Scientists use nuclear fallout software to predict spread of devastating crop disease

STEM RUST: The once-devastating disease is staging a comeback

The disease in question is stem rust, a fungal disease which blighted farm land and caused famine for centuries before being tamed by fungicides and resistance genes.

At the turn of the last century, new aggressive strains of stem rust began emerging, including Ug99, so named because it was detected first in Uganda 18 years ago. Since then, these strains have begun infecting widely grown varieties of wheat resulting in a global economic loss of approximately $1 billion every year.

The team of scientists from Cambridge University, the UK Met Office and the International Maize and Wheat Improvement Centre (CIMMYT) published the results of their study in the journal Nature Plants.

It quantifies for the first time the circumstances – routes, timings and outbreak sizes – under which dangerous strains of stem rust pose a threat from long-distance dispersal out of East Africa to the large wheat-producing areas in India and Pakistan.

The results highlight the role of Yemen as a potential ‘stepping stone’ for the transmission of the disease between continents. The key scenario for disease spread is from Yemen directly to Pakistan or India. In case of a large outbreak in Eastern Yemen results indicate a 30 per cent chance for transmission to occur.

Another important scenario for wheat rust spread is from Yemen through Middle Eastern countries, in particular Iran, to Central and South Asia. If Iran were to suffer a moderate outbreak of Ug99 – more than 1000 hectares – then spores would likely spread to Afghanistan, and from there potentially further to the northern plains of Pakistan and India. However, transmission along this route is restricted to a relatively short time-window in March and April, before wheat is typically harvested in South Asia.

“New races of wheat rust are threatening wheat worldwide, and we need to know which areas are at risk,” said senior author Prof Chris Gilligan, from Cambridge’s Department of Plant Sciences.

“From our work, we now believe that if we start to see Ug99 or other new wheat rust strains take hold in Yemen in early spring then action must be taken immediately to mitigate the risk of further spread.”

However, the modelling work also offers some encouraging news: the airborne transmission of the disease from East African countries directly to South Asia is highly unlikely, with transmission events possible only on less than one day a year.

The scientific team used field disease surveys from the International Maize and Wheat Improvement Centre and weather data from the UK Met Office as key input for the modelling framework.

“This research has allowed us to obtain the first quantitative estimates of long-term airborne spore transmission frequencies for different outbreak scenarios. We compiled risk assessments for pathogen dispersal from key disease locations to important wheat-producing countries. These assessments can effectively inform surveillance and control strategies,” said Cambridge’s Marcel Meyer, the study’s first author.

The team said their work, including 3-D spore dispersal animations and a catalogue of dispersal trends (indicating likely directions, frequencies, pathogen loads), provides new ways to raise awareness, communicate risks, and inform agricultural stakeholders.

Their modelling framework can be applied as a tool to analyse risks in case new disease strains should be uncovered in other geographic areas. This has already helped in estimating dispersal risks from sites of other diseases in Europe and Siberia. In ongoing work the team is developing an Early Warning System forecasting rust risk in Ethiopia, East Africa’s largest wheat producing country.

“The combined expertise from plant sciences and atmospheric dispersion sciences has delivered ground breaking tools that highlight the risks, and support the management of the devastating potential of these diseases,” said Dr Matthew Hort, co-author from the UK’s Met Office.