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The Influence of Host and Pathogen Genotypes on the Apparent Infection Rates of Potato Late Blight Epidemics. R. X. Latin, Graduate assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802, Current address of senior author: Department of Plant and Soil Sciences, University of Idaho, Moscow 83843; D. R. MacKenzie(2), and H. Cole, Jr.(3). (2)(3)Associate professor, and professor, respectively, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 71:82-85. Accepted for publication 15 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-82.

The capacity of potato genotypes to reduce the apparent infection rate of potato late blight epidemics was investigated by studying different isolates of Phytophthora infestans. In 1977, the rate-reducing capacity of seven host genotypes was assessed with two pathogen isolates and, in 1978, five host genotypes were tested with four isolates. The isolates were selected only for ability to overcome vertical resistance conditioned by the R1 gene. Epidemics were monitored in the field and apparent infection rates were calculated for each of the 14 cultivar/isolate combinations in 1977 and the 20 cultivar/isolate combinations in 1978. The apparent infection rates were subjected to analysis of variance. An orthogonal comparison procedure revealed cultivar isolate interactions with probability levels of P≤ 0.07 and P≤0.01 for 1977 and 1978, respectively. The interactions imply that the level of rate-reducing capacity of host genotypes differs for different isolates of the pathogen. It is suggested that several different isolates of the pathogen be employed in screening host genotypes in order to assess their rate-reducing capacity.