Xylella fastidiosa has recently been identified in the Apulian province of Lecce (south-eastern Italy) in olive trees affected by a devastating disease denoted Olive Quick Decline Syndrome (OQDS), that appeared suddenly in 2010. Symptoms of OQDS consist of withering and desiccation of scattered terminal shoots, which rapidly expands to the rest of the canopy, and results in the collapse and death of the tree. The identification of X. fastidiosa in OQDS-affected trees represents the first confirmed detection of this bacterium in the European Union (EU), but its exact role in the aetiology of this disease is yet to be determined. Since X. fastidiosa is a regulated quarantine pathogen in the EU, upon request of the Apulian Plant Protection Service, surveys were initiated in order to delineate the contaminated area. To this effect, diagnostic protocols based on ELISA and conventional PCR for X. fastidiosa detection in olive samples were compared and validated via an interlaboratory ring-test in which three accredited laboratories, all located in Italy, participated. Both procedures proved to be equally effective but, due to the simplicity of sample preparation, ELISA was chosen for the large-scale X. fastidiosa monitoring programme now in progress.