back to Country Profiles

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)?

Late blight is the main fungal disease of potato. It is particularly important in the super-humid zone where cool and humid conditions favor its development throughout the potato-planting season (May–August). There are two planting seasons — the first is April–June and the second is July–August.

Tomato and Solanum nigrum are also hosts.

The sources of the disease are not known but it is postulated that the main one is tomato, which is grown around the year.

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

Defoliation can reach 100% if fields of susceptible varieties are not treated. Yield losses of 20% or more can be expected. On the other hand, tuber rot is rarely encountered.

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

Fungicide treatment is an important component in the management of late blight. The most effective fungicide remains metalaxyl and guidelines have been issued to avoid its misuse. Mixtures of metalaxyl (75 g a.i/ha) and mancozeb (2 400 g a.i/ha) are used preventatively early in the crop cycle during the period of active growth and not more than four sprays are recommended during one crop cycle. Following the metalaxyl treatments, a range of other fungicides can be applied up to the end of the growth cycle for the control of late blight. These are (per hectare): mancozeb only (2400 g), mancozeb + cymoxanil (2100 g + 180 g), mancozeb + fosetyl-Al (780 g + 1320 g), mancozeb + propamocarb-HCl (1206 g + 992 g). Growers are also advised to consider factors such as growth stage, susceptibility of the variety and prevailing weather conditions when deciding on the treatment to be applied.

Trials have been conducted to identify fungicides that could be substituted to avoid the over-use of metalaxyl, which could lead to a build-up of metalaxyl-resistant P. infestans strains. Melody Duo (iprovalicarb+propineb - 165 g + 1838 g/ha) followed by Dithane M45 (mancozeb - 2400 g/ha) was found to be as effective as the standard treatment of Ridomil Gold (metalaxyl M + mancozeb - 75 g + 1200 g/ha) followed by Dithane M 45. Blight incidence was at 0.1% with both treatments compared to 100% in untreated plots 11 weeks after plantation. Melody Duo followed by Dithane M45 appears to be an appropriate non-phenylamide alternative against late blight in a strategy to limit the development of metalaxyl-resistant strains of P. infestans.

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

The most popular variety is Spunta, which occupies 35% of the area planted. Most of the seed of this variety is produced locally and is not enough to satisfy the demand. Spunta has a good tolerance to late blight in Mauritius despite the fact that it is reported to be susceptible in many parts of the world. Even though it is resistant to late blight, variety Stirling is still being discovered by growers and is yet to occupy a significant area. The varieties cultivated and their reaction to late blight is given in the table below.

In 1989, the Mauritius Sugar Industry Research Institute and the International Potato Center (CIP) initiated a collaborative project for the development of potato varieties adapted to lowland tropical conditions. Four varieties (Belle Étoile, Belle Mare, Belle Rive and Belle Vue) have been released from the program. All the four varieties are resistant to late blight, although under circumstances extremely favorable to the disease, moderate infection has been occasionally been observed on Belle Mare and Belle Rive. These varieties are yet to be adopted by growers.

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

Occurrences of host resistance failures are unknown. When varieties with good resistance have become infected, they were more likely to have been exposed to very high disease pressure (see 4 above).

Fungicide failures have been occasionally encountered. Possible reasons are: high disease pressure, poor spraying technique, use of sub-standard fungicides, pathogen resistance.

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

Population structure of Phytophthora infestans

The presence of the A2 mating type has not been demonstrated. Thus, presently only the A1 mating type is known. Molecular characterization carried out by Cornell University on isolates from potato and tomato has shown that isolates belong to the US-1 genotype.

Metalaxyl sensitivity

Isolates of Phytophthora infestans were collected from potato, tomato and Solanum nigrum in 1999-2000 and grown on pea agar supplemented with antimicrobial compounds. Metalaxyl resistance of strains was assessed in vitro by measuring radial growth of the colony on pea agar containing metalaxyl. Variation in metalaxyl sensitivity was observed among the strains. Only one isolate was highly sensitive and did not grow in the presence of the fungicide.  Nine other isolates were affected to various extents. Although they all grew less than the control at 5 ppm metalaxyl, six isolates grew more than 40% of the control and by convention, they can be considered as resistant to the fungicide. This preliminary study shows that a proportion of the local population of P.infestans could be resistant to metalaxyl.

7. Disease risks and/or major needs?

Late blight infection is high under cool and wet environment in Mauritius. There is a need to investigate the population structure of the pathogen and the role of host plants in the perpetuation of the disease. It would be interesting to analyze whether the implementation of a forecast system would be helpful to the grower.

8. Any publications on late blight in the country?

Felix, S. Ricaud, C. 1975. Chemical control of late blight of potatoes in Mauritius. Revue Agricole et Sucriere de L'Ile Maurice (Mauritius). ISSN 0370-3576. 1975. 54:205-210.

Ganoo, S. Saumtally, S. 2000. Potato late blight: in vitro studies and development of a fungicide programme for disease control. pp 309-315. In: 5^th Triennial Congress of the African Potato Association, 29 May-2 June 2000, Kampala, Uganda

Ganoo, S. Saumtally, S. 2001. In vitro resistance of Phytophthora infestans to metalaxyl and control of late blight using non-phenylamide fungicides. African Crop Science Conference Proceedings 5: 487-491.

9. Is tuber blight an increasing problem?

Tuber blight is not important and is very rarely observed

10. Other comments

Current areas of research on Late Blight

• Screening of varieties in the field
• Screening of new clones using a leaf-disks method
• Island wide collection of isolates and determination of their sensitivity to metalaxyl
• Evaluation of fungicides for the late blight control
• Sources of the disease

Scientists involved  

Amongst their other duties, two scientists are involved in late blight research, which is carried out at the Mauritius Sugar Industry Research Institute

 Potato production and utilization

The local seed scheme was started in 1973 with the bulking of certified seed stocks. Presently, varieties Mondial, Spunta (Netherlands), Stirling (Scotland) and Mauritian varieties (Belle Étoile, Belle Mare, Belle Rive and Belle Vue) are included in the seed program. The basic seed (100 tons) of the Dutch and Scottish varieties are imported from Europe and Australia. Nuclear stock of the Mauritian varieties (5 tons) are either produced locally or bulked overseas. The annual seed production is about 1250 tons, of which around 350 tons are retained for a second bulking. The remaining 900 tons are used for ware potato production during the first planting season (April–June). Eighty-five percent of the seed produced is of the variety Spunta. The second season (July–August) relies solely on the imported varieties Delaware, Mondial and Up to date.

The country annual production is about 17,000 tons of potato, and a further 5000 tons is imported from various countries