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Abstract. The interaction between Phytophthora infestans (Mont.) de Bary and Solanum was examined cytologically using a diverse set of wild Solanum species and potato (S. tuberosum L.) cultivars with various levels of resistance to late blight. In wild Solanum species, in potato cultivars carrying known resistance (R) genes and in nonhosts the major defense reaction appeared to be the hypersensitive response (HR). In fully resistant Solanum species and nonhosts, the HR was fast and occurred within 22 h. This resulted in the death of one to three cells. In partially resistant clones, the HR was induced between 16 and 46 h, and resulted in HR lesions consisting of ®ve or more dead cells, from which hyphae were occasionally able to escape to establish a biotrophic interaction. These results demonstrate the quantitative nature of the resistance to P. infestans. The eectiveness of the HR in restricting growth of the pathogen diered considerably between clones and correlated with resistance levels. Other responses associated with the defense reaction were deposition of callose and extracellular globules containing phenolic compounds. These globules were deposited near cells showing the HR, and may function in cell wall strengthening. Key words: Hypersensitive response ± Nonhost resistance ± Phenolic compound ± Phytophthora ± Resistance (R) genes ± Solanum (disease resistance) Introduction Late blight, caused by the oomycete Phytophthora infestans, is the most devastating disease of potato (Solanum tuberosum) world-wide. To limit chemical control, breeding potato to incorporate durable forms of genetic resistance is needed. The genus Solanum comprises an extensive gene pool, in which a broad spectrum of pathogen resistance has accumulated throughout evolution (Ross 1986). In some wild Solanum species, resistance to P. infestans that may be of a durable nature has been identi®ed (Colon and Budding 1988). Also some old potato cultivars have a seemingly durable resistance Resistance responses to pathogens are traditionally classi®ed as race-speci®c, race-nonspeci®c, and nonhost resistance (Agrios 1997). In this concept, race-speci®c resistance is based on the presence of major resistance genes (R), which are conserved among plant species. The R genes are thought to encode speci®c receptors that upon triggering by elicitors initiate signal transduction pathways leading to the hypersensitive response (HR; Hammond-Kosack and Jones 1997). In the gene-forgene mode