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To investigate the impact of induced host defenses on the virulence of a compatible Peronospora parasitica strain on Arabidopsis thaliana, we examined growth and development of this pathogen in nim1-1 mutants and transgenic salicylate hydroxylase plants. These plants are unable to respond to or accumulate salicylic acid (SA), respectively, are defective in expression of systemic acquired resistance (SAR), and permit partial growth of some normally avirulent pathogens. We dissected the P. parasitica life cycle into nine stages and compared its progression through these stages in the defense-compromised hosts and in wild-type plants. NahG plants supported the greatest accumulation of pathogen biomass and conidiophore production, followed by nim1-1 and then wild-type plants. Unlike the wild type, NahG and nim1-1 plants showed little induction of the SAR gene PR-1 after colonization with P. parasitica …