The big American suddenly stood up, leaned over the table and said to the Colombian in a low voice: "You'd better be careful not to talk to the press!" Dr. David Sands, scientist and entrepreneur, was meeting with advisors to the Colombian environment ministry last March to push a new drug-war weapon marketed by his company: a special toxic fungus which would kill coca plants. The Colombian scientist who raised Sands' hackles had pointed out that the fungus could also attack humans with weakened immune systems - a condition common among the often undernourished and generally unhealthy poor coca farmers and workers in the tropical rain forests of Colombia, where Sands wants to carry out a massive spraying programme. "He didn't care" said the Colombian, who asked not to be named.

Sands, a professor at Montana State University in Bozeman and the vice-president of Ag/Bio Con (agricultural and biological control), a company that markets the fungus, is not the only party pushing this new biological weapon. The US congress is demanding Colombia apply the controversial fungus to receive $1.6bn in emergency funds for Colombia's anti-drug/counter-insurgency strategy called Plan Colombia. Last March, New York Repupublican congressman Benjamin Gilman tacked on an amendment to the pending aid bill requiring President Clinton to certify that the Colombian government "has agreed to and is implementing a strategy to eliminate Colombia's total coca and opium poppy production" using, among other means, "tested, environmentally safe mycoherbicides". (Myco means fungus; herbicide means plant killer).

Steve Peterson, from the state department's international narcotics and law enforcement unit says it wants to see mycoherbicides used because they would be "more cost effective and more environmentally friendly" than herbicides. The trouble is that abundant evidence indicates that the only mycoherbicide being considered for this purpose, Fusarium oxysporum may, in massive application, pose serious dangers to the environment and human health. Florida has put an indefinite hold on its plans to test the fungus for its own anti-drug efforts after environmentalists and a state official warned that it could mutate, spread rapidly and kill off other plants including food crops. And for over a decade, coca growers in Peru have accused the US of secretly applying the fungus to attack coca plants - harming food crops and farm animals. Moreover, the fungus can, in certain circumstances, cause lethal infections in humans with weakened immune systems. None of this, however, has dimmed US government enthusiasm for the project - nor that of Sands' corporation, which stands to profit if the fungus is adopted for widespread use.

While the concept of using herbicides against weeds and camouflaging foliage (such as Agent Orange in Vietnam) is not novel, using them against crops is. Ironically, the great majority of research on Fusarium focuses on combating it as a major food-crop killer. The soil borne mould infects crops by secreting toxins into their roots, which then putrefy and dissolve the plants cells, often eventually killing them, or worse, poisoning humans or animals who feed on contaminated plants or plant products. The fungus can survive in soil for years.

The idea of using a fungal herbicide to kill drug plants began in the 1970s after a fungus later identified as EN-4, began to kill off the coca at a soft drink research plantation in Hawaii. In 1986, the US department of agriculture's Agricultural Research Service (ARS) began a full blown research project to find a biological agent to kill coca. By 1991 the government had invested at least $14m. Congress has now given the state department $23m originally slated for mycoherbicide development in the US, which it plans to pass on to the UN. By getting the UN to take on the fungus project, the US not only gets political cover, but makes it harder to get data about the programme. Unlike the US federal government, the UN has no freedom of information act.

The US Congress's arm-twisting to make Colombia use the fungus even before it has been tested raises the fundamental issue of informed consent by the Colombian people. The programme could easily be construed as having a non peaceful purpose, thus breaching the international Biological Weapons convention and morphing it from "biocontrol" into "bio-warfare". While both the US and UN stridently object to the latter term, the secrecy surrounding the project leaves serious questions unanswered.

By late March, the UN proposal had already landed in the environment ministry, which must approve its use. But at a meeting with officials, it became clear the ministry had precious little to go on in making their decision. Ministry staffers were reduced to trying to cull information from the internet. what they found there included evidence that Fusarium could mutate to gobble other plants and could endanger animal and human health.

Ministry advisers also told us that Peruvian groups had not responded to queries on the fungus epidemic that affected coca fields there. Since 1991 Peruvian coca growers claimed that they have seen helicopters fly over their coca fields emitting a brown or white cloud that caused their coca and food crops to die and sickened their farm animals. Many of the farmers believe these helicopters are part of an American anti-drug campaign, a charge the US denies. Research in 1993, by a US funded Peruvian scientist showed that many of the food crops were infected by the same fungus species that had killed the coca.

This no small matter in Colombia, home to the world's second most diverse biosystem - one that is uniquely vulnerable to the potential threat posed by the massive spraying of a toxic, mutative fungus in vast swathes of jungle. Colombia is no stranger to Fusarium, a genus that includes several strains besides EN-4. "There's a group of scientists who've been working [to combat] Fusarium here for a long time", said Colombian agriculture vice-minister Claudia Martinez. In fact, a major epidemic of one Fusarium strain hit the flower growers in the plains of Bogota a few years ago, and as a result, growers could no longer plant in the contaminated earth - they were forced to switch to soiless hydroponics systems.

Eduardo Posada, head of the Colombian Centre for International Physics, believes Fusarium can be devastating to people with lowered resistance due to immunological diseases or malnutrition - common conditions among the farmers who often live near the coca fields set to be sprayed. "The mortality rate for people infected by Fusarium is 76%", wrote Posada. He lists the scientific literature indicating that Fusarium toxins are "highly toxic" to animals and humans, and that the use of ants to spread the fungus (research actually done by ARS scientists), could cause the ecosystem to be affected much faster than imagined. None of that however, appears to trouble Sands.

Vice-minister Martinez was ordered by the Colombian ambassador to Washington to receive him. Sands is listed as a major researcher of the fungus in the UN proposal, and it was he who first isolated EN-4 in Hawaii. Yet now he seems to be more appropriately classified as a freelance businessman, hawking his compamy's version of a fully developed fungus ready for a "precision delivery from high altitude" application by large C-130 cargo planes - as a picture in his literature shows.

Sands has no shortage of influential contacts. Ag/Bio Con has retained a prominent Washington DC consulting firm to lobby on bills related to mycoherbicide development. The company's officials include a retired air force general with a background in research: Sands has received a navy research award and has traveled with high ranking US government personnel to a similar fungus project in Kazakhstan and Russia. Through his congressional connections, he arranged a face-to-face meeting with President Andres Pastrana in Washington last January. sands did not return repeated phone calls for comment on this article.

Sands received nationwide attention for Ag/Bio Con in spring and summer of last year, when he tried a similar sales job to use another strain of Fusarium to control Florida'a burgeoning marijuana industry. David Struhs, the head of Florida's department of environmental protection, reacted with a strongly cautionary letter saying: "...It is difficult, if not impossible, to control the spread of Fusarium species. The mutated fungi can cause disease in a large number of crops, including tomatoes, peppers, flowers, corn and vines, and are normally considered a threat to farmers as a pest, rather than a pesticide. Fusarium species are more active in warm soils and can stay resident in the soil for years. Their longevity and enhanced activity under Florida conditions are of concern, as this could lead to an increased risk of mutagenicity."

Having been rebuffed by the state of Florida - failing even to convince the state authorities to initiate a simple experiment in a quarantined test site - Sands apparently targeted Colombia. The Colombian environment ministry has come up with a preliminary counter-proposal, calling for back to basic research on "native micro-organisms with bio-control potential" in the coca zones. The proposal does not rule out the unpredictable and dangerous Fusarium, as some scientists demand. But it does call for a long, meticulous study. After all, why should the people of Colombia expose themselves to a risk that the people of Florida refuse to run?