Distinguishing major-gene from field resistance to late blight (Phytophthora infestans) of potato (Solanum tuberosum) and selecting for high levels of field resistance.

Potato cultivar Stirling, which has a Solanum demissum derived R-gene and quantitative field resistance to late blight, was crossed with the susceptible cultivar Maris Piper to produce an F1 population from which three genotypes (94B13A29, 57 and 61) were backcrossed to Maris Piper. The F1 and backcross populations were assessed for resistance to simple race 1,4 (incompatible with Stirling's R-gene) and complex race 1,2,3,4,6,7 of Phytophthora infestans (compatible with R-gene) in whole plant glasshouse tests. The segregation results in the F1 generation with the simple race confirmed the presence of a single copy of the R-gene in Stirling, and the results with the complex race were consistent with Stirling having a high level of quantitatively inherited field resistance. Comparisons of the results with the simple and complex races apparently enabled F1 clones to be classified for the presence or absence of the R-gene and to be assessed for their level of quantitative field resistance. However, two out of the three backcrosses done to check classifications revealed unexpected findings: 94B13A29 had two copies of the R-gene as a result of double reduction, but was, as expected, susceptible to the complex race; and 94B13A57 had the R-gene (one copy) and it, and its offspring with the R-gene, had some resistance to the complex race, whereas those offspring without the R-gene were susceptible. Clone 94B13A61, as expected, lacked the R-gene and had moderate quantitative field resistance to both races. The implications are discussed for breeding potatoes with durable resistance to late blight.