Massing weapons of destruction

Rot, blight, wilt and pestilential viruses - Irish potato growers will be sure to have experienced some if not all of these damaging disease types. One of the most feared, late blight, Phytophthora infestans, can wipe out a potato crop in a few weeks. Phthorimaea operculella, the potato tuber moth, causes damage to growing plants by mining leaves in the field and infesting potato tubers held in storage allowing bacterial infection to get a hold. Bacterial wilt, Ralstonia solanacearum, is possibly the most important plant bacterial disease in the world, entering plants through their roots to mount a deadly assault on their fluid and nutrient transport networks. From biotech laboratory to down on the farm, no wonder every means at their disposal is being explored by scientists to control these and other diseases.

credit: Cabi Bioscience

It is estimated by CIP that $3.25 billion is lost to late blight in developing countries every year with $740 million spent on pesticides alone. The potential risk to users, to say nothing of the cost, from this level of pesticide use can only be imagined. And would more farmers take advantage of the buoyant potato market if they could be more confident of a blight free season? In the lab, CIP scientists use DNA fingerprinting to detect variation in the disease strains. This helps them to understand how pathogen populations evolve and so provide farmers with advice on how best to control outbreaks. Plant breeders are incorporating partial blight resistance genes from wild and cultivated potatoes in a process called recurrent selection to develop new blight resistant varieties. Meanwhile, down on the farm, farmers need access not only to resistant varieties but to knowledge of how best to complement resistance with effective management and use of fungicides. Farmers Field Schools have been established in several countries. In one example, Bolivian potato farmers used their field school knowledge to earn each year $2,400 per hectare more than their neighbours.

By using toxin genes from Bacillus thuringiensis (Bt) inserted into potato germplasm, scientists at Michigan State University hope to develop effective resistance to the potato tuber moth. Potato has a narrow genetic base and reproduces asexually. It is therefore difficult to introduce desirable traits, such as pest resistance, by conventional breeding. Genetic engineering offers new routes to resistance.

Hopes have also been raised for finding a mechanism to defeat bacterial wilt. The genome of Ralstonia solanacearum, the bacteria that causes the disease, has now been sequenced by a French team at the National Agronomic Research Institute in Toulouse. Knowing the genes may allow scientists to work out how to switch them off, a trick that Ralstonia can do itself, allowing it to live in plants without harming them. Another avenue that is being explored is to engineer benign forms of the bacterium with which to inoculate plants against the lethal form as in a biocontrol strategy. Kenya Agricultural Research Institute [KARI] in partnership with CABI Bioscience has had some promising early results with such non-pathogenic biocontrol agents [BCA] and hopes eventually to run trials in Kenya and possibly Uganda. Current biosafety legislation relating to crop-related GM technologies makes this a challenging task, but provisional approval for testing the BCA has been obtained for contained-use in Kenya and an application is being discussed for Uganda.

Importantly this molecular based genetically modifying research has not gone on without visulising the cropping system within which it is to be applied. Practical, 'down on the farm', novel seed systems have been piloted that are suitable for application of the BCA and other good management practices such as crop rotation. In the early phase of this work farmers in the village of Njabini in Kenya have been testing a nursery seed production system that breaks away from traditional systems of on-farm seed maintenance over many generations without recourse to healthy new stocks. These farmers have now formed themselves into the Jitegemee Agricultural and General Self Help Group and continue to practice the seed plot system independently of the piloting project. This method is also now being promoted in Uganda through two new DFID CPP projects in Kabale and Kapchorwa through farmer field schools, and amongst much enthusiasm from the farmers.

Another interesting aside to this GM research on BCAs' has come from the development of ways to assess their potential environmental impact. Such studies are a pre-requisite to the eventual acceptance of the technology and have necessitated very exacting analysis of the BCAs' population dynamics in the soil. These studies have produced some of the first quantitative data on the Ralstonia-supporting potential of rotation crops to potato and, by inference, some of the most robust recommendations on what may be an effective rotation system for control. So, curiously, whereas the BCA still remain in the laboratory at CABI, a peripheral aspect of the project has led to significant new knowledge about the control of this serious disease.

As more countries sign up to biosafety legislation based on the Cartagena Protocol that covers transboundary movement of living modified organisms, more technologies will become available as a means of mitigating the effects of potato pests and diseases - provided that the necessary funds are also made available for research and development.

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