The effect of plant age and autumn growth on development of resistance in first-year alfalfa seedlings to winter crown rot caused by the low-temperature basidiomycete (LTB) was evaluated under field and controlled-environment conditions during 1991 and 1992. Early-seeded alfalfa exhibited greater survival and had greater dry shoot weights than late-seeded treatments. Alfalfa plants, seeded in May of 1991 and 1992, were sampled from the field on three dates between September and November, which represented three different levels of cold hardiness in the first-year alfalfa. Plants exposed to the longest hardening conditions prior to inoculation exhibited the highest percentage survival and shoot dry weight and the lowest crown-rot rating. These results demonstrated that early planting and exposure of plants to prolonged hardening conditions in the autumn reduce injury due to winter crown rot in first-year alfalfa. Key words:Medicago sativa, alfalfa, low temperature, winter crown rot, Coprinus psychromorbidus, cold hardening