Aggressive and Non-aggressive Strains ofCeratocystis ulmiin North America

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Abstract
Examination of the culture morphologies and growth rates of 300 isolates of C. 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 2 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.degree. 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. The aggressives strain is migrating into the northeast; this phenomenon is discussed in relation to current theories on the origins of the 2 strains of C. ulmi.