Migration and hydraulic disruption by a bacterial pathogen in oak tree xylem

Xylella fastidiosa (Xf) is a xylem-limited plant pathogen that causes leaf scorch symptoms in numerous host plant species.  Xf currently threatens woody plants in natural, urban and agricultural ecosystems over an expanding geographic range.   In a survey of the oak species on the Saint Joseph's University campus, we found that ~41% tested positive for Xf-infection and exhibited severe leaf scorch symptoms in this urban ecosystem.  To better understand how the bacteria systemically colonize host xylem and induce scorch symptoms, we studied xylem structure and function in infected and healthy Red and Pin oaks.  Hydraulic conductivity in Xf-infected petioles was lower than that of healthy trees throughout the season and plummeted with the onset of scorch symptoms.  Early in the season, % embolism was significantly higher in Xf-infected petioles compared to healthy trees, likely resulting from degradation of pit membranes during Xf systemic colonization of new host xylem.  Air seeding and microsphere injections on individual vessels showed that pit membrane pores are smaller than individual Xf, thus requiring pit membrane degradation in order to systemically colonize host xylem.  Understanding the exact mechanism of long distance Xf movement in hosts will allow us to better predict susceptibility of hosts under a range of conditions.