Crop diseases cause economic losses amounting to over $200 billion each year. One of the most devastating crop failures was caused by the oomycete pathogen Phytophthora infestans that led to the Irish Potato Famine in the 1840s. The oomycete Hyaloperonospora arabidopsidis (Hpa) is a useful pathogen to study oomycete Hyaloperonospora arabidopsidis (Hpa) is a useful pathogen to study oomycete diseases as it is a natural pathogen of the model plant Arabidopsis and a draft genome sequence of the Hpa race Emoy2 is available. Three approaches have been used in this study to identify host and pathogen genes involved in the interaction between Arabidopsis and Hpa. A cDNA library of Hpa race Noco2 transcripts expressed in plant a was generated.