Microbial secondary metabolites not only can deter the invasion of pathogens but also have key roles in the development of agricultural antibiotics. Microbial antibiotics have a high biodegradability since they are naturally biosynthesized antibiotics. Thus, they lower the toxicity damaging to ecosystem as well as decrease harmful effects, such as destruction of ecosystem and pollution.
Further, the use of naturally accruing microbial secondary metabolites reduces appearances of chemical-resistant strains of pathogens and casualties to soil, humans and animals associated with organic-synthesized agricultural fungicides having various chemical structures and biological functions.
Blasticidin S is the first agricultural antibiotic isolated from actinomycete strain Streptomyces for the control of rice blast. Since the production of Blasticidin S, the use of antibiotics in plant protection has highly involved the production of microorganic secondary metabolites. As a result, kasugamycin, validamycin A, polyoxin derivatives, mildiomycin and the like have been developed and being used for the control of various plant diseases.
Microbial antibiotics themselves have been used as fungicides and as a lead for developing agricultural fungicides. Examples of such microbial antibiotics include commercialized β-methoxyacrylates azoxystrobin and kresoxim-methyl developed from strobilurin which is an antibiotic isolated from Basidiomycota. Other agricultural fungicides, feniclonil and fludioxonil, are developed from an antibiotic pyrrolnitrin isolated from Pseudomonas.
Among microbial secondary metabolites, some recently exhibit agricultural antifungal activity against various plant pathogen infections which are not confirmed. For example, gopalamycin, tubercidin, manumycin-type antibiotics, oligomycin A, streptimidone, daunomycin, and phenylacetic acid are confirmed to have control efficacy against various plant diseases.
Studies on secondary metabolites isolated from microorganisms inhabiting various different environments have been conducted. Particularly, since actinomycetes produce antibiotic compounds having diverse chemical structures, such as aminoglycosides, anthracyclines, glycopeptides, β-lactams, macrolides, nuclocides, peptides, polyenes, polyethers, tetracyclines, and the like, developments of physiologically active compounds including antibiotic compounds have been carried out.
Some species of Saccharothrix, one of the rare actinomycete genera designated as non-Streptomyces, have been renamed Lechevalieria due to recent changes in classification. Actinomycetes of Lechevalieria and Saccharothrix have been recently developed as sources of various physiologically active compounds. Even though there are high possibilities to produce a variety of antibiotic compounds, antibiotic production from Lechevalieria species active against oomycete pathogens has not yet been reported.
It has been known that actinomycetes of Lechevalieria are mainly isolated from land soil containing high contents of organisms. Examples of antibiotics from Lechevalieria species including Saccharothrix include galacardin and saccharomicin having an antibacterial activity, karnamicin and formamicin having an antifungal activity, tetrazomine having an anticancer activity, and phosphonothrixin having a herbicidal activity.
Some butanoic acid derivatives having a physiological activity have been isolated from natural products or synthesized. For example, 3-(3-indolyl)-butanoic acid has been reported to have an antibacterial activity against Ralstonia solanacearum causing bacterial wilt. Further, 2-amino-2-methyl-butanoic acid has been reported to have an anticancer activity. However, so far, antifungal and antioomycete activities of butanoic acid derivatives have not yet been reported.
Phytophthora blight is a primary factor for decreases in the pepper production. In order to protect from such diseases, cultivation of resistant cultivars, crop rotation, biological control using an antifungal bacteria and the like have been performed. So far, the most effective control is a chemical control involving spraying of fungicides.
Examples of fungicides for control of Phytophthora blight in Korea include metalaxyl, fosetyl-Al, oxadixyl, propamocarb, copper oxychloride, chlorothalonil, dithianon and the like. Further, metalaxyl-copper oxychloride, metalaxyl-dithianone, oxadixyl-chlorothalonil and the like are used in combination. However, even though such active compounds have excellent efficacy, development of new compounds having different chemical structures and novel activities have been highly demanded due to appearances of chemical resistant strains of pathogens.
Therefore, development of environmentally friendly antifungals with low toxicity for control of phytophthora blight is urgently needed to replace such synthetic fungicides.