Plant pathogens cause huge crop losses worldwide and cause much human suffering. Recent studies on plant pathogen interactions have revealed that pathogens introduce proteins (pathogenicity effectors) into the host plant to suppress host immune responses. These effectors allow the pathogen to grow and damage the plant. Analysis of the genomes of a group of very important pathogens called the oomycetes, which includes Phyptophthora infestans the causal agent of potato blight, has revealed that they contain massive arsenals of these effector proteins. We have identified and isolated 100 such effector genes from Hyaloperonospora arabidopsidis (Hpat) which infects the model plant Arabidopsis. By delivering these one at a time to the plant we have shown that more than 50% are capable of altering the hosts ability to defend against pathogens. We are now selecting a small number of these with which to identify the host targets to understand how they allow pathogen growth and developement on the host plant. The long term goal of the project would be to develop novel methods to block pathogen growth.