Survey of Lettuce Downy Mildew (Bremia lactucae) in Arizona to Identify Race and Fungicide Resistance

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. In order to ensure that breeders are developing varieties with resistance against virulent phenotypes currently in Arizona, they must know what phenotypes occur in Arizona.

The current project objectives are to determine which resistance genes protect cultivars against Bremia lactucae in Yuma by planting field trials with cultivars containing known resistance genes; to identify pathogen races by sending samples to Richard Michelmore at the University of California, Davis for race typing; and to evaluate commercial desert lettuce cultivar performance against the current population of B. lactucae in a field trial to confirm what cultivars are resistant to the current population.

Long-term objectives of this project are to establish a program for evaluating the B. lactucae population in Arizona resulting in durable DMR varieties to be developed. You can help us learn more about the Arizona pathogen population by providing leaf tissue for analysis by Dr. Michelmore’s laboratory at UC Davis. The results of this analysis will be publicly available. Samples can be dropped off at the two Yuma locations listed below or shipped directly to UC Davis. Download the sample submission form and shipping instructions by clicking on the link in the attachment section.

Click Here to Reommend Field Trial Entries

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)