Association of Induced Disease Resistance of Rhododendron Seedlings with Inoculation of Streptomyces sp. R-5 and Treatment with Actinomycin D and Amphotericin B to the Tissue-culture Medium

Seedlings of rhododendron were treated by adding Streptomyces sp. strain R-5, actinomycin D and/or amphotericin B to the tissue-culture medium. HPLC analysis showed that all of the treated seedlings contained these antibiotics at concentrations higher than the suppressive levels to mycelial growth of Pestalotiopsis sydowiana, a major pathogen of rhododendron. Occurrence of disease caused by this fungus in the seedlings was suppressed by treatment of the medium surface with strain R-5, but not by treatment with these antibiotics, suggesting that growth of strain R-5, an antibiotic producer, could be essential for induction of disease resistance in tissue-cultured seedlings of rhododendron.