Study On Pathogen Identification And Occurrence Regularity Of Tobacco Target Spot In Hunan

Tobacco target spot disease has occurred in some tobacco-growing areas in Hunan Province in recent years,which is caused by Rhizoctonia solani.It spreads rapidly after the disease,causing considerable economic losses to tobacco farmers.In this paper,the pathogen of tobacco target spot disease in seven tobacco growing areas in Hunan was isolated,identified by molecular and fusion groups,and the occurrence regularity of tobacco target spot disease in each ecological tobacco growing area in Hunan was investigated.The infection characteristics such as variety resistance,disease conditions,sporulation conditions and overwintering methods were studied.The results are as follows :(1)Sixteen tobacco target spot pathogens were isolated from Hunan tobacco-growing areas.After Koch ’ s rule verification and molecular detection,the phylogenetic tree was constructed,and the pathogen was identified as R.solani,belonging to AG-3 fusion group.The fusion group types of 16 strains were further determined by slide positioning fusion method.Among them,XY1,XY2,XY3,XY4 and XY5 were completely fused,belonging to the same fusion group;XL1,HJ and CS are completely fused and belong to the same fusion group;XL2,ZG1,ZC1-1 and ZC1-2 are completely fused,belonging to the same fusion group;the incomplete fusion of XH,ZC2-1,ZC2-2 and YJ belongs to the same fusion group.(2)The occurrence regularity of tobacco target spot disease in ecological tobacco areas of central south and northwest Hunan was investigated.In 2022,the field onset period of tobacco target spot disease in central and southern Hunan was April 30,and the field onset period in northwestern Hunan was May 29.About 7 days after the disease onset,it entered the peak period,which lasted for about 14 days,and then entered the slow growth period of about 10 days.The second peak period in the field was different due to the difference in temperature and humidity.The incidence rate of systematic investigation field was 27.15-30.99 %,and the disease index was 5-6.The incidence rate of general investigation field was 0.85-11.48 %,and the disease index was 0.8-2.6.(3)The resistance of main varieties of tobacco target spot in Hunan was investigated,and the disease condition and sporulation condition were determined.The incidence of tobacco target spot disease of four main tobacco cultivars in Hunan,namely,Yunyan 87,Xiangyan 7,GZ36 and K326,exceeded 90%,and the disease index was higher than 20,indicating that they were not resistant to tobacco target spot disease.The optimum humidity for pathogen infection was 100% and the temperature was 25-30 °C;the optimum temperature and humidity for lesion expansion were 20-25 °C and 90-100 %,respectively.Under the conditions of 20-25 °C and 100%humidity,the lesion produced aerial hyphae and basidiospores after 78 h;the pathogen overwinters in the soil through the diseased residue and sclerotium.