PHOTO COURTESY OF CARLOS PAES/SXC.HU.
The precut lettuce industry has suffered instability in recent years due to highly publicized concerns about food safety, and less publicized grower challenges with crop-devastating diseases. Scientists from a variety of disciplines have been busy addressing these issues. Unfortunately, one of the remedies actually turned out to be the cause of another industry problem: spoilage during transportation.
Short shelf life in lettuce isn't inevitable; in some cases, it has been the result of breeding for disease resistance. One disease in particular, lettuce dieback disease, has frustrated romaine growers in California and Arizona since the early 1990s. California and Arizona are the two largest lettuce-producing states, together accounting for over 95 percent of the lettuce grown in the United States. Lettuce dieback disease often leads to crop loss of 60 percent or more. Meanwhile, verticillium wilt has been attacking California's iceberg crops in the last decade. It seems the country's salad bowl is under assault.
Lettuce dieback disease is caused by two closely related, soilborne viruses from the family Tombusviridae: tomato bushy stunt virus (TBSV) and lettuce necrotic stunt virus (LNSV). Symptoms include mottling, yellowing and death of older leaves, and the stunting and eventual death of plants. Previous studies have provided no evidence that either chemical treatment or rotation with non-host crops can effectively reduce, remove or destroy the virus in infested soil. This makes genetic resistance the only option for disease control. Verticillium wilt is caused by the soilborne pathogenic fungus Verticillium dahliae (see last month's column for more information: http://www.growingmagazine.com/article-8122.aspx).
PHOTO COURTESY OF JAVIER RAMOS/SXC.HU.
Preventing disease causes problems
The focus of research at the USDA's Salinas station has been developing disease-resistant cultivars. In recent years, scientists have discovered that the source of the resistance genes that prevent lettuce dieback disease and verticillium wilt may also possess genes for short shelf life. The sources of such genes used in breeding may be cultivars, landraces or wild species. The cultivars with poor shelf life start breaking down immediately after processing, ruining a crop before it even hits market.
Dr. Ryan Hayes and Dr. Ivan Simko conducted research to determine how to combine good shelf life and disease resistance into a single cultivar at the ag research station in Salinas. What they developed were four lines with shelf life around 30 days longer than the short shelf life varieties. SM09A and SM09B are eighth-generation romaine breeding lines with resistance to lettuce dieback disease. RH08-0472 and RH08-0475 are ninth-generation iceberg breeding lines with resistance to verticillium wilt caused by V. dahliae. The shelf life of these new cultivars matches that of common commercial cultivars and rivals those that lack resistance to lettuce dieback and other diseases.
In the romaine types, resistance to dieback disease stems from PI 491224, a primitive romaine-type lettuce from Greece that is not used commercially in the U.S. PI 491224 is highly perishable when processed for salad. Crossing the Grecian landrace with more modern varieties (with 'Darkland' to form SM09A and with 'Green Towers' to produce SM09B) produced the cultivars with the necessary shelf life. 'Darkland' is a romaine cultivar with a strong bolting tolerance, smooth midribs and very dark green color developed by Central Valley Seeds, Inc. (Salinas, Calif.). 'Green Towers' is a large and tall romaine cultivar with an erect habit and intense dark green color developed by Harris Moran Seed Co. (Modesto, Calif.). The shelf life of 'Green Towers' and 'Darkland' is acceptable to the industry; however, both cultivars are highly susceptible to the dieback disease.
Putting cultivars to the test
In replicated field trials, both SM09A and SM09B showed complete resistance to dieback. To ensure disease resistance, the researchers tested the new cultivars in fields infested with TBSV and LNSV. Using this approach, single plant selections were made through seven generations of self-pollinations. Hayes and Simko also relied on marker-assisted breeding to ensure they were on the right track.
Testing shelf life needed to be both lab scale and practical. In Salinas, Hayes worked with salad processing companies to develop a lab scale method to test the salad-cut lettuce. Testing the lettuce in "modified atmosphere packaging," which allows oxygen and carbon dioxide to pass between the inside and outside of the package, the researchers rated the lettuce on a scale of 0 percent decay to 100 percent decay. They found slower decay in SM09A and SM09B compared with other dieback-resistant romaine varieties.
Dr. Yaguang Luo, of the USDA's ARS station in Beltsville, Md., assessed the arrival quality of whole heads of several cultivars of lettuce. Twenty-four heads per breeding line or cultivar were harvested on August 10, 2010, packed in cartons, vacuum-cooled and transported in a refrigerated truck to Beltsville. Forced to exclude RH08-75 due to an insufficient number of heads, Luo assessed Salinas 88, an iceberg variety similar to RH08-0472 and RH08-0475, as well as cultivars that have poor shelf life when processed into salad. According to Hayes, the short shelf life cultivars he and his colleague tested are generally fine when stored as whole heads. On arrival, the heads were stored at 5 degrees Celsius. On August 24, 2010, three trained evaluators rated each head for decay, using a five-point scale.
As consumer demand for healthy convenience foods increases, there is potential for growth of lettuce as a value-added product. Key to the growth of the cut-lettuce market is the ability to protect that product and deliver it in good condition. "In the cultivars and breeding lines we tested with Dr. Luo, short shelf life only occurs when the lettuce is processed and packaged into salad," explains Hayes. "This trait is so important for California and Arizona lettuce producers and processors." Hayes and Simko released the new varieties of romaine in 2010 and the improved varieties of iceberg in 2011.
The author is a freelance writer based in Massa- chusetts and a monthly contributor to Growing.
In the May Growing South issue, there is an error in the article "A Growing Market with Avocados." The price per pound figures on page B2 should have been in cents, not dollars. The 1971-72 figures should read, "at a price per pound of 47.58 cents." The 2010-11 and (2009-10) price per pound should be 152.10 cents (75.35 cents). We apologize for any confusion this may have caused.