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Etiology

Varnish Spot, Destructive Disease of Lettuce in California Caused by Pseudomonas cichorii. R. G. Grogan, Department of Plant Pathology, University of California, Davis, CA 95616; I. J. Misaghi(2), K. A. Kimble(3), A. S. Greathead(4), D. Ririe(5), and R. Bardin(6). (2)(3)Department of Plant Pathology, University of California, Davis, CA 95616; (4)(5)Farm Advisors, Monterey County, Cooperative Extension Service, Salinas, CA 93901; (6)Plant Pathologist, Agricultural Commissioner’s Office, Monterey County, Salinas, CA 93901. Phytopathology 67:957-960. Accepted for publication 21 February 1977. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-957.

A destructive disease caused by Pseudomonas cichorii on mature head lettuce has occurred sporadically during the past 4 yr in the Salinas Valley of California. The disease, which is referred to as “varnish spot,” is characterized by dark-brown, shiny, firm, necrotic spots a few millimeters in diameter, that occur on the blades and petioles of leaves underneath the two or three outermost leaves of the head. Isolations from the lesions consistently yielded fluorescent pseudomonads. The disease was reproduced consistently within 24 to 36 hr at 23 C in mature Calmar lettuce plants that were inoculated by spraying with bacterial suspensions (10⁵ cells/ml). Significantly (P = 0.01) more infection occurred in the field on Calmar lettuce plants that were inoculated 3 wk before harvest than on those inoculated 1, 2, 4, or 5 wk before harvest. The pathogen was identified as P. cichorii by comparison of some of its biochemical and physiological characteristics with P. cichorii and other nomenspecies, and by the similar pathogenicity of the varnish spot pathogen and P. cichorii strains on lettuce and several other hosts. The pathogen was isolated from soil and root samples from fields in the Salinas Valley with a history of the disease.