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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)?

With access to markets in India, potato became the most important cash crop for the higher regions of Bhutan. Potato is cultivated in all Dzongkhas (administrative units, equivalent to states or counties). In some Dzongkha it is cultivated on kitchen garden scale while in some Dzongkha it is the main cash crop. Figure 1 below shows the major potato growing Dzongkhas.

Figure 1: Major Potato growing Dzongkhas in Bhutan.

Various pests have been recorded in potato in different parts of the country. In the highlands, Late Blight, viruses and scab are common. At medium to low elevations, red ants and early blight are of importance.

Late blight sometimes causes economic losses and occurs sporadically. In some years with early spring rains it takes off at an epidemic scale but only in specific conducive environments. In high altitude areas, early potato crops planted in the period January - March normally escape the disease. Low altitude potato is planted in September/October, and normally escapes the disease due to the approach of the dry winter season.

So far only potato has been affected by LB in Bhutan; no serious effort is made to study infection of LB in other solanaceaous crops. No scientific data are available, but it is assumed that disease sources are the soil in high altitude areas, and probably left over infected tubers in the field. In major potato areas in high altitude volunteer plants cannot survive frost in winter; therefore they are not considered important in LB epidemiology.

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

The total harvested potato area in Bhutan is around 2900 ha and the average yield amounts 11.2 tons/ha. Total potato production is 32200 metric tons (FAOSTAT, average data over 1999-2003). No economic figures on impact of Late Blight are currently available.

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

In Government run seed farms Mancozeb is sprayed when LB is first detected and repeated sprays normally effectively contain LB. Most potato farmers in Bhutan do not spray fungicides. The Ministry of Agriculture supplied fungicides at subsidized rate until 1995. Since then pesticides subsidy is removed to encourage safe and judicious use of pesticides.

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

An overview of the main potato cultivars in Bhutan and their susceptibility to Late Blight is given in Table 1. All of them are varieties introduced from outside; they are preferred by farmers and markets in India. Desiree is a red skin variety that fetches a good price in the market. The other two are white skin varieties. None of these varieties are good for making chips. Evaluation of potato varieties is a regular activity in some of the research centers in Bhutan. Varieties are evaluated especially on yield potential, resistance to late blight and early maturity. Early maturity is important because in some parts of Bhutan potato is cultivated before rice and thus those farmers prefer early maturing varieties. Germplasm potato exchange within the region and with CIP should be encouraged.

Table 1. Commonly cultivated potato varieties and their reaction to late blight.

*S = susceptible, MR = moderately resistant

Forecasting

No information about possible deployment of forecasting systems is available.

Cultivation practices

The main cultivation practice contributing to the reduction of Late Blight damage is escaping the period with the highest infection pressure. This can be done early cropping with early varieties (see Table 1), or, in the lowlands, by planting in September or October, so that the crop is growing during the dryer winter period.

Late blight resistant cultivars

Highly resistant cultivars are not grown at a significant scale (see Table 1)

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

Fungicides that can lead to development of resistant pathogens (e.g. metalaxyl) are not deployed at significant scale.

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

Not much is known about the characteristics of the population of Phytophtora infestans in Bhutan and the presence of mating type A2 has not yet been confirmed. More research on the pathogen is needed to aid in effectively controlling the disease.

7. Disease risks and/or major needs?

Introduction of varieties with durable resistance, such as the B3C1 clones that are currently being evaluated, could contribute to reducing the damage caused by late blight.v

8. Any publications on late blight in the country?

9. Is tuber blight an increasing problem?

LB infection of tuber is a major problem in the farmer’s field. It is one of the major causes of storage rot of tubers. Disinfection after harvest is not practiced by farmers. Cultivated potato cvs. in Bhutan is attacked by LB suggesting lack of resistance.

10. Additional comments

From: Thinlay. 2004. Overview of late blight in Bhutan 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. Prepared by Sander De Vries, International Potato Center, Indonesia.