Identification of major quantitative trait loci controlling field resistance to downy mildew in cultivated lettuce (Lactuca sativa)

Authors: PARRA, LORENA - University Of California; Simko, Ivan; MICHELMORE, RICHARD - University Of California

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2020
Publication Date: 3/21/2021
Citation: Parra, L., Simko, I., Michelmore, R.W. 2021. Identification of major quantitative trait loci controlling field resistance to downy mildew in cultivated lettuce (Lactuca sativa). Phytopathology. 111(3):541-547. https://doi.org/10.1094/PHYTO-08-20-0367-R.
DOI: https://doi.org/10.1094/PHYTO-08-20-0367-R

Interpretive Summary: Lettuce downy mildew cause by Bremia lactucae primarily affects the foliar tissue thus reducing the quality and marketability of the crop; under severe infection, it can result in major yield losses. Lettuce plants can become infected by B. lactucae at any developmental stage, but often show more disease severity in the field when they are close to harvest maturity. Downy mildew is managed in commercial production by the combined use of fungicides and agronomic practices that minimize conditions favorable for the pathogen, such as excess irrigation. The use of fungicides, however, is constrained by high costs and the development of fungicide-insensitive strains of the pathogen. The use of resistant lettuce varieties containing resistance genes is the most viable method for controlling this disease. We found a major resistance locus in chromosome 4, and two additional significant QTLs in chromosomes 2 and 5. Marker-assisted gene pyramiding of multiple resistance genes in provides the opportunity to develop cultivars with more durable resistance to B. lactucae.

Technical Abstract: 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, but the high pathogen variability leads to the defeat of plant resistance conferred by Dm genes only by new isolates of the pathogen. Some lettuce cultivars exhibit field resistance that is only manifested in adult plants. Two populations of recombinant inbred lines originated from crosses between field resistant cultivars Grand Rapids and Iceberg and susceptible cultivars Salinas and PI491224 were evaluated for downy mildew resistance under field conditions. We found a major QTL in chromosome 4 that is present in both Grand Rapids × Salinas and Iceberg × PI491224 populations, and two additional significant QTLs in chromosomes 2 and 5 in Iceberg × PI491224. 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.