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Disease Detection and Losses

Yield Losses to Iceberg Lettuce Due to Corky Root Caused by Rhizomonas suberifaciens. R. Douglas O’Brien, Department of Plant Pathology, University of California, Davis 95616; Ariena H. C. van Bruggen, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 82:154-159. Accepted for publication 10 September 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-154.

Plant growth and yield variables of closely related resistant and susceptible lettuce cultivars (440-8 and Salinas, respectively) were compared in microplots at Davis, CA, in soil infested with Rhizomonas suberifaciens, the causal agent of corky root of lettuce. To validate the results for large scale lettuce production, the same variables were compared for 440-8 and Salinas in fumigated soil and soil naturally infested with R. suberifaciens at Salinas, CA. Most plant growth variables for 440-8 in infested microplots were not significantly different from those for 440-8 or Salinas in uninfested microplots. The most important effect of corky root on Salinas lettuce was to reduce fresh weights of shoots and marketable yield. Reductions in shoot dry weight and plant area were less significant. Marketable yield losses ranged from 34 to 92%, depending on season and location, with the highest yield loss in the warmest growing season. Significant yield reductions did not occur until 20–30 days after the appearance of the first symptoms. Taproots affected by corky root were wider, had a lower moisture content, and higher dry weight. A linear regression model predicted shoot fresh weight based on the amount of disease at the nine-leaf stage in microplots. Data from the large scale field experiment did not fit to the same model, but final yield loss fell within the range of losses obtained in microplots.