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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)?

Phytophthora infestans is an important pathogen on potato and tomato crops during the winter season. 

Late blight is the most important disease of potato in Vietnam in all potato production areas. In the Red River Delta (RRD) in the north, where 98% of the production takes place, the disease usually occurs late during the second half of the main growing season (November to February) starting some time in late December. Severe epidemics often break out during January and February when the weather is cool and wet. In Sapa, located in the northern highlands at 1,500 masl, late blight is a major threat to any potato crop during the growing season. Disease incidence and severity can be as high as 100% with susceptible varieties.

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

Foliar destruction is the main impact of LB on potato. Crop losses can often range from 5 to 50%.

With highly susceptible cultivars, losses of up to 100% have been observed in some instances during the rainy season at Dalat and Sapa. Tuber infection occurs mostly during the rainy season at Dalat and Sapa and during spring growing season in the RRD. This causes considerable losses during the storage of seed or ware potatoes. Seed tuber loss may reach over 13 % in the diffused light store during the rainy season at Dalat (Nen et al., 1993).

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

With the moderate level of resistance of the widely used varieties, application of effective fungicides is the major measure for controlling the disease and protecting the crop. The most commonly used are cymoxanil (Curzate M-8), mancozeb (Mancozeb, Dithane M-45, Manzate, etc.), metalaxyl (Ridomil Gold 68 BHN, Fortazeb, Apron 35 SD, etc.), zineb (Zodiac 80 WP, Zinforce 80 WP) and copper compounds (copper hydroxide, copper oxychloride), as a single fungicide or in combinations. Every year, new preparations continue to be tested and introduced into practical application. Recent results from field-testing in Dalat of Anzeb 80 WP and Dizeb M45 80WP indicate that both have reliable protective effect on late blight of potato. The effective rates of application are 1.5 – 2 kg/ha with Dizeb M45 80WP and 2-3 kg/ha with Anzeb 80 WP. Under rainy conditions with higher air humidity and higher disease pressure, it is advisable to apply the higher rate at shorter intervals between sprays (5-7 days).

In the RRD, fungicides are used more on the spring crops, as the disease is more prevalent and serious during the season. When the main crop is planted earlier in late October or early November, the need for chemical sprays is considerably reduced to just a few during the first half of the growing season as protective measures. However, chemical treatments can become important practice during the later stage when the weather becomes cold and wet. A recent survey showed that copper compounds and zineb have been the most and traditionally used by farmers. These chemicals are cheap but effective enough to check the pathogen population in the area.

In Dalat, potato can be grown all year round. However, late blight renders the rainy season (May to October) the off-season for potato cultivation. While the high relative humidity during the rainy season is highly conductive to late blight infection, continual rains set off almost all effort to control the disease by fungicide sprays, which are sometimes as frequent as three times a week.

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

Other control measures include mainly the use of resistant cultivars and escape of disease. In the RRD, with earlier planting in October, the crop may escape the disease or may be attacked with a minimal incidence. In Dalat, potato crops grown during the dry season have minimal LB infection and ensure high tuber yields.

The use of highly resistant cultivars is considered the most important measure to ensure a successful potato crop in both regions. In Dalat, however, it is probably the prerequisite for potato production as plantings can be done at any time during the year.   

Since the early 1980’s, in collaboration with the International Potato Center (CIP), the National Potato Breeding Project started introduction, evaluation and selection of resistant germplasm. A large number of the introductions have been done during the last 20 years’ including in vitro materials, tuber families and true potato seed progenies (TPS). The outcome of this project has been the resistant varieties Atzimba, CFK-69.1, B-71.290.2 an P-3 that have been grown widely in Dalat area during the 1980’s in replacement for the traditional European susceptible varieties and the latter Red Skin 07 which is a favorite of the local farmers and consumers. In the RRD, the varieties P-3 and KT-3 are the resistant selections highly appreciated by the farmers for their high level of resistance and adaptation to the local growing and storage conditions. It appears that in this breeding project, the resistant varieties selected so far all have the S. demissun  (P-3) and Solanum andigena (Atzimba, CFK-69.1, B-71-240-2, 07, and KT-3) genetic background.

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

Chemical control can be effective except under the conditions of continual rains. To assure more effective sprayings, Dalat farmers tend to use alternatively several types of fungicides during a growing season. Spraying a single chemical during the whole season seems to be less effective.

Host resistance is a major measure to fight against the disease, but evidence during the last 20 years has shown that host resistance may gradually fail with time. The cultivars CFK-69.1 and B 71-240.2 had shown high level of resistance to LB in Dalat for some time after their introduction in 1980. This resistance, however, has gradually been lost and CFK-69.1 is now considered as moderately susceptible or B 71-240.2 is no longer commercially cultivated.

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

All data so far is from the north of Vietnam. Only mating type A1 is found, and preliminary tm DNA haplotype tests indicate that the strains belong to the the “old” population of P. infestans, which was distributed worldwide outside central Mexico before 1970’s. Reduced sensitivity to metalaxyl is common, but few isolates are resistant to this compound. Virulence factors 2,3,4,7,8,10,11 are found, but races with three or four virulence factors are most common. Data about isolates from 2002/2003, which also includes other parts of Vietnam, are in progress (Hermansen A, personal communication).

7. Disease risks and/or major needs?

As late blight is the most destructive disease of potato, which can cause considerable crop losses or even can destroy a potato crop prematurely, further research is needed. The areas of priority are:

·         Development and implementation of a system of integrated management for control of the disease both in the field and in seed and table potato storage with training of personnel and research in blight forecasting as an important element.

·         Breeding for resistant varieties with more emphasis on field resistance, which is considered the core element in the system of integrated management for controlling the disease. For this field of research, participation in the international network for testing resistant germplasm is a key activity.

More research on biology and epidemiology of the disease, including race identification and composition changes and other important epidemiological aspects of the pathogen.

8. Any publications on late blight in the country?

Chien D. H. and My N. V., 1995. Preliminary results from research and development of potato production in SAPA, Laocai. In: Root Crops Research and Development: Results from 1991–1995. Vietnam Agricultural Research Institute.

Chien D. H. and Ho T. V., 1993. Evaluation of late blight (Phythophtora infestans ) resistance germplasm at Sapa, Vietnam in 1990. pp. 61–74 in: Potato Research and Development in Vietnam (A collaborative experience from 1989 TO 1993). Ministry of Agriculture and Rural Development, Vietnam and CIP ESEAP, Philippines.

Ho T. V., Thuan L. T. and Nga D. B., 1993. Potato Germplasm evaluation in North Vietnam in 1989–1990. pp. 48–60 in: Potato Research and Development in Vietnam (A collaborative experience from 1989 to 1993). Ministry of Agriculture and Rural Development, Vietnam and CIP ESEAP, Philippines.

Le V. H., Nguyen V. K., Bruberg  M.B. and Hermansen A., 2002. Characterization of Phytopthora infestans populations in the north of Vietnam. Pp 136 in: Lizárraga C (ed), Late blight: Managing the global threat. Proceedings of the Global Initiative on Late Blight Conference, Hamburg, Germany, 11–13 July 2002. International Potato Center, Lima, Peru.

Nen N. T., Lan N. T., Quang T. K., Dam N. D. and Tung P. X., 1993. Potato Germplasm evaluation in Dalat: I. Results from replicated trails from 1987 to 1992. pp 12–22 in: Potato Research and Development in Vietnam (A collaborative experience from 1989 to 1993). Ministry of Agriculture and Rural Development, Vietnam and CIP ESEAP, Philippines.

Tung P.X., 2002. Breeding potatoes for late blight resistance in Vietnam. pp 154 in: Lizárraga C (ed), Late blight: Managing the global threat. Proceedings of the Global Initiative on Late Blight Conference, Hamburg, Germany, 11–13 July 2002. International Potato Center, Lima, Peru.

Tung P.X., Hau N.T. and Hung L.T., 2001. Effect of fungicides Anzeb 85 WP and Dizeb M45 WP on late blight (Phytophthora infestans (Mont.) de Bary) incidence on potato during the rainy season in Dalat. pp 10–15 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.

Tung P.X., 2001. The late blight disease of potato in Vietnam. Journal of Agricultural University of Hebei, China Vol. 24 (2): 16–19. Proceedings of the GILB East and Southest Asia Linkage Group workshop on late blight, 16–20 August, Baoding Hebei, China.

Viet N. V., 1999. Research on occurrence, development of some fungal diseases and the curly leaf disease of tomato and controlling practices in Hanoi and surrounding areas. Dissertation, Hanoi University of Agriculture. # I. (in Vietnamense).

9. Is tuber blight an increasing problem?

Yes. 

Submitted by Pham Xuan Tung, Potato, Vegetable and Flower Research Center, Thaiphien, Dalat, Lamdong, Vietnam. Email: tung.ctp(at)hcm.vnn.vn