Xylella fastidiosa, a harmful bacterium, was found in olive trees in southern Italy suffering from olive quick decline syndrome (OQDS). Researchers isolated the bacterium and tested its ability to cause disease in olive, oleander, and myrtle-leaf milkwort plants. They found that the bacterium was present in all diseased olive trees tested and was able to infect and spread in different plant species, causing symptoms similar to those seen in the field. The severity and timing of symptoms varied depending on the plant species.