Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past *50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance. Keywords

Key message Eleven new major resistance genes for lettuce downy mildew were introgressed from wild Lactuca species and mapped to small regions in the lettuce genome. Abstract Downy mildew, caused by the oomycete pathogen Bremia lactucae Regel, is the most important disease of lettuce (Lactuca sativa L.). The most effective method to control this disease is by using resistant cultivars expressing dominant resistance genes (Dm genes). In order to counter changes in pathogen virulence, multiple resistance genes have been introgressed from wild species by repeated backcrosses to cultivated lettuce, resulting in numerous near-isogenic lines (NILs) only differing for small chromosome regions that are associated with resistance. Low-pass, whole genome sequencing of 11 NILs was used to identify the chromosome segments introgressed from the wild donor species. This located the candidate chromosomal positions for resistance genes as well as additional segments. F2 segregating populations derived from these NILs were used to genetically map the resistance genes to one or two loci in the lettuce reference genome. Precise knowledge of the location of new Dm genes provides the foundation for marker-assisted selection to breed cultivars with multiple genes for resistance to downy mildew.

Lettuce downy mildew caused by Bremia lactucae Regel is the most economically important foliar disease of lettuce (Lactuca sativa L.). The deployment of resistant cultivars carrying dominant resistance genes (Dm genes) plays a crucial role in integrated downy mildew disease management; however, high variability in pathogen populations leads to the defeat of plant resistance conferred by Dm genes. Some lettuce cultivars exhibit field resistance that is only manifested in adult plants. Two populations of recombinant inbred lines (RILs), originating from crosses between the field resistant cultivars Grand Rapids and Iceberg and susceptible cultivars Salinas and PI491224, were evaluated for downy mildew resistance under field conditions. One hundred and sixty RILs from the Iceberg × PI491224 and 88 RILs from the Grand Rapids × Salinas RIL populations were genotyped using genotyping by sequencing, which respectively generated 906 and 746 high quality markers that were used for quantitative trait locus (QTL) analysis. We found a QTL in Chromosome 4 that is present in both Grand Rapids × Salinas and Iceberg × PI491224 populations that has a major effect on field resistance. We also found two additional significant QTLs in Chromosomes 2 and 5 in the Iceberg × PI491224 RIL population. Marker-assisted gene pyramiding of multiple Dm genes in combination with QTLs for field resistance provides the opportunity to develop cultivars with more durable resistance to B. lactucae.