The vegetative and physiographic site characters of jack pine (Pinus banksiana Lamb.) stands, and their relation to the hazard of infection by the sweetfern rust disease (Cronartium comptoniae Arth.), were investigated. Cankers caused by the disease were prevalent only when the alternate host, sweetfern (Comptonia peregrina [L.] Coult.), was present, and the percentage of pine with cankers was principally a function of the abundance of sweetfern. Models for predicting sweetfern abundance indicated an affinity for dry sites that are low in certain soil nutrients. Models for rating disease hazard were dominated by sweetfern abundance or by characters that functioned to predict sweetfern abundance. Disease hazard models, however, indicated additional epidemiological roles for certain nutrient and soil characters. Sweet gale (Myrica gale L.), another alternate host for the rust, was recorded in only two stands. The percentage of pine cankered was low in both stands; hence, this host seems unimportant in northern Ontario. Forest Sci. 29:771-778.