| Southern blight of peanut is a worldwide soil-borne plant fungal disease and caused by Sclerotium rolfsii.In recent years,due to the expansion of peanut planting area,continuous cropping and cropping and returning of straw to the field have become increasingly common,coupled with the improvement of water and fertilizer conditions and the reduction of organic fertilizer application,resulting in the widespread occurrence of southern blight of peanut in major peanut producing areas in China.At present,southern blight of peanut has become an important threat to peanut production in Henan Province.In this study,the five-point sampling method was used to investigate the incidence of southern blight of peanut in 23 peanut fields randomly selected from Nanyang,Zhumadian,Shangqiu,Zhengzhou,and Xinxiang,Henan Province,and the pathogens were isolated from the diseased peanut plants;the mycelial compatibility group(MCG)and morphological characteristics of 123 Sclerotium rolfsii isolates collected from 5 regions were analyzed in vitro,and the pathogenicity of 37 representative isolates was determined;the mycelial growth rate method was used to determine the sensitivity of 37 representative Sclerotium rolfsii isolates to 8 fungicides;the baseline of the sensitivity of 123 Sclerotium rolfsii isolates to thifluzamide was established by the mycelial growth rate method,and the resistance risk of Sclerotium rolfsii to thifluzamide was evaluated.The main findings are as follows:1.The five-point sampling method was used to investigate the incidence of southern blight of peanut in 23 peanut fields randomly selected from Nanyang,Zhumadian,Shangqiu,Zhengzhou,and Xinxiang,Henan Province.The results showed that southern blight of peanut occurred in the 23 investigated fields(13 spring sown peanut fields and 10 summer peanut fields),and the incidence of southern blight of peanut in different fields was significantly different.Among them,the field 5(Nanyang)had the highest incidence rate(50.40%),while the field 16(Shangqiu)had the lowest incidence rate(3.60%).In addition,it was also found that the incidence of southern blight of peanut in spring-sown peanut fields(8.80%to 50.40%)was higher than that in summer-sown peanut fields(3.60%～8.40%).2.123 Sclerotium rolfsii isolates from 5 different regions of Henan Province were selected to determine the mycelial compatibility group(MCG).The results showed that 123 Sclerotium rolfsii isolates were divided into 6 different MCGs.MCG1 is the largest and most widely distributed MCG,consisting of 71 isolates from 5 regions;MCG2 is second only to MCG1 and contains 32 isolates from Nanyang,Xinxiang and Zhumadian;the remaining MCGs contain 2-10 isolates respectively.Except for Shangqiu,the isolates in other regions were divided into different MCGs.The Shannon-Wiener diversity index(H)of 123 Sclerotium rolfsii isolates was 1.13,and the Shannon-Wiener diversity indexs(H)of different regions were different,which indicated that the diversity of MCG of Sclerotium rolfsii was inconsistent in the 5 regions of Henan Province.3.Based on MCG classification,the differences in colony characteristics,mycelial growth rate,sclerotia number,sclerotia dry weight and sclerotia diameter among the MCG isolates were compared.The results showed that there were significantly different in colony characteristics,the number of sclerotia,the dry weight of sclerotia and the diameter of sclerotia among 6 MCGs,but there were not significantly different in mycelial growth rate among 6 MCGs.37 representative isolates were selected from 123 isolate of Sclerotium rolfsii,which were compared for differences in pathogenicity among different MCG isolates.The results showed that there was no significant difference in pathogenicity among different MCG isolates.Correlation analysis of mycelial growth rate,number of sclerotia,dry weight of sclerotia,diameter of sclerotia and pathogenicity among different MCG isolates was carried out.The results showed that a significant negative correlation was found between the number of sclerotia and dry weight of sclerotia in these MCG isolates(r=-0.848,P<0.05),but there was no correlation between other characteristics in these MCG isolates.4.The mycelial growth rate method was used to determine the sensitivity of 37 representative Sclerotium rolfsii isolates to fludioxonil,tetraconazole,thifluzamide,boscalid,fluxapyroxad,bixafen,carbon and tebuconazole.The results showed that the 8 fungicides had good inhibitory activity to Sclerotium rolfsii,but the mean EC50 values of different fungicides against Sclerotium rolfsii isolates were significantly different.Among them,carbon,thifluzamide and fluxapyroxad were most effective in inhibiting mycelia growth,with mean EC50 values of 0.2034,0.3493 and 0.4264 mg/L respectively;followed by tebuconazole and fludioxonil and bixafen had mean EC50 values of 1.0724,1.6099,and 1.8161 mg/L,respectively;tetraconazole and boscalid had the least inhibitory effects,with average EC50 values of 2.9232 and 3.7164 mg/L,respectively.In addition,the variables ratio of Sclerotium rolfsii isolates to the 8 fungicide were less than 4,which indicated that Sclerotium rolfsii isolates to the 8 fungicide was sensitive.Comparing the sensitivity of different MCG isolates to the same fungicide,the results showed the mean EC50 values of the other 7 fungicides had no significant differences among the MCGs,with the exception of boscalid.5.The mycelial growth rate method was used to determine the sensitivity of 123 Sclerotium rolfsii isolates to thifluzamide.The results showed that all tested isolates had high sensitivity to thifluzamide,the EC50 value range was between 0.1686～0.8276 mg/L,and the mean EC50 value was 0.3897±0.1204 mg/L.the sensitivity frequency of isolates to thifluzamide distributed as a unimodal curve,and no sub-group with declining sensitivity was detected.Therefore,the mean EC50 value could be considered as the sensitivity baseline of Sclerotium rolfsii to thifluzamide and could be used for the field resistance monitoring in Henan Province.Five isolates of Sclerotium rolfsii which were randomly selected as the wild parent isolates.Two stable genetic thifluzamide-resistant isolates(ZZ-3-1 and ZZ-3-2)were obtained by drug domestication.The mutation frequency was 0.2%and the resistance factor of two resistant isolates 40.3 and 79.8-fold compared with their parental isolates,respectively.The results of biological fitness analysis showed that there was no significant difference in the growth rate of mycelium,the dry weight of sclerotia and the diameter of sclerotia between the thifluzamide-resistant isolates ZZ-3-1 and the parent sensitive isolate,but the number of sclerotia and the pathogenicity were significantly lower than that of the parental sensitive isolate;there was no significant difference in the number of sclerotia,the dry weight of sclerotia,the diameter of sclerotia and the pathogenicity between the thifluzamide-resistant isolates ZZ-3-2 and the parent sensitive isolate,but the growth rate of mycelium was significantly lower than that of the parental sensitive isolate;The results of cross-resistance showed that there was no cross-resistance among thifluzamide and tebuconazole,tetraconazole,carbon and fludioxonil.The above analysis results show that Sclerotium rolfsii had a moderate resistance risk to thifluzamide. |
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