Pectobacterium chrysanthemi as the dominant causal agent of bacterial soft rot in Oncidium “Grower Ramsey”

Abstract

Oncidium orchids are important ornamental flowers for export in Taiwan, but their production is limited by bacterial soft rot disease. The common pathogens of bacterial soft rot include Pectobacterium carotovorum subsp. carotovorum (Pcc) and P. chrysanthemi (Pch). Pch is a unique pathogen that can be isolated from Oncidium. Thus, the possible reasons for that are need to be studied. In this study, we sought to determine the possible factors affecting the pathogenicity of Pcc and Pch in Oncidium Gower Ramsey. Results showed that both Pcc and Pch are pathogenic to Oncidium orchid and two other common hosts of soft rot bacteria, potato and radish. However, Pch is more virulent in Oncidium orchid than Pcc. Moreover, temperature is not the dominant factor contributing to differences in soft rot development in Oncidium caused by Pcc and Pch, suggesting that host selection is more important than temperature. We further found that Pcc growth is not inhibited by Oncidium leaf extract and that the inhibitory effect of Oncidium on Pcc may be attributed to the defense response of living cells. In addition, stronger reactive oxygen species accumulation was triggered by Pcc rather than Pch. This defense response against Pcc is related to plant immune signal pathways, including calcium influx and NADPH oxidase activation. Through this study, we have provided evidences that Pch, not Pcc, is the dominant soft rot pathogen in Oncidium. The immune response triggered by bacterial recognition explains the inhibition of Pcc growth in Oncidium Gower Ramsey.