Effectiveness of the monitoring of X. fastidiosa subsp. pauca in the olive orchards of Southern Italy (Apulia)

In 2013, an outbreak of Xylella fastidiosa was identified for the first time in Europe, in the extreme South of Italy (Apulia, Salento territory). The locally identified subspecies turned to be lethal for olive trees, starting an unprecedented phytosanitary emergency for one of the most iconic cultivations of the Mediterranean area. The Apulian-monitoring program of the epidemic amassed data on several hundreds of thousands of laboratory screening for the bacterium presence, jointly with georeferenced sample information. Starting from these data, it is possible to show that Xylella fastidiosa spreads by forming new, tightly clustered groups of infected plants (epidemic hotspots), with 98% of the infected trees separated by less than 100 mt from another infected tree. Surprisingly, more than three quarters of the newly detected epidemic hotspots are farther than 1 km from any previously known infected plants. Considering this finding, either long-range spreading of the bacterium is underestimated, or the current monitoring strategy must be called into question. In both cases, however, it can be anticipated that, under the current monitoring protocol, yearly epidemic spreading 1–15 km far from olive trees currently labeled as infected will be more common than previously thought.