Unsinkable Icebergs

Unsinkable Icebergs

Researchers at USDA’s Agricultural Research Service  (ARS) in Salinas, CA, are on a roll this year, as they’ve developed several new parent iceberg lettuces with the ability to ward off such destructive disorders as Verticillium wilt, corky root, and mosaic virus. Here’s a closer look at these promising developments, which should prove quite a boon to growers in the coming years.

Verticillium Wilt Resistance

Iceberg lettuce is renowned for its big, firm, round heads, so it was with no small measure of dismay when a dozen years ago growers in Coastal California started seeing their beauties collapsing like deflated balls. The cause was determined to be a soil-dwelling, root-rotting fungus called Verticillium dahlie, which causes the disease Verticillium wilt.

Symptoms of the disease first appear on the basal leaves as areas of chlorosis and angular necrotic lesions along the leaf margins prior to wilting, says Ryan Hayes, an ARS research geneticist. Other key foliar symptoms include stunting and defoliation.

The disease symptoms appear first on the older leaves, progressing inward and eventually leading to the plant’s death. What makes the disease even more insidious is that plants that succumb to Verticillium dahlie produce large quantities of microsclereotia that can persist in the soil for 10 to 15 years. The fungus can also infect and kill hundreds of other kinds of plants, including strawberries and tomatoes.

Hayes teamed up with Krishna Subbarao of the University of California-Davis to develop three parent lettuces with Verticillium wilt resistance. They crossed the Pacific variety with La Brillante, a Batavia-type lettuce cultivar with resistance to race 1 of Verticillium dahlie. Batavia-type is a primitive version of iceberg, says Hayes, a predecessor to today’s iceberg. They have had requests for seed from more than a half-dozen seed companies, who he expects will cross the parent lines with consumer-friendly lettuces. “It will be nice to get Vert resistance in commercial varieties,” says Hayes. “And we’ll get there, I have no doubt about that.”

Corky Root And Mosaic Virus

These two microbial enemies are not as devastating as Verticillium wilt in that they don’t kill the lettuce plants, but they cause plenty of damage nonetheless. For example, in California and Florida fields severely infested with corky root, yield losses from reduced head size can reach 30% to 70%, says Hayes. Corky root has been observed in all the major lettuce-producing regions in the world, including Western Europe, Australia, and New Zealand, as well as North America. Its name derives from the fact that the lettuce roots develop ugly, yellow-to-brown lesions that harden to a cork-like texture. It is caused by the bacterium Sphingomonas suberifaciens.

Lettuce mosaic is generally not nearly such a problem as corky root, as it is generally controlled in California and Arizona through seed indexing, i.e. planting virus-free seed. But there is only one cultivar with resistance to both mosaic virus and corky root, and it is protected by patent, which greatly restricts breeding research. Like corky root, mosaic results in stunted growth. In addition, the leaves develop an unattractive mottling, hence its name. The virus is generally spread by green peach aphids.

Hayes worked with fellow ARS plant geneticists Edward J. Ryder, who is now retired, and Beiquan Mou, who was the project leader, to develop a total of seven kinds of parent iceberg lettuces that shrug off attack by the two microbial enemies. The parent plants are the first publicly available iceberg lettuce parents with resistance to both corky root and lettuce mosaic. The lettuces were developed specifically for California climates and soils, and the sooner they are crossed with consumer-friendly icebergs, the better. “I know growers are really interested in resistant varieties,” says Hayes.

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