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Corky Root of Lettuce in California Caused by a Gram-Negative Bacterium. Ariena H. C. van Bruggen, Assistant professor, Department of Plant Pathology, University of California, Davis 95616; R. G. Grogan(2), C. P. Bogdanoff(3), and C. M. Waters(4). (2)(3)Emeritus professor, Postgraduate research assistant, Department of Plant Pathology, University of California, Davis 95616; (4)Research associate, Campbell Institute for Research and Technology, Davis, CA 95616. Phytopathology 78:1139-1145. Accepted for publication 8 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1139.

In greenhouse and field experiments, corky root of lettuce was shown to be caused by a gram-negative bacterium with one lateral flagellum. In the greenhouse, seedlings of iceberg lettuce inoculated with gram-negative bacteria isolated from corked lettuce roots developed symptoms similar to those observed in the field. Bacteria isolated from the roots of these seedlings caused corky root symptoms in subsequent pathogenicity tests. These bacteria were identical to those isolated originally from field samples. Corky root symptoms appeared initially on the taproot and later on secondary roots, as observed in root observation boxes. Small field plots were infested with bacterial suspensions at three concentrations. Even at the lowest concentration (about 108 bacteria per square meter), the plants became severely diseased, while plants in control plots remained healthy. Identical bacteria were reisolated from corked roots. Head dry weight and marketable yield were progressively decreased at increasing inoculum levels.