Identification of Races and Host Resistance to Lettuce Downy Mildew

Oct 2023 to Sep 2025

Collaborating Researcher(s)

Dr. Richard Michelmore, University of California, Davis

Project Supporter(s)

USDA/Arizona Department of Agriculture Specialty Crop Block Grant Program

Lettuce downy mildew caused by Bremia lactucae is an important disease that can significantly reduce the quality and marketability of lettuce. This disease is controlled using fungicides, by modifying the environment to reduce humidity, and by planting varieties containing downy mildew resistance (Dm) genes. B. lactucae evolves rapidly resulting in new races leading to the breakdown of resistance in downy mildew resistant (DMR) lettuce varieties, and fungicide resistance, threatening the available control methods. To ensure that breeders are developing varieties with resistance against virulent phenotypes currently in Arizona, they must know what phenotypes occur in Arizona. This project will help to identify the B. lactucae phenotypes in the Yuma growing region, and make this information widely available, and to screen commercial cultivars to identify cultivars resistant to the current B. lactucae population.

You can help us learn more about the Arizona pathogen population by providing leaf tissue for analysis by Dr. Michelmores' laboratory at UC Davis. The results of this analysis will be publicly available. Samples can be dropped off at two Yuma locations or shipped directly to UC Davis. Please download the 2023 sample submission form and shipping instructions attached under the "More Information" section.

Yuma sample drop off locations:

1. Yuma Agricultural Center (6425 W. 8th Street)

2. Yuma Center of Excellence for Desert Agriculture (899 E. Plaza Circle, Suite 2)