The hypersensitive response is associated with host and nonhost resistanceto Phytophthora infestans

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 lat e blight. In wild Solanum species, in potato cultivars carrying known resis tance (R) genes and in nonhosts the major defense reaction appeared to be t he hypersensitive response (HR). In fully resistant Solanum species and non hosts, 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 b etween 16 and 46 h, and resulted in HR lesions consisting of five or more d ead 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 effectiveness of the HR in restrict ing growth of the pathogen differed considerably between clones and correla ted with resistance levels. Other responses associated with the defense rea ction were deposition of callose and extracellular globules containing phen olic compounds. These globules were deposited near cells showing the HR, an d may function in cell wall strengthening.