Specific recognition events, defined by the perception of pathogen elicitors by plant receptors, trigger defense responses including the hypersensitive response (HR), a form of programmed cell death in plants. Phytophthora infestans, an oomycete plant pathogen and the causal agent of late blight of potato and tomato, produces INF elicitins, a diverse family of extracellular proteins that induce the HR in a restricted number of plants, particularly in the genus Nicotiana within the Solanaceae. Resistance of Nicotiana to P. infestans is always associated with a rapid HR of epidermal and mesophyll cells, which restricts the pathogen to the infection site. Our studies aim at determining to which extent this response is mediated by the recognition of INF elicitins. P. infestans strains deficient in the major elicitin INF1 induced disease lesions on Nicotiana benthamiana, suggesting that INF1 functions as an avirulence factor that conditions resistance in this species (1). In contrast, INF1 deficient strains remained unable to infect other Nicotiana species, such as tobacco. Tobacco may therefore respond to additional elicitors, perhaps other members of the complex INF elicitin family (2). To dissect the response of Nicotiana to elicitins, functional expression of inf genes in plants was conducted using a recombinant Potato Virus X (PVX) vector (3). These PVX-inf constructs triggered a number of responses such as local and systemic HR lesions, allowing a quantitative evaluation of the response of Nicotiana to the various INF elicitins. This study shows that species-specific elicitors can be used to dissect genetically complex resistance processes, such as nonhost resistance, into discrete components. Identification of the plant genes involved in nonhost resistance to economically important oomycete pathogens, such as P. infestans, remains an important challenge (4).