States, Bangladesh, and the International Maize and Wheat Improvement Center in Mexico. (Molecular analysis by an independent group of researchers based in Europe, OpenWheatBlast, came to the same conclusion.) Officially, the question of how blast arrived in Bangladesh remains unresolved, but some outside observers believe that grain imported for food was partly diverted for planting.