APS Homepage

Poster: Biology & Disease Mgmt: Genetics of Resistance


Genetic characterization of quantitative resistance to Bremia lactucae, the causal organism of lettuce downy mildew.
L. Parra (1), I. Simko (2), R. Michelmore (3) (1) UC Davis- Genome center, U.S.A.; (2) USDA-ARS, U.S.A.; (3) UC Davis-Genome Center, U.S.A.

Lettuce (Lactuca sativa) is one of the most valuable vegetable crops in the United States. Downy mildew (DM), caused by Bremia lactucae, is the most important foliar disease of lettuce worldwide, which decreases the quality of the marketable portion of the crop. The use of resistant varieties carrying dominant genes (Dm genes) is the most effective method for controlling this disease. However, the high pathogen variability leads to the defeat of cultivars containing resistance genes by new isolates of the pathogen. Some lettuce varieties such as Iceberg and Grand Rapids exhibit field resistance that cannot be attributed to Dm genes. This resistance trait, which is manifested in adult stages of development, has shown to be quantitatively inherited. To determine the genetic basis of field resistance, two populations of recombinant inbred lines originated from crosses between a field resistant cultivar and a susceptible cultivar (Grand Rapids x Salinas and Iceberg x PI491224) have been evaluated for DM severity and genotyped using Genotyping-By-Sequencing for quantitative trait loci (QTL) analysis. Marker-assisted gene pyramiding of multiple Dm genes in combination with QTLs for field resistance provides the opportunity for more durable resistance to B. lactucae.