| Tobacco black shank is one of the most serious diseases damaging world tobacco production and a important disease of tobacco in China. In this study, the resistances to two unsystemic fungicides of chlorothalonil, mancozeb and four systemic fungicides of metalaxyl, cymoxanil, fosetyl-Al, propamocarb of the 38 strains of Phytophthora nicotianae var. nicotianae collected from tobacco plants in Yunnan, Guizhou, Shandong, China in 2005 were determined with the method of colonial growth-rate; the baselines-sensitivity to above six fungicides of P. nicotianae var. nicotianae were established with the analytic method of frequency; the low and high resistant-metalaxyl strains of P. nicotianae var. nicotianae were gained by the method of metalaxyl inducement; the hereditary stabilities of all the resistant-metalaxyl strains were determined; the resistance levels of all the resistant-metalaxyl strains were determined; the colonial growth-rate, the number of sporangium, the pathogenicity and fitness of the low and high resistant-metalaxyl strains and the sensitive strain were compared with the method of colonial diameter, the method of count, the method of inoculation in vivo and the method of computation, respectively; the conductivity changing of metalaxyl solution between different concentrations were compared, in which the mycelia of the low and high resistant-metalaxyl strains and the sensitive strain were dipped; the polyacrylamide gel electrophoretical patterns of the esterases and the soluble proteins of the low and high resistant-metalaxyl strains and the sensitive strain of P. nicotianae var. nicotianae were compared; the cross-resistances to above five fungicides(except metalaxyl) of resistant-metalaxyl strains in P. nicotianae var. nicotianae were determined. The results showed as below:1. The EC50 values of chlorothalonil, mancozeb, metalaxyl, cymoxanil, fosetyl-Al and propamocarb to the 38 strains of P. nicotianae var. nicotianae ranged from 2.3325μg/mL to 49.471μg/mL, from 3.5098μg/mL to 82.055μg/mL, from 0.0867μg/mL to 1.6781μg/mL, from 24.042μg/mL to 62.663μg/mL, from 60.468μg/mL to 239.51μg/mL and from 2493.3μg/mL to 19625μg/mL. The mean EC50 values to the 38 strains of above six fungicides were 9.3509±2.7721μg/mL, 10.590±4.0556μg/mL, 0.2991±0.0843μg/mL, 35.891±2.7823μg/mL, 105.73±11.154μg/mL and 9688.7±1396.4μg/mL. P. nicotianae var. nicotianae had low resistance to all above six fungicides and there were no resistant subcolony in P. nicotianae var. nicotianae.2. The mean EC50 values of chlorothalonil, mancozeb, metalaxyl, cymoxanil, fosetyl-Al and propamocarb to the 30, 37, 34, 37, 37 and 25 strains, which were all from sensitive colony and composed the continuous and approximately normal distributions-resistance, were 5.9856±0.7817μg/mL, 8.6588±1.0941μg/mL, 0.2389±0.0292μg/mL, 35.167±2.4317μg/mL, 102.11±8.6497μg/mL and 7136.1±929.93μg/mL. They could be used as the baselines-sensitivity of P. nicotianae var. nicotianae to above six fungicides.3. Ten low resistant strains and one high resistant strain were gained through metalaxyl inducement. The occurrence frequency of resistant strains increased when metalaxyl concentration increased within the range of from 0μg/mL to 20μg/mL. There was high positive correlationship between the occurrence frequency of resistant strains and the logarithm of metalaxyl concentration.4. Although there was complicated change of colonial inhibition of metalaxyl to the eleven resistant strains between different generations, the colonial inhibition began to stabilize from second or fourth or sixth generation, which showed that the resistances- metalaxyl of all the resistant stains could inherit stably.5. The EC50 values to ten low resistant strains of metalaxyl ranged from 1.1257μg/mL to 4.4157μg/mL and the mean EC50 value was 2.7267μg/mL; the resistance levels of ten low resistant strains ranged from 3.54 to 13.90 and the mean resistance level was 8.59. The EC50 value of metalaxyl to and resistance level of the high resistant strain, 10M-004, were 161.77μg/mL and 509.38 respectively. There was no middle resistant strains in all the eleven resistant strains, which implied that the resistant-metalaxyl mutation might be the mutation of quality character.6. The fitness indexes in vitro, fitness indexes in vivo and compound fitness indexes of all the resistant strains tested were lower than those of the sensitive strain, ZC002, and those of the high resistant strain, 10M-004, were highest within all the resistant strains tested. There was no correlationship between above three fitness indexes and resistance levels of all the resistant strains tested.7. Metalaxyl might damage the membrane of P. nicotianae var. nicotianae. The membrane permeability of the sensitive strain, 10M-006, and the low resistant strain, ZC002, increased after metalaxyl treatment. The high resistant strain, 10M-004, had stronger ability to adapt to metalaxyl resulting in smaller membrane damage after metalaxyl treatment, which decreased when metalaxyl concentration increased, than the sensitive strain, 10M-006, and the low resistant strain, ZC002 had.8. There were significant special differences on the polyacrylamide gel electrophoretical patterns of the esterases between the sensitive-metalaxyl strain, ZC002, the low resistant- metalaxyl strain, 10M-006, and the high resistant-metalaxyl strain, 10M-004, in P. nicotianae var. nicotianae. Only the sensitive strain, ZC002, had the esterase- zones of Rf 0.705 and Rf 0.849 while only the high resistant strain 10M-004, had the esterase-zone of Rf 0.487. The quantity of the esterase-zone of Rf 0.026 in the high resistant strain, 10M-004, and in the low resistant strain, 10M-006, was higher than that in the sensitive strain, ZC002.9. There were no significant differences on the polyacrylamide gel electrophoretical patterns of the soluble proteins between the sensitive-metalaxyl strain, ZC002, the low resistant-metalaxyl strain, 10M-006, and the high resistant-metalaxyl strain, 10M-004, in P. nicotianae var. nicotianae. The quantity of the soluble proteins in the low resistant strain, 10M-006, and in the high resistant strain, 10M-004, was higher than that in the sensitive strain, ZC002.10. The resistant-metalaxyl strains in P. nicotianae var. nicotianae had negative cross-resistance to mancozeb, which belongs to the unsystemic fungicide of organic sulphide, and had no cross-resistance to chlorothalonil, which belongs to the unsystemic fungicide of substituted-phenyl, to cymoxanil, which belongs to the systemic fungicide of acetamide, to fosetyl-Al, which belongs to the systemic fungicide of alkyl phosphate, and to propamocarb, which belongs to the systemic fungicide of carbamate .
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