Features Production Research
New lettuces resist corky root, mosaic virus

Crispy, crunchy, and low in
calories, iceberg lettuce commands a starring role as North America’s
favourite lettuce for salads, burgers, and even wraps. In the lettuce
field, however, iceberg faces attack from an array of microbes that are
harmless to humans but deadly to this vegetable.

Crispy, crunchy, and low in calories, iceberg lettuce commands a starring role as North America’s favourite lettuce for salads, burgers, and even wraps. In the lettuce field, however, iceberg faces attack from an array of microbes that are harmless to humans but deadly to this vegetable. That’s why plant geneticist Edward J. Ryder, now retired, and colleagues Ryan J. Hayes and Beiquan Mou at Salinas, California, developed seven kinds of parent iceberg lettuces that shrug off attack by two microbial enemies. Corky root causes lettuce roots to develop ugly, yellow-to-brown lesions that harden to a corklike texture. Corky-root-infected plants may produce stunted heads 30 to 70 per cent smaller than normal. The bacterium Sphingomonas suberifaciens causes the disease. Lettuce mosaic is caused by a virus of the same name. It results in stunted growth as well as unattractive mottling of leaves. Green peach aphids, about one-eighth-inch long, can spread the virus from an infected plant to an uninfected one as they move about a lettuce field, sipping plant juices. The parent plants debuted in 2006 as the first publicly available crisphead lettuces that come equipped with powerful natural resistance to both microbes. The researchers’ work isn’t over yet. For example, Mou and co-investigator Carolee T. Bull, a plant pathologist at Salinas, are searching for corky-root-resistance genes that are different from the one already working inside the new parent lettuces. Why the need to hunt for more genes? Giving lettuce additional corky-root-resistance genes may bolster the plant’s defences, according to Mou.