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 The Threat to the Potato

Ranked fourth in the list of 20 staple crops worldwide, the potato is an essential part of our diets and economy. Its history has been marked by disease, and breeders have struggled to produce varieties less vulnerable to fungal attack. Public hostility to the use of chemicals – currently the only effective weapon against ‘late blight’ – and the genetic modification of crops makes it increasingly difficult to pursue these ways. Therefore, an approach in the spirit of sustainable agricultural development would be more suitable. An EU-funded project aims to improve organic production methods in order to manage potato blight more successfully.   Graphic element
     
A major staple crop: the potato
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Our food supply depends worldwide on a mere 20 staple crops. Among them, following the three cereal giants (rice, wheat, maize), is the potato. More than 270 million tonnes are produced every year in the world, 48.5 million tonnes of those in the EU alone (1999), almost equal to China, the main producer.

In fact, the potato mentioned here is only one single species, Solanum tuberosum, among many others which are cultivated or growing wild in the Andes region. In 1570, the Spaniards brought some cultivars of S. tuberosum from there to Europe, from where the potato then began its conquest of the world. Today it is a very important staple crop in various regions, e.g. Africa and South-east Asia as well as Europe.

The threat: late blight
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Over the centuries, farmers have been collecting other Solanum subspecies to overcome the lack of genetic variability. Despite these efforts, our potato remained vulnerable to diseases. The most dangerous enemy arrived in Europe around 1840 and came from Latin America: the fungus Phytophthora infestans, also called ‘late blight’. It swept through Europe in a matter of a few years, causing crop losses on a wide scale. Potato late blight was the primary cause of the great Irish “potato famine” in 1845. It seems from genetic fingerprinting studies that until recently the late blight fungus in Europe was very uniform. This suggests that all were descended by clonal reproduction from one or a very few original isolates.

The late blight fungus usually strikes under humid, cool conditions just before harvest and can destroy the foliage in 24 hours. It causes mottled, dark lesions on leaves and stems which produce a white, velvety growth that kills the plant and can cause rotting in potatoes in storage. The annual losses for agriculture globally are still very high: 14 million tonnes, representing a value of more than €3 billion.

However, over time, breeders have identified sources of resistance. In its homeland, the fungus is present as two distinct types (A1 and A2) which can sexually reproduce and there is thus the potential for gene flow between populations of the fungus, and for faster breakdown of host plant resistance. Only one type (A1) arrived in Europe originally. Unfortunately, since the late 1970s other strains, including type A2 have been found in various European countries, probably following the importation of potatoes from Central America An increase in virulence has also been observed. Although asexual spores are unable to persist in the environment for any length of time, the sexually produced ones have a protective coat and can survive in the soil for years. With the introduction of more aggressive strains, sexual recombination (and increasingly wet weather), late blight is now a major threat again.

The response: improved host resistance
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To date, the only effective weapons farmers have against this fungus are chemical fungicides which have to be sprayed prophylactively. However, not only can these products be harmful for the environment, but the fungus is developing resistance to some of them. Moreover, the use of copper-based fungicides in organic agriculture is being phased out. An alternative, would be to genetically engineer anti-fungal defence into potatoes. Such techniques might have other advantages, too, such as achieving defined sugar or starch contents better adapted to industrial processing to obtain, for instance, high-quality chips and crisps.

The growing public awareness of the need to shift towards more sustainable agriculture, however, makes it difficult to promote the use of chemicals; and there is some hostility and reluctance to accept genetically modified crops. Plant researchers have the alternative of understanding the natural defences of the plant and its wild relatives, and improving the crop by making crosses in the traditional way. In 1996, the Lima-based International Potato Centre (CIP) launched a world-wide effort, called the Global Initiative on Late Blight (GILB), to try to solve the problem of late blight, through world-wide collaborative research. Already more durable sources of resistance have been found among potato varieties from Mexico, and among old European varieties, and new resistant varieties are being produced. European scientists are collaborating in this global initiative, contributing their skills, for example in decision support systems for integrated control strategies (from a FAIR project FAIR-1-0206 “European Network for development of an integrated control strategy of potato late blight (EU.NET.ICP)”, in assessing variation in populations of the Phytophthora infestans fungus, and in assessing resistance to late blight in potato varieties and species. Research is focussing on integrated control strategies and in the longer term, on resistance breeding.

The European approach: organic farming
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Resistant potato varieties are, of course, welcomed by organic farmers who avoid fungicides and genetically modified crops. In the United Kingdom alone, the market for organically grown potatoes was £9 million in 2000, representing 5% of the total potato market. Facing substantial losses, these farmers have to identify the most efficient approaches to cope with the threat of late blight.

This is where an EU-funded project (QLK5-1999-31065) promises important help. It aims to develop improved organic production systems in which potato blight can be managed with or without minimum inputs of alternative intervening treatments. This will be achieved by the development of regionally adapted blight management strategies.

The project focuses on the following tasks:

  • Identification of best current practices in different regions of the EU;
  • M anagement of resistance in different varieties;
  • Identifying diversification strategies within the same field;
  • Developing the concept of preventative agronomy;
  • Development and improvement of formulation and application technologies for alternative treatments to copper fungicides.

A complementary project on co-ordinating breeding activities for blight resistance is currently under negotiation. In the long run, farmers might finally win the race against late blight, although total eradication will remain a dream.

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CO-ORDINATOR
Carlo Leifert
University of Newcastle TESCO
Centre for Organic Agriculture, Department of Agriculture
Newcastle, UK
Tel: +44 192 222 6867
E-mail: cleifert@ncl.ac.uk

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