Lettuce dieback is a disease that has appeared in some romaine lettuce plantings in California, including southeastern Imperial County, as well as in Yuma, Arizona. Initial symptoms on infected plants consist of extensive yellowing of the outermost leaves, with younger inner leaves usually remaining dark green in color. Dead spots on older leaves can develop into widespread areas of brown necrotic tissue. As the disease progresses, plant stunting and death can occur. Rotted roots may also be present, but whether this is caused by the pathogens or is a secondary issue is not clear. Lettuce dieback is caused by the Tomato bushy stunt virus and the closely related Moroccan pepper virus. The disease is primarily a problem on romaine lettuce, although some green leaf, red leaf and butterhead cultivars also can be affected. Symptoms are not observed in commercial plantings of crisphead lettuce, as all modern cultivars contain a resistance gene that provides a very stable source of resistance. Lettuce dieback is usually found in fields near rivers or low-lying areas on soil that drains poorly. High salinity and plant stress can enhance disease symptoms. The viral pathogens can be dispersed by contaminated soil and water and can survive for a long period of time. No vectors for Tomato bushy stunt virus and Moroccan pepper virus currently are known. Soil fumigation or crop rotation does not reduce disease severity in subsequent plantings of susceptible lettuce varieties. Romaine, leaf, and other specialty lettuce types are being developed that contain the gene conferring dieback resistance to crisphead lettuce.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.