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Histopathology of a Symptomless Chrysanthemum Cultivar Infected by Erwinia chrysanthemi or E. carotovora subsp. carotovora. Barbara White Pennypacker, Research associate, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; Constance M. Smith(2), Robert S. Dickey(3), and Paul E. Nelson(4). (2)(3)Former graduate assistant and professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, (2)Present address: Biochemicals Department, E. I. du Pont de Nemours and Co., Wilmington, DE 19898; (4)Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 71:141-148. Accepted for publication 20 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-141.

A histological study was made on rooted cuttings of Chrysanthemum morifolium ‘White Marble’ infected with either Erwinia carotovora subsp. carotovora (E. c. subsp. carotovora) or E. chrysanthemi to determine why infected plants of that cultivar remain symptomless under normal growing conditions. Both Erwinia species were distributed rapidly in the host vascular system and caused breakdown of the xylem parenchyma cells and formation of hypertrophied, hyperplastic and chromophilic xylem parenchyma cells around the infected vascular bundle. Vascular plugging also was present in the later stages of host reaction. Histochemical tests revealed suberin in the host cells surrounding infected vascular bundle and a decrease in pectic material in infected xylem parenchyma cells. Host reaction to isolates of E. c. subsp. carotovora was more intense than that caused by E. chrysanthemi, E. c. subsp. carotovora, but not E. chrysanthemi, was restricted to the initially infected vascular bundle. Suberin was present earlier and in greater amounts in plants infected with E. c. subsp. carotovora. The host reaction of hypertrophied, hyperplastic, and chromophilic cells around the infected vascular bundle and the formation of suberin may explain, in part, the lack of symptoms in cultivar White Marble infected with E. c. subsp. carotovora and E. chrysanthemi.