Cytological Comparison of Specific ( R3 ) and General Resistance to Late Blight in Potato Leaf Tissue

Abstract

In comparisons of several cultivars possessing different levels of general resistance to Phytophthora infestans as well as differing by the presence or absence of the specific gene R3 it was noted that (1) penetration frequency did not differ among the cultivar-pathogen combinations; (2) up to 90% of penetrations of leaf tissue occurred in the stoma complex; (3) the time required for lesions to start expanding tended to increase in cultivars containing the R3 gene; (4) speed of tissue colonization following this latent period was fairly uniform in all cultivars where growth beyond the inoculation site occurred; (5) all cells initially penetrated by the pathogen turned brown during incompatible reactions, 31-98% turned brown during compatible reactions; (6) sporulation expressed as numbers of sporangia formed/unit area of infected tissue was similar in all cultivars; and (7) high general resistance may have a phenotype indistinguishable from a hypersensitive reaction evoked by specific resistance genes.