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Spread and Increase of Ceratocystis ulmi with Cultural Characteristics of the Aggressive Strain in Northeastern North America. D. R. Houston, USDA Forest Service, Northeastern Forest Experiment Station, 51 Mill Pond Rd., Hamden, CT 06514. Plant Dis. 69:677-680. Accepted for publication 23 January 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, 1985. DOI: 10.1094/PD-69-677.

Elm trees with Dutch elm disease in the state of Vermont and in the town of Millinocket, ME, were systematically sampled for infection by Ceratocystis ulmi in 1980 and 1983. Isolates of C. ulmi were classified as either the aggressive or nonaggressive strain on the basis of growth rate and cultural morphology. Results indicated that these two subgroups are isolated in nature. Compared with an earlier survey in 1977, the relative number of nonaggressive isolates declined with each successive survey in both areas, suggesting that this strain will soon disappear from these areas.