Sweet Potato Virus Disease (SPVD) Comparison Of Resistance Identification Methods And Variety Screening

Sweet potato[Ipomoea batatas L.(Lam)],as an important food crop in China and even the whole world,has received increasing attention and recognition from the market due to its extremely high nutritional value and industrial processing value.With the highest sweet potato cultivation area in the world,China has played a very important role in the sweet potato industry.However,in recent years,with the deterioration of environmental conditions and the high incidence of various diseases,sweet potato production has decreased significantly in some planting areas.Sweet potato virus disease(SPVD)is infested especially by sweet potato feathery mottle virus(SPFMV)and sweet potato chlorotic stunt virus(SPCSV).SPVD is a devastating disease on sweet potato.Sweet potato infected with SPVD is characterized by dwarfing of plants,chlorosis,distortion,and deformity of the leaves,and the yield is also seriously decreased.The occurrence of SPVD in sweet potato production areas,such as Guangdong,Jiangsu and Sichuan,shows that SPVD has been widely distributed in China and shows a trend of increasing year by year.It is particularly important for planters to strengthen the prevention and control of SPVD.At present,there are no special chemical preparations for the prevention and treatment of SPVD.Screening and planting disease-resistant varieties is the most practical and economical effective method.The breeding resource traits have a great influence on efficiency of crop breeding.And it is imperative to screen and foster confrontation SPVD sweet potato varieties.The SPVD resistance identification method usually is the sweet potato material infected with SPVD used as the scion,the species material identified used as the rootstock,and the grafting technique used for identification.Due to the short duration of SPVD in China,there is still a lack of uniform,standardized and accurate SPVD resistance identification methods.This study compared and optimized two common methods of grafting and screened the more suitable method for SPVD disease resistance.Through investigation and analysis of the disease index of sweet potato varieties,and combination with serological identification and fluorescence quantitative RT-PCR,the following results were verified and acquired.1.Comparative optimization of two grafting methodsIn 2016 and 2017,the three dominating sweet potato varieties were tested by flanking and plugging methods separately.By comparing the survival rates of the three varieties at each stage,the survival rate of each species of the plug-in treatment group was significantly higher than that of the side-effect treatment group.The study found that in 2016,the survival rate of the three varieties of cleft-grafing treatment group was lower relatively,and even the lowest one was only 50%,which indicated that the grafting results were not satisfactory.In 2017,the survival rate of the cleft-grafing treatment group did not change greatly but showed a slight increase.Considering the improvements in grafting practices.The survival rate of the three varieties of the plug-in treatment group in 2016 was higher than that of the side-effect treatment group during the same period.After improving and perfecting the plug-in method in 2017,the survival rate of each species was higher than 85%.In addition to the higher survival rate after grafting,the plugging method is easy to grasp in actual operation,the wound connection is more easily fixed and the wound is easy to heal.To sum up,in the present study,the plugging method is easy to operate,and the survival rate of the grafted varieties is high and easy to popularize.It can be applied to the SPVD resistance identification of sweet potato.It is more suitable for the artificial induction method to identify the disease resistance of sweet potato SPVD.2.Identification and screening of SPVD resistance of sweet potato varietiesDuring the period of 2016,86 varieties of sweet potato were subjected to artificial induced infection with SPVD,and the number of diseased plants was counted after grafting.According to the symptomatology of plants,the disease index was calculated and graded.At the same time,the leaves of the plant treatment group were collected and subjected to NCM-ELISA for detection of SPFMV and SPCSV.Combining the disease index of each cultivar with the qualitative results of serological tests to classify the test varieties into the following categories.Category I: Predisposition,its disease index was greater than 27,and the NCM-ELISA was positive for both SPCSV and SPFMV,including 11 varieties such as Ningzihu-1,Ning 4-6,ZheShu-602 etc.The second category: No flu-like illness,including 30 such as CT3,and their disease index ranged from 8 to 27,After detection by NCM-ELISA,the results of SPFMV and SPCSV were negative in some varieties,and they were not infected with SPVD.Individual species such as Sushu-1 and Xiangshao have detected SPFMV,no SPVSV has been detected,and no SPVD infection has been determined.The third category: disease resistance.The disease index of these breeds was less than 8,NCM-ELISA test results showed that the test samples were negative for both viruses and were not infected with SPVD.In 2017 a second grafting trial was conducted on 23 species,which were susceptible and disease-resistant in 2016.The test results showed that Yushuwang,Ningzishu-1 and Ning 4-6 had higher disease index than 65 after 30 days of grafting.NCM-ELISA showed that the two viruses were positive and the performance was very weak to SPVD.Wanzi-51,Enshu-2 and Chaoshu-1 were all found to have a disease index of less than 5 after 30 days,and they were identified by NCM-ELSA without infection from SPFMV and SPCSV,which all indicated that they had strong resistance to SPVD.3.Verification of disease resistance of extreme speciesIn term of SPVD resistance screening,three quantitatively highly susceptible cultivars and three pre-and post-grafted varieties were screened by fluorescence quantitative RT-PCR.Virus signatures were used to detect the content of SPCSV and SPFMV from samples.Among the 12 subjects,6 pre-grafted materials were not infected with SPCSV and SPFMV by NCM-ELISA.No fluorescence signal was detected by fluorescence quantitative RT-PCR,and it was determined that SPVD was not infected.The NCM-ELISA results of three highly susceptible cultivars were positive for SPFMV and SPCSV.Fluorescence quantitative RT-PCR detected viral signals in all three materials,and the expression level was high,which was consistent with the results of NCM-ELISA.The syndrome index,serology qualitative test,and fluorescence quantitative RT-PCR viral expression results all confirmed that Yushuwang,Ningzishu-1 and Ning 4-6 were more susceptible cultivars.No co-infection of SPFMV and SPCSV was detected in the other three highly resistant varieties after grafting with NCM-ELISA and fluorescence quantitative RT-PCR.It was determined that SPVD was not infected,and two viral signals were not detected by fluorescence quantitative RT-PCR.The results were consistent with the symptoms and NCM-ELISA results,indicating that these three varieties showed strong resistance to SPVD and were highly resistant.In this study,through comparing different grafting methods,the optimum grafting method for sweet potato was selected and optimized to apply SPVD resistance identification.By combining the symptom index and serology qualitative identification,the SPVD resistance of sweet potato cultivars was classified from resistant to susceptible.At the same time,the viral expression level in the cultivars was detected by fluorescence quantitative RT-PCR,and the screening results of varieties were verified to improve the resistance of SPVD varieties.Accuracy of screening will provide reference for the future sweet potato production and breeding of good varieties,and provide important information for the resistance breeding of sweet potato.