Symptoms of corky root as seen above ground are poorly formed, discolored heads or tops with uneven, stunted growth depending upon the severity and timing of the infection.

Above ground, the plant develops poorly because the root system is unable to support the normal vegetative development. Severely infected plantings remain small and unmarketable (Fig. 1). In those cases where the infection occurs relatively late, the above ground symptoms may be minimal. At times, the plant may compensate for loss of a tap root with extended feeder roots and maintain sufficient root area to support the tops.

Below ground, the roots appear stunted, darkened in color with lesions and cracking (Fig. 2). In severe cases, roots are severely stunted and deformed with marked darkening, internal necrosis and, at times, with an army green coloration (Fig. 3.). The exterior tap root surface becomes pitted or corky.

Figure 2.Stunting, deformation, pitting, corkiness
and discoloration due to corky root disease.



Figure 3. Corky root susceptible Parris Island leaf lettuce (left),
and resistant Ultega leaf lettuce from side-by-side rows.

Management of corky root disease in lettuce revolves around the use of resistant varieties where available and cultural practices. Some varieties are available commercially with resistance to corky root and the USDA lettuce breeding program in Salinas has an active effort ongoing to develop varieties with resistance to this disease. The Dutch varieties Ultega and Athena (Arden Zaden Seed Co.) showed marked resistance in corky root infested fields in Santa Barbara County in 2000. A number of other resistant varieties are also available.

Cultural practices for corky root management involve rotating lettuce crops, and soil and water management practices to improve aeration and drainage. Lettuce residue management is also important, and management of other residues may also contribute to the chances of infestation by corky root. The causal bacteria will survive for three years even in the absence of lettuce, but fields may be managed to minimize disease pressure. Excess moisture and poor soil aeration may contribute to corky root development, and soil ammonium ion levels may also be a factor. Compacted or poorly drained soils and extended or frequent irrigation also contribute to disease development.

It may be that a number of these factors interact in the field to create favorable conditions for disease development. Higher organic matter levels normally improve drainage and aeration in compacted soils. But the higher organic matter may affect the relationships between soil nitrogen dynamics and moisture to favor conditions for disease development. Organic growers or growers using reduced tillage to grow leafy crops should be especially aware of these factors. In these circumstances, high accumulated organic matter in surface soil may increase corky root incidence. Growers, working with fields with a history of corky root, should take care to rotate out of lettuce, allow time for residues to decompose completely, carefully manage irrigation timing and duration, and use resistant varieties wherever possible. Anecdotal information suggests that the preceding crop may also affect corky root disease development. In side-by-side plantings in late summer 2000, leaf lettuce following cilantro showed little corky root, while leaf lettuce following spinach suffered nearly complete loss to corky root.

Dr. van Bruggen has evaluated several potential bacterial biocontrol agents for effectiveness in controlling corky root. In research, financed by the Iceberg Lettuce Advisory Board, Dr. van Bruggen reported significant differences in effectiveness among bacterial biocontrol strains, but no effect of application technique of these biocontrol agents. Although one or more of the bacterial strains showed a significant effect for controlling corky root disease, none of the biocontrol agents are available commercially at this time. For more information on corky root management in lettuce also see the UC Publication #3307, titled "Integrated Pest Management for Lettuce and Cole Crops," available from this office (805.934.6240) or online at http://www.ipm.ucdavis.edu/PMG/crops-agriculture.html

The USDA Agricultural Marketing Service has published a Fruit and Vegetable Market News User’s Guide to assist anyone interested in using data collected through their extensive Market News Service network. Of particular interest to many are the price and volume reports for fruit, vegetables, and ornamentals for a number of shipping points and terminal markets in the US. Data is also available for several foreign markets for the past several seasons.

The guide describes the programs, reports, and services available through Market News Service and provides details of reading and interpreting these reports. The publication lists field offices that collect market information as well as those which also provide telephone-recorded information for daily updates. The guide provides addresses of field offices where hard copy reports may be ordered, or in some cases subscriptions are available for hard copy reports or daily reports via fax. Market reports are also available via the Internet at the AMS web site at:

Some historical fruit, vegetable and ornamental price and volume data is available on the AMS web site. Additional historical records of the AMS data—as far back as 1990—are available through a web site maintained by the University of Florida at:

I have a few copies of the guide available through this office or copies may be ordered or downloaded from the USDA-AMS web site.