Need for protective measures to combat potential outbreaks of Homalodiscia coagulata and Pierce's disease in European viticulture
Pierce's Disease (PD) is a lethal disease of grapevines (Vitis vinifera) caused by a strain of the bacterium Xylella fastidiosa. At the end of the 1990s, PD spread rapidly in California after accidental introduction of the non-native vector glassy-winged sharpshooter (Homalodisca coagulata). The new vector, which prefers citrus as a host for reproduction and overwintering, transmits the bacteria very efficiently to a large variety of plants and is a threat not only to the state's production of grapes, but also to large-scale cultivation of e.g., almonds and peaches. In order to stop the spread of the new vector and protect the wine industry from PD, quarantine measures have been put into effect and several research projects concerning pest management strategies have been initiated. From a European perspective, interesting results have been achieved by the use of a climate model. This model predicts that cold stress accumulation would exclude PD within wine-producing regions of Europe. However, some reports suggest the existence of cold-tolerant X fastidiosa PD-causing strains, which could exhibit an invasion potential. Because of global trade and the fact that both the bacterium and the vector are associated with a large number of plant families, other temperate areas such as those in parts of Europe where grape and citrus production dominate are now under threat. The European and Mediterranean Plant Protection Organization (EPPO) has decided to consider H. coagulata a serious threat and is currently asking member countries to pay particular attention to this organism. From experience gained in California, H. coagulata needs to be combated using a palette of measures. Suggested initiatives to prevent introduction to Europe could involve quarantine and/or phytosanitary regulations and consideration of tourist traffic regulation between, for instance, Europe and North America and between Europe and Polynesia. However, current phytosanitary regulatory policies in California indicate that such regulations are not sufficient to prevent outbreaks. Preparation of action plans to be followed in the event of an outbreak should therefore carefully take note of the experience gained so far in deciding which eradication measures should be implemented and at which level of infestation. In the following, the spread of PD is presented and discussed with respect to previous experiences. Knowledge generated within California, such as such use of clay particle sprays and physical barriers and biological control with egg parasitoids, is suggested to be an important tool for European viticulturalists to combat potential outbreaks.