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Ecology and Epidemiology

Aggressive and Non-aggressive Strains of Ceratocystis ulmi in North America. J. N. Gibbs, Forestry Commission Research Station, Farnham Surrey, England, GU10 4LH; D. R. Houston(2), and E. B. Smalley(3). (2)USDA Forest Service, Forest Insect and Disease Laboratory, Northeastern Forest Experiment Station, Hamden, CT 06514; (3)Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 69:1215-1219. Accepted for publication 23 May 1979. 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, 1979. DOI: 10.1094/Phyto-69-1215.

Examination of the culture morphologies and growth rates of 300 isolates of Ceratocytis ulmi collected during 1977 from a range of locations in eastern and central North America revealed that all but one could readily be assigned to the two strains (“aggressive” and “non-aggressive”) originally defined in Britain. A sample of 70 isolates collected in 1970 and maintained on autoclaved elm twigs at –20 C also was classified similarly. In the North Central states of the USA the aggressive strain was predominant, but in northern New England and the adjacent provinces of Canada, and in a small sample from Kansas, the non-aggressive strain was detected more frequently. Available evidence indicates that the aggressive strain is migrating into the northeast and this phenomenon is discussed in relation to current theories on the origins of the two strains of C. ulmi.