Assessment of horizontal resistance to late blight of potatoes

SUMMARY Ten potato clones were assessed for horizontal resistance to late blight in the foliage, in replicated plots for 4 years. Plots were scored on several occasions each year and this paper explores the most reliable ways of using the data to detect differences between clones. Using the genetic component of variance (heritability in the broad sense, h2) as criterion, mean resistance over all scoring dates was found to be the best measure out of several tried. Though good discrimination was achieved within years (mean h2 > 90%), there were large clones × years interactions and changes of rank order such that h2 fell to about 65% over seasons, and many years of testing would be required to discriminate between similar clones. There was no reason to think that the interactions were caused by the erratic expression of R genes, though one case of vertical resistance was detected. Some results from parallel testing of diverse potato breeding materials are described, and implications for breeding methods and objectives are briefly discussed.