Identification And Biological Characteristics Of The Pathogen Causing Fig Leaf Spot And Screening Of Effective Agents

Fig leaf spot is a new disease of fig caused by Lasiodiplodia theobromae,which has not been reported yet.An unknown leaf disease of fig was discovered in Guanting Town,Hefei City in September 2019.Four strains were isolated from the infected leaves of fig.It can be divided into Fusarium oxysporum and Lasiodiplodia theobromae.According to Koch’s rule,the pathogen of fig leaf spot was identified as Lasiodiplodia theobromae.By studying the biological characteristics of the pathogen and measuring the inhibitory effect of five fungicides on the mycelia and spores of the pathogen,it provided a reference for the occurrence,epidemic and prevention and control of the fig leaf spot.The main research results are as follows:1.Isolation,identification and pathogenicity determination of pathogenic bacteriaThe pathogen was purified by conventional tissue isolation method and formed white cotton flocculent colony on PDA medium.The colony was white at the initial stage,then deepened gradually,and turned black at the later stage.The conidiophores were half buried in the culture medium and were in lead black globular or irregular shape.Microscopically,the conidia are oval or oval in shape,with a single spore about 21.5-31.5μm in lengthμm.Width 13-17μm.In the early stage,the conidia are colorless,transparent,without septum,with granules in them.In the later stage,they gradually turn dark brown,with a septum in the center.Cluster analysis of the internal transcribed spacer（ITS）and the elongation factor-1 alpha（EF-1a）gene of the pathogen showed that it was closely related to several strains of Lasiodiplodia theobromae（MK460232、MK370843、MH447076 and MK530011）were in the same branch,and the support rate was 99%.Combined with morphological observation and pathogenicity test,the pathogen was Lasiodiplodia theobromae.2.Biological characteristics of fig leaf spotLight,medium,p H value,temperature,mycelium lethal temperature,and the utilization of carbon and nitrogen sources were determined in this experiment.The results showed that light had no obvious effect on the mycelial growth of Lasiodiplodia theobromae mycelium.PDA medium was the most suitable medium for the growth of P.figs.When the p H value was 5-7,the mycelial dry weight of Lasiodiplodia theobromae mycelium reached the maximum.The optimum temperature was 28℃for the growth of Lasiodiplodia theobromae in the range of25-35℃.Water bath at 54℃for 10min could not grow.In the liquid medium with sucrose as the carbon source,Lasiodiplodia theobromae mycelium grew best,the dry weight of mycelia was the largest,and the growth in the medium without carbon source was the worst,indicating that this pathogen had certain requirements for carbon source.In the liquid medium with sodium nitrate as the nitrogen source,the growth was the best,and the dry weight of mycelia was the largest,while in the medium without nitrogen source,the growth was the worst,indicating that the pathogen had certain requirements for nitrogen source.3.Determination of virulence of fungicides against fig leaf spotThe inhibitory effects of propiconazole,carbendazim,prochloraz,fluosilazole and tebuconazole on Lasiodiplodia theobromae were studied by laboratory toxicity assay.The results showed that fluosilazole had the best inhibitory effect on the growth of mycelia,with the EC₅₀value of 1.0554μL/m L.The EC₅₀values of propiconazole and carbendazim were 1.3282μL/m L and 1.8213μL/m L,respectively.The EC₅₀value was5.6652μL/m L.The bacteriostatic effect of tebuconazole was the worst,with EC₅₀value of 11.4610μL/m L.Propiconazole had the most obvious inhibitory effect on conidial germination,the conidial germination rate was only 32.93%,the relative inhibitory rate of conidial germination reached 67.07%.The conidial germination rate was 45.26%,and the relative inhibition rate was 54.57%;The inhibition of carbendazol on spore germination was general,the conidial germination rate was57.14%,and the relative inhibition rate was 42.86%;The inhibition of prochloraz on conidial germination was poor,the conidial germination rate was 63.73%,and the relative inhibition rate was 36.27%;Tebuconazole had the worst inhibition on conidial germination,with the conidial germination rate of 72.06%and the relative inhibition rate of 27.94%.