The number of colony forming units (CFU) inside xylem vessels of leaf petioles were evaluated in tobacco by scanning electron microscopy followed by an image analysis. Symptom expression of Nicotiana tabacum, cultivars TNN, Havana and RP1, were correlated with the presence of bacteria inside leaf petiole vessels. The symptom expression were evaluated in terms of colony forming units per gram of colonized tissue and percentage of colonized vessels. No statistical differences were found among the varieties tested. Havana expressed the most intense symptoms, best indicating a better experimental host. We also observed that leaf symptoms could be reversed with the application of ammonium sulphate along with pruning. This routine was effective in delaying symptom development. In summary, colonization efficiency was similar in tobacco varieties. Fertilization may affect Xylella fastidiosa symptom expression and pruning may be used as an aid to diminish X. fastidiosa advance within the plant.