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Rate and Extent of Colonization of Naturally and Artificially Inoculated American Elms by Ceratocystis ulmi. John H. Hart, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823; W. Ronald Landis, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823, Present address of junior author: Niagara Chemical Division, FMC Corporation, Middleport, New York 14105. Phytopathology 61:1456-1458. Accepted for publication 12 July 1971. DOI: 10.1094/Phyto-61-1456.

When current season shoots of Ulmus americana in close proximity to main branches were inoculated with Ceratocystis ulmi, systemic infection resulted more frequently than when similar inoculations were made in the apical portion of the tree or in the terminal portion of lateral branches. The fungus could be readily recovered from leaves which had been taken from trees previously inoculated at the base with a large number of spores. However, the frequency of recovery of the fungus was much lower from leaves taken from naturally infected trees. Symptomless leaves harvested from infected shoots from naturally infected trees rarely contained the pathogen, and hence it is doubtful if leaf insects are important agents of fungal transmission in nature.