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This disease has appeared in some romaine plantings in southeastern Imperial County,
California and in Yuma, Arizona over the past several 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 will 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 are known. Soil fumigation or crop rotation
does not reduce disease severity in subsequent plantings of susceptible lettuce
varieties. Active research is in progress in California to develop commercial romaine
varieties that will have resistance to these two soil-borne viruses.
To contact Mike Matheron go to: matheron@ag.arizona.edu.
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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
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