Summer black stem and leaf spot
Also called Cercospora black stem, this disease is caused by the fungus Cercospora medicaginis. It is fairly common after first cutting in warm, moist weather. Small brown spots appear on leaves, enlarging to form larger lesions. Bad cases will cause early defoliation. Stem and petiole lesions are elongate, elliptical and reddish-brown, and may expand and girdle stems and petioles. The fungus overwinters in crop residue and may be seed-borne.
Spring black stem
Caused by the fungus Phoma medicaginis, this disease is similar to summer black stem, but is favored by cool, wet weather in spring and affects first cutting. Dark green, water-soaked lesions develop in the stems and petioles, turning brown to black with age. This fungus also overwinters in crop residue and may be seed borne.
See the first article of this series, “Be familiar with root rot diseases of alfalfa” for more information on certain root rot diseases. The third, and final, article in this series will describe common alfalfa leaf diseases and recommendations for control.
- Alfalfa Diseases in Illinois, Department of Crop Sciences, University of Illinois
- Leaf and Stem Diseases of Alfalfa, RPD #301, University of Illinois Extension
- Alfalfa Diseases, Bulletin 631-98, The Ohio State University Extension
- MSU Extension Bulletins (archived) E-1422 “Alfalfa Diseases I” and E-1423 “Alfalfa Diseases II”
For more information, contact MSU Extension educator Jim Isleib at 906-387-2530.
Source: Michigan State University