Biological agents of micro-destruction

Below ground, invisible to the naked eye, yet causing huge economic losses, root knot nematodes are a hidden enemy to growers of horticultural crops throughout the tropical world. Not only can they by themselves cause yield losses of up to 30% but the tiny burrowing holes made by these microscopic worms let in soil fungi and bacteria causing diseases such as bacterial wilt. Wherever horticultural crops are grown intensively, root knot nematodes are a problem.

Credit:University of Reading

Pesticides - organophosphates - are the usual method of control but these are falling out of favour as regulatory authorities respond to consumer concerns about food safety. And, however carefully such chemicals are applied, there is no discounting the hazards they present to workers obliged to use them on a regular basis. So what are the alternatives? Some control can be achieved by growing crops that nematodes are less likely to attack. Cabbages, for example, are not such good hosts as tomatoes and some growers in Kenya alternate crops of tomatoes and cabbages to reduce the build up of nematode populations in their soil. Some crop varieties are less susceptible than others but, by and large, growers are conservative in their choice of varieties - or perhaps they recognize that their consumers are conservative in their tastes (See Food in fashion). There are some plants that are believed to be actively antagonistic to nematodes, for example Tagetes minuta from the marigold family, but growers are unlikely to welcome infestations of what they consider to be a weed among their bean and tomato crops. Perhaps more promising, although only in the longer term, will be the use of biological control agents such as naturally occurring diseases. These should not attract the same censure as manmade, more generally toxic, chemical pesticides.

Microscopic fungi and bacteria that are natural pathogens of nematodes are the most promising biocontrol agents. They work by affecting the organism's ability to reproduce. However, both a strength and a weakness of a biocontrol agent is its specificity. A chemical insecticide may well kill a wide range of insects, whether pest or not. A biopesticide will control only the pest for which it has been developed. This is, of course, a good thing because it makes them far less hazardous environmentally. Less conveniently, nematodes, though a universal problem, may be sufficiently different, even from one hectare to another, to mean that a biopesticide developed for one place will be ineffective in another. The answer will probably lie in applying a combination of different types of biopesticide, for example a fungi and a bacteria. Pochonia chlamydosporia (formally named Verticillium chlamydosporium) is a promising example of the former and Pasteuria penetrans of the latter. But far more research is needed before recommendations can be offered to growers.

Currently the University of Reading, working with funding provided by the UK Government's Department for International Development under its Crop Protection Programme, is running field trials in Kenya with farmers who are committed to growing horticultural crops organically. It is too early as yet to show very definite results. Nematode damage is insidious and hidden - unless the plant is dug up and the roots examined - and natural enemies work more slowly than a zap-all chemical insecticide. But long term, this is undoubtedly the future simply because there are no other practical alternatives on the horizon.

Credit:University of Reading

Another challenge to overcome will be the manufacture and distribution of nematode biocontrol agents. Agrochemical companies are unlikely to be interested, at least in the near future. Nor is village production likely to take off, as it has with the nuclear polyhydrosis virus that is used to control podborers in chickpea and pigeon pea (see From darkness into light). Although it is possible to develop a crude product from the roots of plants infected with pathogenic bacteria, and applying this in a powder form to seedbeds and around transplanted bean or tomato plants, the process is time consuming and unlikely to appeal to any but the most dedicated organic growers. But a company in Kenya, Dudutech, which is part of Flamingo Holdings and a sister company to the major vegetable exporter, HomeGrown, has programmes reproducing biocontrol agents for a number of major pests. University of Reading scientists are collaborating with Dudutech on a programme to tackle nematodes.

Another development that will be taking place this month (May 2003) is a meeting in Kenya to which scientists from Cuba have been invited. Cuba produces thousands of tons of biopesticides each year, their development stimulated by the collapse of the former Soviet Union, which cut its access to subsidized agrochemicals, and the US embargo. The Cuban system is based on a network of high-tech laboratories responsible for quality control and production of 'mother cultures' of pest-killing fungi etc. These then go out to local field laboratories for further bulking up, before distribution to small-scale farmers. Kenyan scientists are hoping to learn from their experience, not least in how to overcome problems of quality control.

While nematodes will always be with us, a greater awareness by growers of their impact, coupled with a range of safe, organic methods of control, should help to increase the quality and quantity of horticultural crop production, reduce the hazards of chemical pesticides and safeguard the livelihoods of the many thousands of outgrowers feeding their production into the horticultural export business.