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Variation in Virulence of Botryodiplodia hypodermia to Ulmus pumila. J. M. Krupinsky, Plant pathologist, USDA, Agricultural Research Service, Northern Great Plains Research Center, P.O. Box 459, Mandan, ND 58554; Phytopathology 73:108-110. Accepted for publication 17 June 1982. 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, 1983. DOI: 10.1094/Phyto-73-108.

Variation in spore type and virulence of 218 isolates of Botryodiplodia hypodermia was examined. Seven percent of these isolates were considered to be atypical. Spores from atypical isolates were slightly narrower and longer than spores from typical isolates; however, the two types could not be differentiated by spore size. Approximately 50% of the spores from mature cirrhi of atypical isolates were septate; spores of typical isolates were aseptate. Atypical isolates were less virulent than typical isolates. Branches above the point of inoculation were killed on 20% of 132 branches inoculated with atypical isolates and on 73% of 266 branches inoculated with typical isolates. Atypical isolates should not be used in evaluating germ plasm. Because there is some variation in virulence among typical isolates, several typical isolates should be used to evaluate the resistance of Ulmus pumila germ plasm.