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The following points highlight the four main modes of action suggested for biocontrol activity of antagonistic microorganisms. The modes are: 1. Antibiotic Production 2. Nutrient Competition between Antagonist and Pathogen 3. Direct Parasitism 4. Induction of the Host Defence Mechanism.
Mode # 1. Antibiotic Production:
It is appears to be a major mode of action for many antagonists. The antibiotic substances secreted by microorganisms inhibit the pathogen. The Singh and Deverall, 1984 found antibiotics produced by Bacillus subtilis and Pseudomonas cepacia able to inhibit the development in culture of main pathogens of harvested citrus fruits.
These facts indicated that living cells of the Bacillus may act on the fruits through production of antifungal metabolites by antagonizing the pathogen and competition for nutrients. The bacterium P. cepacia inhibits Botrytis cinerea and Penicillium expansum growth in apples via the pyrrolnitrin antibiotic compound.
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Various antibiotic compounds which control human diseases have been tested against plant diseases i.e., chlortetracycline, cycloheximide, fungicidin, griseofulvin, mycostatin and streptomycin were found to be effective in controlling serious Post-harvest diseases like bacterial soft rot caused by Erwinia carotovora, gray rot caused by B. cinerea and brown rot caused by M. fructicola.
However, none of these compounds is used commercially to control Post-harvest diseases, in addition to their high cost of application on fresh fruits and vegetables.
Mode # 2. Nutrient Competition between Antagonist and Pathogen:
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A fresh wound is a good source of nutrients for invading microorganisms. A delicate balance apparently exists at the wound site between the propagules of the antagonist and the pathogen which affects the interaction between them and will determine whether the wound becomes the site of infection.
The antagonist should be better adapted to various environmental and nutritional conditions, using low concentrations of nutrients, grow more rapidly, compete successfully at the wound site with the pathogen and able to survive under conditions that are unfavourable to the pathogen.
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The antagonist, which grows rapidly in the wound by utilizing nutrients located there, will deprive the pathogen of available nutrients, inhibit spore germination and germtube elongation, which are the stages prior to pathogen establishment in the tissues.
Thus, it was found that the rapid development of the antagonistic yeast Pichia guilliermondii on grapefruit peel at a broad range of temperatures and at various levels of relative humidity enables it to populate in the wound within 24 hour while P. digitatum spores are still at their initial stages of germination.
Several findings support the hypothesis that the main mechanism by which P. guilliermondii inhibits B. cinerea and other Post-harvest pathogens, is competition for nutrients at the wound site.
Mode # 3. Direct Parasitism:
Various studies with species of Trichoderma, which is known as effective biocontrol agents of several important plant pathogenic fungi highlighted the role of lytic enzymes including glucanases, chitinases and proteinases in the capability of the antagonist to attack the pathogen and thus to reduce disease incidence.
Trichoderma harzianum isolates were found to produce chitinases and glucanases when grown on live mycelium of Sclerotium rolfsii and Rhizoctonia solani in soil.
The levels and the expression patterns of these enzymes are specifically affected by the pathogenic fungus attacked by the Trichoderma. All the Trichoderma species reduced spore germination and inhibited germtube elongation of pathogen.
Microscopic investigation demonstrated direct parasitism and coiling of T. harzianum and T. viride around the hyphae of fungal pathogens. In addition to direct parasitism, the deformation of some cells of the pathogenic fungus, granulation of the cytoplasm and disintegration of the pathogen hyphal walls also occurred without intimate contact between the hyphae.
It was, therefore, suggested that the two Trichoderma species could produce antifungal metabolites that contributed to their antagonistic activity.
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Antibiosis and direct parasitism have been suggested to be the modes of action of T. harzianum in reducing the incidence of Post-harvest stem-end rot (Botryodiplodia theobromae), anthracnose (Colletotrichum gloeosporioides) and brown spot (Gliocephalotrichum microchlamydosporum) on rambutan fruits.
Mode # 4. Induction of the Host Defence Mechanism:
Biocontrol agents stimulate the host defence mechanism by the production of enzymes phytoalexins, proteins wound healing preformed inhibitory compounds. Antagonistic yeast, P. guilliermondii showed its ability of wound healing by stimulated ethylene production, raised levels of enzyme phenylalanine ammonia lyase (PAL) and enhanced defence mechanisms in inoculated citrus fruit.
Electron microscopy revealed a rapid colonization of the fungal mycelium at the wound sites with numerous yeast cells strongly attached to hyphae exhibiting lytic activity and rapid alterations. Furthermore, when the yeast was inoculated into artificial wounds alone or with the pathogen, stimulated the fruits to produce phytoalexins scoparone and scopoletin at the wound site.
Increased resistance to infection can result not only from enhanced production of phytoalexins but also from preformed inhibitory compounds. It was thus found that a non-pathogenic mutant of Colletotrichum magna was capable of enhancing the preformed antifungal diene compound in avocado fruit peel and that protect the fruit against infection by C. gloeosporioides.
