ARS Releases New Leaf Lettuce Breeding Lines with
Corky Root Resistance By
July 7, 2009
Three new leaf lettuce breeding lines with resistance to corky root, a
serious disease of lettuce, have been released by the
Agricultural Research Service (ARS).
Corky root is caused by a bacterium called Sphingomonas
suberifaciens. It lives in the soil and attacks the plant's roots, causing
them to enlarge and develop yellow to brown lesions and longitudinal cracks,
taking on a cork-like appearance.
Once infected, the roots are unable to effectively absorb water and
nutrients, resulting in smaller lettuce heads and yield loss. Cultural
practices and fumigation techniques used to treat the disease are costly and
labor-intensive. Therefore, developing lines with genetic resistance is still
the most common and preferred method to combat the disease.
Additionally, farmers have increased production of this type of
lettuce, further highlighting the need for leaf lettuces with resistance to
Mou developed three leaf lettuce breeding lines--one red leaf lettuce and
two green leaf lettuces--with resistance to the disease. This follows his and
colleagues' previous work in developing corky-root-resistant iceberg lettuces.
Mou crossed disease-resistant iceberg lettuce Glacier with popular leaf lettuce
varieties. The offspring were tested at the
Crop Improvement and Protection Research Unit's research farm in Salinas,
The new breeding lines are the latest to be released by the Salinas
lab. Their plant weight is comparable to or higher than commercial cultivars.
The breeding lines also showed little to no tipburn in test trials. Tipburn, a
physiological disorder common in lettuce, occurs when calcium is not adequately
transported to the young, growing leaves. It causes brown or black spots that
can coalesce and rot. This spoils the appearance and reduces the shelf life of
The adaptation of these breeding lines to other lettuce-growing
regions has not been evaluated. However, leaf lettuce has less environmental
requirements than iceberg lettuce, allowing leaf lettuce to adapt to a wider
geographical area. The new lines can be used commercially for production of
fresh lettuce or to develop new cultivars.
ARS is the principal intramural scientific research agency of the
U.S. Department of Agriculture.