LINDELL BEACH, B.C. – Two new lines of romaine lettuce have been launched in California that promise longer shelf life and resistance to dieback disease.
Dieback disease is caused by two viruses: tomato bushy stunt virus (TBSV) and lettuce necrotic stunt virus (LNSV).
It causes mottling, yellowing, decay of the older leaves, stunting and ultimately plant death. It affects romaine and leaf-type lettuce. The symptoms usually appear when the plant is six to eight weeks old, rendering it unmarketable.
The new lettuce lines are good news for California and Arizona, where 95 percent of lettuce is grown in the United States.
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Dieback has been affecting the main producing areas of both states since the early 1990s. It can lead to losses of 60 percent or more.
TBSV and LNSV can survive in infested soil and water and are not effectively reduced with chemical treatment or crop rotation. Consequently, genetic resistance appears to be the only real option for disease control.
“There is still a lot that is unknown about the interaction between the TBSV virus and lettuce,” said U.S. Department of Agriculture researcher Ivan Simco.
“We know that the virus infects lettuce through the root system. We don’t know if resistance in lettuce is based on a prevention of infection by the roots (themselves) or if it is based on limiting the effect of a viral infection inside the plant.”
Early research discovered lines of lettuce that were resistant to the disease but they had a short shelf life after being processed into fresh cut salad.
Two romaine lettuce breeding lines were eventually developed that met both criteria of disease resistance and acceptable shelf life.
“The dieback resistance in both breeding lines is derived from PI 491224, which is a primitive, romaine-type lettuce accession collected in Greece that is not used for commercial production in the United States,” Simco said in a report of his team’s findings.
The development of the successful breeding lines came from selections of Green Towers, Darkland and PI 491224.
Simco said the shelf life of Green Towers and Darkland is acceptable to the industry but both are highly susceptible to the disease.
With the introduction of the influence of the PI 491224 line, researchers were able to produce romaine lettuce that met the criteria.
Researchers still don’t know what conditions lead to plant infection: temperature, length of day, ground water levels, abiotic stress or the interaction of several factors?
“So we are working on detection methods to quantify the virus level in soil and plant material samples. Then we can test plants from different growing conditions and study the various relationships between the virus and the lettuce.”
Seed samples are available for research purposes, including the development and commercialization of new cultivars.
The USDA will provide germplasm to industry breeders for crosses with their own material or for seed production.
Simco said 12 seed companies have requested seeds and plan to test the lines, SM09A and SM09B, in their field trials.
Depending on the results, they will either increase seeds for commercial cultivation, use them in their respective breeding programs or both.
Agriculture Canada says lettuce dieback disease is not a problem in Canada.
