Plant Pathology and Microbiology Department, Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK

Over the last few years there has been a significant increase in the number of biological disease control agents registered or on the market worldwide. Currently, there are approximately 30 bacterial and fungal products for control of foliar, soil-borne and post-harvest diseases caused by fungi alone.

Studies of modes of action, selection and screening, physiology, inoculum production, formulation and application processes, have all played a part in the recent successes. However, perhaps the greatest impetus to commercial development has been the realization that biological control agents must be adapted to the environment where biocontrol is required and to survive for long enough for activity to ensue. Thus, the study of the ecology of biocontrol agents carried out against a background knowledge of pathogen biology, plant cultivation and behaviour of the resident microbial community has undoubtedly been a key factor in increasing reproducibility and success of disease biocontrol.

Reappraisal of well-established phenomena of naturally occurring biocontrol including suppressive soils, monoculture decline, use of organic amendments and composts, and physical and chemical practices have been particularly useful. For example, the products Fusaclean and Biofox-C (nonpathogenic Fusaria) for control of Fusarium wilts of several crops, and Mycostop (Streptomyces griseoviridis) for control of soil-borne diseases of several ornamentals and vegetables, were obtained from suppressive soils or peat, respectively. Key environmental limitations for biocontrol have also been identified from such studies.

One area of relative neglect until recently has been the interaction of soil fauna with biocontrol agents. The demonstration of the ability of mites and collembolans to transmit propagules of biocontrol agents such as Coniothyrium minitans highlights one aspect of this phenomenon. Consumption or grazing of pathogen propaguies such as scierotia and subsequent infection by opportunistic mycoparasites has also been demonstrated. Clearly, the whole area of animal-microbe interactions with respect to disease biocontrol deserves further study.

Against this background, selected aspects of work on the use of Pythium oligandrum for the control of damping-off fungi such as Pythium ultimum and Aphanomyces cochlioides, and C. ;minitans for control of Sclerotinia sclerotiorum will be described to illustrate the significance of studies of microbial interactions as a basis for the development of biological control of fungal diseases.