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Internal Rib Necrosis and Rusty Brown Discoloration of Climax Lettuce Induced by Lettuce Mosaic Virus. Stella M. Coakley, NSF Graduate Trainee, Department of Plant Pathology, University of California, Davis 95616; R. N. Campbell(2), and K. A. Kimble(3). (2)(3)Professor, and Staff Research Associate IV, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 63:1191-1197. Accepted for publication 28 March 1973. DOI: 10.1094/Phyto-63-1191.

Internal rib necrosis (IRN) and rusty brown discoloration (RBD) are pre- and postharvest disorders, respectively, that occur only in the crisp-head lettuce cultivar ‘Climax’. IRN is characterized by necrosis of parenchyma cells in the midrib; whereas, RBD is characterized by necrosis of epidermal cells. Climax lettuce was inoculated with lettuce mosaic virus (LMV) at 30, 45, 60, and 75 days of age and usually incubated in growth chambers (at 7-18 C) until harvest. Twenty-one days after inoculation IRN symptoms began to develop; by harvest time, 50-60 days after inoculation, nearly every LMV-infected head had IRN. After harvest the heads were stored at 1 C for 2 weeks and RBD developed on most of the heads with LMV. No IRN or RBD developed on virus-free Climax or on healthy or LMV-infected heads of the lettuce cultivars ‘Vanguard’ and ‘Calmar’. Beet western yellows, cucumber mosaic and alfalfa mosaic viruses did not induce IRN or RBD in Climax lettuce. LMV alone is, therefore, responsible for the development of IRN and RBD on Climax lettuce. ‘Salad Bowl’, a butter head type lettuce, develops a discoloration similar to IRN after infection by LMV.

Additional keywords: postharvest disease, ethylene damage, ammonium damage.