The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

The major source of substrates for microbial activity in the ectorhizosphere and on the rhizoplane are rhizodeposition products. They are composed of exudates, lysates, mucilage, secretions and dead cell material, as well as gases including respiratory CO 2. Depending on plant species, age and environmental conditions, these can account for up to 40% (or more) of the dry matter produced by plants. The microbial populations colonizing the endorhizosphere, including mycorrhizae, pathogens and symbiotic Nz-fixers have greater access to the total pool of carbon including that recently derived from photosynthesis. Utilization of rhizodeposition products induces at least a transient increase in soil biomass but a sustained increase depends on the state of the native soil biomass, the flow of other metabolites from the soil to the rhizosphere and the water relations of the soil. In addition, the phenomena of oligotrophy, cryptic growth, plasmolysis, dormancy and arrested metabolism can all influence the longevity of rhizosphere organisms. With this background, microbial growth in the rhizosphere will be discussed.

Summary The phyllosphere represents the habitat provided by the aboveground parts of plants, and on a global scale supports a large and complex microbial community. Microbial interactions in the phyllosphere can affect the fitness of plants in natural communities, the productivity of agricultural crops, and the safety of horticultural produce for human consumption. The structure of phyllosphere communities reflects immigration, survival and growth of microbial colonists, which is influenced by numerous environmental factors in addition to leaf physico‐chemical properties. The recent use of culture‐independent techniques has demonstrated considerable previously unrecognized diversity in phyllosphere bacterial communities. Furthermore, there is significant recent evidence that plant genotype can play a major role in determining the structure of phyllosphere microbial communities. The main aims of this review are: (i) to discuss the diversity of phyllosphere microbial populations; (ii) to consider the processes by which microbes colonize the phyllosphere; (iii) to address the leaf characteristics and environmental factors that determine the survival and growth of colonists; (iv) to discuss microbial adaptations that allow establishment in the phyllosphere habitat and (v) to evaluate evidence for plant genotypic control of phyllosphere communities. Finally, we suggest approaches and priority areas for future research on phyllosphere microbiology.

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More than 80 disease biocontrol products are on the market worldwide, but none of these contain mycor rhizal fungi. This is despite ample evidence that both arbuscular mycorrhizal fungi and ectomycorrhizal fungi can control a number of plant diseases. A procedure for successful development of disease biocontrol agents in general is used as a background to examine the potential for achieving commercial mycorrhizal biocontrol agents. This includes (i) selection and screening; (ii) characterization involving identification, studies of modes of action and ecophysiology, as well as inoculum production, formulation, application and shelf life; (iii) registration. The last stage is problematic for mycorrhizal fungi, as currently they can be sold as plant growth promoters without any form of costly registration, even though in some instances they may actually function to some extent through biocontrol activity. The significance of this approach is discussed, and some possible ways of enhancing biocontrol by mycorrhizas are considered.Key words: arbuscular mycorrhizas, ectomycorrhizas, biological disease control, soilborne pathogens, modes of action, ecology.