Leaf symptoms on plum, coffee and citrus and the relationship with the extent of xylem vessels colonized by Xylella fastidiosa

Leaf petioles of plum, coffee and sweet orange were examined using scanning electron microscopy (SEM). The presence of Xylella fastidiosa in the samples was confirmed by polymerase chain reaction and gel electrophoresis. The number of vessels colonized by X. fastidiosa was determined by SEM in petiole areas that were transversally sectioned under liquid nitrogen. The percentage of colonized vessels in petioles of coffee was higher than in petioles of plum and citrus whether trees were exhibiting mild symptoms (MS) or severe symptoms (SS). The percentage of vessels colonized varied from 10.9 (MS) to 38.0% (SS), 26 (MS) to 51.6% (SS), and 8 (MS) to 11.8% (SS) for plum, coffee and citrus, respectively, and did not vary by position within the petiole. Severity of symptoms consistently reflected higher proportion of colonized vessels in coffee and plum, but not in citrus.