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Efficacy of Criteria to Identify Aggressiveness in Ophiostoma ulmi and Resistance in American Elm Germ Plasm. LAWRENCE R. SCHREIBER, Research Plant Pathologist, USDA-ARS, U.S. National Arboretum, Delaware, OH 43015. SUBHASH C. DOMIR, Research Plant Physiologist, USDA-ARS, U.S. National Arboretum, Delaware, OH 43015. Plant Dis. 78:629-632. Accepted for publication 14 March 1994. 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, 1994. DOI: 10.1094/PD-78-0629.

Criteria were evaluated to identify levels of aggressiveness of the Dutch elm disease pathogen, Ophiostoma ulmi, and resistance in American elms. Twenty-year-old American elms were inoculated with putative nonaggressive strains PG442 and TN and aggressive strains PMPI, H961, and 16K. Strains PMPI and H96I were confirmed lo be aggressive and strains TN, PG442, and 16K to be nonaggressive. Six-month-old American elm seedlings were inoculated with the five O. ulmi strains, and aggressiveness was judged by disease symptoms and fungus multiplication. Vascular discoloration and numbers of colony-forming units most closely correlated with in vivo results. By most in vitro criteria, PG442 was categorized as aggressive. Callus tissues from susceptible and resistant American elm selections were challenged with the O. ulmi strains. Aggressiveness was differentiated by growth on callus from both a susceptible and a resistant elm. Differences in resistance between susceptible and resistant elm selections were determined by growth of strains H961, PMPI, and PG442 but not 16K or TN. Growth and mycelial habit on agar were not reliable in identifying 16K as nonaggressive.