U.S. Department of Agriculture breeders have released three new lines of leaf lettuce that are resistant to corky root, a serious lettuce disease.

Corky root is caused by the soilborne bacterium, Sphingomonas suberifaciens, according to a news release.

Infected roots enlarge and develop yellow to brown lesions and longitudinal cracks, taking on a corky appearance.

Infected roots can't absorb water as readily, resulting in stunted heads and yield losses.

Although cultural techniques and fumigation can help manage the disease, they are costly.

Agricultural Research Service geneticist Beiquan Mou, who's based in Salinas, Calif., developed the lines—one red leaf lettuce and two green leaf lettuces.

He crossed the disease-resistant iceberg lettuce Glacier with popular leaf lettuce varieties.

The new lines have similar plant weights to commercial cultivars and show little or no tipburn in trials.

They have not been tested in production areas outside of the Salinas Valley.

The new lines can be used commercially for fresh lettuce production or to develop new cultivars.