Index >> Microbial Products Influencing Plant Growth >> Some Examples of Antibiotics Produced by Actinomycetes Fungi and Bacteria

melon canker caused by Mycosphaerella and powdery mildews caused by Erysiphe. Griseofulvin is a systemic fungicide since the antibiotic permeates plant tissues uniformly and forms a barrier to the penetration by pathogenic fungi.

Aureofungin is a broad spectrum antifungal antibiotic with the unique property of inhibiting growth of a large number of phytopathogens. It was once commercially produced in India at the Hindustan Antibiotics, Pimpri, Poona. The antifungal activity of the antibiotic has been demonstrated in pure cultures on Pyricularia oryzae, Helminthosporium oryzae, H. turcicum, H. nodulosum, Alternaria tenuis, Curvularia lunata, Verticillium alboatrum, Phytophthora citrophthora, Aspergillus niger, A. Fumigatus, Candida albicans, Cryptococcus neoformans, Trichophyton mentagrophytes and T. rubrum. The minimum inhibitory concentration of the antibiotic ranges from 0.005 to 1 mg/ml. The antibiotic is insoluble in water but soluble in alcohols and can be made soluble in water at alkaline pH. Its effectiveness against citrus gummosis incited by Phytophthora sp., powdery mildew of apple caused by Podosphaera leucotricha and diseases of grapes such as powdery mildew, downy mildew and the anthrachnose is well established. It has also been reported that aureofungin has potentialities in the control of seed-borne infection and seedling blight caused by Helminthosporium oryzae and Pyricularia oryzae disease of ragi, and many other disease including post­harvest and storage diseases.

Some antibiotics are of limited application such as cycloheximide (ac­tidione), blasticidin-S, kasugamycin and streptomycin. Cycloheximide has been shown to be active against leaf spots, powdery mildews and blister rust of pine while blasticidin-S has shown promise against P. oryzae. Strep­tomycin has proved effective against diseases caused by Erwinia, Xan­thomonas, Pseudomonas, Corynebacterium, Agrobacterium, Pseudoperonospora, Peronospora and Sphaerotheca.

In Japan, agriculturally useful antibiotics have, been widely used to protect plants against diseases and pests. Some of the examples are cited below:
(1) Cycloheximide (from Streptomyces griseus) as a wettable powder against onion downy mildew and shoot blight of Japanese larch,
(2) Kasugamycin (from Streptomyces kasugaensis) as dust against rice blast,
(3) Polyoxins (produced by Streptomyces cacaoi var asoensis) as dust, wettable powder and emulsion against rice sheath blight and fungal diseases of fruits and vegetables,
(4) Validamycin A (from Streptomyces hygroscopicus var. limoneus) as dust against rice sheath blight,
(5) Streptomycin (from Streptomyces griseus) as wettable powder or liquid against bacterial diseases of fruits and vegetables,
(6) Oxytetracycline (from Streptomyces viridifaciens) as wettable powder against citrus canker and peach bacterial leaf spot and
(7) Tetranactin as emulsion against carmine mite of fruits and tea. The primary sites of action of these antibiotics relate to chitin synthesis of cell wall, cation leakage from mitochondria, biosynthesis of inositol and protein and DNA synthesis. Limitations in the use of antibiotics in agricul­ture are difficulties in analysis because they are used in small amounts and there is a likelihood of development of plant pathogens resistant to antibiotics. Therefore, the use of chemicals and antibiotics in alternate years has been recommended to overcome such a possibility.

Antibiotics are also widely used as growth stimulants in poultry and livestock feeds. The use of aureomycin, terramycin and penicillin at the rate of 5-20 g/ ton of wheat increases the rate of growth of farm animals from 5 to 50%. Although the mechanism behind such stimulating effect of antibiotics is not known, it is believed that they may kill unwanted bacteria in the intestine and also increase the availability of vitamin B12.