Lettuce Dieback (January 20, 2016)

What is lettuce dieback? This is a disease that has appeared in some romaine plantings in southeastern Imperial County, California and Yuma, Arizona in past years. Lettuce dieback occurs in other lettuce production regions in California as well. Initial symptoms on infected plants consist of extensive yellowing of the outermost leaves, with the younger inner leaves usually remaining dark green in color. Dead spots on older leaves can develop into extensive 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 Lettuce necrotic stunt virus. The disease is primarily a problem on romaine lettuce, although some green leaf, red leaf and butterhead cultivars can be affected as well. To date, symptoms have not been observed in commercial plantings of crisphead lettuce. Lettuce dieback is usually found in fields near rivers or low-lying areas that drain poorly. High salinity and plant stress can enhance lettuce dieback 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 Lettuce necrotic stunt virus currently are known. Soil fumigation or crop rotation does not reduce disease severity in subsequent plantings of susceptible lettuce varieties. Breeding lines have been developed in California that have genetic resistance to lettuce dieback.