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Abstract In recent years， sweet potato virus disease （SPVD） has severely affected the production of sweet potato in China. In order to select chemical agents to prevent sweet potato from being infected with SPVD， 14 pesticides were sprayed on the plants of sweet potato infected with SPVD， and the relative mRNA level of the viruses in the leaves and physiological indicators of sweet potato plants were detected. The results showed that after the application of most of the pesticides， the relative mRNA level of the two viruses that caused SPVD decreased， and the chlorophyll content and biomass of sweet potato infected with SPVD increased significantly or extremely significantly. Among the pesticides， BASF virus liquid， 0.1% physcion and Aolike powder for cucumber virus were proved to be the best， as the chlorophyll content and stem length of sweet potato increased significantly. The experiment indicated that the pesticides could reduce the harm of SPVD to sweet potato.
Key words Sweet potato； SPVD； Pesticide treatment
When sweet potato feathery mottle virus （SPFMV） and sweet potato chlorotic stunt virus （SPCSV） both infect sweet potato and interact with each other， the sweet potato virus disease （SPVD） will be produced to affect the production of sweet potato severely and is one of the most harmful diseases to sweet potato industry worldwide[1]. After sweet potato is infected with SPVD， its leaves will be contorted， deformed and faded， and its plants will be dwarfed[2]. The yield loss of sweet potato infected with SPVD in China is more than 30%， and even up to 80% for some varieties of sweet potato[3].
At present， the main methods for the prevention and control of the sweet potato virus disease include cutting off the source of infection， killing biological media， breeding for disease resistance， establishing a sweet potato detoxification system， and applying antiviral pesticides[4]， of which there are few studies on the application of antiviral pesticides in the prevention and control of sweet potato SPVD. However， studies have shown that chemical agents can inhibit the infection of other crops with viruses. Tobacco mosaic virus[5]， tomato virus disease[6]， apple chlorotic leaf spot virus disease and stem grooving virus disease[7]， wheat yellow dwarf disease[8] and cucumber green mottle mosaic virus disease[9] can be prevented and controlled by chemical agents. In order to select chemical agents that can inhibit SPVD， the effects of different pesticides on the prevention and control of SPVD were analyzed， and the pesticides that had good effects on the prevention and control of SPVD were selected according to the fresh weight of underground and overground part， stem length， chlorophyll content and the relative mRNA level of the viruses in sweet potato after the application of pesticides. Materials and Methods
Experimental materials
In 2015， a variety of sweet potato Xuzishu No. 8 with chlorotic dwarfing and leaf malformation， which had been infected with SPVD， was collected from the Sweet Potato Research Center in Xuzhou， Jiangsu， and stem segments were cut. They were planted in insect nets on June 17， 2015.
Extraction of total RNA in samples and RT??PCR detection
Total RNA was extracted from samples using the Huayueyang ultra??fast new plant RNA extraction kit， and genomic DNA was removed with matched DNases. The Takara Reverse Transcriptase M??MLV （RNase H） kit[10] and the CWBIO RNAse inhibitor CW0596 were used in reverse transcription to obtain cDNA. PCR detection was conducted using CWBIO 2??Taq Master Mix， and primers[11] were synthesized by Shanghai Shenggong Bioengineering Co.， Ltd. （Table 1）. Electrophoresis analysis was performed， and DNA fragments were recovered using the CWBIO rapid sepharose DNA recovery kit and sent to Shanghai Shenggong Biological Engineering Co.， Ltd. for sequencing.
Pesticide treatment and sampling
The leaves of diseased plants of sweet potato were sprayed with different pesticides （Table 2） on July 13， 20 and 28， and August 4， and there were five plants in each treatment. On July 11， August 11， and September 15， the top fourth leaf was collected， and RNA was extracted and transcribed into c??DNA.
Real??time fluorescence quantitative PCR detection
Viral real??time fluorescence quantitative primers[12] were synthesized by Shanghai Sangon Biotech Co.， Ltd. （Table 3）. Real??time fluorescence quantitative PCR detection of cDNA of samples treated by the pesticides was performed using Toyobo SYBR?k Green Realtime PCR Master Mix QPK201. The instrument used was ABI Stepone plus. The reaction process was as follows： it was conducted for 10 min at 95 ??， 15 s at 95 ??， 15 s at 60 ??， and 20 s at 72 ??， and there were 40 cycles. Determination of primer melting curves was conducted at 60-95 ??. The gene of sweet potato IbARF （ADP??ribosylation factor）[13] was used as an internal control， the relative content of virus was calculated using the 2???¤?¤Ct method （Table 3）.
Determination of biomass and chlorophyll content
On August 11， the chlorophyll content of the top fourth leaves of plants in various pesticide treatments was detected using an SPAD??502 chlorophyll meter. Moreover， the length of the longest stem of each plant in each treatment was measured. On September 15， the total fresh weight of overground and underground parts of each plant in each treatment was measured. Results and Analysis
RT??PCR and cDNA sequence detection of diseased seedlings of sweet potato
From the electrophoresis results of PCR products， it can be seen that based on the detection primers of the two viruses， bands that were consistent with the target genes of SPFMV and SPCSV in length were amplified （Fig. 1）. After the unreliable sequences before and after the final sequence were removed， BLAST was performed by NCBI. It was found that the consistency between the amplified SPFMV target gene sequences and the SPFMV sequences of JN131584.1， JN131586.1 and JN131574.1 in GenBank was97.92%， 97.80% and 97.48% respectively. The consistency between the amplified SPCSV target gene sequences and the SPCSV sequences of JN131558.1， KC146843.1 and HQ291260.1 in GenBank was 99.20%， 99.20% and 98.93% respectively. The above results indicated that the selected sweet potato plants had been infected with SPVD.
Effects of pesticide treatment on SPCSV and SPFMV content in sweet potato
Real??time fluorescence quantitative PCR detection was performed to obtain the relative mRNA level of SPCSV and SPFMV in each sample. The initial content of the viruses in each sample was 1， and the relative mRNA level of SPCSV and SPFMV in each sample was compared with that of CK in the same sampling time. The results showed that the effects of different pesticides on the relative mRNA level of SPFMV and SPCSV in sweet potato were different （Fig. 2）. According to the relative mRNA level of SPFMV and SPCSV in sweet potato leaves after treatment， the pesticides can be divided into the following groups.
The first group： the relative mRNA level of the viruses in sweet potato leaves decreased after the application of the pesticides， but did not increase after the application of the pesticides was stopped. For SPCSV， this group of pesticides included numbers 7 and 12， of which the relative mRNA level of SPCSV decreased more greatly. For SPFMV， only No. 7 pesticide was included in this group. This group of pesticides could effectively reduce the relative mRNA level of the viruses for a long time.
The second group： the relative mRNA level of the viruses in sweet potato leaves declined six days after the application of the pesticides （August 11）， and increased 40 days after the application of the pesticides was stopped （September 15）， but accounted for less than 50% of that of CK. For SPCSV， this group of pesticides included numbers 2， 3 and 11. For SPFMV， this group of pesticides included numbers 2， 11 and 14. Among them， the relative mRNA level of the viruses in sweet potato leaves increased more slightly after the application of the pesticides 2 and 11 was stopped， accounting for only 30% （SPCSV） and 15% （SPFMV） of that of CK. The second group of pesticides could also effectively decrease the relative mRNA level of the viruses for a long time. The third group： the relative mRNA level of the viruses in sweet potato leaves decreased after the application of the pesticides， and increased 40 days after the application of the pesticides was stopped， accounting for more than 50% of that of CK. For SPCSV， this group of pesticides included numbers 4， 8， 13 and 14. For SPFMV， this group of pesticides included numbers 1， 3 and 12. The third group of pesticides could effectively reduce the relative mRNA level of the viruses for a short time.
The fourth group： the relative mRNA level of the viruses in sweet potato leaves declined after the application of the pesticides， and rose after the application of the pesticides was stopped， exceeding that of CK. For SPCSV， this group of pesticides included numbers 1， 5， 6， 9 and 10. For SPFMV， this group of pesticides included numbers 4， 5， 6， 8， 10 and 13. To a certain degree， the fourth group of pesticides could decrease the relative mRNA level of the viruses for a short time.
Besides， after the application of No. 9 pesticide， the relative mRNA level of SPFMV in sweet potato leaves did not decrease compared with CK.
In summary， the No. 2， 7 and 11 pesticides could effectively reduce the relative mRNA level of the viruses in sweet potato leaves （Fig. 2）.
Agricultural Biotechnology 2018Effects of pesticide treatment on chlorophyll content and biomass of sweet potato plants infected with SPVD
Chlorophyll content After the application of the pesticides， the chlorophyll content of sweet potato increased in most treatments compared with CK （Fig. 3）. In comparison with CK， the chlorophyll content of sweet potato increased extremely significantly after the application of nine kinds of pesticides （numbers 1， 2， 3， 5， 6， 7， 9， 10 and 11）， and rose significantly after the application of No. 4 and 13 pesticides. After the application of No. 8 and 12 pesticides， there was no significant change in the chlorophyll content of sweet potato.
Stem length After the application of the pesticides， the stem length of sweet potato increased in most treatments compared with CK （Fig. 4）. In comparison with CK， the stem length of sweet potato increased extremely significantly after the application of sev?? en kinds of pesticides （numbers 1， 2， 4， 7， 8， 9 and 13）， and increased significantly after the application of No. 3， 6 and 11 pesticides. After the application of No. 5， 10， 12 and 14 pesticides， there was no significant change in the stem length of sweet potato. Biomass After the application of the pesticides， the fresh weight of underground part of sweet potato increased in most treatments compared with CK （Fig. 5）. In comparison with CK， the fresh weight of underground part of sweet potato increased extremely significantly after the application of 11 kinds of pesticides （numbers 1， 2， 3， 4， 7， 8， 9， 10， 11， 12 and 13）， and rose significantly after the application of No. 6 and 14 pesticides. After the application of No. 5 pesticide， there was no significant change in the fresh weight of underground part of sweet potato.
After the application of the pesticides， the fresh weight of overground part of sweet potato also increased in most treatments compared with CK （Fig. 6）. In comparison with CK， the fresh weight of overground part of sweet potato increased extremely significantly after the application of eight kinds of pesticides （numbers 3， 4， 7， 8， 9， 11， 12 and 13）， and rose significantly after the application of No. 2， 6 and 14 pesticides. After the application of No. 1， 5 and 10 pesticides， there was no significant change in the fresh weight of overground part of sweet potato.
After the application of the pesticides， the total biomass of sweet potato increased in most treatments compared with CK （Fig. 5）. In comparison with CK， the total biomass of sweet potato increased extremely significantly after the application of eight kinds of pesticides （numbers 3， 4， 7， 8， 9， 11， 12 and 13）， and rose significantly after the application of No. 1， 2 and 6 pesticides. After the application of No. 5， 10 and 14 pesticides， there was no significant change in the total biomass of sweet potato.
Comprehensive evaluation of various pesticides
After most of the pesticides were sprayed on the leaves of sweet potato infected with SPVD， the fresh weight of underground and overground part， total biomass， stem length and chlorophyll content of sweet potato were significantly or extremely significantly higher than those of CK. The physiological indicators of sweet potato and the relative mRNA level of the viruses in sweet potato after the application of the pesticides were compared to select the pesticides with certain effects on the prevention and control of SPVD. After the application of No. 2， 3， 4， 6， 7， 9， 11 and 13 pesticides， the physiological indicators of sweet potato infected with SPVD recovered better （Table 4）. After the application of No. 2， 7 and 11 pesticides， the relative mRNA level of the viruses in sweet potato infected with SPVD decreased more greatly （accounting for less than 30% of that of CK）， and their effect was long （the relative mRNA level of the viruses did not increase to a higher level after the application of these pesticides was stopped）. After the application of No. 3， 4， 6， 9 and 13 pesticides， the physiological indicators of sweet potato increased （especially the fresh weight of the underground part）， but the relative mRNA level of the viruses declined slightly and even increased. This may be because these pesticides could inhibit the symptoms of SPVD but could not kill the viruses. Sweet potato is an asexually propagated crop， in which the relative mRNA level of the viruses might affect that of the seedlings used in the next year and then the production of sweet potato in the next year. Therefore， it is believed that the effects of No. 3 （2% amino??oligosaccharin）， 4 （8% ningnanmycin）， 6 （virus II）， 9 （Aolike powder for watermelon virus） and 13 （oxyenadenine， isoamyl alkenyl adenine and moroxydine hydrochloride） pesticides on the prevention and control of SPVD were general， but it is feasible to use them to inhibit the harm of the viruses to sweet potato and then reduce its yield loss. No. 2 （BASF virus liquid）， 7 （0.1% physcion） and 11 Aolike powder for cucumber virus） pesticides had better effects on the prevention and control of SPVD. The three pesticides could not only reduce the yield loss， but also effectively decrease the relative mRNA level of the viruses in sweet potatoes.
Conclusions and Discussion
SPVD has a huge impact on the production of sweet potato. After sweet potato is infected with SPVD， the overground part of sweet potato is dwarfed， and the leaf area index and chlorophyll content decrease[14]， which inhibits photosynthesis and affect the accumulation of assimilate， eventually resulting in the reduction of biological yield[15]. At present， the effects of SPVD on the yield of sweet potato is mainly reduced by removing the diseased seedlings and killing biological media. Studies have shown that chemical agents can inhibit wheat yellow dwarf disease[8]， tobacco mosaic virus disease[5]， apple chlorotic leaf spot virus disease and stem grooving virus disease[7].
In this study， the effects of the 14 pesticides on the relative mRNA level of the viruses in sweet potato leaves and physiological indicators of sweet potato were analyzed. The results showed that after the pesticides were sprayed on the leaves of sweet potato infected with SPVD， the fresh weight of underground and overground part， total biomass， stem length and chlorophyll content of sweet potato were significantly or extremely significantly higher than those of CK， and the relative mRNA level of the viruses decreased. That is， after the application of the pesticides， the relative mRNA level of the viruses declined， and the dwarfing of sweet potato was alleviated； the chlorophyll content was restored to a certain extent， thereby reducing the effect of SPVD on the yield of sweet potato.
According to the results， three pesticides that could reduce the relative mRNA level of the two viruses in SPVD to a certain extent， improve the physiological indicators of infected sweet potato， and restore the yield of infected sweet potato were selected， including No. 2 （BASF virus liquid）， 7 （0.1% physcion） and 11 （Aolike powder for cucumber virus） pesticides. After the application of the three pesticides， the yield of infected sweet potato increased by 23.85%， 140.11% and 22.22% compared with that of CK. After the application of 0.1% physcion， the yield of infected sweet potato rose greatly， indicating that it had a good effect on the prevention and control of SPVD. This laboratory is dedicated to the research of conventional breeding and molecular breeding of sweet potato. However， due to the severe damage of SPVD， many breeding materials with good traits have been seriously infected with SPVD in recent years and cannot be preserved， which is a huge loss for the breeding of sweet potato. In the production of sweet potato in fields in 2016， after the research group sprayed BASF virus liquid and Aolike powder for cucumber virus on the leaves of sweet potato infected with SPVD， the symptoms of sweet potato infected with SPVD were significantly alleviated.
In this study， the three pesticides that could prevent and control SPVD to some extent were selected， but the optimal application period and concentration were not determined. In the future research， it is necessary to find out the most suitable method of application. Further field production tests should be conducted， and a negative control group of virus??free seedlings should be set to better verify the effectiveness of pesticide treatment. At the same time， discussing the action mechanism of the pesticides in this experiment is also a good research direction.
References
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[2] ZHANG ZC， QIAO Q， QIN YH， et al. 2012， First evidence for occurrence of sweet potato virus disease （SPVD） caused by dual infection of sweet patato feathery mottle virus and sweet potato chlorotic stunt virus in China[J]. Acta Phytopathologica Sinica， 2012， 42（3）： 328-333. （in Chinese）.
[3] ZHAO YQ， ZHANG CL， SUN HJ， et al. Effects of viruses （SPVD） on yield of sweet potato[J]. Southwest China Journal of Agricultural Sciences， 2012， 25（3）： 909-911. （in Chinese）.
[4] DONG F， ZHANG CF. Progress and prospects on prevention and control measures of virus disease in sweet potato[J]. Crops， 2016（3）： 6-11. （in Chinese）.
[5] WANG J， WANG KY， ZHANG Q， et al. Inhibition of laminarin against TMV and effect on protective enzymes in tobacco[J]. Acta Phytophylacica Sinica， 2011， 38（6）： 532-538. （in Chinese）.
[6] GAO JF. Field efficacy trial of 2% amino??oligosaccharin aqueous solution against tomato virus disease[J]. Modern Rural Technology， 2012（13）： 53. （in Chinese）.
[7] QIAO XH， GUO C， SHAO JZ， et al. Detection of apple latent virus in seeds of Malus robusta and effect of physicochemical treatment on the virus[J]. Journal of Fruit Science， 2013， 30（3）： 489-492. （in Chinese）. [8] YAN JH， YAO Q， GUO QY. Effects of ten kinds of plant virus inhibitors on controlling wheat yellow dwarf， 2015，42（1）： 238-242. （in Chinese）.
[9] CAI MY， CHEN HB， YE JR. Control effect of soil treatment on cucumber green mottle mosaic virus disease[J]. Primary Agricultural Technology Extension， 2016，4（7）： 19-21. （in Chinese）.
[10] LIU F， LI T， YU NT， et al. Molecular detection and sequence analysis of Cucumber mosaic virus on tomato in Hainan[J]. Genomics and Applied Biology， 2015， 34（8）： 1723-1728. （in Chinese）.
[11] QIAO Q， ZHANG ZC， ZHANG DS， et al. Serological and molecular detection of viruses infecting sweet potato in China[J]. Acta Phytopathologica Sinica， 2012， 42（1）： 10-16. （in Chinese）.
[12] LU HX， LV CW， WU ZD， et al. Development of detection method for sweet potato feathery mottle virus （SPFMV） and sweet potato chlorotic stunt virus （SPCSV） through fluorescence quantitative RT??PCR[J]. Scientia Agricultura Sinica， 2016， 49（1）： 90-102. （in Chinese）.
[13] PARK SC， KIM YH， JI CY， et al. Stable internal reference genes for the normalization of real??time PCR in different sweet potato cultivars subjected to abiotic stress conditions[J]. PLoS One， 2012， 7（12）： e51502.
[14] XU Z， ZHAO YQ， SUN HJ， et al. Effects of viruses （SPVD） grafting on chlorophyll content and Yield of Sweet Potato[J]. Southwest China Journal of Agricultural Sciences， 2012， 25（5）： 1681-1684. （in Chinese）.
[15] ZHOU QL， ZHANG YJ， HUANG YD， et al. Effect of sweet potato virus disease （SPVD） on sweet potato yield formation[J]. Jiangsu Journal of Agricultural Sciences， 2014， 30（1）： 42-46. （in Chinese）.
Key words Sweet potato； SPVD； Pesticide treatment
When sweet potato feathery mottle virus （SPFMV） and sweet potato chlorotic stunt virus （SPCSV） both infect sweet potato and interact with each other， the sweet potato virus disease （SPVD） will be produced to affect the production of sweet potato severely and is one of the most harmful diseases to sweet potato industry worldwide[1]. After sweet potato is infected with SPVD， its leaves will be contorted， deformed and faded， and its plants will be dwarfed[2]. The yield loss of sweet potato infected with SPVD in China is more than 30%， and even up to 80% for some varieties of sweet potato[3].
At present， the main methods for the prevention and control of the sweet potato virus disease include cutting off the source of infection， killing biological media， breeding for disease resistance， establishing a sweet potato detoxification system， and applying antiviral pesticides[4]， of which there are few studies on the application of antiviral pesticides in the prevention and control of sweet potato SPVD. However， studies have shown that chemical agents can inhibit the infection of other crops with viruses. Tobacco mosaic virus[5]， tomato virus disease[6]， apple chlorotic leaf spot virus disease and stem grooving virus disease[7]， wheat yellow dwarf disease[8] and cucumber green mottle mosaic virus disease[9] can be prevented and controlled by chemical agents. In order to select chemical agents that can inhibit SPVD， the effects of different pesticides on the prevention and control of SPVD were analyzed， and the pesticides that had good effects on the prevention and control of SPVD were selected according to the fresh weight of underground and overground part， stem length， chlorophyll content and the relative mRNA level of the viruses in sweet potato after the application of pesticides. Materials and Methods
Experimental materials
In 2015， a variety of sweet potato Xuzishu No. 8 with chlorotic dwarfing and leaf malformation， which had been infected with SPVD， was collected from the Sweet Potato Research Center in Xuzhou， Jiangsu， and stem segments were cut. They were planted in insect nets on June 17， 2015.
Extraction of total RNA in samples and RT??PCR detection
Total RNA was extracted from samples using the Huayueyang ultra??fast new plant RNA extraction kit， and genomic DNA was removed with matched DNases. The Takara Reverse Transcriptase M??MLV （RNase H） kit[10] and the CWBIO RNAse inhibitor CW0596 were used in reverse transcription to obtain cDNA. PCR detection was conducted using CWBIO 2??Taq Master Mix， and primers[11] were synthesized by Shanghai Shenggong Bioengineering Co.， Ltd. （Table 1）. Electrophoresis analysis was performed， and DNA fragments were recovered using the CWBIO rapid sepharose DNA recovery kit and sent to Shanghai Shenggong Biological Engineering Co.， Ltd. for sequencing.
Pesticide treatment and sampling
The leaves of diseased plants of sweet potato were sprayed with different pesticides （Table 2） on July 13， 20 and 28， and August 4， and there were five plants in each treatment. On July 11， August 11， and September 15， the top fourth leaf was collected， and RNA was extracted and transcribed into c??DNA.
Real??time fluorescence quantitative PCR detection
Viral real??time fluorescence quantitative primers[12] were synthesized by Shanghai Sangon Biotech Co.， Ltd. （Table 3）. Real??time fluorescence quantitative PCR detection of cDNA of samples treated by the pesticides was performed using Toyobo SYBR?k Green Realtime PCR Master Mix QPK201. The instrument used was ABI Stepone plus. The reaction process was as follows： it was conducted for 10 min at 95 ??， 15 s at 95 ??， 15 s at 60 ??， and 20 s at 72 ??， and there were 40 cycles. Determination of primer melting curves was conducted at 60-95 ??. The gene of sweet potato IbARF （ADP??ribosylation factor）[13] was used as an internal control， the relative content of virus was calculated using the 2???¤?¤Ct method （Table 3）.
Determination of biomass and chlorophyll content
On August 11， the chlorophyll content of the top fourth leaves of plants in various pesticide treatments was detected using an SPAD??502 chlorophyll meter. Moreover， the length of the longest stem of each plant in each treatment was measured. On September 15， the total fresh weight of overground and underground parts of each plant in each treatment was measured. Results and Analysis
RT??PCR and cDNA sequence detection of diseased seedlings of sweet potato
From the electrophoresis results of PCR products， it can be seen that based on the detection primers of the two viruses， bands that were consistent with the target genes of SPFMV and SPCSV in length were amplified （Fig. 1）. After the unreliable sequences before and after the final sequence were removed， BLAST was performed by NCBI. It was found that the consistency between the amplified SPFMV target gene sequences and the SPFMV sequences of JN131584.1， JN131586.1 and JN131574.1 in GenBank was97.92%， 97.80% and 97.48% respectively. The consistency between the amplified SPCSV target gene sequences and the SPCSV sequences of JN131558.1， KC146843.1 and HQ291260.1 in GenBank was 99.20%， 99.20% and 98.93% respectively. The above results indicated that the selected sweet potato plants had been infected with SPVD.
Effects of pesticide treatment on SPCSV and SPFMV content in sweet potato
Real??time fluorescence quantitative PCR detection was performed to obtain the relative mRNA level of SPCSV and SPFMV in each sample. The initial content of the viruses in each sample was 1， and the relative mRNA level of SPCSV and SPFMV in each sample was compared with that of CK in the same sampling time. The results showed that the effects of different pesticides on the relative mRNA level of SPFMV and SPCSV in sweet potato were different （Fig. 2）. According to the relative mRNA level of SPFMV and SPCSV in sweet potato leaves after treatment， the pesticides can be divided into the following groups.
The first group： the relative mRNA level of the viruses in sweet potato leaves decreased after the application of the pesticides， but did not increase after the application of the pesticides was stopped. For SPCSV， this group of pesticides included numbers 7 and 12， of which the relative mRNA level of SPCSV decreased more greatly. For SPFMV， only No. 7 pesticide was included in this group. This group of pesticides could effectively reduce the relative mRNA level of the viruses for a long time.
The second group： the relative mRNA level of the viruses in sweet potato leaves declined six days after the application of the pesticides （August 11）， and increased 40 days after the application of the pesticides was stopped （September 15）， but accounted for less than 50% of that of CK. For SPCSV， this group of pesticides included numbers 2， 3 and 11. For SPFMV， this group of pesticides included numbers 2， 11 and 14. Among them， the relative mRNA level of the viruses in sweet potato leaves increased more slightly after the application of the pesticides 2 and 11 was stopped， accounting for only 30% （SPCSV） and 15% （SPFMV） of that of CK. The second group of pesticides could also effectively decrease the relative mRNA level of the viruses for a long time. The third group： the relative mRNA level of the viruses in sweet potato leaves decreased after the application of the pesticides， and increased 40 days after the application of the pesticides was stopped， accounting for more than 50% of that of CK. For SPCSV， this group of pesticides included numbers 4， 8， 13 and 14. For SPFMV， this group of pesticides included numbers 1， 3 and 12. The third group of pesticides could effectively reduce the relative mRNA level of the viruses for a short time.
The fourth group： the relative mRNA level of the viruses in sweet potato leaves declined after the application of the pesticides， and rose after the application of the pesticides was stopped， exceeding that of CK. For SPCSV， this group of pesticides included numbers 1， 5， 6， 9 and 10. For SPFMV， this group of pesticides included numbers 4， 5， 6， 8， 10 and 13. To a certain degree， the fourth group of pesticides could decrease the relative mRNA level of the viruses for a short time.
Besides， after the application of No. 9 pesticide， the relative mRNA level of SPFMV in sweet potato leaves did not decrease compared with CK.
In summary， the No. 2， 7 and 11 pesticides could effectively reduce the relative mRNA level of the viruses in sweet potato leaves （Fig. 2）.
Agricultural Biotechnology 2018Effects of pesticide treatment on chlorophyll content and biomass of sweet potato plants infected with SPVD
Chlorophyll content After the application of the pesticides， the chlorophyll content of sweet potato increased in most treatments compared with CK （Fig. 3）. In comparison with CK， the chlorophyll content of sweet potato increased extremely significantly after the application of nine kinds of pesticides （numbers 1， 2， 3， 5， 6， 7， 9， 10 and 11）， and rose significantly after the application of No. 4 and 13 pesticides. After the application of No. 8 and 12 pesticides， there was no significant change in the chlorophyll content of sweet potato.
Stem length After the application of the pesticides， the stem length of sweet potato increased in most treatments compared with CK （Fig. 4）. In comparison with CK， the stem length of sweet potato increased extremely significantly after the application of sev?? en kinds of pesticides （numbers 1， 2， 4， 7， 8， 9 and 13）， and increased significantly after the application of No. 3， 6 and 11 pesticides. After the application of No. 5， 10， 12 and 14 pesticides， there was no significant change in the stem length of sweet potato. Biomass After the application of the pesticides， the fresh weight of underground part of sweet potato increased in most treatments compared with CK （Fig. 5）. In comparison with CK， the fresh weight of underground part of sweet potato increased extremely significantly after the application of 11 kinds of pesticides （numbers 1， 2， 3， 4， 7， 8， 9， 10， 11， 12 and 13）， and rose significantly after the application of No. 6 and 14 pesticides. After the application of No. 5 pesticide， there was no significant change in the fresh weight of underground part of sweet potato.
After the application of the pesticides， the fresh weight of overground part of sweet potato also increased in most treatments compared with CK （Fig. 6）. In comparison with CK， the fresh weight of overground part of sweet potato increased extremely significantly after the application of eight kinds of pesticides （numbers 3， 4， 7， 8， 9， 11， 12 and 13）， and rose significantly after the application of No. 2， 6 and 14 pesticides. After the application of No. 1， 5 and 10 pesticides， there was no significant change in the fresh weight of overground part of sweet potato.
After the application of the pesticides， the total biomass of sweet potato increased in most treatments compared with CK （Fig. 5）. In comparison with CK， the total biomass of sweet potato increased extremely significantly after the application of eight kinds of pesticides （numbers 3， 4， 7， 8， 9， 11， 12 and 13）， and rose significantly after the application of No. 1， 2 and 6 pesticides. After the application of No. 5， 10 and 14 pesticides， there was no significant change in the total biomass of sweet potato.
Comprehensive evaluation of various pesticides
After most of the pesticides were sprayed on the leaves of sweet potato infected with SPVD， the fresh weight of underground and overground part， total biomass， stem length and chlorophyll content of sweet potato were significantly or extremely significantly higher than those of CK. The physiological indicators of sweet potato and the relative mRNA level of the viruses in sweet potato after the application of the pesticides were compared to select the pesticides with certain effects on the prevention and control of SPVD. After the application of No. 2， 3， 4， 6， 7， 9， 11 and 13 pesticides， the physiological indicators of sweet potato infected with SPVD recovered better （Table 4）. After the application of No. 2， 7 and 11 pesticides， the relative mRNA level of the viruses in sweet potato infected with SPVD decreased more greatly （accounting for less than 30% of that of CK）， and their effect was long （the relative mRNA level of the viruses did not increase to a higher level after the application of these pesticides was stopped）. After the application of No. 3， 4， 6， 9 and 13 pesticides， the physiological indicators of sweet potato increased （especially the fresh weight of the underground part）， but the relative mRNA level of the viruses declined slightly and even increased. This may be because these pesticides could inhibit the symptoms of SPVD but could not kill the viruses. Sweet potato is an asexually propagated crop， in which the relative mRNA level of the viruses might affect that of the seedlings used in the next year and then the production of sweet potato in the next year. Therefore， it is believed that the effects of No. 3 （2% amino??oligosaccharin）， 4 （8% ningnanmycin）， 6 （virus II）， 9 （Aolike powder for watermelon virus） and 13 （oxyenadenine， isoamyl alkenyl adenine and moroxydine hydrochloride） pesticides on the prevention and control of SPVD were general， but it is feasible to use them to inhibit the harm of the viruses to sweet potato and then reduce its yield loss. No. 2 （BASF virus liquid）， 7 （0.1% physcion） and 11 Aolike powder for cucumber virus） pesticides had better effects on the prevention and control of SPVD. The three pesticides could not only reduce the yield loss， but also effectively decrease the relative mRNA level of the viruses in sweet potatoes.
Conclusions and Discussion
SPVD has a huge impact on the production of sweet potato. After sweet potato is infected with SPVD， the overground part of sweet potato is dwarfed， and the leaf area index and chlorophyll content decrease[14]， which inhibits photosynthesis and affect the accumulation of assimilate， eventually resulting in the reduction of biological yield[15]. At present， the effects of SPVD on the yield of sweet potato is mainly reduced by removing the diseased seedlings and killing biological media. Studies have shown that chemical agents can inhibit wheat yellow dwarf disease[8]， tobacco mosaic virus disease[5]， apple chlorotic leaf spot virus disease and stem grooving virus disease[7].
In this study， the effects of the 14 pesticides on the relative mRNA level of the viruses in sweet potato leaves and physiological indicators of sweet potato were analyzed. The results showed that after the pesticides were sprayed on the leaves of sweet potato infected with SPVD， the fresh weight of underground and overground part， total biomass， stem length and chlorophyll content of sweet potato were significantly or extremely significantly higher than those of CK， and the relative mRNA level of the viruses decreased. That is， after the application of the pesticides， the relative mRNA level of the viruses declined， and the dwarfing of sweet potato was alleviated； the chlorophyll content was restored to a certain extent， thereby reducing the effect of SPVD on the yield of sweet potato.
According to the results， three pesticides that could reduce the relative mRNA level of the two viruses in SPVD to a certain extent， improve the physiological indicators of infected sweet potato， and restore the yield of infected sweet potato were selected， including No. 2 （BASF virus liquid）， 7 （0.1% physcion） and 11 （Aolike powder for cucumber virus） pesticides. After the application of the three pesticides， the yield of infected sweet potato increased by 23.85%， 140.11% and 22.22% compared with that of CK. After the application of 0.1% physcion， the yield of infected sweet potato rose greatly， indicating that it had a good effect on the prevention and control of SPVD. This laboratory is dedicated to the research of conventional breeding and molecular breeding of sweet potato. However， due to the severe damage of SPVD， many breeding materials with good traits have been seriously infected with SPVD in recent years and cannot be preserved， which is a huge loss for the breeding of sweet potato. In the production of sweet potato in fields in 2016， after the research group sprayed BASF virus liquid and Aolike powder for cucumber virus on the leaves of sweet potato infected with SPVD， the symptoms of sweet potato infected with SPVD were significantly alleviated.
In this study， the three pesticides that could prevent and control SPVD to some extent were selected， but the optimal application period and concentration were not determined. In the future research， it is necessary to find out the most suitable method of application. Further field production tests should be conducted， and a negative control group of virus??free seedlings should be set to better verify the effectiveness of pesticide treatment. At the same time， discussing the action mechanism of the pesticides in this experiment is also a good research direction.
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