Study On Controlling Potato Lste Blight With Seed Coating And Fungicide Resistance In Phytophthora Infestans

Virulence of four fungicides to Phytophthora infestans was tested by a radical growth assay on fungicide-amended agar. Virulence of flumorph and metalaxyl was strongest. EC50 of the two fungicides was 1.55 i?g ?mL and 1.63 i?g mL? EC50 of propamocarb was 5.43 i?g. mL? Virulence of fosety-A I was weakest. EC50 of the fungicide was 246.58 i1 g mL? But flumorph seriously affected emergence of potato. So, two fungicides of metalaxyl, propamocarb were selected as seed coating effective ingredient of seed coating controlling potato late blight. Seed coating of the concentration tested on seed pieces could inhabit but not kill pathogen in seed pieces in the laboratory. Persistent period of seed coating was more than one month. The sensitivity to metalaxyl of P. infestans was assessed by an in vitro radical growth assay on fungcide-amended agar media. The sensitivity to metalaxyl of 46 isolates collected from 4 main potato production provinces differed significantly and highly metalaxyl-resistant and metalaxyl-sensitive isolates existed generally in China. Range of 5:0 i?g mL?was 0.03?.20. Floating leaf-disk assay also could be used to assess resistance to matalaxyl of P infestans, but a radical growth assay in vitro was more accurate than .floating leaf-disk. Leaf-disk assay in viva was used to assess fitness of metalaxyl-resistant and metalaxyl-sensitive isolates. It showed fitness of metalaxyl- resistant isolates were higher than metalaxyl-sensitive ones. The ability of P. infestans to overwinter in infected potato tuber, Atlantic, was studied. Recovery of infective sporangia from tuber 12 weeks after inoculation with 3 metalaxyl-sensitive (MS) or with 3 metalaxyl-resistant (MR) isolates were significantly higher for the MR than the MS isolates. The type of metalaxyl-resistance of single-zoospore cultures of different isolates did not change and the sensitivity of single-zoospore cultures of the same isolate was significantly different. Metalaxyl did not exist cross-resistance with propamocarb and cymoxanil.