The occurrence, hosts and symptoms of a hitherto undescribed root rot of Melilotus, Medicago and Trifolium, and the relation of temperature and the reaction of substrate to growth of the pathogen Plenodomus meliloti are discussed, and its pathogenicity demonstrated. It is suggested that the disease be called "brown root rot".Evidence is that the hosts mentioned are susceptible only during the winter and early spring dormancy stage. Normal roots of sweet clover, when frozen at −4 °C. for four days and subsequently kept at 2–3°, 9° and 16 °C., did not become susceptible. The brown root-rot disease is distinct from true winter injury resulting from insufficient hardiness to cold.The temperature range for vegetative growth and pycnidia of P. meliloti is from 0° to 27 °C., with optimum between 15° and 17 °C. Increasingly good growth occurs from 2 °C. to optimum temperature. Severe lesions are produced at 2–3°, 9° and 16 °C. The optimum pH value for growth in potato dextrose decoction is about 6.2, the other limits being approximately pH 3.2 and 8.2. Soils with an alkaline reaction apparently are unfavorable.The disease is characterized by brown lesions, on or within which are an abundance of black to dark brown pycnidia. These bodies, 0.5 to 2 mm. in longest diameter, may have one or more spore bearing chambers. Each chamber may have one to several ostioles, through which the one-celled spores, averaging 5.2 × 2.84 μ, exude. The hyphae do not bear spores.Dissemination of the pathogen by seed would not seem to be of practical importance. Control by crop sanitation is recommended, at least until varieties more resistant are available.