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Abstract. Aouar L, Boukelloul I, Benadjila A. 2020. Identification of antagonistic Streptomyces strains isolated from Algerian Saharan soils and their plant growth promoting properties. Biodiversitas 21: 5672-5683. To produce new bioactive substances of agricultural interest, extreme ecosystems can be a source of unexplored microorganisms. Accordingly, in this study, twenty-two actinobacteria strains were obtained from rhizospheric arid soils of palm groves collected from Biskra and El Oued in the Algerian Sahara. All isolates were examined for the in vitro antifungal potential towards phytopathogenic fungi: Aspergillus flavus, Verticillium dahlia, Rhizoctonia solani, Botrytis cinerea and Fusarium oxysporum as well as for their antibacterial property toward phytopathogenic bacteria: Streptomyces scabiei, Pectobacterium carotovorum and Agrobacterium tumefaciens. The three isolates (13%) that inhibited at least five pathogens were then selected, identified and assessed for their attributes to produce indole-3-acetic acid (IAA) and siderophores, to solubilize phosphate, and to antagonize Streptomyces scabiei in vivo. According to phylogenetic analysis performed with 16S rDNA sequence, chemotaxonomy and phenotypic characteristics, the strain SO1, which inhibited all tested pathogens, was assigned to Streptomyces flaveus. While, strains SO2 and SB1 were affiliated to Streptomyces enissocaesilis and Streptomyces albidoflavus, respectively. All strains produced IAA but only SO1 and SB1 were able to elaborate siderophores catecholate-type. Two strains SO1 and SO2 exhibited a capacity to solubilize phosphate and SO1 was able to suppress the pathogenic effect of Streptomyces scabiei on radish seedlings. The findings indicate that SO1 strain may reveal the potential for use as a biocontrol agent and plant growth promoter.

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This work aimed to explore an antagonistic actinobacterial strain isolated from roots of Ziziphus lotus in bioformulation processes and the biocontrol of Rizoctonia solani damping-off of tomato seedlings. The strain Streptomyces lincolnensis SZ03 was investigated for the principal in vitro biocontrol mechanisms and then formulated in three different biofungicides Wettable Talcum Powder (WTP), Sodium-Alginate Propagules (SAP) and Clay Sodium-Alginate Propagules (CAP). Compared to a marketed control products (Serenade® and Acil 060FS®), the formulated biofungicides were investigated against the R. solani damping-off of tomato cv. Aïcha seedlings. The strain S. lincolnensis SZ03 produced chitinases, cellulases, β-1,3-glucanases, cyanhydric acid and siderophores and it showed strong antagonistic effect on the mycelial growth of R. solani. Bioautographic and HPLC analysis revealed the production of a single antifungal compound. The biofungicide WTP showed an attractive biocontrol effect by reducing significantly the disease severity index (DSI) compared to untreated seeds. No significant differences were obtained compared to the chemical treatment with Acil 060FS®. The viability of spores and biocontrol efficacy of the WTP were confirmed after one year storage. S. lincolnensis strain SZ03 has never been reported in the biocontrol of phytopathogenic fungal diseases and it open up very attractive prospects in the fields of biocontrol and crop improvement.