Changes in the distribution of Ophiostoma ulmi subgroups
Surveys in the New England states of the USA Maine and Vermont showed that non-aggressive subgroups of Ophiostoma ulmi (EPPO A2 organism) are continuously declining and have virtually disappeared from Vermont. Aggressive subgroups, however, have increased and infect increasingly young trees and saplings.
Sources
Houston, D.R. (1991) Changes in nonaggressive and aggressive subgroups of Ophiostoma ulmi within two populations of American elm in New England.
Plant Disease 75, 720-722.