Induction of Disease Resistance in Tissue-Cultured Seedlings of Mountain Laurel after Treatment with Streptomyces padanus AOK-30

Agrochemicals are invaluable and effective for increasing food production to stave off hunger throughout the world, although they have fallen into disfavor because of associated environmental pollution and detrimental effects in a variety of nontarget organisms1). Because of such disfavor, the use of microbe-based biological control agents has increased in agricultural importance as a replacement or supplement for agrochemicals. In 1978 Hasegawa et al.2) isolated a new genus of actinomycete, Actinosynnema, from a grass blade. As far as we know, this report is the first to describe the isolation of an actinomycete from plant materials. In the 1990s, a number of research groups isolated several actinomycetes from surface-sterilized leaves and roots of a variety of plants, thereby showing the presence of endophytic actinomycetes3–6). Since then, more reports have described the use of actinomycetes in biological control7–10). Based on these reports, Kunoh1) assumed that a symbiotic actinomycete that is able to produce antibiotics in plants without any adverse effects on plant growth could be used as a biological control agent and that such an endophytic actinomycete could efficiently colonize in a plant when applied to tissue-cultured seedlings in axenic flasks, in which the applied actinomycete does not have to compete with other microbes. We therefore started to isolate endophytic actinomycetes from a variety of field-grown plants and successfully isolated a number of strains from rhododendron. An isolate of Streptomyces galbus R-5, which had the most intense antagonistic activity against a variety of microbes including Phytophthora cinnamomi and Pestalotiopsis sydowiana, major pathogens of rhododendron was selected as a potential biocontrol agent11). Shimizu et al.12) spread a suspension of R-5 on the surface of the multiplication medium in glass flasks in which rhododendron seedlings were growing, and successfully induced seedling resistance against Pestalotia disease. Their success suggested a novel way for producing disease-resistant tissue-cultured seedlings without using agrochemicals. Nishimura et al.13) isolated a strain of Streptomyces padanus AOK-30 (hereafter AOK-30) from potted mountain laurel by the method of Shimizu et al.11) and found that this strain also had broad antimicrobial activities similar to those of the R-5 isolate reported previously11). In this paper, we describe the control of Pestalotia disease caused by P. sydowiana in seedlings of mountain laurel tissue-cultured in glass flasks after treatment of the medium surface with a spore suspension of AOK-30. The suppression of expansion of Rhizoctonia root rot in a cell-tray with transplanted mountain laurel seedlings treated with AOK-30 in different ways is also described. Actinomycetologica (2004) 18:48–53 VOL. 18, NO. 2