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Aggressiveness, Competitiveness, and Stability of Tolerance of Benzimidazole-Tolerant Strains of Ceratocystis ulmi. L. R. Schreiber, Supervisory Plant Pathologist, USDA-ARS, Nursery Crops Research Laboratory, Delaware, OH 43015. E. E. Conaway, Former Graduate Research Assistant, Department of Plant Pathology, Ohio State University, Columbus 43210; and J. W. Peacock, Research Entomologist, USDA-FS Northeastern Forest Experiment Station, Delaware, OH 43015. Plant Dis. 70:154-158. Accepted for publication 12 August 1985. 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, 1986. DOI: 10.1094/PD-70-154.

Benzimidazole-tolerant (T) variants of Ceratocystis ulmi were as aggressive or more aggressive than the sensitive (S) wild-type strains from which they were selected when inoculated into nursery-grown American elms. T strains reisolated 34 mo later retained their aggressiveness when tested by reinoculation into both nursery and potted elms. The competitiveness of T and S strains was compared in American elms infested with the smaller European elm bark beetle (Scolytus multistriatus), vector of C. ulmi. The pathogenic phase of the fungus in the elm xylem and the saprophytic phase in beetle galleries were examined as possible sources of contamination of emerging beetle progeny. T and S strains were both readily transmitted from either the xylem of infected trees or from the bodies of breeding beetles in brood galleries to beetle progeny emerging from elm bolts. Tolerance of T strains to chemicals was stable in vivo and in vitro. Tolerance levels were undiminished when the pathogen was isolated from diseased trees 34 mo after inoculation, after storage on unamended or on benzimidazole-amended PDA for 20 mo, or after 43 serial transfers.