Surveillance and distribution of the emergent Sri Lankan cassava mosaic virus in China

Cassava (Manihot esculenta Crantz) is a major staple food crop for more than a billion people in the world. Cassava mosaic virus (CMV), belonging to the Geminiviridae family, is a primary threat to cassava production. Sri Lankan cassava mosaic virus (SLCMV) is the only emergent CMV prevalent in South Asia and Southeast Asia since its identification in 2002. We reported the identification of two invasive strains of SLCMV, Col and HN7, in China in 2018. However, the occurrence and distribution of these known SLCMV strains and the presence of unknown geminivirus in China are still elusive. In this study, we firstly reported an improved CMV detection system based on molecular and serological methods, which was further used to determine the distribution of CMV in major cassava plantations in China. Two optimized PCR primer pairs based on the conserved regions of AV1 and AC1 genes were designed to detect different CMV species and distinguish SLCMV simultaneously. For a serological method, a polyclonal antibody against SLCMV AV1-encoded capsid protein was raised and used for enzyme-linked immunosorbent assay (ELISA). Consistent detection results were achieved by PCR- and ELISA-based methods. Among 62 examined samples collected in 2018, 10 were SLCMV positive, with 4 coinfection cases of two strains (HN7 and Col) in the same cassava plant. Two primer pairs could also be used to detect the presence of CMV in whitefly (Bemisia tabaci) sensitively. All positive samples were from Fujian and Hainan Provinces, indicating a limited distribution of SLCMV in cassava plants in China. Our detection methods could be used for future surveillance system to control and manage cassava mosaic disease in China and other countries.