Fire blight is a bacterial disease, caused by the gram-negative enterobacterium Erwinia amylovora, that mainly infects plants of the Rosaceae family. It can spread very quickly and cause enormous damage, which is particularly serious for the economically important fruit crops apple and pear. But also quince and ornamental plants like cotoneaster, hawthorn, pyracantha and mountain ash are affected (Zwet and Beer, 1991). E. amylovora can infect all parts of the plant including flowers, leaves, branches, stems, fruits and roots (Vanneste, 2000). It overwinters within infected tissue of annual cankers, formed on branches diseased in the previous season. In spring, bacterial ooze, which is composed of millions of bacteria, polysaccharides and plant sap, exudes from these areas and serves as primarily inoculum (Zwet and Beer, 1991). Insects attracted by the ooze, but also wind or rain disseminate the bacteria to blossoms (Schroth et al., 1974). There it multiplies initially at the stigma and later it migrates down to the floral nectaries by moistness, resulting in infection and the production of new bacterial ooze, which serves as secondary inoculum (Thomson et al., 1986). In the secondary phase the pathogen infects also shoots, fruits and rootstocks by entering through the stomata or wounds caused by a number of biotic or abiotic factors. From the infected tissue the pathogen moves through the xylem into the parenchyma and down to the root system (Bogs et al., 1998). Typical symptoms of a shoot blight infection are shown in the right picture of figure 1.1 on page 20. Leaves wilt and turn dark brown, but usually remain attached to the tree. The end of the shoot is bended to a so-called “shepherd’s crook” and droplets of the amber coloured bacterial ooze exude from the infected tissue. Symptoms of fire blight were first reported by Denning (1794) in North America in the Hudson Valley of New York. However, only a hundred years later initiating research by T.J. Burrill, J.C. Arthur, and M.B. Waite suggested that a bacterium could be the reason for these symptoms (Griffith et al., 2003). Finally, the pathogen was isolated at the beginning of the 20th century as the first proven bacterial plant pathogen and termed E. amylovora (Burrill) Winslow et al. (Baker, 1971; Winslow et al., 1920). Since the discovery of fire blight the pathogen had moved in a period of approximately 135 years into every region of the USA and from there it finally spread over to Japan and New Zealand and reached Europe in 1957, where it was observed for the first time in pear orchards in Kent, England (Bonn and van der Zwet, 2000; Peil et al., 2009). Around ten years later E. amylovora was introduced to the mainland of the European continent by infected plant material from Great Britain and reached four years later West Germany (Van der Zwet et al., 1970; Schroth et al., 1974). Today the disease is present in more than 47 countries around the world and with the exception of Portugal it can be found all over Europe (Van der Zwet et al., 2012; EPPO, 2016). To prevent further introductions and the spread of fire blight, the bacterium is classified in many of these countries, also in Germany as a quarantine disease.weiterlesen