Eastern spruce dwarf mistletoe (Arceuthobium pusillum) is a parasitic flowering plant that causes the most serious disease of black spruce (Picea mariana) throughout its range. The parasite occurs in the Canadian provinces of Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland; in the Lake States of Minnesota, Wisconsin, and Michigan; and the Northeastern states of New York, Pennsylvania, Vermont, New Hampshire, and Maine. (Figure 1). The eastern spruce dwarf mistletoe is rarely found in Rhode Island, Connecticut, New Jersey, and Massachusetts.