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Potato late blight

Strategies to control a devious pathogen

Phytophthora infestans, the pathogen responsible for potato late blight, is one of the most devastating potato diseases of all. The tiny fungus causes annual harvest losses of around 5 billion euros. Attempts to breed resistant varieties have so far failed to bring lasting success, because the pathogen always manages to breach the bred-in resistance. Genetic engineering methods can be used to transfer resistance genes from wild potatoes to cultivated potatoes. A potato of this kind, developed by BASF under the brand name Fortuna, was submitted for approval in 2011 following several years of greenhouse and field trials. Another GM potato developed at the University of Wageningen is being tested in the open in trials that started in 2008. There are plans for a deliberate release in Ireland in 2012.

Phytophthora spreads like wildfire, particularly during warm, wet weather. Grey-green patches appear initially on the leaves and stems, turning brown later. A white fungal coating forms on the underside of the leaves. The leaves eventually dry up or rot. The fungus is spread mainly by spores carried on the wind.

From 2006 – 2010 genetically modified potatoes which are resistant to Phytophthora are being field tested in Sweden, the Netherlands, Britain, Germany and Ireland.

Up until now, Phytophthora has been controlled almost exclusively with the use of chemical fungicides. Potato crops in Germany are sprayed up to 16 times during a single growing season. In organic farming, environmentally harmful copper compounds are used to control Phytophthora.

Because Phytophthora is so versatile and adaptable, conventional plant breeders throughout the world are currently working on a resistance type which is determined by several genes and is not race specific. Although this does not result in absolute protection against infestation, the protection is long-lasting because it cannot be breached as easily by new forms of the pathogen, due to the large number of genes involved.

The genes responsible for the race-nonspecific resistance are crossed from wild potatoes to cultivated varieties. However, the undesirable traits from the wild potatoes, which are transferred at the same time, must then be bred out again, without losing the resistance traits. The complexity of this hereditary transmission makes the process difficult and time-consuming.

Transferring resistance genes from Mexican wild potatoes

BASF has developed a promising fungus-resistant potato in recent years. It contains two genes transferred from Mexican wild potatoes. The scientists took the same approach they would have used in conventional plant breeding: they looked for wild potatoes that are naturally highly resistant to the pathogen that causes potato late blight. They found what they were looking for in a Mexican wild potato called Solanum bulbocastanum. Using molecular biological methods it is possible to identify the genes responsible for the resistance. They can then be isolated and transferred into the plant using genetic engineering methods.

In 2006, following greenhouse trials that successfully tested the resistance of the genetically modified Fortuna potatoes, field trials started in Sweden, the Netherlands, the UK, Germany and Ireland. In 2011, BASF Plant Science applied to the relevant EU authority for approval of the Fortuna potato both for cultivation and for food and feed use. In January 2012, however, the group withdrew its biotechnology activities from Europe and relocated BASF Plant Science from Germany to the USA. The company is no longer seeking to introduce the Fortuna potato onto the European market.

The University of Wageningen has also developed potatoes that contain two or more resistance genes from wild potatoes. The scientists in Wageningen are following a cisgenic approach, in other words they only transfer gene sequences from potato plants. This means, for instance, that they did not use a marker gene, which would normally have come from another organism. Initial release experiments with the Wageningen potatoes took place in 2008, and the potatoes are due to be tested in the field in Ireland in 2012.

Further genetic approaches

In addition to the possibility of transferring resistance genes from wild potatoes, research has also been carried out on a number of other strategies:

  • Transferring plant or bacterial genes for substances which destroy the fungal cell walls, e.g. chitinase or glucanase.
  • Inserting genes for specific proteins which are produced by plants to protect them against fungi.
  • Increasing natural resistance using two genes from a soil bacterium.

These approaches have not yet produced any genetically modified fungus-resistant potato varieties that are likely to be brought onto the market.