The effect of the presence of R‐genes for resistance to late blight ( <i>Phytophthora infestans</i> ) of potato ( <i>Solanum tuberosum</i> ) on the underlying level of field resistance

Abstract

The differential genotypes R 1 , R 10 and R 11 , as originally defined by Black, were crossed with R‐gene‐free cultivars and the progenies divided into two subpopulations comprising those which had inherited the R‐gene and those which had not. The underlying level of field resistance of the two groups was compared in a field trial in which they were inoculated with an isolate that could overcome the relevant R‐genes. The R‐gene‐bearing group was significantly ( P &lt; 0·001) more resistant than the R‐gene‐free group, with mean scores over four dates in 2000 of 4·86 and 4·09, respectively, on a 1–9 scale of increasing resistance, and of 4·10 and 2·35 on one date in 2001. However, the magnitude of the effect depended on the R‐gene and the year of the trial. Data from a progeny of cv. Stirling (with an R‐gene and a high level of field resistance) were examined and the same effect of an R‐gene found. Fewer of the R‐gene‐bearing group of clones were highly susceptible, and more were resistant. The most resistant clones always bore the R‐gene. It is concluded that increased resistance is conferred by the defeated R‐gene or linked genes for field resistance.