Similarities and differences in the genetic and physiological control of resistance to stemphylium leafspot in winter-active and semi-dormant lucernes

Previously, it has proven difficult to improve resistance to some biotypes of Stemphylium vesicarium in selections from lucerne cv. Sequel (highly win ter active) in contrast to selections from cv. Trifecta (semi-winter dorman t). Stemphylium leafspot is a disease of the cooler months, and there are 2 possible explanations for the differing reactions: either the winter-activ e genetic backgrounds do not contain the requisite resistance genes, or the winter-active lines are physiologically disadvantaged through their winter activity. A diallel cross was made between clones from Trifecta and Sequel representing a range of disease reactions. Progeny were grown either under summer or winter conditions before inoculation. For both environments, var iation was entirely additive. There was no interaction between crosses (or parents) and the 2 environments, and there was a close relationship (r = 0. 93) between the breeding values (g(i)) of parents in the 2 environments. Th e data indicated that variation in both backgrounds was entirely additive, although there were indications that phenotypic expression of resistance wa s stronger in clones from the Trifecta background. To increase resistance i n winter-active material, it would be necessary to introgress the resistanc e genes from non-winter-active material.