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Resistance

Genetic Analysis of Lactuca Accessions with New Major Gene Resistance to Lettuce Downy Mildew. F. J. M. Bonnier, Centre for Plant Breeding and Reproduction Research (CPRO-DLO), P.O. Box 16, NL-6700 AA Wageningen, Netherlands; K. Reinink, and R. Groenwold. Centre for Plant Breeding and Reproduction Research (CPRO-DLO), P.O. Box 16, NL-6700 AA Wageningen, Netherlands. Phytopathology 84:462-468. Accepted for publication 2 December 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-462.

The short-lived effectiveness of major gene resistance in lettuce to downy mildew (Bremia lactucae) means that plant breeders are in constant need of new resistance genes. In this study, the genetics of resistance to B. lactucae in nine accessions of Lactuca sativa or L. serriola was analyzed using segregating F₂ populations. This analysis was aimed at establishing the number of resistance factors in these accessions, their phenotypic effects, and the mapping of these factors in linkage groups. The cultivar Mariska (R18) and the line CS-RL both carry a widely effective resistance factor, which is located in linkage group 1. In seven other accessions, eight novel resistance factors, dissimilar to recognized major genes, were detected and designated R23 to R30. Evidence was obtained for a new, fifth, linkage group, which contains resistance factors R23 and R25. Resistance factors R27, R28, and R29 could not be assigned to a linkage group. Many of the new resistance factors identified in this study provide resistance to races that are important in lettuce-growing areas. Therefore, they are a valuable addition to the set of resistance genes available to lettuce breeders.