Potato common scab (PCS), caused by pathogenic Streptomyces spp., is a serious disease in potato production worldwide. Cultural practices, such as optimizing the soil pH and irrigation, are recommended but it is often difficult to establish stable disease reductions using these methods. Traditionally, local farmers in southwest Japan have amended soils with rice bran (RB) to suppress PCS. However, the scientific mechanism underlying disease suppression by RB has not been elucidated. The present study showed that RB amendment reduced PCS by repressing the pathogenic Streptomyces population in young tubers. Amplicon sequencing analyses of 16S ribosomal RNA genes from the rhizosphere microbiome revealed that RB amendment dramatically changed bacterial composition and led to an increase in the relative abundance of gram-positive bacteria such as Streptomyces spp., and this was negatively correlated with PCS disease severity. Most actinomycete isolates derived from the RB-amended soil showed antagonistic activity against pathogenic Streptomyces scabiei and S. turgidiscabies on R2A medium. Some of the Streptomyces isolates suppressed PCS when they were inoculated onto potato plants in a field experiment. These results suggest that RB amendment increases the levels of antagonistic bacteria against PCS pathogens in the potato rhizosphere.

Seeds of common buckwheat usually mature while leaves are still green, causing problems for harvest machinery and limiting yields. We developed an early leaf-senescence line, Kyushu 1, from spontaneous mutants. To characterize this mutant line physiologically and genetically, we measured the change in leaf chlorophyll content in two regions of Japan with different environments, and performed genetic analyses with self-compatible lines. The chlorophyll content of Kyushu 1 was lower than that of the wild type at seed maturity in both regions. F 1 progeny of the cross between wild-type plants and Kyushu 1 showed a wild-type phenotype. F 2 progeny segregated into wild-type and early leaf-senescence phenotypes in a 3:1 or 13:3 ratio, suggesting that the early leaf senescence of Kyushu 1 is controlled by either a single recessive gene or an epistatic interaction between two loci.