• The Plant Pathology Journal
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• v.16 no.4
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• pp.222-226
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• 2000

In disease surveys from 1986 ti 1998, disease incidence of tobacco black shank was gradually increased in burley tobacco from 1996. To study the causes of the disease occurrence, one hundred and fourteen isolates of Phytophthora parasitica var. nicotianae (Ppn) were collected from burley tobacco-growing areas in the southern part of Korea during 1996-1997, and tested in vitro for meatlaxyl sensitivity which was determined by measuring the mycelial growth on corn meal agar (CMA) amended with metalaxyl. Of the tested isolates, 78.1% showed sensitive to metalaxyl, having $\textrm{ED}_{50}$ values less than 1.0 $\mu\textrm{g}/$\textrm{ml}, while 1.7% was resistant weth $\textrm{ED}_{50}$ greater than 100 $\mu\textrm{g}/$\textrm{ml}. Ppn isolates from three provinces, Chungnam, Chonbuk and Chonnam showed similar distributions of metalaxyl sensitivity. Metalaxyl-resistant isolates were not significantly different from metalaxyl-sensitive ones in mycelial growth rate, chlamydospore formation capacity and size of the spore, and pathogenicity on tobacco plant (cv. Burley 21). These results suggest that the metalaxyl-resistant Ppn in burley tobacco may be one of the major factors to cause the higher occurrence of the tobacco black shank in the burley tobacco-growing area.

• The Korean Journal of Mycology
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• v.31 no.2
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• pp.98-102
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• 2003

Metalaxyl sensitivity related with distribution feature of mating type was characterized far Phytophthora capsici population, totally 433 isolates of the red-pepper pathogen collected from 75 pepper fields in Korea from 1995 to 1998. At the concentration of metalaxyl $2{\mu}g/ml$, inhibition rate of mycelial growth of P. capsici isolates was 68.2% in average compared to control, and 28.6% isolates in average were estimated as resistance to the chemical. Isolates of field unit with a single mating type revealed similar level of sensitivity to metalaxyl and showed sensitive or resistant in most field units. However, isolates of field units with both mating types revealed diverse sensitivity level to the chemical and various occurrence ratio of metalaxyl sensitive : resistant in each field unit. Results indicated that different levels of metalaxyl sensitivity of P. capsici population in Korea seem to be closely related with occurrence ratio of A1 : A2 mating type of each field.

• The Plant Pathology Journal
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• v.19 no.3
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• pp.143-147
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• 2003

Sensitivity of Phytophthora infestans isolates to fungicides metalaxyl and ethaboxam in Korea was examined with 260 isolates for 3 years (9 isolates in 2000,93 isolates in 2001, and 158 isolates in 2002). Both Al and A2 mating types were found from the isolates collected for 3 years. Al mating type was dominant in the population with 8 isolates (88.9%) in 2000, 84 isolates (89.4%) in 2001, and 138 isolates (87.3%) in 2002. Only some isolates from diseased tomatoes in Buyergun and diseased potatoes in Pyeongchanggun were of the A2 mating type. As for metalaxyl sensitivity, 77.0% of the isolates were moderately resistant with 8 isolates (88.9%) in 2000, 73 isolates (77.7%) in 2001, and 120 isolates (75.9%) in 2002. Meanwhile, those found resistant were 1 isolate (11.1%) in 2000, 16 isolates (17.0%) in 2001, and 33 isolates (20.9%) in 2002. Only 5 isolates (3.2%) were sensitive to metalaxyl in 2002. There was no significant difference in the sensitivity among years. As for ethaboxam, no isolate was able to grow at 5.0 $\mu\textrm{g}$ /ml, and only four isolates (1.5 %) grew at 1.0 $\mu\textrm{g}$ /ml with heavy retardation compared with the untreated control. Based on these 3-year results, the minimum inhibitory concentration (MIC) of ethaboxam to p. infestans was determined to be 0.2-1.0 $\mu\textrm{g}$ /ml. Results indicate that resistance development by p. infestans to ethaboxam is not likely to occur in the natural condition. furthermore, there was no indication of cross resistance between metalaxyl and ethaboxam because all the isolates, regardless of classification for their sensitivity to metalaxyl, were not able to grow at 5.0 $\mu\textrm{g}$ /ml of ethaboxam.

• 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.4 no.1
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• pp.59-63
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• 2000

Isolates of Phytophthora infestans obtained from several locations of Kangwon area in 1998 and 1999 were examined on mating types and sensitivity to metalaxyl. Both A1 and A2 mating type isolates were isolated in 1998 and 1999. The majority of the P. infestans isolates were A1 mating type. About 64.3% of the isolates collected in 1998 and 99.1% in 1999 were determined as A1 mating type. Sensitivity of the P. infestans to metalaxyl was examined by measuring mycelial growth on V8 juice agar amended with $10{\mu}g/mL$ matalaxyl. About 44.6% of the isolates examined in 1988 were resistant to metalaxyl, 55.4% of the isolates were intermediate resistant, but sensitive isolate was not isolated. However, 10.5% of the isolates examined in 1999 were sensitive, 88.6% of the isolates were intermediate resistant, and 0.9% of the isolates were resistant to metalaxyl. This studies indicate that A1 mating type is displacing A2 mating type and metalaxyl sensitivity of the P. infestans isolates of Kangwon area is increasing. This result is quite different from trends of early in 1990s.

• The Plant Pathology Journal
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• v.25 no.4
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• pp.317-321
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• 2009

Sensitivity to the new carboxylic acid amide fungicide, mandipropamid, of Phytophthora capsici causing pepper Phytophthora blight was determined on 187 isolates collected in Korea over 3 years, from 2005 to 2007. All isolates were sensitive to mandipropamid, with $EC_{30}$ values for growth of mycelia ranging from 0.001 to $0.037\;{\mu}g/ml$. Among the isolates, 147 (79.0%) isolates were sensitive to metalaxyl, whereas others were resistant to this fungicide. Mandipropamid had the same effect on mycelium growth of both metalaxyl-sensitive and metalaxyl-resistant isolates, indicating an absence of cross-resistance between these two fungicides. Comparison of the sensitivities of P. capsici isolates showed a positive correlation between sensitivity to mandipropamid and dimethomorph ($r^2$=0.8533). The results of this study indicate that there is no evidence for development of resistance to mandipropamid in this population of P. capsici isolates collected in Korea.

• 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.

• Proceedings of the Korean Society of Plant Pathology Conference
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• 2003.10a
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• pp.88.2-89
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• 2003

Changes of control efficacy of metalaxyl to potato late blight caused by Phytophthora infestans were examined in potato fields from 2001 to 2003. In 2001 and 2002, control efficacy of metalaxyl was similar to those of dimethomorph and ethaboxam. However, the control efficacy of metalaxyl were decreased to 50.3％ in 2003. Total 366 isolates of P infestans obtained from several areas in Korea from 2001 to 2003 were examined for changes of sensitivity to metalaxyl. About 6.8％ of fungal isolates examined in 2001 were sensitive, 84.1％ were intermediate resistant, 9.1％ were resistant to metalaxyl. Among the isolates collected in 2002, 3.9％ were sensitive, 75.6％ were intermediate resistant, 20.6％ were resistant to the chemical. However, among the isolates obtained in 2003, 55.9％ were intermediate resistant, 44.1％ were resistant, but none of the isolates tested were sensitive. Both A1 and A2 mating type isolate were isolated in 2002∼2003. However, all isolates collected in 2001 were A1 mating type. About 87.5％ of the isolates collected in 2002, 89.8％ In 2003 were determined as A1 mating type. The majority of the p. infestans isolates were A1 mating types. Changes of control efficacy of metalaxyl to potato late blight might be caused by the occurrence rate of moderately resistant isolates within A1 mating type.

• Mycobiology
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• v.31 no.4
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• pp.229-234
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• 2003

A total of 24 isolates of Phytophthora infestans were tested and analyzed for their resistance to metalaxyl fungicides. Sensitivity to metalaxyl was determined by growing isolates on 20% V8 medium amended with 0, 5, and 100 ${\mu}g/ml$ metalaxyl. Four isolates among the 24 tested were resistant to metalaxyl. Eleven isolates were intermediate and nine isolates were sensitive. Amplified fragment length polymorphism(AFLP) assay was used to identify the amplification products of resistant isolates. As a result, selected fragments were cloned, sequences and primer pairs were developed which linked to metalaxyl insensitivity in P. infestans using competitive PCR.

• 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.