Types of biological agents, modes of action, and conditions of use. Types of organisms The microorganisms useable for biocontrol against pathogens targeted by copper belong to three major biological groups: 1) fungi, yeasts, and oomycetes – eukaryotic organisms, generally multicellular, that produce a thallus bearing sexual or asexual reproductive organs; 2) bacteria and actinomycetes – prokaryotic, unicellular organisms that generally multiply by asexual cellular division; and 3) viruses, which multiply by making use of the cellular machinery of the infected host. The specific biological properties of each group determine their manner of use (formulation, application methods, shelf life, ability to be combined with other crop protection materials or strategies, etc.) as well as their potential for practical use under crop production conditions. These factors are necessarily taken into account by the crop protection firms working to develop microbial biocontrol products. Information as to the specific composition of these products is scarce, however (nature of the formulations, etc.), since such details constitute a form of intellectual property. Modes of action Three main modes of action have been identified, involving diverse biological mechanisms: the destruction or direct inhibition of the pathogen; competition with the pathogenic agent; and interaction with the process of pathogenesis. For some biological control organisms, the mode of action has not been precisely determined. Direct inhibition or destruction of the pathogenic agent This mode of action can involve two mechanisms: - antibiosis, in which the antagonist organism produces secondary metabolites that are toxic to the pathogen, inhibiting its germination, mycelium growth and/or sporulation. Antibiosis is the most extensively studied mode of action, because it is readily identified in the laboratory. However, the production of antibiotic compounds depends on numerous environmental factors (water potential, environmental pH, temperature, etc.). There are many examples of bacteria and fungi that produce antibiotic compounds, notably Bacillus subtilis, Pseudomonas fluorescens, Streptomyces sp., and Trichoderma sp. Substances responsible for antibiosis have been identified in several species, and the genes involved in the production of some of these substances have been identified. This mode of action is the closest to that of chemical control; it can thus be very effective in inhibiting the development of a microbial pathogen, but can also present similar risks ...
