Brazilian actinobacteria with antagonistic activity against Rhizoctonia solani and Xanthomonas euvesicatoria pv. perforans in tomato

Abstract Tomato is affected by economically important diseases caused by Rhizoctonia solani and Xanthomonas euvesicatoria pv. perforans , which reduce yield and require sustainable management strategies. This study evaluated the antagonistic potential of Brazilian actinobacteria against these pathogens in tomato. Eight isolates obtained from rhizospheric soils of sugarcane and tomato were characterized based on cultural and micromorphological traits, together with multilocus phylogenetic analyses using partial sequences of the 16S rRNA, atpD , and rpoB genes. The isolates were affiliated with Streptomyces spp. and a Amycolatopsis sp. In vitro assays showed that most isolates inhibited the mycelial growth of R. solani , with NPF62 ( S. noursei ) and NPF59 ( S. lydicus ) showing the highest antagonistic activity in dual-culture and non-volatile metabolite assays. Against X. euvesicatoria pv. perforans , isolates NPF57 ( S. eurocidicus ) and NPF58 ( Streptomyces sp.) exhibited the highest in vitro antibacterial activity, with inhibition zones reaching up to 0.43 cm and exceeding the antibiotic control under certain conditions. Under greenhouse conditions, none of the actinobacterial treatments significantly reduced collar rot severity caused by R. solani compared with the corresponding pathogen-inoculated controls. In contrast, bacterial spot severity was affected by the interaction among isolate, actinobacterial preparation, and application method, with specific treatment combinations differing significantly from their corresponding controls. NPF60 ( S. aquilus ) applied as a spore suspension by soil drenching significantly reduced disease intensity relative to the corresponding pathogen-inoculated control. These findings highlight the variable correspondence between in vitro antagonistic activity and greenhouse disease suppression and indicate that actinobacterial performance was pathogen- and treatment-dependent under the conditions evaluated.