An endophytic actinomycete, Streptomyces padanus AOK-30, isolated from a potted mountain laurel (Kalmia latifolia L.) plant, was used to protect tissue-cultured seedlings of mountain laurel from Pestalotia disease and Rhizoctonia root rot. AOK-30, which has a broad antagonistic spectrum against various microbes, did not adversely affect the seedlings in glass flasks. A spore suspension of AOK-30 was spread on the surface of the rooting medium in glass flasks in which seedlings were growing. Ten days later, the 4th upper leaves were inoculated with Pestalotiopsis sydowiana and incubated for 20 days. In controls untreated with AOK-30, substrate mycelia of this fungus grew on all leaves and stems above and below the 4th leaves within 7–10 days after inoculation. Such growth resulted in the wilting death of 13 of 20 seedlings by the 20th day. In contrast, only the inoculated leaves and some neighboring leaves turned brown in 17 of 20 seedlings growing on medium treated with AOK-30. Thus, treatment of the medium surface with AOK-30 efficiently protects the seedlings from infection by P. sydowiana. The treatment of seedlings with AOK-30 in flasks before transplanting in soil and transplanting of untreated seedlings in soil mixed with suspension of AOK-30 significantly and marginally, respectively, slowed the expansion of root rot caused by Rhizoctonia sp. in seedlings in the cell trays.