Scleroderris canker (Scleroderris lagerbergii Gremmen) has damaged conifers in Europe for nearly a century. The disease has intensified in the Great Lakes region of North America during the past several decades, and the fungus could spread throughout the North Temperate Zone. Attacks by S. lagerbergii are most severe in topographical depressions and the resulting disease was thought to be associated with frost damage. Observations in depressions that resembled frost pockets indicated that extensive damage to regeneration of Pinus resinosa Ait. could be attributed primarily to amplified spore deposition in the depressions and rapid intensification of the disease in the presence of prolonged periods of high relative humidity. Topographic projections, photographs, temperature profiles, and spore collection data support this thesis. Vertical restriction to foliage within ca. 1.5 m of the ground is attributed to radiation fog that prolongs the incubation period afforded the fungus. Effects of edaphic variation, ground vegetation, and snow cover are evaluated. Snow is accorded importance in modifying extremes of winter and early spring temperatures to the advantage of the fungus. Frost damage and infection by S. lagerbergii both result in plantation failures in topographic depressions.