Chrysanthemum phloem necrosis: onset of systemic ultrastructural changes

Vascular tissues in root and shoot apices, lateral buds, leaves, and petioles of florists' chrysanthemum Dendranthema grandiflora cv Pink Marble affected by chrysanthemum phloem necrosis were studied by light and electron microscopy for early evidence of disease. Specimens were taken from field-, glasshouse-, and growth-chamber-grown plants. Additionally, vascular tissues of initially uninfected cv Fanfare scions were examined after they were inoculated by grafting to infected Pink Marble stocks. Ubiquitous cytopathological features of affected tissues included (i) strongly autofluorescent clusters of obliterated young phloem; (ii) severely hypertrophied, resembling nacre, and/or collapsed mature sieve-tube elements and protophloem cells; (iii) prominent cell-wall ingrowths, similar to those of transfer cells, in phloem parenchyma and companion cells; (iv) distinct apposition formation along phloem parenchyma walls; (v) end-wall disruption of immature sieve-tube elements; and (vi) abnormally dense and darkly stained cell protoplasts. Intracellular bodies, some of which resembled mycoplasma-like organisms, were seen consistently in infected plants but infrequently in anatomically affected cells.