Haitao Wang,Haohao Hao,Xiaojing Xu*,Haitang Huang,Xueli Song,Lijun Yang

1.Institute of Tobacco Research,Henan Academy of Agricultural Sciences,Xuchang 461000,China;2.Zhumadian Branch of Henan Provincial Tobacco Company,Zhumadian 463000,China

Abstract [Objective]The paper was to study the relationship between spatial distribution characteristics of soil pH and bacterial wilt in tobaccogrowing area in Henan Province.[Method]The 21 tobacco-growing fields infected by bacterial wilt and 91 tobacco-growing fields with no cases were selected from Henan Province,and the pH of fields was determined.[Result]Bacterial wilt mainly occurred in the tobacco-growing fields in Luoshan,Pingqiao,Queshan,Biyang,Suiping,and Xiping,and the soil pH mainly ranged from 5.5 to 6.5(accounted for 84.75%of total surveyed tobacco fields);bacterial wilt had not been found in tobacco-growing fields in Tanghe,Zhengyang,Shangcai,Sheqi,Wuyang and Chenqu,and the soil pH mainly ranged from 6.5 to 7.0(accounted for 81.25%of total surveyed tobacco fields).There were significant differences in soil pH between the fields with and without bacterial wilt,and soil pH 6.5 could be regarded as the threshold for the occurrence of tobacco bacterial wilt in Henan Province.There was extremely significantly positive linear correlation between soil pH and latitude (y=0.737 5x-17.799,R2=0.386,P＜0.01).The geographical boundary of soil pH 6.5 was roughly 33°N,and bacterial wilt was less likely to occur in the north tobacco-growing areas of the boundary.[Conclusion]The study will provide a scientific basis for the prevention and control of tobacco bacterial wilt in Henan.

Keywords Soil pH;Bacterial wilt;Tobacco;Henan

Tobacco bacterial wilt,one of the most important soil-borne diseases,is a bacterial vascular disease caused by Rastonia solanacearum in the rhizosphere soil of tobacco.Tobacco bacterial wilt is a major destructive disease threatening tobacco production,and it is very difficult to control the disease[1].

Henan is an important flue-cured tobacco-growing area in China,with a planting area of about 47 000 hm2in 2018.Bacterial wilt has occurred for a relatively short history in tobacco-growing area of Henan Province.Suspected strains of bacterial wilt were found in Suiping in 2011,and then the damage was gradually aggravated.In 2014,tobacco bacterial wilt was successively found and confirmed in Queshan,Xiping,Suiping and Biyang of Zhumadian City,and Pingqiao and Luoshan of Xinyang City[2-3].

pH is an important attribute of soil,which affects the occurrence and damage degree of tobacco bacterial wilt[4-6].Wang et al.[6]suggested that soil pH and temperature were the decisive factors affecting the incidence of tobacco bacterial wilt,and the optimal pH was 6.0.Wang et al.[7]put forward that pH 5.5 and 6.5 were the best acid-alkali environment for the expression of tobacco resistance genes.It is an effective way to control tobacco bacterial wilt in tobacco-growing areas in southwest China by applying alkaline substances to increase soil pH[8-11].

Since 2015,Henan tobacco system has effectively controlled the occurrence and spread of bacterial wilt in Zhumadian and Xinyang cities by taking measures such as forced crop rotation.However,whether bacterial wilt has occurred in other tobacco-growing areas and the geographical boundaries of bacterial wilt are not clear.Since R.solanacearum can survive in soil or compost for 2-3 years or even longer[1],the harm of bacterial wilt may re-occur under suitable climatic conditions.Therefore,it is necessary to study and define the environment of producing area where tobacco bacterial wilt easily occurs in Henan,so as to strengthen the prevention and control of bacterial wilt.Therefore,the spatial distribution characteristics of soil pH in tobacco-growing areas and its relationship with the occurrence of bacterial wilt in Henan Province were studied,in order to determine the range of tobacco-growing area where bacterial wilt easily occurs and its geographical boundary.

1 Materials and Methods

1.1 Survey region A total of 12 counties (districts),including Luoshan and Pingqiao counties (districts)in Xinyang City,Queshan,Biyang,Suiping and Xiping counties,as well as adjacent Chengqu(Yancheng County)and Wuyang counties(districts)in Luohe City,Shangcai and Zhengyang in Zhumadian City,Sheqi and Tanghe counties in Nanyang City,were chosen as the survey regions(Fig.1).

Fig.1 Survey region and sampling sites

1.2 Soil sample collection and pH measurement Soil samples of infected tobacco fields were collected from 21 tobacco fields attacked by bacterial wilt from July to August 2014.Soil samples of disease-free tobacco fields were collected after flue-cured tobacco harvest in 2014.According to the principle of one repre-sentative tobacco field each village(town),soil samples were collected from 91 tobacco fields,with plain tobacco area＞30 hm2and hilly tobacco area＞15 hm2in tobacco planting area of sampled villages and towns.The soil samples were collected from S-shaped 5-8 points at the arable layer of 0-20 cm in the same tobacco field,and mixed thoroughly.Approximately 1.5 kg of soil samples were kept by quartering method for later use.

After indoor air drying and impurity removal,the collected soil samples were ground through a 10-mesh sieve and extracted by 2.5∶1 water and soil mass ratio,and the pH was determined by potentiometer method[12].

1.3 Data processing and analysis Microsoft Excel 2016 was used for data description and statistics,and DPS 15.1 program was used for correlation analysis,cluster analysis and difference significance test.

2 Results and Analysis

2.1 General situation of infected tobacco fields In 2014,tobacco bacterial wilt was successively found and confirmed in Luoshan,Pingqiao,Queshan,Biyang,Suiping and Siping in pest investigation in tobacco fields[2-3],and the soil pH of affected tobacco fields ranged from 5.58 to 6.59,with an average of 6.13(Table 1).

Table 1 Information of surveyed fields with tobacco bacterial wilt(2014)

2.2 Statistical characteristics of soil pH It is generally believed that acid(pH≤5.5)or weak acid (pH 5.5-6.5)soil is benefit for the occurrence of tobacco bacterial wilt,while neutral(pH 6.5-7.5)or alkaline(pH＞7.5)soil is not conducive to the propagation of R.solanacearum.As shown in Table 2,the soil pH of six counties(districts)that had attacked by bacterial wilt ranged from 5.15 to 7.77,with an average of 6.06.The proportion of weakly acidic soil was the highest,reaching 84.75%;followed by acidic and neutral soil,accounting for 6.78%;and the proportion of alkaline soil was the least,only 1.69%.The soil pH of six counties with no cases was 6.40-8.10,with an average of 7.09.The proportion of neutral soil was the highest,reaching 81.25%;followed by alkaline soil,accounting for 15.63%;and the proportion of weakly acidic soil was the least,only 3.14%.The results demonstrated that the biggest difference between the tobacco fields with and without the disease was the classification and distribution frequency of soil pH;pH 5.5-6.5 occupied an absolute advantage in the infected tobacco fields,while pH 6.5-7.5 occupied an absolute advantage in disease-free tobacco fields.As shown in Table 3,the average soil pH of tobacco-planting areas in 12 counties(districts)successively were Luoshan＜Pingqiao＜Biyang＜Queshan＜Suiping＜Siping＜Tanghe＜Zhengyang＜Shangcai＜Sheqi＜Wuyang＜Chengqu;the average soil pH of Luoshan,Pingqiao,Biyang,Queshan,Suiping and Xiping counties(districts)was 5.87-6.27,belonging to weak acid soil;the average soil pH of Tanghe,Zhengyang,Shangcai,Sheqi and Wuyang ranged from 6.83 to 7.41,belonging to neutral soil,but the average soil pH of Wuyang was 7.41,close to alkaline soil;the average soil pH of Chengqu was 7.78,belonging to alkaline soil.The coefficient of variation of Suiping was the largest,up to 11.85%,indicating that there were great changes in soil pH and both acidic and alkaline soils existed in the area;followed by Shangcai,Luoshan and Wuyang,with coefficients of variation≥7%,and the coefficients of variation of other counties(districts)were small.The results of 5%difference significance analysis showed that the average soil pH of Luoshan,Pingqiao,Biyang,Queshan,Suiping and Xiping counties(districts)was the lowest,although there were differences among the six counties,the difference was not significant.The average soil pH of Tanghe and Zhengyang were 6.83 and 6.85,respectively,which were significantly higher than those of the six counties that had been attacked by bacterial wilt.The average soil pH of Shangcai and Sheqi were 6.98 and 7.06,slightly higher than those of Tanghe and Zhengyang,but the difference was not significant.The average soil pH of Chengqu was the highest,significantly higher than those of other counties (districts)except Wuyang.The average soil pH of Wuyang was between those of Sheqi and Chengqu,but had no significant difference with those of Shangcai,Sheqi and Chengqu.

Table 2 Classification and frequencies of soil pH of surveyed tobacco-planting fields

Table 3 Statistics of soil pH of tobacco-planting fields in different regions

2.3 Spatial distribution characteristics of soil pH The correlation analysis of soil pH and corresponding geographical location parameters(Fig.2)showed that the pH of tobacco-growing soil had very significant negative correlation with the longitude of sampling plots,very significant positive correlation with the latitude of sampling plots,and significant binomial correlation with the altitude.

Fig.2 Correlation between soil pH and geographic parameters of tobacco-planting fields

2.4 Clustering analysis of soil pH The 12 counties(districts)were roughly divided into three groups by dynamic clustering analysis of their average pH.The first group included Luoshan, Pingqiao,Biyang,Queshan,Suiping and Xiping,which were the six counties(districts)infected by bacterial wilt;the average soil pH was 6.10,with a range of 5.87-6.27.The second group included Wuyang and Chengcheng counties(districts),and the average soil pH was 7.60,with a range of 7.41-7.78.The third group included Tanghe,Zhengyang,Shangcai and Sheqi,and the average soil pH was 6.93,with a range of 6.83-7.06.Statistical analysis showed that there were extremely significant differences in soil pH among the three groups,indicating that the first group was the area most easily attacked by tobacco bacterial wilt,while the second group had the largest difference with the first group,with a relatively high pH,so it was the area less likely to be infected by bacterial wilt.

The average pH values of the 12 counties(districts)were fuzzy clustering based on the Euclidean distance method(Fig.3),which could be roughly divided into two types:the six counties(districts)with confirmed cases were all classified as the first type,and the other six counties(districts)with no cases were all classified as the second type.The difference of average soil pH between the two types of tobacco areas was 1.03 units(Table 1),which further indicated that low pH(more acidic)of soil in the first type of tobacco area promoted the proliferation of R.solanacearum,easily leading to the occurrence of tobacco bacterial wilt.However,high pH (neutral,alkaline)of soil in the second type of tobacco area inhibited the proliferation of R.solanacearum,leading to no or low incidence of tobacco bacterial wilt.

Fig.3 Fuzzy clustering analysis of soil pH of tobacco-planting fields

3 Discussion

The tobacco fields that have ever been attacked by bacterial wilt have relatively low soil pH.The infected tobacco fields are mainly distributed in southern part of Henan Province,with high rainfall and relatively strong base substances leaching.Moreover,some tobacco fields are yellow brown soil,so the soil pH is low.However,due to low rainfall or brown soil,the soil pH of surrounding tobacco areas is relatively high[13-15].

Soil acidification will affect the activity of mineral nutrient elements such as nitrogen,phosphorus and potassium,and heavy metal elements,which further affects the normal growth and development of plant roots and easily leads to the occurrence of tobacco bacterial wilt,but alkaline environment inhibits the occurrence of tobacco bacterial wilt[4].There is a traditional practice of applying quicklime to improve soil to control bacterial wilt in low pH tobacco-planting area in southern China.Wang et al.[16]also suggested that increased secretion of extracellular polysaccharide of R.solanacearum under acidic conditions promoted the recognition and adsorption of R.solanacearum and tobacco root,which was conducive to the incidence of bacterial wilt,while neutral and alkaline conditions were conducive to the secretion of lipopolysaccharide and reduced the probability of occurrence of the disease.Our research results showed that there were significant differences in soil pH between six counties(districts)infected by bacterial wilt and six disease-free counties (districts)in Henan Province.The average soil pH of six counties(districts)infected by bacterial wilt had the largest differences with those of Chengqu and Wuyang counties(districts),but had relatively small differences with those of Shangcai,Zhengyang,Sheqi and Tanghe counties.Wuyang and Chengqu are separated with Xiping by a river,respectively.The three counties(districts)have basically the same climatic conditions and the same planting varieties,but the soil pH of Xiping is obviously lower than that of the other two counties(districts).Tobacco bacterial wilt has not been found in Wuyang and Chengqu,which further illustrates that soil pH is one of the key factors determining the occurrence of tobacco bacterial wilt.The soil pH of Luoshan,Pingqiao,Queshan,Biyang,Suiping and Xiping counties(districts)attacked by tobacco bacterial wilt in Henan Province mainly ranged from 5.5 to 6.5.Tobacco bacterial wilt had not been found in the six counties(districts)bordering in east,west and north directions,and the soil pH was mainly distributed from 6.5 to 7.5.Therefore,it could be roughly inferred that pH 6.5 could be used as an important indicator to judge the occurrence of tobacco bacterial wilt.

There was significant positive correlation between soil pH and latitude in Henan tobacco-planting soil.With the increasing elevation of latitude from south to north,pH showed an obvious upward trend.According to the regression equation between pH(y)and latitude(x):y=0.737 5x-17.799,it is found that the fitting value of latitude was 32.97°when the pH was 6.5.The latitude line of 33°N falls just near the boundary between Suiping and Xiping,which extends to Sheqi in the west and Shangcai in the east.The fitting value of latitude was 33.63°when the pH was 7.0,and the boundary of Xiping with Wuyang and Chengqu is just in the vicinity of 33.5°.

It is of great significance for diagnosis,prevention and control and spread prevention of tobacco bacterial wilt by clearing soil pH condition and approximate range of its occurrence.Certainly,the occurrence range of tobacco bacterial wilt is not only affected by soil pH,but also related to the accumulation of pathogens,variety resistance,temperature and soil texture.There are two important reasons for the rapid and effective control of bacterial wilt in tobacco-planting areas of Henan Province.First,there is a short history of bacterial wilt in tobacco-planting areas, and the accumulation of pathogens in soil is relatively small.Second,tobacco companies have reduced the transmission rate of bacterial wilt by opti-mizing the distribution of tobacco fields,prohibiting continuous cropping of fluecured tobacco in affected plots,and tobacco farmers have consciously implemented crop rotation out of their own interests.In terms of the resistance of planting varieties,Yunyan 87,a medium resistant variety[17-19],is the main variety planted in the tobacco-planting area of southern Henan Province,and acidic soil conditions lead to the incidence of bacterial wilt.However,susceptible cultivar Zhongyan 100 is mainly planted in the tobacco-planting areas of central and eastern Henan with high soil pH[20],so it has not been affected by bacterial wilt.

4 Conclusions

(i)The tobacco fields infected by bacterial wilt in Henan Province were mainly weakly acid soil,while the tobacco fields without bacterial wilt infection were mainly alkaline and neutral soil.The soil pH 6.5 could be used as the threshold value to determine the occurrence of bacterial wilt in Henan Province.

(ii)There was significant positive correlation between soil pH and latitude.33°N is the geographical boundary of soil pH 6.5,which could also be used as the geographical boundary for the occurrence of tobacco bacterial wilt.