OPPORTUNITIES AND PROBLEMS OF CONTROL OF FOLIAR PATHOGENS WITH MICROORGANISMS

Three different strategies in biological control of diseases of above-ground plant parts are discussed. (1) Microbial suppression of infection. Undisturbed naturally occurring yeast populations will create unfavourable nutritional conditions for leaf infection by necrotrophic pathogens. Application of biological control agents, operating through nutrient competition, to healthy leaves, however, is generally not very effective, because the pathogen rapidly penetrates the leaf and escapes competition. In contrast, microbial protection of man-made wounds on fruits looks promising. Field applications of bacterial preparations or aqueous compost extract to leaves reduce disease by necrotrophs as well as biotrophs. The mechanisms are not clear, but involvement of induced resistance may make the establishment of the biological control agents in the phyllosphere less important. (2) Microbial suppression of pathogen sporulation. Suppression of the dissemination of the pathogen is, in principle, effective against diseases with many infection cycles per season. This approach allows a long interaction period between the antagonist and the pathogen. Research on the use of mycoparasites for the control of mildews and rusts is well known in this respect. Promising, and relatively new, is this approach in relation to necrotrophic pathogens sporulating on dead plant material. (3) Microbial suppression of pathogen survival. Mycoparasites may also interfere with the formation and vitality of sclerotia in infected above-ground plant tissue and crop remains. Depending on the importance of initial inoculum, originating from sclerotia, treatment of infected plant material may reduce the severity of the disease in the following season.