Symptoms of the leaf scorch disease on the leaves of oleander infected by Xylella fastidiosa appearing for the first time in Lebanon were confirmed by ELISA tests and anatomical observations of sections of petioles by a scanning electron microscope. Xylella fastidiosa was detected in the xylem forming bacterial aggregates in the lumen of tracheary elements. The bacterium colonizes and invades the conducting tissues degrading pit membranes (PMs) of tracheary elements. Dense networks of fibrillar material and tyloses, originating from the plant, as a defense mechanism form around the bacterial aggregates to limit the movement of the pathogen. The bacterial aggregates that plug the tracheary elements and the tyloses produced by the infected oleander plants cause water stress and consequently leaf scorch symptoms. To the best of our knowledge, this is the first report of Xylella fastidiosa in Lebanon.
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