Identification Of The Pathogen Of Southern Corn Leaf Blight In Different Areas Of Shandong And Effects Of Combined Infection With Maize Dwarf Mosaic Disease On Resistance Of Corn

Southern corn leaf blight（SCLB）occurs in all corn-producing areas in China and is one of the main fungal diseases infecting corn.Bipolaris maydis is the pathogen causing SCLB.Due to changes in climate and cultivation habits,etc,the population of B.maydis might be changed.B.maydis often mixed infected with other pathogens,e.g.,sugarcane mosaic virus,caused significant losses in corn production.In this study,we identified the pathogen of SCLB from different areas of Shandong province through biological and molecular biological methods,determined their pathogenicity,tested their sensitivity to different fungicides,analyzed the effect of B.maydis and SCMV mixed infection on corn cultivars against SCLB based field and lab experiments.The main results are shown as following:（1）Obtained 23 strains of B.maydis in Shandong province and analyzed their pathogenicity and sensitivity to difenoconazole and tebuconazole.Based on morphology and molecular biology,23 strains of B.maydis were identified from 8 regions in Shandong province.Those 23 B.maydis strains’growth rate was between 0.37 and 1.38 cm/day;their spore production rates were between 0.70×10⁴and 45.30×10⁴m L^-1;the conidia length was between 53.94 and 81.96μm.With nine polymorphism ISSR primers,a total of 72polymorphic bands were amplified from those 23 strains;they have a polymorphism ratio of93.51%;UPGMA cluster analysis using NTSYS2.10e software showed that the genetic similarity coefficient of those strains was between 0.50 and 0.97,indicating a high genetic variation.Detached leaves assays were used to determine the pathogenicity of different strains.Results showed that strains TA2,TA4,BZ3,JNi1,JNi2,DZ1 were more virulent than the rest strains;strains TA3,WF2 were the weakest one.The sensitivity of different strains to the two fungicides was analyzed using mycelial growth rate assays.Results showed that the EC₅₀of difenoconazole was between 0.6720 and 18.2497μg/m L,the EC₅₀of tebuconazole was between 0.6160 and 10.7171μg/m L,and the sensitivity of different strains to difenoconazole and tebuconazole were varied.（2）Screened the resistance of 332 corn lines against SCLB and SCMV and obtained 37 lines resistant to both pathogens.Among 332 corn lines inoculated with B.maydis,13 lines were high resistant（HR）,203 lines were resistant（R）,89 lines were middle resistant（MR）,22 lines were susceptible（S）,five lines were high susceptible（HS）.The resistance of 332 corn lines against SCMV under natural infection condition,48 lines did not show any SCMV symptoms;among them 10 lines were HR to SCLB,27 lines were R,11lines were MR.Analysis on 284 corn lines showing typical SCMV symptoms showed that one line was originally HR to SCLB but changed to R in the presence of SCMV;two lines were originally HR to SCLB but changed to MR in the presence of SCMV;41 lines were changed from R to MR,118 lines were changed from R to S,and 11 lines were changed from R to HS.After being infected by the SCMV,the susceptibility of corn to B.maydis is increased.（3）Definituded the resistance of corn lines to SCLB decreased after being infected by SCMV.With SCMV CP specific antibody,we analyzed the presence of SCMV in leaves with no symptoms of SCMV or SCLB,leaves only showing typical SCLB symptoms,leaves showing symptoms of both SCMV and SCLB.We analyzed the number and length of the necrotic area in SCLB and SCLB/SCMV-infected leaves and found that the number and length of the necrotic area are increased in the mixed infection leaves.Field survey showed that mixed infection SCLB and SCMV enhanced the disease index.With corn cultivar Denghai605,we found that mixed infection with SCMV and SCLB enhanced the number and length of the necrotic area under lab condition.