| Resistance to fungicides in plant pathogens is outstanding problem in chemical control of palnt diseases. Metalaxyl is now the fungicide used most widely for control potato late blight. In the thesis, the management strategy of resistance in Phytophthora infestans was established through testing sensitivity to 6 systemic fungicides including metalaxyl, screening the fungicides with high control efficacy to potato late blight, no positively correlated resistance with metalaxyl and low risk of rensistance, screening the fungicide combinations synergistic against metalaxyl-resistant isolates of P. infestans, and studies on effects on the development of resistance to metalaxyl in P. infestans. The major result were reviewed as followed.1. The isolation method of potato slices claping leaves infected by late blight was improved, which made the ratio of successfully isolation up to 100%. The colonial spreading and sporulation of P. infestans on 3 media (RSA①, RSA②and CA) was tested. The result showed that P. infestans grows faster on paltes of RSA①and CA than on plates of RSA②. The sporulation capacity is biggest on plates of RSA①than on plates of CA and RSA②. So, RSA①is more suitable for cultivation of P. infestans. Besides, speed of colonial extension of P. infestans on CA is almost the same as that on RSA①, and the raw material of CA is easily acquired, so CA can be used for testing inhibitory activity of fungicides against mycelial growth and in vitro testing sensitivity to fungicides in P. infestans.2. Sensitivity to metalaxyl in 380 isolates and 227 isolates collected from five provinces of northern China during 2007~2009 with mycelaial growth rate on CA amended with metalaxyl of discriminatory concentration and leaf disc floating test, respectively, the result showed that the percentages of metalaxyl-resistance isolates of P. infestans was 80% and 74%, respectively. It suggests that metalaxyl-resistance of P. infestans occurred widely in the detected areas.3. Sensitivity to mefenoxam, cymoxanil, dimethomorph, flumorph and azoxystrobin of P. infestans collected from five provinces of northern China during 2007~2009 was tested through leaf disc floating test. The result showed that (1) EC50 values of mefenoxam in 96 field isolates of P. infestans were 0.002 9~395.867 5μg/mL, mefenoxam-resistant isolates accounts for 54.2%, intermediate isolates accounts for 44.8%, only 1.0% is sensitive isolates. (2) The baseline-sensitivity to cymoxanil of P. infestans was (0.673 6±0.090 2)μg/mL. 70% were resistant to cymoxanil among in 110 field isolates, 1.8% is high-resistant to cymoxanil with resistance level up to 1 024 folds. (3) The baseline-sensitivity of dimethomorph, flumorph and azoxystrobin to P. infestans was (0.215 5±0.011 9)μg/mL, (0.337 4±0.019 9)μg/mL and (0.218 8±0.011 1)μg/mL, respectively. Isolates sensitive to dimethomorph, isolates sensitive to flumorph and isolates sensitive to azoxystrobin were dominated in the population of P. infestans, account for 92.2%, 93.7% and 88.4%, respectively. A few isolates lowly resistant to the three fungicides exist, no moderately or highly resistant isolates were detected. (4) Isolates double resistant to metalaxyl and cymoxanil (MRCRDSAS) , or double resistant to metalaxyl and azoxystrobin (MRCSDSAR) or triple resistant to metalaxyl, cymoxanil and azoxystrobin (MRCRDSAR) exist in the field, accounting for 78.0%, 1.1% and 3.3%, respectively.4. The fungitoxicity of 19 fungicides to P. infestans was assessed by a leaf disc spray test. The result showed that azoxystrobin-difenoconazole, azoxystrobin, dimethomorph, mandipropamid, pyraclostrobin, pyraclostrobin-metiram and pyrametostrobin showed higher toxicity to metalaxyl-resistant isolates, Theirs EC90 values were lower than 10μg/mL. EC90 values of fluopicolide-propamocarb, azoxystrobin-chlorothalonil, dimethomorph-mancozeb, famoxadone-mancozeb, mancozeb, cymoxanil-mancozeb, famoxadone-cymoxanil, chlorothalonil and cupric hydroxide were 10~100μg/mL. And fluazinam, propamocarb, propineb showed a lower toxicity to metalaxyl-resistance isolates, theirs EC90 values were higher than 100μg/mL.5. The result of the trial on control efficacy done in Weichong showed that: control efficacy of fluopicolide-propamocarb and mandipropamid to potato late blight in field was 88.9% and 82.7%, respectively, which was significantly higher that of cymoxanil-mancozeb, dimethomorph, azoxystrobin, mancozeb and chlorothalonil. The result of the trial on control efficacy done in Chongli showed that control efficacy of fluopicolide-propamocarb to potato late blight in field was 93.8%, control efficacy of cymoxanil-mancozeb and dimethomorph was 84.7% and 80.2%, respectively, which was significantly higher that of mancozeb, azoxystrobin and chlorothalonil.6. The resistance risk of mandipropamid and pyrametostrobin to P. infestans was preliminarily assessed. (1) The result of early detection of resistance to mandipropamid showed that among 232 isolates, sensitive isolates account for 94.4%, lowly resistant isolates account for 5.6%, no moderately resistant or highly reisitant isolates were found. No positively correlated cross resistance between mandipropamid, metalaxyl and cymoxanil, but positively correlated cross resistance between mandipropamid, dimethomorph and flumorph. Resistance to mandipropamid of resistant isolates from the fields not stable and their fitness index were significantly lower than the sensitive isolates. These suggests that P. infestans has a lower risk of resistance to mandipropamid. (2) The result of early detection of resistance to pyrametostrobin showed that among 127 isolates, sensitive isolates account for 91.3%, 8.7% is lowly resistant isolates, no moderately resistant or highly reisitant isolates are found. No positively correlated cross resistance exists between pyrametostrobin, metalaxyl, cymoxanil and dimethomorph, but positively correlated cross resistance exists between pyrametostrobin and azoxystrobin. Resistance to pyrametostrobin can keep stable of resistant isolates, and their fitness index were not significantly different from that of the sensitive isolates. It suggested that P. infestans has a high risk of resistance to pyrametostrobin.7. The joint fungitoxicity of fungicides was tested by detached leaf spray test and whether the synergistic action exists between fungicides was judged by Horsfall formula and Wadley formula. The results showed that synergistic effects (SR 2.8 and 2.0) against metalaxyl isolates exist between compound A and pyraclostrobin and between compound A and chlorothalonil as mixing suspension of compound A and pyraclostrobin at ratio of 5:5, and the mixing suspension of compound A and chlorothalonil at ratio of 4:6 were applied to the detached leaves of potato. Synergistic action (SR 1.6 and 2.2) to metalaxyl-resistant isolates exists while mixing suspension of compound B and chlorothalonil at ratio of 9:1 and mixing suspension of compound B and mancozeb at ratio of 3:7 were applied to the detached leaves of potato.8. Alternate application of cymoxanil-mancozeb 72 WP (1 080 g a.i./hm2) , mefenoxam-mancozeb WG (1 224 g a.i./hm2), fluopicolide-propamocarb 68.75 SC (1 031 g a.i./hm2) and dimethomorph 50 WP (375 g a.i./hm2) in demonstration plots exhibited good control efficacy (>90%). And development of resistance to metalaxyl or azoxystrobin in the population of P. infestans was delayed through detecting the sensitivity to metalaxyl or azoxystrobin in the alternate application areas in the population of P. infestans. 9 Based on the results of detection of field resistance to fungicides, screening of fungicides with high control efficacy, low risk of resistance to the novel fungicides, screening of fungicide combinations synergistic to metalaxyl-resistant isolates, and studies on effects of alternate application of fungicides with different modes of action on development of resistance to fungicides, a strategy of management strategy of resistance to metalaxyl in P. infestans was established, that is, selecting and applying novel fungicides (such as mandipropamid, dimethomorph, pyrametostrobin, azoxystrobin, fluopicolide-propamocarb) without positive correlated cross resistance with metalaxyl in alternation, or applying the mixing suspension of compound A with pyraclostrobin or chlorothalonil, or the mixing suspension of compound B with chlorothalonil or mancozeb;limiting metalaxyl-mancozeb or mefenoxam-mancozeb within the 2 applications per season of growth.
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