The U.S. Department of Agriculture has released seven iceberg lettuce breeding lines with resistance to bacterial leaf spot.

The disease, caused by the bacteria Xanthomonas campestris pv. vitians, causes block spots on lettuce leaves, according to a USDA news release. Eventually, the spots merge to form papery, brown or black patches.

Harvest crews have to peel and remove the damaged leaves, resulting in less profitable smaller heads.

It can attack both leaf and head lettuce varieties and is prevalent in Califorina and Arizona.

The organism thrives in wet, cool conditions and is the most serious in early spring and late fall during California's rainy season.

Agricultural Research Service geneticist Ryan Hayes says creating disease-resistant breeding lines is the most efficient and cost-effective tool to manage the disease.

Hayes, along with geneticist Edward Ryder, who has since retired, and plant pathologist Carolee Bull, developed the breeding lines at the ARS Crop Improvement and Protection Research Unit in Salinas, Calif.

Similar in appearance to the famous 'Salinas' variety created by Ryder, these lines are the first western shipping-type icebergs with commercially useful levels of resistance to bacterial leaf spot, according to the news release.

ARS will provide the releases to seed companies, which in turn will use them to develop commercial iceberg varieties.