Almond leaf scorch disease (ALSD), caused by Xylella fastidiosa, has been reported from some regions of Iran. Biological traits of isolates the pathogen from almond were investigated in pot and orchard conditions. ALSD killed trees of susceptible cultivars during 3 to 4 years outdoors in pots, but overwintered in root tissues in orchards with winter temperatures below -15 degrees C. Xylella fastidiosa was not detected by culturing or DAS-enzyme-inked immunosorbent assay (DAS-ELISA) in almond leaves until early summer and peaked in early autumn. However, root samples taken in winter (January and February) and early spring (April) reacted positively in DAS-ELISA, culture media and polymerase chain reaction assays. This demonstrates that X. fastidiosa survives in root tissues of almond trees under orchard conditions in very cold (-28 degrees C) winters. Trunk injection of oxytetracycline into leaf scorched almond trees reduced symptoms of the disease, while penicillin applications also reduced symptoms but to a lesser degree.