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Dutch elm disease can be caused by two species of fungi. The
species Ophiostoma ulmi is the more weakly
pathogenic of the two species and is now believed to be responsible for the
first pandemic of Dutch elm disease that occurred in Europe and North America
in the 1920's-40's. Ophiostoma
novo-ulmi is a highly pathogenic and aggressive subgroup,
responsible for the current pandemics of Dutch elm disease in Europe and North
America. The fungus is transmitted chiefly by two elm bark beetles - Hylurogopinus rufipes (Eichh.), a native species, and
Scolytus multistriatus (Marsh.), a species
indigenous to Europe but widely distributed in North America.
All species of native North American elm are susceptible and the disease is
now occurs in most of the natural range of Ulmus
americana from Manitoba to the Maritimes. Dutch elm disease is not
known to occur in the three provinces where native elms do not grow:
Newfoundland, Alberta and British Columbia. In these provinces, Ulmus is generally confined to landscape plantings around
human settlement.
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