Comparative Study on the Efficacies of 11 Chemical Agents against Taro Soft Rot Caused by Pectobacterium aroidearum

Dongkun Wang,Xiuting Ji,Huaixu Zhan,Chengpeng Zheng,Hao Zong,Xiaolei Tan,Xiuchun Du,Weimin Wang,Jingjing Wu,Fenglong Wang,Xiaoqiang Wang*

1.Tobacco Research Institute,Chinese Academy of Agricultural Sciences,Qingdao 266101,China;2.Shandong Linyi Tobacco Co.,Ltd.,Linyi 276000,China;3.Jiaozhou Branch of Qingdao Tobacco Co.,Ltd.,Qingdao 266300,China;4.China Tobacco Zhejiang Industry Co.,Ltd.,Hangzhou 310000,China

Abstract [Objective]The paper was to screen the effective agents for effectively controlling the occurrence and prevalence of taro soft rot.[Method]The fungicidal effects of 11 fungicides on taro soft rot were determined by inhibition zone method.[Result]0.3% Tetramycin,25%bromothalonil,3%Zhongshengmycin and 30%zinc thiazole had better antifungal effect on Pectobacterium aroidearum at recommended concentration.The indoor toxicity of four fungicides showed that the EC50of 0.3%tetramycin AS was the smallest of 0.6 μg/mL,indicating that tetramycin had the largest toxicity and the best inhibitory effect against P.aroidearum;followed by 25%bromothalonil EC and 3%Zhongshengmycin WP,with the EC50 of 2.57 and 97.72 μg/mL,respectively;the inhibitory effect of 30%zinc thiazole SC against P.aroidearum was the poorest.[Conclusion]The study provides a reference for screening out new and efficient chemical agents against taro soft rot.

Keywords Taro soft rot;Pectobacterium aroidearum;Efficacy;EC50;Chemical agents

Soft rot caused by Pectobacterium carotovorum subsp.carotovorum is one of the most important diseases in taro production.The disease is widely distributed in taro planting areas in China,mainly damaging petiole base,plant base and underground corm.The diseased tissue become brown,soft and even rot,or the whole plant dies in severe cases,and the diseased part gives off a bad smell.P.carotovorum spreads via rainfall,irrigation water,insects and agricultural activities,and invades from wounds.Continuously cropping land,low wetland,or high temperature and heavy rainfall easily cause the occurrence and prevalence of the disease.When the conditions are suitable,the disease will occur in large area,causing serious economic losses or even no harvest.The disease occurs in the growth,transportation and storage process of taro,which seriously affects the quality of taro.

In 2020,serious soft rot occurred in taro planting area in Jimo area of Shandong Province.A new strain,Pectobacterium aroidearum,was isolated from diseased taro,and the pathogen could cause soft rot of various crops.

Chemical control is still the major measure against taro soft rot,and copper preparation(such as thiediazole copper),antibiotic fungicides(such as embamycin)are mainly used.Studies have shown that embamycin has certain control effect against coriander soft rot[1],konjac soft rot[2],carrot soft rot[3]and sorghum soft rot[4],and thiediazole copper can effectively control banana soft rot[5].Currently,there are relatively few reports on the screening of agents against taro soft rot,and little reports on comparative studies of efficacy against taro soft rot.The agents were mainly screened for the prevention and control of P.carotovorum,while the agents specially for the prevention and control of P.aroidearum had not been screened.Therefore,in order to effectively control the occurrence and prevalence of taro soft rot,it is necessary to further screen the effective control agents against taro soft rot.In this study,the indoor inhibitory effects of 11 commonly used agents on P.aroidearum were determined,and the efficacy of four agents with higher inhibitory effects were compared,in order to screen out new and efficient chemical agents for the prevention and control of taro soft rot.

1 Materials and Methods

1.1 Materials

1.1.1 Strains.P.aroidearum was preserved in Tobacco Research Institute,Chinese Academy of Agricultural Sciences.

1.1.2 NA medium.NA medium:peptone 10.0 g,beef extract 3.0 g,sodium chloride 5.0 g,agar 15.0 g,distilled water 1L,pH 7.3±0.1.The medium was sterilized in sterilization pot.

1.1.3 Agents.The agents used in the study included 25% bromothalonil EC,20% thiosen copper SC,20% thiediazole copper SC,30%zinc thiazole SC,garlicin EC,30%hymexazol AS,qingkulike AS,33.5% copper quinolate SC,0.3% tetramycin AS,3%zhongshengmycin WP,and 80%bordeaux mixture WP.The name,recommended concentrations and manufacturers of the tested agents are shown in Table 1.

Table 1 Agents used in the test

1.2 Methods

1.2.1 Primary screening of indoor agents.The indoor toxicity of agents was preliminarily determined by inhibition zone method[6].P.aroidearum stored at-80℃were activated and cultured on NA solid medium,and then performed extended culture.The bacterial suspension with the concentration of 105CFU/mL was prepared with sterile water.Approximately 15 mL of melted solid NA medium was poured into a petri dish with the diameter of 9 cm and made into a plate.Approximately 100 μL of bacterial suspension were evenly spread on the cooled NA plate.After the bacterial suspension was dried,subequilateral triangle holes were punched by a puncher(ϕ9 mm),and the interval of each hole was 32 mm.The 11 test agents were diluted according to the recommended concentrations. Approximately 50 μL of diluents and original solutions of test agents were added into holes,respectively,and those added with sterile water were used as blank control(CK).After cultured in an incubator at 28℃for 48 h,whether there was obvious inhibition zone around holes was observed.According to the presence and size of inhibition zone,the agents with good fungicidal effects were screened.The test was repeated three times.

1.2.2 Indoor toxicity determination of effective agents.The agents with good antifungal effects were dissolved in sterile deionized water,and prepared into 10 000 μg/mL mother liquids for later use(prepared immediately before use).The mother liquid of each agent was diluted with sterile deionized water according to the following gradients(Table 2).Approximately 50 μL of fluid were added into holes of the plate containing NA medium,and those added with sterile water was used as blank control(CK).Each concentration was repeated three times.After cultured at 28℃for 48 h,the inhibition zone diameter(mm)was measured by crossing method.The inhibition rate of colony growth was calculated according to the method described by Sun et al.[7].With the logarithm of mass concentration of agents(denary logarithm)as the abscissa and the probability value corresponding to inhibition rate as the ordinate,the plots were drawn,and the toxicity regression equation and median effective concentration(EC50)were obtained.

Table 2 Final concentration of four effective fungicidal agents(active ingredients)

Inhibition rate of colony growth(%)=(inhibition zone diameter in treatmentinhibition zone diameter in control)/inhibition zone diameter in treatment×100%

2 Results and Analysis

2.1 Primary screening of indoor agents The antifungal test by horizontal diffusion method showed that at recommended concentrations,the test agents had no fungicidal effect on P.aroidearum except for 0.3%tetramycin,25%bromothalonil,3%zhongshengmycin and 30%zinc thiazole(Fig.1).The determination results of original liquids of agents showed that thiosen copper,copper quinolate,thiediazole copper and garlic oil had certain fungicidal effect,but their fungicidal effects in the test were very poor,with the inhibition zone diameters of 0.4,12.6,4.2 and 4.7 mm,respectively.Therefore,tetramycin,bromothalonil,zhongshengmycin and zinc thiazole were selected for indoor toxicity determination.

Fig.1 Inhibitory effects of four agents against Pectobacterium aroidearum at recommended concentrations

2.2 Indoor toxicity determination of effective agents The EC50of 0.3%tetramycin AS was the smallest of 0.6 μg/mL,indicating that tetramycin had the largest toxicity and the best inhibitory effect against P.aroidearum;followed by 25%bromothalonil EC and 3%zhongshengmycin WP,with the EC50of 2.57 and 97.72 μg/mL,respectively;the inhibitory effect of 30%zinc thiazole SC against P.aroidearum was the poorest(Table 3 and Fig.2).The inhibitory effects of various agents against P.aroidearum successively were 0.3%tetramycin AS ＞25%bromothalonil EC＞3%zhongshengmycin WP＞30%zinc thiazole SC.

Table 3 Toxicity of four effective agents against Pectobacterium aroidearum

Fig.2 Inhibitory effects of four effective agents against Pectobacterium aroidearum

3 Discussion

Taro soft rot occurs in various growth periods of taro,which harms root,stem and leaf of plants,especially underground corm,greatly affecting the yield or even leading to no harvest in severe cases.In addition,taro can also be infected in the process of transportation and storage,resulting in huge economic losses.In order to prevent and control taro diseases effectively,the toxicities of 11 agents against P.aroidearum,a new pathogen of taro soft rot,were determined,which would lay a foundation of the prevention and control of P.aroidearum.The results showed that 0.3%tetramycin,25%bromothalonil,3%zhongshengmycin and 30%zinc thiazole had good inhibitory effects against P.aroidearum.The inhibitory effect of 0.3%tetramycin AS was the best,with the smallest EC50of 0.6 μg/mL.

The inhibitory effects of 11 chemical agents on P.aroidearum,a new pathogen of taro soft rot,were determined.The indoor toxicity test showed that the inhibitory effects against P.aroidearum varied with agents.Except for 0.3%tetramycin,25%bromothalonil,3%zhongshengmycin and 30%zinc thiazole,the other seven agents had no inhibitory effects against P.aroidearum at recommended concentrations,and the original liquids of seven agents also had poor inhibitory effect against P.aroidearum.Within the test concentrations,the inhibitory effects of 0.3%tetramycin,25%bromothalonil,3%zhongshengmycin and 30%zinc thiazole against P.aroidearum increased with the increase of mass concentration.Therefore,it is of important reference value to determine the EC50of various agents for rational and accurate medication.The toxicity determination results demonstrated that the EC50of 0.3%tetramycin AS was the smallest,indicating that 0.3%tetramycin had the best control effect against P.aroidearum;followed by 25%bromothalonil EC and 3%zhongshengmycin WP,with the EC50of 2.57 and 97.72 μg/mL,respectively;the inhibitory effect of 30%zinc thiazole SC against P.aroidearum was the poorest.The results of the study lays a foundation for the control of P.aroidearum and the follow-up screening of agents against taro soft rot in the field.