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Histopathology of A Susceptible Chrysanthemum Cultivar Infected with Fusarium oxysporum f. sp. chrysanthemi. Gary Emberger, Former graduate research assistant, Fusarium Research Center, The Pennsylvania State University, University Park 16802, Present address of senior author: Department of Plant Pathology, North Carolina State University, Raleigh 27650; Paul E. Nelson, professor of Plant Pathology, Fusarium Research Center, The Pennsylvania State University, University Park 16802. Phytopathology 71:1043-1050. Accepted for publication 26 January 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-1043.

Chrysanthemum morifolium ‘Yellow Delaware’ rooted cuttings grown in steam-treated peat-perlite-soil mix at 18–24 or 29–32 C were inoculated after 2 wk of growth with 100 ml of a conidial suspension of one of two isolates of Fusarium oxysporum f. sp. chrysanthemi. Terminal leaves of infected plants grown at soil temperatures of 29 to 32 C became chlorotic and twisted after 2 wk and eventually plants wilted and died. The lower temperatures delayed or prevented development of severe foliage symptoms and anatomical changes. Light microscopy of stained sections of stem tissue, together with results of fungus isolations showed the fungus to be systemic in the stem from the stem base to the upper limit of fungus colonization. Conidia were not observed in advance of mycelia in xylem vessels, indicating that culture-indexing should detect the fungus. Infection by either isolate resulted in plugging of xylem vessel elements with gum and pectinaceous materials, hypertrophy and hyperplasia of xylem parenchyma cells, abnormal activity of the vascular cambium derivatives, the formation of cavities within xylem tissue, and eventual colonization of phloem and cortex parenchyma cells, which resulted in their collapse. The collapse and necrosis of these cells resulted in a black streak appearing on the stems of severely wilted plants.