Monitoring Of Metalaxyl-resistance In Phytophthora Sojae And Sieving Of Effectual Fungicides For Control

Phytophthora sojae cause root rot of soybean,one of the most devastating disease ofsoybean, which can causes approximately1to2billion USD in the world．At present,metalaxyal was used widely to control the disease, but the resistance was engenderingeasily for the single function site of metalaxyl. In this dissertation，the sensitivity tometalaxyl, the mutagenizing of metalaxyl-resistant mutants, the biological character ofmetalaxyl-resistant mutant in P. sojae and the sifting of effective fungicides for the of thedisease caused by it were studied，respectively. The results are as follows：The sensitivity of57Phytophthora sojae isolates obtained from Anhui, Shandong andHenan to metalaxyl was tested by the methods of mycelial growth inhibition．The resultsshowed that EC50values of57isolates ranged from0.0362g·mL^-1to0.2408g·mL^-1, andthe mean EC50value was0.1043g·mL^-1. So the population of P. sojae from the threeprovinces has been still sensitive to metalaxyl. However, sensitivity of the populations in3provinces to metalaxyl was different. Sensitivity of the populations in Anhui to metalaxylwas the lowest and the mean EC^-150value was0.1077g·mL. Sensitivity of the populationsin Henan to metalaxyl was higher than that in Anhui, the mean EC50value was0.0966g·mL^-1. Sensitivity of the populations in Shandong to metalaxyl was the highestand the mean EC50value was0.07308g·mL^-1.A metalaxyl-resistant mutant was obtain from the wide-type parent isolate by the drugtaming method and EC50value of metalaxyl to it was158.222g·mL^-1. The rate of hyphalgrowth and sporangium, the sizes of sporangium and row-spore-hole of metalaxyl-resistantmutant were the same as those of wide-type parent, but oospore production ofmetalaxyl-resistant mutant were less than that of wide-type parent. Pathogenicity ofmetalaxyl-resistant mutant were very high and the same as that of the wide-type parent.The toxicity of the5fungicides to P. sojae were tested by the methods of mycelialgrowth inhibition, and the result indicated that the EC50value of Anke（dimethomorph）tothe mycete was0.11792g·mL^-1, which was simlar to metalaxyl．The interaction ofdimethomorph and metalaxyl was tested, and the result showed that the two fungicideshave a synergy，so the inhibitory effect of the different ratios of the mixture to P. sojaewere tested. The EC50values of differents mixing of the two fungicides ranged from0.0377g·mL^-1to0.117g·mL^-1. When the ratiowas1:1, it has the most obvious effect: theEC50was0.0377g/mL, CTC was946, and the synergistic effect was obvious. When theratio was2:3, the mixture also has a high obvious synergy with the EC50of0.0387g/mL and CTC of858. The synergy of the ratio of1:2,3:1and3:2were all less than that of theratio of1:1and2:3. The lowest CTC were the ratio of2:1and1:3, but they were also highmore than100, which is the lowest threshold of synergy. The results also showed that therewas obvious correlativity between CTC and the ratio. The sporangium production ofPhytophthora sojae treated by the mixtures at different ratios were also tested, and the resultsshowed that the sporangium production were restrained seriously. When the concentrationof0.5g/mL, the inhibition of all the ratios were reached50%; the ratio of2:3has thehighest inhibition, and its inhibition rate was77.41%; the lowest was1:1, the inhibitionrate was54.84%.