| Summary: | Of ten actinobacterial isolates, <i>Streptomyces cellulosae</i> Actino 48 exhibited the strongest suppression of <i>Sclerotium rolfsii</i> mycelium growth and the highest chitinase enzyme production (49.2 U L<sup>-1</sup> min<sup>-1</sup>). The interaction between Actino 48 and <i>S</i>. <i>rolfsii</i> was studied by scanning electron microscope (SEM), which revealed many abnormalities, malformations, and injuries of the hypha, with large loss of <i>S</i>. <i>rolfsii</i> mycelia density and mass. Three talc-based formulations with culture broth, cell-free supernatant, and cell pellet suspension of chitinase-producing Actino 48 were characterized using SEM, Fourier transform infrared spectroscopy (FTIR), and a particle size analyzer. All formulations were evaluated as biocontrol agents for reducing damping-off, root rot, and pods rot diseases of peanut caused by <i>S</i>. <i>rolfsii</i> under greenhouse and open-field conditions. The talc-based culture broth formulation was the most effective soil treatment, which decreased the percentage of peanut diseases under greenhouse and open-field conditions during two successive seasons. The culture broth formulation showed the highest increase in the dry weight of peanut shoots, root systems, and yielded pods. The transcriptional levels of three defense-related genes (<i>PR-1, PR-3</i>, and <i>POD</i>) were elevated in the culture broth formulation treatment compared with other formulations. Subsequently, the bio-friendly talc-based culture broth formulation of chitinase-producing Actino 48 could potentially be used as a biocontrol agent for controlling peanut soil-borne diseases caused by <i>S</i>.<i> rolfsii</i>.
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