Fine structural observations on substances attributable to Ceratocystis ulmi in American elm and aspects of host cell disturbances

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Abstract
Plugging of certain vessels may occur in elm shortly after inoculation with the Dutch elm disease pathogen, Ceratocystis ulmi (Buism.) C. Moreau. Plugging components include fibrillar material of varying density and fungal cells traceable mostly to inoculated spores. Some material is similar to fungal cell contents, and indications of extrusion of the latter through ruptured or unruptured walls were obtained. Other material is also attributable to disintegrating fungal walls. Radioautographs obtained from samples treated with [6-3H]thymidine indicate significant labeling of fungal cell contents and of similar material, free.Similar fibrillar material, some labeled, is present within pit membranes, in adjacent parenchyma cell walls, and in periplasmic areas associated with retraction of the plasmalemma and with other cytoplasmic disturbances. Host vessel walls are also altered in the presence of some fibrillar material but apparently release only limited amounts of disintegration products into vessels.The possible implications of these observations are discussed in relation to current hypotheses on wilt diseases.

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