Mortality from Armillaria root rot is a major concern of forest management. Field experiments were conducted in Minnesota to evaluate interspecific differential susceptibility and to assess whether density or species composition, specifically the proportion of conifers in a plot, influences seedling mortality from Armillaria spp. Seedlings of 10 tree species (six conifers and four hardwoods) were planted at four densities in several species mixtures on recently logged sites. Species differed significantly in susceptibility (p < 0.0001); balsam fir (Abies balsamea (L.) Mill.), tamarack (Larix laricina (Du Roi) K. Koch), and black spruce (Picea mariana (Mill.) BSP) showed the greatest infection and mortality. Hardwood species and eastern white pine (Pinus strobus L.) showed negligible mortality. There was a trend (p = 0.1) toward increased root infection with increasing density of the three species that showed significant mortality. Their mortality rates were 5.6, 8.7, 10.2, and 10.8% in plots with 2-, 1-, 0,5-, and 0.25-m spacing, respectively. Mortality increased significantly (p = 0.001) with an increase in the proportion of conifers in a plot. Mortality in the three most susceptible species was reduced by 75% when grown in aspen-rich rather than in conifer-rich plots. Thus, seedling mortality was related to species identity, planting density, and proportion of conifers, suggesting that selected diversity (mixtures of conifers with hardwoods rather than conifer monocultures) reduces disease impact.