Identification, solid-state fermentation and biocontrol effects of Streptomyces hygroscopicus B04 on strawberry root rot

Root rot is one of the major constraints on strawberry production worldwide, and no effective control measure is available yet. Therefore, the objective of this study was to develop a new bio-agent for controlling this disease. The antagonistic strain B04 was isolated from the rhizosphere soil of a healthy strawberry plant, and it strongly inhibited the growth of phytopathogenic fungi, especially that of Fusarium oxysporum. Strain B04 was identified as Streptomyces hygroscopicus by morphological and cultural traits and by 16S rRNA gene sequence analysis. Strain B04 could utilize numerous carbon sources and produce extracellular fungal cell wall-degrading enzymes including chitinase, beta-1,3-glucanase, cellulase and protease. Agro-industrial waste materials, specifically wheat bran and vermicompost, were used for solid-state fermentation with strain B04 to produce bioorganic fertilizer. On the seventh day of fermentation, the spore number peaked at 5.3 x 10(10) CFU g(-1). Pot and field experiments were performed to evaluate the effects of bioorganic fertilizer B04 (BOF-B04) on the infested soils. Each experiment included three groups: (1) control treatment, (2) organic fertilizer, and (3) bioorganic fertilizer B04. BOF-B04 significantly increased the fresh weight of strawberry plants compared to the control and also decreased the incidence of root rot. The biocontrol efficiencies of BOF-B04 were 63.35% and 56.77% in pot and field experiments, respectively. Moreover, application of BOF-B04 also altered the microbial community structure of the rhizosphere soil. The number of fungi in rhizosphere soil decreased markedly, by 82.43% compared to the control. In contrast, bacterial and actinomycete densities were dramatically higher than in the control. Application of BOF-B04 decreased the levels of stress-related enzyme activity and of malondialdehyde compared to the control. Consequently, this study showed that bioorganic fertilizer B04 is a potential biocontrol agent for controlling strawberry root rot. (C) 2016 Elsevier B.V. All rights reserved.