Based on 3–13 yr of observations of 25 permanent plots in infection centers of Arceuthobium pusillum in stands of black spruce (Picea mariana) in Minnesota, an average of 8% of the infected trees died each year, and each year 11.1 healthy trees became infected per 100 trees that had been infected. In one plot white spruce (Picea glauca) had an annual mortality rate of 2.7% of the infected trees, and newly infected trees appeared at the rate of 0.9 trees per year per 100 infected trees. The lateral rate of spread of A. pusillum was 0.74 m/yr.