Endophytic Bacterium Lysobacter firmicutimachus Strain 5-7 Is a Promising Biocontrol Agent Against Rice Seedling Disease Caused by Pythium arrhenomanes in Nursery Trays

Rice root rot disease caused by spp. is a highly destructive disease in rice nurseries. Biocontrol with endophytic bacteria was developed in this study to control rice seedling diseases. An in planta screening assay revealed that two bacterial endophytes, strains 5-7 and 6-4, displayed strong protection of rice seedlings from attack by . Phylogenetic analysis indicated that strain 5-7 is , while strain 6-4 belongs to the genus. To quickly evaluate the disease severity of the root system damaged by spp. in nursery trays, a root surface area measurement assay was developed. By using this measurement, the control efficacy in nursery trays was evaluated, and 5-7 showed promising biocontrol activity against Pythium disease. In a field trial, the two endophytes exhibited significant disease control efficacy on rice brown spot disease caused by naturally occurring in a commercial nursery field. The two endophytes exhibited multiple enzymatic activities and broad-spectrum antagonistic activities against multiple rice pathogens. The two endophytes colonized the root surface and inside of the root. 5-7 primarily colonized the intercellular space and aerenchyma. Antibiosis is the major mechanism used by strain 5-7 to cause hyphal swelling and inhibit zoospore germination and sporangium formation, while a hyperparasitism-like phenomenon was found in the interaction of strain 6-4 with and hyphae. In conclusion, we report the promising biocontrol agent 5-7 and the potential biocontrol agent sp. 6-4 for disease control of rice seedlings in commercial nursery trays and their possible mechanisms of action.