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Head of iceberg lettuce.
Iceberg lettuces bred several years ago by ARS plant geneticists still offer the best-available natural resistance against attack by Mirafiori lettuce big vein and lettuce mosaic viruses. Image courtesy Produce Marketing Association.

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Superior Lettuces Fend Off Two Destructive Viruses

By Marcia Wood
February 12, 2007

Iceberg lettuce ranks as one of America's top-five favorite veggies. However, this delicately flavored, slightly sweet crisphead and its relatives—the romaine of Caesar salads, the softer textured leaf lettuces, or the creamy butterheads like Boston and bibb—are vulnerable to attack by an impressive array of stealthy viruses and other natural enemies.

But five kinds of superior iceberg lettuces, developed several years ago by Edward J. Ryder of the Agricultural Research Service (ARS), are today still holding their own against attack by two of these daunting villains: big vein virus and lettuce mosaic virus. A world-renowned lettuce breeder, Ryder, now retired, did the work while based at the ARS Crop Improvement and Protection Research Unit in Salinas, Calif.

In 2004, the plants became the first publicly available iceberg lettuces to boast resistance to both diseases. That's why lettuce breeders and seed companies in California and elsewhere were quick to request samples of the tiny black seeds.

Dual-resistance enhances survival because a lettuce field can easily be besieged by both viruses at once. That's according to research horticulturist James D. McCreight, who is in charge of ARS research at Salinas.

Equipping lettuce plants with genes that enable strong, natural resistance is still the most economical, eco-friendly way to defend vulnerable plants from the viral diseases.

Bert Robinson and Edward Ryder harvest seeds from a potted Lactuca virosa plant in a greenhouse: Link to photo information
ARS technician Bert J. Robinson (left) and plant geneticist Edward J. Ryder (retired) harvest seeds of Lactuca virosa—a wild relative of cultivated lettuce—for greenhouse and field tests. Click the image for more information about it.

Lettuce big vein gets its name from the unhealthy, enlarged appearance of veins in infected lettuce leaves. These lettuces may be bushy-looking or undersized.

The likely culprit? Mirafiori lettuce big vein virus, which makes its way to lettuce roots via a soil-dwelling, funguslike microbe.

Lettuce mosaic, caused by a virus of the same name, results in stunting and unattractive mottling. Green peach aphids can spread the virus as they move about a lettuce field, sipping plant juices.

Read more about the research in the February 2007 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.