Irrigation can create new green bridges that promote rapid intercontinental spread of the wheat stem rust pathogen

Abstract

Wheat stem rust epidemics caused by the obligate pathogenic fungus Puccinia graminis f.sp. tritici have historically driven severe yield losses on all wheat growing continents and, after many decades of control, stem rust is re-emerging as a disease of concern. In 1998, a highly virulent race able to overcome 90% of world wheat cultivars, Ug99, was identified in Uganda. Since initial detection, the pathogen has evolved many new variants and spread to many countries. The original variant spread from East Africa to the Middle East with three years between detection in Ethiopia and subsequent detection in Yemen. In 2014, another Ug99 variant (TTKTT), with one of the most complex virulence profiles, was detected in Kenya. This variant also spread from East Africa to the Middle East, but with only one year between detection in Ethiopia and subsequent detection in Iraq. Here we investigate potential airborne migration routes to account for the rapid spread of TTKTT in East Africa and beyond to the Middle East by using an integrated model combining the outputs from a meteorology-driven fungal spore dispersion model with epidemiological models to account for seasonal availability of susceptible crops and conditions for spore release and infectivity. We find viable pathways in the 2018/19 season that incorporate critical stepping-stone locations in Yemen or Saudi Arabia, but only in the presence of newly irrigated regions in Ethiopia. Our results indicate the potential and increasing importance of irrigated wheat areas in Ethiopia, Yemen and Saudi Arabia for inter-regional stem rust movements. Future movement of stem rust races out of East Africa is considered likely as irrigated areas expand. Targeted surveillance and the use of mitigation strategies including the use of durable resistant varieties in regions of irrigation are required to reduce the risks of enhanced dispersal of stem rust to other regions.