Stem Blight of Alfalfa in Utah: A Complex of Two Distinct Diseases

In 1908, Sackett described for the first time what appeared to be a new bacterial disease of alfalfa in Colorado. To this particular disease he applied the name “stem blight and attributed the cause to a bacterium, Pseudomonas medicaginis n. sp. For some two decades or more Sackett\u27s work on this disease was accepted. During this time stem blight, which was first thought to be indigenous to the western United State, was reported from nine states east of the Rocky Mountains. In 1933, Johnson and Valleau described a disease of alfalfa, characterized by stem lesions very similar in appearance to those of stem blight as described by Sackett. These authors determined the cause of this new disease to be a fungus, Phoma medicaginis Malbr. and Roum., and gave to it the name black stem . Richards in 1934, in reporting on the resistance of certain alfalfa varieties to stem blight in Utah, states in a foot note that a species of Phoma was isolated with considerable uniformity from bacterial stem blight lesions. This author raises the question as to the possible dual etiology of stem blight in Utah. Research work by Remsberg and Hungerford in Idaho, more definitely suggested this dual etiology of\u27 the stem blight complex. These authors determined the black stem lesions on alfalfa to be caused by Phoma medicaginis, and indicated that the darker lesions caused by this fungus were distinct from the lighter colored lesions characteristic of the bacterial stem blight described by Sackett. Because of the similarity of the late stage of bacterial stem blight to the insipient stage of black stem, much confusion has arisen as to their identity and etiology It has been the policy in Utah during recent years to refer to this wide range of stem lesions as stem blight, recognizing that the exact nature and etiology is not known. This disease complex presents a problem in Utah of major economic importance. It is the purpose of this thesis to bring together the known facts regarding stem blight; to determine the specific etiology of the stem blight lesions on alfalfa as they occur in Utah; and if possible to establish definite criteria by which the components of the stem blight complex might be separated