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Changes in Nonaggressive and Aggressive Subgroups of Ophiostoma ulmi Within Two Populations of American Elm in New England. D. R. Houston, USDA Forest Service, Northeastern Forest Experiment Station, 51 Mill Pond Road, Hamden, CT 06514. Plant Dis. 75:720-722. Accepted for publication 14 January 1991. 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, 1991. DOI: 10.1094/PD-75-0720.

Surveys of elms with Dutch elm disease in Vermont (1989) and in Millinocket, ME (1986 and 1989), revealed the continued decline of the nonaggressive (NA) subgroup of Ophiostoma ulmi. This subgroup has virtually disappeared from Vermont; only two of 200 isolates were NA. The same trend was apparent for the isolated elm population in Millinocket, where less than 10% of the 1989 isolates were NA. The pattern of spread of the aggressive (AG) subgroup within Vermont appears to support the concept that this subgroup entered the state from the west and south along elm-rich valley corridors. Since 1980, the structure of the diseased elm population has shifted from scattered, large trees to abundant, small trees and saplings.