FUNGAL DISEASES are one of the worst threats to cultivated plants. Both the amateur gardener and the arable farmer know to their cost that fungal infections can damage or destroy their plants. For a century or more the urgent economic need to understand and control diseases of crops has driven the science of plant pathology. But what the scientists had overlooked was the extent to which wild plants are also vulnerable to fungal infections, presenting an opportunity to turn fungal diseases to the farmer's advantage - as weedkillers. Mycoherbicides, fungi that kill weeds, promise to deliver what other herbicides cannot: a highly specific treatment aimed at a single species of weed that will leave other plants untouched.