Pathological anatomy of the Dutch elm disease. Distribution and development of Ceratocystis ulmi in elm tissues

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Abstract
Extensive histological studies carried out, during 3 years, on more than 200 white elms (Ulmus americana L.), young and old, artificially or naturally inoculated with Ceratocystis ulmi (Buis.) C. Moreau, add more precision on the spread and location of the fungus in host tissues and the site of the pathogenesis. With new and improved techniques, characteristic spores and hyphae of the pathogen were clearly observed in vessels and never in other tissues and were sharply differentiated from cell contents and artifacts. Double bordered pits or the thin primary wall between secondary thickenings is the only means of passage of the fungus from one vessel to another. All fungus elements in culture and in tree are 1.0 μ or more in diameter, and no living structure of this species passed through a millipore filter of 1.2 μ. Hyphae and spores, observed in fibers and parenchyma cells in dead wood or close to a dead area of the stem, were attributed to other fungi.Clusters of spores and hyphae, frequently found in vessels of all types and sizes, do not alone explain leaf wilting.

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