Submitted to: Forage Focus
Publication Type: Trade Journal
Publication Acceptance Date: 2/22/2017
Publication Date: N/A
Technical Abstract: Alfalfa producers count on the first harvest in late spring to deliver the highest tonnage and best quality of forage of the year. A late frost can significantly reduce both yield and quality. Losses are due not only to the physical damage from freezing of the alfalfa stem and leaves but also from damage caused by a fascinating bacterial pathogen called Pseudomonas syringae. When an alfalfa plant breaks dormancy, the leaves rapidly become sensitive to frost and freezing injury. Exposure to 25°F for as little as two hours will often result in frost injury. The degree of injury depends on many factors such as soil moisture, soil type, field location, residue on surface, and presence of P. syringae on the leaves and stems. The contribution of P. syringae to frost damage has largely been overlooked in the past, although the disease has been reported as widespread in many parts of the United States and forage losses from first crop harvests have been reported as large as 40 to 50% for some cultivars. Recognizing bacterial stem blight disease is important for managing the crop to reduce damage and to reduce the populations of P. syringae and protect the field from future damage.