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Histopathology of Chrysanthemum Stems Artificially Inoculated with Erwinia chrysanthemi. Barbara White Pennypacker, Research Assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; Paul E. Nelson(2), and Robert S. Dickey(3). (2)Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; (3)Professor, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 64:1344-1353. Accepted for publication 18 May 1974. DOI: 10.1094/Phyto-64-1344.

Chrysanthemums, cultivar ‘Giant #4 Indianapolis White,’ were inoculated in a young leaf midrib with a chrysanthemum isolate of Erwinia chrysanthemi and placed in a mist chamber. The stems of these plants were examined histologically at selected intervals during disease development. The pathogen caused extensive breakdown of xylem parenchyma, pith, and phloem tissue, as well as separation of cells in the xylem and pith. Cell walls in the affected areas were frequently thinner than the walls of cells in nonaffected areas. Erwinia chrysanthemi moved vertically in the host through xylem vessel elements and some limited vertical movement probably occurred in the pith tissue. Vascular plugs of pectic substances and bacterial masses occasionally were present in the xylem vessel elements, but mechanical rearrangement of xylem vessel elements and subsequent interruption of the transpiration stream, appeared to be a major cause of the wilt symptom.