GMOs and Disease

and Pest Resistance




by Professor Martin Wolfe
Retired Chair of Plant Pathology,
Swiss Federal Institute of Technology (Zurich),
Research Director, Elm Farm Research Centre (U.K.) and
Wakelyns Agroforestry research farm (U.K.)





an occasional paper of

The Edmonds Institute
20319-92nd Avenue West
Edmonds, Washington 98020




Note: This occasional paper of The Edmonds Institute first appeared in the Elm Farm Research Center Research Bulletin and is reproduced with the permission of the author.

Elm Farm Research Centre is an international research, advisory and educational organisation based in the UK. The business of EFRC is to change agricultural practice and policy to bring about a greater uptake of organic farming as a way of contributing towards sustainability in farming and land use in the UK and internationally.

There are many valid reasons for a healthy suspicion of the supposed benefits from the introduction of genetically modified organisms (GMOs) into agriculture. On the plant side, unknown side effects of the novel breeding system, possible contamination of natural and agricultural plant populations and the non-democratic imposition of a major change in agriculture, are only some. Another aspect is the introduction of genes for herbicide, pathogen and pest resistance into crop plants, under the stated objective of providing a reduction, or elimination, of pesticide use on the farm. Achieving this laudable objective appears distinctly remote, except, perhaps for some short-term but non-sustainable gains. The reasons can be complex.

Recently, concern has been raised over the introduction of crop varieties which incorporate the gene for expression of Bt (Bacillus thuringiensis) toxin. The gist of the argument is that Bt toxin is widely used as a spray (acceptable under organic regulations) which means that it need only be used when there is a problem. When it is used, the toxin is effective for only a short period so that it does not force selection for pest genotypes resistant to it. However, if resistance to the toxin does start to develop in the pest population, it is then possible to change to use of a different form of the toxin. In other words, Bt used as a spray is relatively flexible, requiring minimal use and causing relatively little selection for resistance.

Where Bt toxin expression is incorporated into a host such as potato or maize, the gene is 'switched on' throughout the life of the plant whether or not the pest is present (- and what happens to passing beneficial insects?). If pest resistance develops, which is more likely under these conditions, it is difficult to switch among varieties expressing different toxin molecules. If an appropriate range of varieties is available (very doubtful), switching among them is both slow and expensive. In other words, the use of crop varieties that express Bt toxin is very likely to reduce the effective life of these useful, natural insecticides, both for the industrial and for the organic farmer.

The likelihood of this negative scenario is heightened strongly by the attitude of the chemical companies delivering these Bt varieties, which is to push the varieties as hard as possible in the market place in order to recoup the expense of breeding them and to provide a profit for the disinterested shareholders.

There are discussions in progress to try to develop strategies for use of the Bt varieties to try to minimise the problem, principally by introducing refugia into the crops. This means having a small part of the crop that lacks the toxin expression and is not sprayed. This allows survival of pest genotypes that are sensitive to the toxin and which then mate with any resistant ones that survive on the rest of the crop: this should dilute Bt resistance in the pest gene pool so that it remains unimportant. It seems to me that the task of developing the range of strategies necessary for the spectrum of environmental conditions under which the crops are grown is nigh impossible. Even if it were possible, the task of implementing those strategies quickly and correctly on all farms really is beyond belief.

This particular example, of Bt toxin manipulation across a number of major crops, helps to highlight major generalisations that we can make about some of the problems of introducing pest and fungal disease resistance into crop plants through genetic manipulation. In the chemical companies, there is a major, and justified, fear about the potential lack of durability of crop resistances developed through genetic engineering. This is principally because of the two reasons noted above with Bt. First, the gene that is built into the crop is permanently switched on, and second, because of the expense of the whole business, there is a continuous need to push for monoculture of the new varieties, which, of course, magnifies selection in the pathogen or pest for lack of sensitivity to the new variety. One worrying consequence of this problem is that the chemical companies prefer to look for manipulable genes that have broad spectrum activity and which affect some fundamental process in the target pathogen or pest. Using such genes may help to control the target organism on a large scale, but this raises serious questions about side-effects in the environment generally of large amounts of such genes being expressed over large areas.

On a number of occasions, critics have rightly posed many questions about such developments, the first often being, do we actually need this kind of technology? A fundamental problem here, in my view, is that the developments that we are seeing arise out of the massive movement towards monoculture, particularly in the second half of this century - and they are designed to allow that movement to continue. Such industrialisation of agricultural production is obviously not sustainable, for the many reasons widely discussed elsewhere. Genetic manipulation could be of interest in a much more diversified agricultural system provided, first, if it was considered only for those disease or pest problems for which no other simpler, cheaper and environmentally benign answer were available, second, if it was limited to development of a strategy which involved only those host genes that operate against the target organism and no other, and third, if it was used in an overall strategy that integrated all appropriate methods of control of the target organism. At that time, the organic movement might then need to look again at the addition of appropriate gene manipulation into its regulations - but not before.