Abstract
Orymycin, a new antifungal antibiotic, has been obtained from Streptomyces albochromogenes isolated from the soil collected in Tokyo. The antifungal spectrum of Orymycin against several plant pathogenic fungi and bacteria was determined by the agar streak dilution method as well as the cup method. Potato-sucrose agar was used in these tests with the exception of its test with shoot blight fungus of larch which was grown on yeast-starch agar.
1. The antifungal spectrum of Orymycin against plant pathogenic fungi was considerably wide, being especially inhibitory to rice blast fungus (Piricularia oryzae), anthracnose fungi of vegetables and fruits (Colletotrichum atramentarium, C. glycines, C. lagenarium, Gloeosporium kaki, G. laeticolor and Glomerella cingulata) and shoot blight fungus of larch (Guignardia laricina). The minimum inhibitory concentrations of Orymycin against rice blast fungus, anthracnose fungi and shoot blight fungus of larch were detected less than 0.1 or 0.5ppm, 0.5 or 1.0ppm, and 0.5ppm, respectively.
2. It was observed that the resistance to Orymycin varied with different species or isolates of anthracnose fungi and black spot fungi, and with different isolates of rice blast fungus.
3. In rice blast fungus, the isolates which have been cultured for a long time after the isolation from the host plants seemed to be sensitive to Orymycin, while the isolates cultured for a short period appeared to be resistant. On the other hand, no correlation was found between the pathogenicity or appressorial formation type of the isolate and the resistance to Orymycin.
4. It appeared that rice blast fungus was suitable as the test fungus for the cup method, but many isolates of this fungus produced relatively small number of conidia, and accordingly it needed considerably a long incubation period to observe the results. Therefore, it is necessary to find out a better test fungus for the cup method against Orymycin.
