Streptomyces sp., strain No. T-7545 was found to produce new antibiotics, validamycins A and B that are effective for the control of sheath blight of rice plants. The validamycins exhibit no in vitro activity. A taxonomic study of strain No. T-7545 was carried out and it was characterized as follows: It forms gray to gray and yellow aerial mycelium, bright yellow to yellow ocher vegetative mycelium and faint, brownish yellow diffusible pigment, good growth and development of abundant aerial mycelium with the formation of coiled chains of spores at 25-45°C, and black moist areas in the aerial mycelium on certain media. As a result of comparison of strain No. T-7545 with known species, the name Streptomyces hygroscopicus WAKSMAN et HENRICI, 1948 var. limoneus nov. var. is proposed.
Japan Antibiotics Research Association