Microbial Metabolites In Use As Biofungicides

The most important antifungal metabolites in commercial use are listed below, which are applied to control fungal diseases on rice, vegetable, and fruits. The relative importance of the microbial compounds, when compared to synthetic fungicides, might have been underevaluated because of several reasons such as the limitation in their spectrum of activity and in certain instances, the development of resistance. Nevertheless, the excellent activity of these biofungicides inspired to launch the screening programs for antifungal microbial metabolites, which resulted in profound chemical libraries of natural products (Godfrey 1994; Knight et al. 1997).

Blasticidin S, the first microbial fungicide available for plant protection, has been used practically for the control of rice blast disease caused by Magnaporthe grisea. Blasticidin S is a nucleoside antibiotic discovered from metabolites of Streptomyces griseochromogenes (Takeuchi et al. 1957). It potently inhibits the mycelial growth and conidial germination of M. grisea. The successful use of the compound encouraged further screening of microbial fungicides that eventually brought out kasugamycin, polyoxin, validamycin, and mildiomycin.

Kasugamycin is an amino-sugar compound discovered in the metabolites of Streptomyces kasugaensis and Strepto-myces kasugaspinus (Umezawa et al. 1965). It has in vitro antimicrobial activity against yeast and some plant pathogenic fungi including M. grisea. In vivo data showed that kasugamycin efficiently suppressed the development of M. grisea mycelia on rice plants by both preventive and curative treatments. However, it did not appear to inhibit the spore germination.

Polyoxins were isolated from the culture broth of Streptomyces cacaoi var. asoensis (Suzuki et al. 1965). The excellent in vitro activity and in vivo efficacy led to its commercial use for the control of fungal diseases of fruit trees and vegetables such as black spot of Japanese pear caused by

Alternaria kikuchiana and gray mold diseases caused by Botrytis cinerea (Isono et al. 1965).

Validamycin A produced by Streptomyces hygroscopicus var. limoneus was effective in controlling rice sheath blight caused by Rhizoctonia solani (Iwasa et al. 1970). Validamycin A was found to be a pro-drug, which is converted within the fungal cell to validoxylamine A, an extremely strong inhibitor of trehalase. This mode of action gives validamycin A a favorable biological selectivity, because the hydrolysis of the disaccharide trehalose does not occur in the vertebrates. The structural elucidation and total synthesis of validamycin A were achieved by Ogawa and coworkers (Suami et al. 1980).

Mildiomycin is an aminoacylated nucleoside produced by Streptomyces rimofaciens (Harada and Kishi 1978). It was discovered by the method established to assay the control efficacy of antifungal agents against powdery mildew. Mildiomycin has been known to act as an inhibitor of the fungal protein biosynthesis. Its low toxicity on vertebrates allows it to be an environmentally favorable crop protection agent.

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