Investigation On The Incidence And Analysis Of Influencing Factorsof Tobacco Bacterial Wilt In Guangxi

Tobacco bacterial wilt caused by Ralstonia solanacearum has always been one of the main diseases in Guangxi,which seriously threatens the cultivation of tobacco,and the damage is increasing year by year.Clarify the epidemics of tobacco bacterial wilt has practical significance for guiding disease control.In view of this,according to the tillage method of the main tobacco producing areas in Guangxi,this study set up representative monitoring points,continuously detected the pathogen quantities in soil and tobacco plants,and systematically investigated the diseases occurrence,so as to master the infection dynamics of the pathogen and the diseases dynamics in the field.The effects of soil physical and chemical properties,overwintering pathogen quantities and meteorological conditions on bacterial infection and disease occurrence were analyzed.Based on the screening of fungicide with good control effect on tobacco bacterial wilt in pot experiment,combined with the infection dynamics of the pathogen,the control effect of fungicide application in different periods was determined,and the appropriate application period was explored.The main test results are as follows:（1）According to the nucleotide sequence of R.solanacearum flagellin（fli C）gene,the specific primer flicf2/flicr2 of R.solanacearum was designed.The primer could only amplify 136 bp fragment from the DNA of R.solanacearum,but could not amplify the target fragment from the DNA of other negative control strains.The minimum detection concentration of R.solanacearum quantities in soil by the established real-time quantitative PCR（q PCR）detection system was 5×10² cfu/g.The circulation threshold（CT）of the standard curve had a good linear relationship with the logarithm of the concentration of R.solanacearum in soil（R²=0.9911）,which could be used to detect R.solanacearum quantities in soil samples.（2）In 2021,the investigation of tobacco bacterial wilt was carried out in 8counties and cities in Guangxi,including Jingxi,Nandan and Zhongshan.It was found that the disease was widely distributed in tobacco-growing areas in Guangxi.In Jingxi and Debao,it was common even in paddy-dryland rotation tobacco fieldswhile in other tobacco growing areas,the disease occurred in tobacco and dryland rotation tobacco fields such as corn and sweet potato.Bacterial wilt could seriously occur on the main varieties Yunyan 87 and K326,but more seriously on Yunyan 87.（3）The soil of heavy diseased fields and their adjacent disease-free or light diseased fields were collected from the main occurrence areas of tobacco bacterial wilt in Guangxi,and R.solanacearum quantities and physical and chemical indexes were analyzed.A total of 31 samples were collected,16 soil samples in light diseased fields（with the average disease index of 2.96）and 15in heavy diseased fields（with the average disease index of 46.14）.There were six soil types of standard samples,basically,all of them had the possibility occurring bacterial wilt severely,but the silty loam accounted for the highest proportion in light diseased fields,which was 62.5%.Comparing the light and heavy diseased fields,logarithm of R.solanacearum quantities and p H value reached a very significant level,and the values of exchangeable magnesium,available boron,silty sand particle and sandy particle reached a significant level.Pearson correlation analysis of disease index and each index showed that the disease index had a significant positive correlation with logarithm of R.solanacearum quantities,a very significant negative correlation with p H value,and a significant negative correlation with organic matter and total nitrogen.The Fisher’s discriminant analysis of the incidence degree of tobacco bacterial wilt was carried out with p H value,exchangeable magnesium,available boron,silty sand particle and sandy particle of x₁-x₅,light disease of y₁ and heavy disease of y₂.The discriminant with a reliability of 87.1%could be obtained.The discriminant equation was: y₁=12.962x₁-0.771x₂+2.714x₃+0.025x₄+0.068x₅-63.130.y₂=11.644x₁-1.134x₂+2.153x₃+0.021x₄+0.070x₅-52.317.（4）When the logarithm of initial R.solanacearum quantities in soil was in the range of 3.32～5.82,the higher the initial bacterial quantities,the faster the infection process and the heavier the disease.However,in the range of5.82～8.84,the area under the course progress curve of bacterial wilt did not reach a significant level,the rate of diseased plants in the harvest period was close to 100%,and the disease index reached about 70.Taking the interval days from the transplanting period to the penetration period of the pathogen as the y-axis and the logarithm of initial R.solanacearum quantities in soil as the x-axis,a univariate linear regression equation was obtained:y=50.375-5.21x（n=5,R²=0.783*）,which could predict the penetration period of the pathogen.（5）In monitoring point in Jingxi,the interval from transplanting period to the penetration period of the pathogen was 40 days,the interval from transplanting period to the initial disease period was about 90～100 days,and the disease incubation period was about 50～60 days.The pathogen quantities of rhizosphere soil in heavy diseased field increased sharply 8 days before the onset,reaching the maximum value in the observation period,which was1.11×10⁷ cfu/g.The epidemic time dynamic model and disease index average daily growth model of tobacco bacterial wilt in Jingxi and Nanning were established by using logistic model and univariate cubic model.The epidemic time dynamic model of tobacco bacterial wilt in both places was better simulated by logistic model,the disease index average daily growth model was higher fitting degree by logistic model or univariate cubic model respectively.It could be seen that the disease incubation period of monitoring point in Jingxi was long,and the disease index increased sharply after the onset,while the disease incubation period of monitoring point in Nanning was short,the disease occured early,and the disease index increased steadily after the onset.（6）The average temperature from transplanting period to penetration period of the pathogen in monitoring point in Jingxi was 18.47℃.The average temperature from the penetration period of the pathogen to the initial disease period rose to 22.23℃,and the rainfall increased,with an average daily rainfall of 5.04 mm.The average temperature from the beginning of the disease to the peak was 24.96℃,and the average sunshine was 3.58 hours.In monitoring point in Nanning,the average temperature from transplanting period to penetration period of the pathogen was 21.43℃,the penetration period of the pathogen was close to the initial disease period,and the average temperature from the initial disease period to the peak was 25.60℃.（7）The pot control effect of chloroisobromine cyanuric acid plus copper sulfate on tobacco bacterial wilt was 44.51%,which was not significantly different from Burkholderia cepacian strain JX-1 and Pseudomonas fluorescens,and was significantly higher than other treatments.The optimal application time of chloroisobromine cyanuric acid plus copper sulfate was tested.The best control effect was 82.57%by applying the fungicide in the transplanting period and the penetration period of the pathogen,followed by 59.26%in the penetration period of the pathogen and initial disease period.It is was preliminarily considered that the initial stage of pathogen infection was one of the key periods for applying fungicide to control tobacco bacterial wilt.