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Late blight, caused by Phytophthora infestans, is the major constraint to potato production in Myanmar. Potato late blight appears annually during July to September in Shan State highlands (over 1,200 m above sea level) where rainy season potato is widely cultivated. In this area, 40-50% foliar infection is found yearly causing 21 to 46% loss in yield. Some promising resistant potato varieties and hybrid true potato seed progenies are beginning to be introduced in limited areas. However, most of the growers are used to applying fungicides to control the disease. Recently, there have been complaints that the effect of metalaxyl combined with mancozeb to control the disears is reduced. This might be due to the occurrence of metalaxyl resistant isolates of P. infestans. There is a need to conduct further research such as monitoring of metalaxyl resistant isolates of P. infestans, screening of potato varieties for durable resistance to the disease, and investigation of the best combination of fungicide application, based on simple Decision Support System (DSS) such as rainfall thresholds, and resistant varieties for integrated management of potato late blight.
The potato (Solanum tuberosum) is one of the main culinary crops in Myanmar's households. The area under potato cultivation is 28,000 ha and average yield is 10.5 mt/ha (CSO, 2002). Potato is grown year round, and according to the season of planting, there are four crops: summer crop (January-February to April-May), rainy season crop (April-May to August-September), intermediate season (post monsoon) crop (August-September to December-January) and winter crop (October-November to January-February). The summer crop is grown under irrigation on peaty and clay loam soil in Heho valley, Kalaw Township, Shan State. The main or rainy crop and intermediate crop are grown in Shan State especially Pinlaung, Kalaw and Pindaya Townships. The winter crop is sown on alluvial soil in plains such as Sagaing, Magwe, Mandalay Divisions and Kachin State (MAS, 1999; Thaung Pe, 1978) (Figure 1).
In Myanmar potato is gradually increasing in production, acreage and yield (CSO, 2002). Concomitantly, late blight is a serious issue. Since Up-to-date (locally known as Sitbo), the widely grown cultivar in Myanmar, is susceptible to late blight, the disease appears yearly in Shan State (Thaung Pe, 1978), where Myanmar's annual production is 63% (CSO, 2002).
Disease Impact on the Industry and Consumer
Late blight causes severe damage to the potato industry of Myanmar (MAS, 1999). The disease is one of the constraints to better yield and production of potato. The yield is not only affected, but expansion of the areas for potato cultivation is also restricted due to late blight (Maung Maung Myint, 2001). Annually yield losses due to the disease range from 21% to 46% (Thaung Pe, 1978). Experimental results also showed 23% to 42% yield loss (Maung Maung Myint et al., 2001a and 2001b). Potato plants can be destroyed completely by early infection, resulting in very low yield in certain years, especially during the post monsoon season. However, the economic impact of potato late blight in Myanmar has not been estimated.
Potato is a traditional source of food and a subsistence crop for many of the potato growers in the highlands. Due to severe yield losses in years when weather conditions are favorable for epidemic outbreaks of late blight, potato consumption is also decreased.
Crops affected by late blight
P. infestans attacks not only potato but also tomato mainly in hilly regions. It is a rare occurrence in the plains. Unfortunately, no quantitative accounts of the damage on tomato have been recorded although global damage to tomato due to late blight is immense.
Infected seed tubers are the main sources of infection. Traditionally, farmers from Shan hilly regions store the tubers from their rainy crop harvest in baskets at home in the dark (MAS, 1990). The low temperature and dark conditions are quite favorable for the survival of P. infestans in seed tubers unto the next crop season. Moreover, the inoculum may be carried over in the form of sporangia from one season to another because of the presence of potato crop round the year.
Disease occurrence and severity, and geographical distribution
Potato late blight appears annually during July to September in Southern Shan State highlands where rainy and intermediate crops are widely grown. The altitude of this area is over 1,200 m above sea level between latitude 20-23˚. Precipitation is 660 mm average and, the maximum and minimum temperatures are 32˚C and 10˚C. In this area, 40% to 50% foliar infection is found yearly (Thaung Pe, 1978). However, under favorable conditions for the pathogen, disease severity on susceptible variety Up-to-date is sometimes as high as 100%, especially in post monsoon time (Survey of the author, 2004). In 1967, epidemic outbreaks occurred on 3131 ha out of 3980 ha in Southern Shan State, causing 50% loss in tuber yield (Thaung Pe, 1978). The disease occurs in the plains in certain years when weather conditions are peculiarly favorable for the pathogens, but it is very rare.
Disease Management Successes and Failures
As potato cultivars resistant to late blight have neither been widely available nor adopted in Myanmar to date, growers are accustomed to applying fungicides to control the disease. At present, the fungicides widely used are metalaxyl (Laun 25 WP), mancozeb (Dithane M-45, Agrozeb 80 WP, Indofil M-45, Agromancozeb), metalaxyl + mancozeb (Fortazeb 72 WP), chlorothalonil (Bankonil 75 WP, Fornil 75 WP), maneb + thiophanate methyl (Labilite 70 WP), copper hydroxide (Kocide), copper oxychloride (Cupromax 85 WP) and dimethomorth (Forum 50 WP). Some new fungicides such as iprovalicarp + propineb (Invento 66.8 WP) and propineb (Antracol) are being readied for introduction.
As Ridomil MZ 72 WP (metalaxyl 8% + mancozeb 64%) gave excellent control of potato late blight (Randoll, 1984), the fungicide has been applied widely since 1990s in Myanmar. Recently, there have been complaints that the effectiveness of metalaxyl combined with mancozeb in controlling the disease is reduced. This might be due to the occurrence of metalaxyl resistant isolates of P. infestans (Maung Maung Myint, 2002).
Often due to lack of knowledge about the disease development and technique of timing fungicide application, there is inefficient application (Maung Maung Myint et al., 2001b). Experimentally, the initial outbreak of potato late blight was accurately predicted by Wallin's forecasting based on relative humidity and temperature (Maung Maung Myint et al., 2001b) and by Hyre's system based on daily rainfall and temperature (Maung Maung Myint et al., 2001a). The evidence also indicated that subsequent sprays after each accumulation of 12.7 mm (0.5 inch) rainfall could be used effectively to control potato late blight. There is no need to apply more than once every 7 days (Maung Maung Myint et al., 2001b).
Some growers in Shan State practice early planting at the end of March and early April to avoid epidemics of late blight normally occurring during July to September. Experimental results, however, indicate that early planting can not be relied upon for control of potato late blight in years with early monsoons (Maung Maung Myint, 2002).
Resistant potato varieties grown
CIP 720088 and Kufri Jyoti, introduced in 1992 from Bhutan as resistant varieties, are grown in some areas. However, these varieties are not popular due to severe tuber rotting during storage. Recently, six hybrid true potato seed progenies: BSS 294, 295, 296, 297, 340 and 341 are being introduced as resistant. At present, 11 promising hybrid TPS progenies obtained from the International Potato Center are being evaluated for resistance to late blight under field conditions. However, local growers are unfamiliar with the use of TPS as planting with seed tubers is still the only viable commercial method in Myanmar (MAS, 1990).
Tuber blight problem
Late blight also affects the quality of stored seed potato as mentioned earlier. Although losses in storage have not been recorded systematically, rotted potato tubers are regularly sorted out from the storage (Maung Maung Myint, 2001).
Comments and Conclusions
To minimize crop losses due to late blight, effective management must be implemented. Growers in Myanmar are used to managing the disease almost solely based on fungicide application. In many situations, irrational use of chemicals results in serious economic, social and environmental problems (Mizubuti and Forbes, 2002).
To develop integrated package for the development of resistant cultivars and optimum use of effective fungicide, further research needs to be conducted, including screening of potato varieties for durable resistance, investigation of the best combination of fungicide application based on simple Decision Support System (DSS), such as rainfall thresholds, and resistant varieties for integrated management of potato late blight, and monitoring of metalaxyl resistant isolate of P. infestans.
CSO. 2002. Statistical Year Book 2002. Central Statistical Organization, Yangon.
MAS. 1990. Potato (Solanum spp) research and development in Union of Myanmar (1983-1989). Myanma Agriculture Service. pp.47.
MAS. 1999. Culinary crop production (In Myanmar). Myanma Agriculture Service, Yangon.
Maung Maung Myint. 2001. Potato late blight in Myanmar. Proceedings of the GILB, East and Southeast Asia Linkage Group Workshop on Late Blight, 16-20 August, Baoding, Hebei, China. Journal of Agricultural University of Hebei 24 (2): 24-26.
Maung Maung Myint. 2002. Research on management of potato late blight in Myanmar. pp 163 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.
Maung Maung Myint, Thein Su and Kyaw Win. 2001a. Effect of fungicides based on disease forecasting in controlling of potato late blight in Myanmar. pp 16-21 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.
Maung Maung Myint, Thein Su and San Thein. 2001b. Effects of sowing time, mound burning practice and timing of fungicide applications in the management of potato late blight in Myanmar. Proceedings of the GILB, East and Southeast Asia Linkage Group Workshop on Late Blight, 16-20 August, Baoding Hebei, China. Journal of Agricultural University of Hebei 24 (2): 25-31.
Mizubuti, E.S.G. and G.A. Forbes. 2002. Potato late blight IPM in the developing countries. pp. 93-97 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.
Randoll, C.R. 1984. Fungicide testing for early and late blight control in potatoes. Plant Disease 68:742.
Thaung Pe. 1978. Potato production in Burma. In: Proceeding of the Second Regional Symposium on Potato Production: Pathogens and pests of the potatoes in the tropics. Los Banos, Philippines.
1] In: Proceedings of the Regional Workshop on Potato Late Blight for East and Southeast Asia and the Pacific held 24–25 August 2004 in Yezin, Myamar. Posted on GILB website and in press.
Links to World Potato Atlas (WPA)