Use of Transplants Instead of Direct Seeding to Reduce Corky Root Severity and Losses Due to Corky Root in Iceberg Lettuce. Ariena H. C. Van Bruggen, Assistant Professor, Department of Plant Pathology, University of California, Davis 95616. Vincent E. Rubatzky, Extension Vegetable Specialist, Department of Vegetable Crops, University of California, Davis 95616. Plant Dis. 76:703-708. Accepted for publication 6 February 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0703.

Severity of corky root and reduction in fresh and dry weights of heads were lower in transplanted than in direct-seeded iceberg lettuce (Lactuca sativa) in microplots infested with Rhizomonas suberifaciens. Disease severity was also lower in transplanted than in direct-seeded lettuce in field experiments in the Salinas and Santa Maria valleys in California. Increased yields were associated with the use of transplants when disease severities were high, but not when they were low. In greenhouse experiments, disease severity increased with duration of plant growth in infested soil and decreased with plant age (2, 3, 4, or 5 wk) at the time of transplanting. Use of 4- to 5-wk-old transplants are recommended for reduction of corky root and increase in lettuce yield in fields heavily infested with R. suberifaciens.