Bidirectional colonization and biofilm formation by Erwinia psidii in eucalypt plants

Dieback and wilt caused by Erwinia psidii is an emerging disease that has been causing considerable damage in eucalypt plantations. Because it is a recently emerged disease, several aspects of the bacterial interaction with its host still remain to be elucidated. In this work, we studied the E. psidii colonization and biofilm formation in eucalypt tissues by specific detection using PCR and scanning electron microscopy (SEM). The results indicate that the bacterium is able to translocate in stem tissue mainly acropetally, although movement in the basipetal direction was also observed to a lesser extent, always through the xylem. No colonization of phloem tissues was observed. In addition to colonizing the xylem, E. psidii colonized the parenchymatous tissue. The bacterium formed cell aggregates enveloped by fibrillar material that evolved into complex, well‐structured biofilms in stem and leaf tissues. In contrast, no biofilm formation was observed on abiotic surfaces. These observations suggest that biofilm formation plays an important role in the elicitation of dieback and wilt symptoms caused by E. psidii on eucalypt plants. This study not only shows ultrastructural aspects of the E. psidii communities but also tissue damage in eucalypt plants that was associated with the presence of bacterial aggregates and formation of tyloses.