• The Korean Journal of Pesticide Science
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• v.12 no.3
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• pp.270-276
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• 2008

Through the agar dilution method on V-8 juice agar, sensitivity of Phytophthora capsici causing pepper Phytophthora blight to metalaxyl was investigated by using isolates obtained from infected pepper plants during 3 years from 2005 to 2007. By the lapse of time, $EC_{50}$ value to metalaxyl was decreased, showing 1.45, 0.83, and $0.32{\mu}g\;mL^{-1}$ in 2005, 2006, and 2007. None of 2007 isolates was found to be resistant to metalaxyl. Compared the sensitivity of P. capsici isolates to metalaxyl with those to mandipropamid and dimethomorph, there is not a cross resistance response between metalaxyl and mandipropamid/dimethomorph. The resistance to metalaxyl in pepper Phytophthora blight pathogen was not related with the mycelial growth on V-8 agar medium and the pathogenicity on pepper plants.

• The Korean Journal of Pesticide Science
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• v.7 no.1
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• pp.25-31
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• 2003

Changes of control effect of metalaxyl to potato late blight caused by Phytophthora infestans were examined in potato fields in 2001 and 2002. Control efficacy of metalaxyl was similar to those of dimethomorph and ethaboxam. About 4.9% of the fungal isolates exanlined in 2001 were resistant, 87.8% were intermediate, and 7.3% were sensitive to metalaxyl. Among the isolates collected in 2002, 20.1 % were resistant, 75.5% were intermediate, and 4.3% were sensitive to the chemical. However, most of resistant isolates were isolated from Pyongchanggun Hoenggae area. Eighty eight isolates collected in 2001 were all A1 mating type, while both A1 and A2 mating types were found in 2002. About 85.6% of the isolates collected in 2002 were A1 mating type, but all the rest (A2 mating type) was isolated from Hoenggae area. Increased control efficacy of metalaxyl to potato late blight might be caused by the increase of moderately resistant and sensitive isolates with decreasing the resistant isolate, and the decreased population of A2 mating type which is resistant to metalaxyl.

• The Plant Pathology Journal
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• v.21 no.1
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• pp.33-38
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• 2005

Changes of control efficacy of chemical to potato late blight caused by Phytophthora infestans in potato fields from 2001 to 2004 were examined. Control efficacy of metalaxyl was suddenly decreased from 100% in 2002 to 50% in 2004 and that of dimethomorph also was similar to those of metalaxyl. However, the control efficacy of ethaboxam no great change. Both A1 and A2 mating type isolates were isolated from 2001 to 2004 in several areas in Korea. The majority of the P. infestans isolates were A1 mating type. Total 939 isolates of P. infestans obtained from several areas in Korea from 2001 to 2004 were examined for changes of sensitivity to metalaxyl. Frequencies of metalaxyl resistance isolates were gradually increased from 17% in 2001 to 84.2% in 2004, but isolation frequencies of metalaxyl sensitive and intermediate resistant isolate were decreased. Cause of decreasing control efficacy of metalaxyl was thought by increase of resistance isolates in A1 mating type population according to increasing metalaxyl use. Most isolates were grown at 0.5 ${\mu}g/ml of dimethomorph and isolates grown at 1 ${\mu}g/ml of dimethomorph were approximately 10.2-22.9%. However, no isolate was able to grow at 5.0 ${\mu}g/ml. Based on these results, minimum inhibitory concentrations (MIC) of dimethomorph to P. infestans were determined to be 0.5-1.0 ${\mu}g/ml. Our results indicated that the reason decreasing control efficacy of dimethomorph was not caused by occurrence of resistant isolates. About 5% and 12.1% isolates among the total isolates collected in 2003 and 2004 were grown on V-8 juice rye agar containing 1.0 ${\mu}g/ml ethaboxam. The 2.1 and 25.4% isolates had MICs of 0.2-0.4 ${\mu}g/ml, and MIC values of 87.9% and 74.3% isolates were less than 0.2 ${\mu}g/ml concentrations of ethaboxam. Therefore, resistance development by P. infestans to ethaboxam is not likely to occur in the natural condition.