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

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Abstract
A cytological comparison was made of several potato cultivars inoculated with the late blight fungus Phytophthora infestans to study possible differences in phenotypic expression of general and specific resistance genes. The host cultivars used had different levels of general resistance and, additionally, differed by the presence or absence of the specific resistance gene R3. It was noted that: 1) penetration frequency did not differ among the cultivar-pathogen combinations; 2) up to 90% of the penetrations of the 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 interactions, 31-98% turned brown during compatible interactions; 6) sporulation expressed as numbers of sporangia formed per 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.