Southeast Asia is the source of 95 percent of global cassava exports, and the detection in 2015 in Cambodia of Sri Lankan cassava mosaic virus raised alarm. By 2016, the disease had spread, showing potential to become a major threat to millions of smallholders. The virus's spread over a single growing season was documented by the International Center for Tropical Agriculture. The study is the first systematic baseline evaluation of SLCMV in Southeast Asia.