Pathogen Identification And The Disease Epidemic Surveillance Of Cassava Mosaic Disease In China

Cassava Mosaic Disease（CMD）is one of the most serious diseases in the global cassava cultivation industry.In 2018,the disease was first reported to have caused damage in China.In order to understand the distribution and harm of domestic cassava mosaic virus disease,identify the pathogen and establish its epidemic surveillance network,and evaluate the potential risk of the disease in cassava industry.In this study,the pathogen was identified through the investigation and suspected sample collection of mosaic disease in the main cassava growing areas in China,combined with symptom identification and molecular identification technology;simultaneously,design specific PCR amplification primer pairs,and develop pathogen PCR/RT-PCR,multiplex PCR detection technology;and develop the disease surveillance technical regulations,establish epidemic surveillance sites and networks,monitor and control the disease epidemic,conduct safety assessment on the potential risks of the disease to the healthy development of the industry,the results can be listed as follows:1.2018-2020,completed a survey and sample collection of cassava mosaic disease 18locations in 8 provinces of Guangxi,Guangdong,Hainan,Yunnan,Guizhou,and found that the disease has been found to harm the cassava in many areas in China,SC9,NZ199 and SC205 and other cultivars are susceptible varieties,but the epidemic is also limited to germplasm nursery and germplasm transporting region,it has not spread to the spread of Main region.2.The results of the collection of suspected disease samples and pathogen identification found that there are two field symptoms of cassava mosaic disease in China:the mosaic virus disease caused by the suspected gemini virus and the common mosaic virus caused by the（Potexvirus）virus（Cassava common mosaic virus,Cs CMV）.The DNA-A and DNA-B genome sequences of two suspected mosaic disease isolates were obtained by sequence determination,Genbank accession number is MN688214-MN688217.The sequence similarity homology comparison found that compared with SLCMV-Cambodia（KT861468）,SLCMV-VTN6（LC312131）,SLCMV-Surin1（MN544647）and SLCMV-HN7（MH891840）,the DNA-A sequence similarity of the two isolates exceeded 99%.It is confirmed that the domestic cassava gemini virus is mainly a Sri Lankan cassava mosaic virus strain（Sri Lankan cassava mosaic virus,SLCMV）.Genome sequences of suspected common mosaic disease isolates were obtained by sequence determination found that,compared with that Latin American sequence,clustered to form independent branches,the domestic common mosaic virus sequence similarity is 91%～96%,and it is possible to evolve new strains.3.Investigation on the infection and harm of the disease in the field found that the two domestic cassava viruses can be divided into three modes of single and compound infection,namely SLCMV alone infection,Cs CMV alone infection and SLCMV-Cs CMV combined infection;the three typical symptoms of the three infection methods in the field are:cassava leaves and plants infected by SLCMV alone can show systematic distortion,deformation and mosaic,young and regenerated plants are more susceptible;plants infected by Cs CMV alone show mottled leaves,Observed the cassava leaves through light,and it was found that the cassava leaves had chlorosis symptoms,and has no obvious effect on plant growth;SLCMV-Cs CMV compound infection shows comprehensive plant dwarf,and both old leaves and young leaves have symptoms of chlorosis and mosaic.4.Established a multiplex PCR detection system for SLCMV and Cs CMV viruses:d NTPs concentration 0.20 mmol/L,Mg²⁺concentration 1.5 mmol/L,Taq DNA polymerase concentration 0.02 U/μL,primer ratio 0.24/0.08,the detection sensitivity for both virus templates is 5×10^-3 ng/μL,the detection limit of SLCMV is 12.15×10⁵ copies/μL,and the detection limit of Cs CMV is 13.65×10⁵ copies/μL.Simultaneously,520 suspected cassava mosaic samples collected within three years were tested.A total of 520 DNA samples and276 c DNA samples were extracted;105 samples were found to be SLCMV positive,accounting for 20.19%,and 242 samples were found to be Cs CMV positive,accounting for87.68%;of the 110 samples in 2020,21 samples were both positive for SLCMV and Cs CMV,accounting for 19.09%.5.Researched and established surveillance technology for cassava mosaic virus disease,established cassava mosaic disease monitoring technology,standardized the construction of monitoring points,disease investigation methods,monitoring and data statistics and other technical points,established monitoring points and monitoring networks,implemented effective monitoring and control of disease epidemics.6.With reference to China’s comprehensive assessment standards for the risk of agricultural and forestry pests,the risk assessment of cassava Sri Lanka mosaic disease（SLCMD）and cassava common mosaic disease（Cs CMD）was carried out.The comprehensive evaluation value of cassava Sri Lanka mosaic disease is 2.05,and the risk level is high risk;the comprehensive evaluation value of cassava common mosaic disease is1.89,and the risk level is moderate risk.