The Influence of Host and Pathogen Genotypes on the Apparent Infection Rates of Potato Late Blight Epidemics

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Abstract
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 7 host genotypes was assessed with 2 pathogen isolates and, in 1978, 5 host genotypes were tested with 4 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 .times. isolate interactions with probability levels of P .ltoreq. 0.07 and P .ltoreq. 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. Several different isolates of the pathogen should be employed in screening host genotypes in order to assess their rate reducing capacity.

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