1999; Rehmany et al. 2005; Allen et al. 2004). Amino acid signature motifs (RXLR-dEER) were identified in the first oomycete avirulence genes discovered (Birch et al. 2006; Tyler et al.

2006) which were demonstrated to be translocation signals to move these associated proteins into plant cells (Whisson et al. 2007). The complete genome sequences are now available for three Phytophthora species (Haas et al. 2009; Tyler et al. 2006), for Pythium ultimum (Lévesque et al. 2010) and Hyaloperonospora arabidopsidis (Baxter et al. 2010). The RXLR effectors are very Anlotinib common in Phytophthora and Hyaloperonospora but are absent in Pythium ultimum. Many more genome sequences will become available and we are now reaching a new level of understanding of how species differ from each other. Oomycetes as pathogens Oomycetes pathogens are found on all crops and in many aquatic or terrestrial plants as well as in many animals. All the different impacts of oomycetes as plant or animal pathogens cannot be covered here but a few significant examples deserve to be discussed. The re-emergence of a disease The most famous, or maybe infamous, MLN2238 nmr oomycete is Phytophthora infestans, the species that caused the Irish potato famine in the 1800’s. Until the 1980’s, only a single clonal lineage of the A1 mating type was present outside Mexico or the Andes (Goodwin et al. 1994),

the centre of origin being still GS-4997 debated (Grunwald and Flier 2005; Gomez-Alpizar et al. 2007), eltoprazine and after that the A2 mating type was introduced to both Europe and North America. This caused P. infestans to re-emerge as a very serious threat to potato cultivation by increasing its aggressiveness towards the host, reducing fungicide efficacy, facilitating its survival in soil or debris and broadening its host range to include tomato (Fry et al. 1992; Fry and Goodwin 1997; Gavino et al. 2000; Lee et al. 1999). Because of the significant impact

of this migration, P. infestans has become a model system for population genetics and the basis of international collaborations for population tracking (Cooke and Lees 2004; Goodwin et al. 1992; Forbes et al. 1998; Fry et al. 1992). Forestry Fifty years ago, the number of known species of oomycetes having an impact on forestry was quite low. Phytophthora cinnamomi and P. cambivora were the most notable disease agents (Brasier 2000). More recently the impact of oomycetes on forestry has increased dramatically with wider ranges of known diseases and more importantly the emergence of agents that were not previously known. Prior to 2000, only 20% of Phytophthora species were known to have an impact in forestry whereas 60% of the species described since that time are associated with forestry or natural environments (Brasier 2009). This exponential growth post 2000 is mainly due to new species of Phytophthora being described that are associated with forestry (Fig.