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 Resistance to Potato Late Blight:
A global research priority
By
Marc Ghislain, Rebeca Nelson and Thomas Walker
  
Keywords:  Policies/Programmes; International Potato Center (CIP); Potato/Sweet potato; Disease/pest resistance; Genetic engineering.
Correct citation: Ghislain, M., Nelson, R. and Walker, T. (1997), "Resistance to Potato Late Blight: A global research priority." Biotechnology and Development Monitor, No. 31, p. 14­16.

Late blight, the world's oldest known potato disease, has received fresh attention. Due to changes in the global population structure of the fungus responsible, the rapid expansion of potato production in developing countries, and growing environmental concerns, the burden of late blight is growing. At the same time, new biotechnological means are emerging that may provide a more efficient disease control in the future. Due to these changes, late blight was recently defined as a top priority at the International Potato Centre.

The International Potato Center (CIP) is an international agricultural research centre funded by the Consultative Group on International Agricultural Research (see also Monitor No. 12). CIP is responsible for identifying the changing conditions that affect its mandate crops and agro-ecosystems and for setting priorities accordingly. New emerging opportunities need to be rapidly and coordinately included in the research programme of the Centre. This is particularly true for biotechnology.
CIP has invested in a series of priority setting exercises, aimed at assessing the potential impacts of research activities in relation to research inputs. Priority setting has generated information leading to the design and funding of research projects. To date three exercises have been conducted at CIP head-quarter in Lima, Peru (see box).
As a result of newly set priorities, CIP has convened a large, collaborative research effort known as Global Initiative on Late Blight (GILB). This initiative aims at coordinating efforts to accelerate the development of improved potato varieties with resistance to late blight, and the integrated management of the disease. GILB involves partners from developed and developing countries. Researchers in advanced research institutes will produce knowledge and technologies that will aid national programmes and the implementation of strategies for sustainable potato production in developing countries. This global partnership is sustained by an equal participation in GILB management, and through sharing of financial resources.
The private sector is expected to play an important role in late blight research by providing genes and technologies that can be used for producing disease- resistant varieties. Through GILB, CIP could act as a facilitator to provide these technologies to developing countries. Such arrangements have been made in other cases; for instance, CIP has obtained free licensing from Plant Genetic Systems, Belgium to deploy transgenic potatoes carrying an insecticidal protein in specific developing countries. Further partnerships between private companies, international centres and developing countries need to be established so that developing countries' agriculture could benefit from biotechnology inventions held often by the private sector.

The increasing importance of late blight disease
Late blight, caused by the fungus Phytophthora infestans, first gained notoriety 150 years ago for its role in the Irish potato famine. The disease is now the most important threat to potato production in developing countries. Several factors contributed to this. Firstly, through migrations of P. infestans, presumably carried on contaminated planting material, new strains of the fungus have emerged in many countries. The 'new' pathogen populations are resistant to the commonly used fungicide metalaxyl. The presence of compatible mating types among the 'new' and the 'old' pathogen populations permits greater genetic variability through sexual reproduction. Oospores, resulting from sexual reproduction, represent an additional threat to potato production because they can survive in the soil for several years. The presence of soil-borne inoculum may lead to earlier infection in the growing season.
Secondly, potato production in developing countries is increasing rapidly. Annual potato production grew from 30 million tonnes in the early 1960s to 102 million tonnes in 1996. Output in developing countries is expected to increase by 2.8 per cent each year. By the year 2000, economists predict that developing countries will produce more than a third of the world's potato crop. Late blight epidemics have become worse in developing countries as well. Crop losses from late blight in developing countries are currently estimated at US$ 2.75 billion.
Thirdly, concerns about environment and farmers' health are growing. The excessive, and often ineffective, use of fungicides poses an increasing threat to the sustainability of potato production, especially in developing countries. An estimated US$ 100 million is spent on fungicides in developing countries each year. In the tropical highlands, where inoculum is often present year-round, farmers must begin applying fungicides at a very early stage of the growing season. They may spray as many as 35 times in a season. The emergence of fungicide-resistant strains is almost inevitable under these conditions. The high risk of losses and the need for increased use of fungicides makes potato cultivation unprofitable.

CIP's methodology of priority setting

The first priority setting exercise took place in 1992 was based on a quantitative scoring model. In this one week long activity, the internationally recruited staff from CIP head-quarters participated. Each of the 36 projects of CIP was scored individually with estimated values for several criteria (see table). These criteria were estimated for six agro-ecological zones (arid and Mediterranean, temperate, subtropical lowlands, highlands, humid tropics, semi-arid tropics). 
The second exercise, in June 1996, was more ad-hoc. It was conducted over two afternoons involving all CIP staff, and based on "gut-feeling" responses to a questionnaire eliciting opinions as to which of the 36 projects of CIP warranted greater or lesser investment. This exercise, humorously entitled "Quick participatory priority setting, or the mediocrity of masses", was in reality a prelude to the third exercise held a few months later. 
Prior to the third priority setting-exercise, a new portfolio of 19 self-contained, problem-driven projects was designed. Of these projects, 15 were considered comparable and suitable for prioritization; projects related to the conservation of genetic resources, impact assessment, and global characterization and policy were excluded from the process, because benefits in terms of increased productivity, cost savings, and area expansion could not be clearly estimated for these service projects. Economic impacts were estimated on the basis of target and spill-over countries instead of agro-ecologies. 
The third exercise involved a one week, full-time, meeting of CIP's internationally recruited staff, including selected regional representatives facilitated by a World Bank economist. The highest ranked project was related to dry matter content and adaptation of sweet potato, followed closely by the project of Integrated Control of Late Blight. This unexpected result was explained by the importance of the area coverage in this model which skewed results towards projects involving large countries such as China.

Date Procedure Participants Criteria
September 
1992
Quantitative 
scoring model
Head-quarter staff
  • Potential for increasing production through research; 
  • Importance of the constraint to be addressed by a project; 
  • Extent of technology adoption; 
  • Probability of obtaining research results; 
  • Importance of the research result for other CIP projects; 
  • Importance of research results for the regions; 
  • Effect of the technology(ies) on the environment; 
  • Research costs.
June 
1996
Ad hoc All CIP staff What projects should receive more or less emphasis given budget allocations 
prevailing in 1996?
September 
1996
Economic 
project 
appraisal
- Head-quarter 
  staff; 
- Regional 
  representative.
  • Perceived sources of benefits; 
  • Probability of research success; 
  • Perceptions on adoptions in target countries; 
  • Perceptions on adoptions in spill-over countries .

Opportunities for biotechnology
In industrialized countries, late blight is managed through sanitation measures, sophisticated forecasting and the massive application of fungicides. For both socio-economic and epidemiological reasons, these strategies are not useful in the highland tropics of developing countries. Although host-plant resistance has been little used in industrialized countries, it is currently thought to be the best alternative to extensive use of fungicides both in industrialized and developing countries. In industrialized countries it is difficult to replace potato varieties due to market constraints. However, farmers in many developing countries are demonstrably willing to adopt new late blight resistant varieties.
Breeding late blight resistant varieties is not easy because of the genetic complexity of the crop. Genetic engineering and marker-assisted selection, however, offer new opportunities for developing improved resistance.
Genes can be transferred to cultivated potato, from sources such as native varieties or wild species, by various routes. Three transgenic approaches are currently envisaged for improvement of resistance to late blight.
Firstly, antifungal proteins, including pathogenesis-related proteins such as osmotin and glucanase; plant defensins; lectins; and lysozymes, can be expressed in various ways in the plant.
Secondly, the natural defense mechanisms that plants use to protect themselves against pathogens can be enhanced by modifying the expression of genes involved in this pathway, such as genes of the systemic acquired resistance.
Thirdly, there are strategies based on the activation or stimulation of the hypersensitive reaction. In these strategies, a pathogen gene eliciting the hypersensitive reaction in the plant is engineered. The purpose of this engineering is to carry race non-specific control sequences. Then, it is transferred into the genome of a potato plant carrying the corresponding resistance gene. Hence, the transgenic potato will acquire a race non-specific hypersensitive reaction, which the pathogen cannot overcome.
In addition, in several laboratories around the world, molecular maps of the potato genome have been constructed and genes of agronomic importance have been located on the potato chromosomes. Molecular genetic analysis of quantitative resistance to late blight from a range of sources is an important research objective of GILB and of CIP's late blight project. In the laboratory at the Max Planck Institute, Cologne, Germany, quantitative trait loci for late blight resistance have been identified and mapped on the potato chromosomes. At CIP, Solanum phureja has been identified as a promising source of quantitative resistance to late blight, and molecular markers associated with this resistance have been identified. A series of mapping populations carrying resistance from diverse sources is now under analysis. In collaboration with several other laboratories, CIP pursues a 'candidate gene' approach, in which genes known or suspected to play a role in plant defense are used as markers in molecular genetic analysis. The results of these studies will be used to obtain more efficient quantitative resistance to late blight in potato improvement, through marker-assisted selection, cloning and direct transfer of resistance genes.
Marc Ghislain/Rebeca Nelson/
Thomas Walker
International Potato Center, P.O. Box 1558,
Lima 12, Peru. Fax (+51) 1 435 15 70;
E-mail cip@cgnet.com

This article is based on a paper by Marc Ghislain and Peter Gregory earlier presented at the IBS-CamBioTec Regional Seminar on Planning, Priorities and Policies for Agricultural Biotechnology, October 6-10, 1996, Lima, Peru.

Sources:
CIP/FAO (1995), Potatoes in the 1990s: Situation and prospects of the world potato economy. Rome: CIP/FAO.

M.H. Collion and P. Gregory (1993), Priority Setting at CIP: An indicative framework for resource allocation. Lima, Peru: CIP/ISNAR.



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