Royal Botanic Garden Edinburgh, 20A lnverleith Row, Edinburgh EH3 5LR, UK

With the publication of the first IUCN Red List of Threatened Plants, it is now clear that some 34,000 plant species (12.5% of the world's flora) are facing extinction. Because plants are the primary producers in most natural ecosystems, each species has dependent upon it approximately 30 other species of organism. It follows that for every plant species that becomes extinct, 30 other species will go with it and many of these will be pathogens. However, this is only part of the problem. With the accelerating loss of habitats and ecosystems worldwide, the increased use of fungicides, pesticides and herbicides in agriculture and the release of genetically modified organisms, the threats to pathogen diversity in the wild are immense. Yet, plant pathogens are key components of the biodiversity of all natural ecosystems.

The importance of plant pathogen diversity in the wild will therefore be evaluated, the extent of knowledge of the role of plant pathogens in natural ecosystems will be assessed and the most important lacunae in that knowledge will be identified. Strategies for future research will be outlined and approaches to conservation will be considered. Equal consideration will be given to terrestrial and aquatic ecosystems; in this context it is important to note that the plants of fresh water and the oceans are responsible for around 50% of the earth's photosynthetic productivity.

A second, equally important threat is to the diversity of pathogens of cultivated plants, especially those of major crop species. Such diversity, which is usually held ex situ either in official culture and DNA collections or, very frequently, in personal collections, has a central role in revealing genetic diversity in potential breeding material and provides vital screens for the development of new cultivars. It is, in addition, the raw material for much of the basic scientific research that generates an understanding of pathogen variation, evolution (on crops and in natural plant populations) and population dynamics. Finally, it constitutes a significant biotechnological resource of particular importance to the genetic engineer.

The value (as opposed to cost) of the diversity of pathogens of cultivated plants will be assessed. Then the threats - biological, agronomic, fiscal and sociological - will be considered and research priorities and strategies for the future identified. Finally, conservation needs, policies and strategies will be proposed.