Phytophthora on Alnus spp. (alders)
The genus Alnus (Betulaceae) includes thirty-five species distributed across northern hemisphere (http://www.discoverlife.org). There are four species native to Europe: A. incana (grey alder), A. cordata (Italian or Corsica alder), A. glutinosa (common or black alder) and A. viridis (green alder). In addition, A. rubra native to North America has been extensively planted in some European countries (Claessens, 2003). Alnus sp. plays important ecological roles. It is a pioneer genus, tolerant of high ground water levels and periodic flooding. A. glutinosa is the most common species and is present throughout Europe up to 1800 m. It is well adapted to wet sites and plays a vital role in riparian ecosystems as the root system helps to stabilise riverbanks reducing the effect of erosion (Webber et al., 2004). Black alder has a beneficial effect on soil (porosity, symbiosis with Frankia), on water quality (filtration, purification) and also on fauna. It contributes to increase the biodiversity of birds and insects and its root system allows fish to shelter. Alnus incana is widely distributed in central and eastern Europe. In the south, it grows mainly in mountain areas. As it is a root sprout pioneer that tolerates both dry conditions and flooding, A. incana is very important for improving the stability of slopes and riverbanks (Jung & Blaschke, 2004). A. cordata and A. viridis are much less frequent species and are native to Corsica-Italy and mountains of central Europe, respectively (Claessens, 2003).