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Seasonal Variations in Susceptibility and in Internal Inoculum Densities in Maple Species Inoculated with Verticillium dahliae. Lawrence R. Schreiber, USDA-ARS, U.S. National Arboretum, Ohio Research Site, 359 Main Road, Delaware, OH 43015. James S. Mayer, Former Graduate Research Associate, Department of Plant Pathology, The Ohio State University, Columbus. Plant Dis. 76:184-187. Accepted for publication 1 August 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992. DOI: 10.1094/PD-76-0184.

Differences in susceptibility to Verticillium dahliae between potted Norway (Acer platanoides) and silver (A. saccharinum) maple seedlings were determined by the percentages of infected trees, foliar symptoms, infected stem sections, and height of fungus movement in the stems. Data were pooled for four inoculation times in each growing season in 1983, 1984, and 1987. Norway and silver maples did not differ in percentage of infected trees, but Verticillium was recovered from a higher percentage of stem sections and the fungus grew to a higher percentage of the stem length in Norway maples. Similar comparisons were made in sugar (A. saccharum), Norway, and silver maples in 1987. Sugar maple was the most susceptible species by all criteria. A seasonal decline in susceptibility was observed for sugar and Norway maples but not for silver maples. Verticillium produced foliar symptoms in sugar but not in silver or Norway maple seedlings. Inoculum densities in the stems of silver and sugar maples were compared and expressed as total numbers of colony-forming units. Numbers of both short-lived conidia and mycelial fragments and microsclerotia were determined, and the number of colony-forming units isolated was higher from sugar than from silver maples.