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1. Late blight occurrence and LB severity by major production area in the country. What crops were affected (tomato, potato, others)? Disease sources (seed, cull piles, volunteer plants, soil)? 

Potato late blight (Phytophthora infestans) is a very serious disease in most of the country where potatoes are produced. The disease incidence depends on the weather conditions during the potato-growing season. In the southern part of the country, potatoes are produced in a double cropping system and in the northeastern part of the country most seed potatoes are produced. Tomato late blight is one of the most important diseases in plastic greenhouses. 

The primary inoculum is considered to be seed tubers, cull piles and soil. Although a seed production system for high quality exists, seed potatoes can be infected during the rainy season before harvesting and then be distributed throughout the country.

2. LB impact (foliar damage, yield losses, tuber rot occurrences)?  

Plants can be destroyed completely by early infection, resulting in very low yields in certain years.

Losses in the field are estimated as more than 10% of total annual yields. 

During storage, tuber losses are estimated at 10%. Late blight is responsible for 20% of these losses. (Seed tubers are stored at 4°C during the winter season and distributed in the spring.)

3. Fungicide use (amounts, types, etc.)?

Because most of the cultivars, except cv. 'Jopung', are very susceptible to the pathogen, chemical applications are required 3 to 10 times in areas where crops are planted early and in the alpine areas, respectively. The fungicides used are listed below.

Table 1. Fungicides used for control of potato late blight in Korea (Kim, Byung-Sup. 2001)

1 a. i = active ingredient; Wp=Wettable powder, Sc=Suspension concentrate

4. Other LB control measures (resistant cultivars, forecast systems, none)?

Since 1995, potato late blight has been forecasted by two systems'BLITECAST' and 'NAAESCAST'. When both systems were combined, the accuracy of prediction was 95.2%. Because of its simplicity of utilization, the 'NAAESCAST' system has proved to be a practical method for prediction of disease in potato field based on simple weather information.

5. LB control effectiveness (fungicide or host resistance failures)?

The occurrence of metalaxyl-resistant isolates of P. infestans in potato field in Korea was reported in 1992. Due to the seriousness of the resistance situation the manufacturer voluntarily withdrew metalaxyl single use from the market in Korea, but of mixtures metalaxy+ethaboxam, metalaxyl+dimethomorph, and metalaxy+mancozeb) were used in the late blight control. However, in 2001, metalaxyl effectively controlled potato late blight in two field tests in the far northeastern province of Gangwon indicating a change in the P.infestans populations.

6. Pathogen strains (mating type, fungicide resistance, virulence factors, etc)?

The majority of P. infestans isolates collected from various geographical locations during 1991–1993 were found to be of A2 mating type, which was consistent with a point of view of displacement of the A1 mating type strain by the A2 mating type reported by many countries However, this trend did not continue. Of the P. infestans isolates collected during 1998 in the Gangwon area (far northeastern part of the country) were 64.3% were A1 mating type and all were either resistant or intermediately resistant to metalaxyl. In 1999 the P. infestans isolates collected from potato growing regions throughout the country were 90% A1 mating type and predominantly sensitive or intermediately resistant to metalaxyl. In 2002 the P. infestans isolates collected from both the northern, central and south of the country were 84.7% A1 mating type and the majority were sensitive to metalaxyl.

The reason for the change in the population from predominantly A2 in the early 1990s to predominantly A1 mating type is not clear. As mentioned above in section 5, there was one change in agricultural practice — the withdrawal of metalaxyl alone for potato late blight control. The relationship between the increasing sensitivity to metalaxyl and change of mating type from A2 to A1 has not been determined.

7. Disease risks and/or major needs?

8. Any publications on late blight in the country?

Ahn, Jae-Hoon and Hahn, Young-IL. 2001. Late blight forecasting in Korea. pp 48–61 in: Proceedings of the International Workshop on Potato Late Blight, 15–19 October 2001, Pyongchang, Ganwon, Korea. National Alpine Agricultural Experiment Station, RDA, Pyongchang, Korea.

Ahn J H, Hahm Y I and Park E W. 1998. Development of 'Moving average method' for prediction of initial appearance of potato late blight. Korean Journal of Plant Protection 14:34–40.

Ahn J H, Hahm Y I and Shin K Y. 1998. Modeling for Prediction of Potato Late Blight(Phytophthora infestans) Progress. Korean Journal of Plant Protection 14:331–338.

Ahn J H, Hahm Y I, Park C S, Kim B H and Kim J K. 1994. Modeling for prediction of potato late blight(Phytophthora infestans) by meteorological elements. RDA Journal of Agricultral Science 36:320–325.

Choi G J, Kim B S, Chung Y R, and Cho K Y. 1992. Occurrence of metalaxyl-resistant isolates of Phytophthora infestans in potato fields in Korea. Korean Journal of Plant Pathology 8:34–40.

Hahm Y I, Hahn B H and Fornckowiak J D. 1978. Forecasting late blight of potatoes at the alpine area in Korea. Korean Journal of Plant Protection 17:81–87.

Hahn, Young-IL and Kim, Jong-Tae. 2001. Mating type differentiation of Phytophthora infestans isolates in Korea. pp 62–65 in: Proceedings of the International Workshop on Potato Late Blight, 15–19 October 2001, Pyongchang, Ganwon, Korea. National Alpine Agricultural Experiment Station, RDA, Pyongchang, Korea.

Kim, Byung-Sup. 2001. Changes of mating types and sensitivity to metalaxyl in populations of Phytophthora infestans in Gangwon area of Korea. pp 84–95 in: Proceedings of the International Workshop on Potato Late Blight, 15–19 October 2001, Pyongchang, Ganwon, Korea. National Alpine Agricultural Experiment Station, RDA, Pyongchang, Korea.

Kim B S, Choi J H, Chun W H, Ryu K Y, Hahm Y I and Lee Y S. 2000. Mating type and metalaxyl sensitivity of Phytophthora infestans isolated from Gangwon area in Korea. Korean Journal Pesticide Science 4:59–63.

Koh Y J, Chung H J, and Fry W E. 1994. Changes in frequencies and distribution of A2 mating type and metalaxyl-resistant isolates of Phytophthora infestans in Korea. Korean Journal Plant Patholology 10:92–98.

Koh Y J, Goodwin S B, Dyer A T, Cohen B A, Ogoshi A, Sato N, Fry W E. 1994. Migrations and displacements of Phytophthora infestans populations in East Asian Countries. Phytopathology 84:922–927.

Lee W H, So M S, and Choi I Y. 1994. Fungicide resistance and mating type of Phytophthora infestans. Korean Journal Plant Patholology 10:192–196.

So M S and Lee W H. 1993. Occurrence of the A2 mating type of the potato late blight pathogen, Phytophthora infestans in Korea. Korean Journal Plant Patholology 9:275–279.

9. Is tuber blight an increasing problem?

Yes it is.

Contact: Ryu, Kyoung Yul, National Alpine Agricultural Experiment Station, RDA, Pyongchang, Ganwon, Korea. Email: kyryu@rda.go.kr