Agricultural disease emerged from harmless virus

Scientists have discovered that one of the world’s most important agricultural diseases emerged when two relatively harmless wild grass infecting viruses mutated over 100 years ago.

Maize streak virus causes the main disease of Africa’s most important food crop. (Credit: USDA photo by Keith Weller.)

Maize streak virus (MSV) causes the main disease of Africa’s most important food crop. By comparing the genome of the virus to those of its less harmful relatives, scientists - from the University of Cape Town in South Africa - have discovered how and why MSV became a serious pest and spread so rapidly across Africa.

“Given the fragility of African agriculture and perpetual famine risks with millions of lives at stake, MSV is actually one of the most important plant pathogens worldwide,” said Dr Darren Martin. “We wanted to learn more about how the virus emerged and spread so we can develop new ways to fight the diseases it causes.”

The team compared the genomes of 11 different strains of MSV and discovered that lots of genes had been exchanged between the strains in a process called recombination. They also found that every MSV that causes severe disease in maize has descended from an ancestral virus that was the recombinant offspring of two relatively harmless wild grass infecting viruses. This chance recombination event could be the reason MSV has become such a serious problem.

"Understanding the chain of events that trigger the emergence of novel pathogens is a major goal of epidemiologists worldwide," said Dr Martin. "There is a lot of circumstantial evidence suggesting how evolutionary processes like recombination might trigger the emergence of a pathogen but there are actually very few well documented examples of this having occurred."

All available information suggests that over 100 years ago, two grass adapted MSVs recombined to produce a new "wide-host range" strain that could infect a greater variety of other plants than its parents. This meant the virus could survive the winters in wild grasses more effectively than its relatives - something that potentially increased the speed at which it spread across Africa. The ability of this recombinant strain to infect many different host species may have also enabled it to rapidly adapt to maize - a process that today has led to its emergence as an economically important crop disease.

The next step is to carry out rigorous tests to provide more evidence for the specific biological causes of the emergence of MSV. The researchers also hope to develop different genetic strategies to help the crops resist disease. The work appears in the Society for General Microbiology's Journal of General Virology.