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Initiation and Development of Systemic Necrosis in Relation to Virus Concentration in Tobacco Ringspot Virus-Infected Cowpea. M. C. Edwards, Graduate research assistant, Department of Plant Pathology, University of Massachusetts, Amherst 01003, Present address of senior author: Department of Plant Pathology, Cornell University, Ithaca, NY 14853; G. N. Agrios, professor of plant pathology, Department of Plant Pathology, University of Massachusetts, Amherst 01003. Phytopathology 71:7-11. Accepted for publication 23 October 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-7.

Tobacco ringspot virus-infected cowpea plants developed local lesions 34 days after inoculation of the primary leaves and systemic stem necrosis, often on one side of the upper stem, 5 days after inoculation. Internally, stem necrosis originated in the immature primary phloem fibers in the protophloem of a main vascular bundle and then spread laterally and vertically to the primary phloem fibers of the interfascicular regions. Subsequently, discoloration and necrosis appeared in the xylem, xylem parenchyma, and cambium and (in later stages) in the metaphloem, epidermis, collenchyma, and pith. The frequency and severity of systemic infection increased with inoculum concentration. Appearance, extent, and severity of systemic necrosis closely followed or coincided with the rise in concentration of virus in the stem. In plants exhibiting asymmetric symptom development virus distribution also was asymmetric.