GENETIC-ANALYSIS OF LACTUCA ACCESSIONS WITH NEW MAJOR GENE RESISTANCETO LETTUCE DOWNY MILDEW

The short-lived effectiveness of major gene resistance in lettuce to d owny mildew (Bremia lactucae) means that plant breeders are in constan t need of new resistance genes. In this study, the genetics of resista nce to B. lactucae in nine accessions of Lactuca sativa or L. serriola was analyzed using segregating F-2 populations. This analysis was aim ed at establishing the number of resistance factors in these accession s, their phenotypic effects, and the mapping of these factors in linka ge groups. The cultivar Mariska (Rig) 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, dissimil ar 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 f actors identified in this study provide resistance to races that are i mportant in lettuce-growing areas. Therefore, they are a valuable addi tion to the set of resistance genes available to lettuce breeders.