Phenotypic Expression of Resistance to Phytophthora infestans in Processing Potatoes in Argentina

Ten potato cultivars were evaluated in Argentina for foliage and tuber susceptibility to Phytophthora infestans, the oomycete pathogen causing late blight. Pathogen isolates collected from two major potato production regions in the country were polymorphic for specific virulence and isozyme banding patterns, although diversity was found among and not within specific locations. Four of the cultivars were found to be highly resistant in foliage, having little disease in the field and greenhouse and race-specific resistance is suspected. One of these is Russet Burbank, which is highly susceptible in the United States. Cultivar Shepody was the most susceptible and was assigned a value of 8, which was used together with area under disease progress curve values to calculate scale values of the other cultivars on a 1 to 9 susceptibility scale (where 9 is highly susceptible). Cultivar Ranger Russet had values of about 2.5 and resistance could be race-non-specific. The other cultivars for which race incompatibility did not appear to be a problem had susceptibility values between 4 and 8. Foliage susceptibility values for greenhouse and field were correlated. Foliage and tuber susceptibility were also correlated, although with exceptions and only one cultivar, Innovator, was highly resistant in tubers. The two isolates chosen for assessment of foliage susceptibility in the greenhouse differed for aggressiveness in separate tests where components were measured; however, no cultivar-by-isolate interaction was measured. Cultivars with high levels of putative race-specific foliage resistance should be used with constant monitoring in Argentina.