Diseases of alfalfa (
Anthracnose
Anthracnose of alfalfa is caused by the fungus Colletotrichum trifolii. It causes lesions on stems and leaves, and advanced stages result in crown rot and plant death. Disease is most active in warm, wet conditions such as occurs during the summer monsoons or with excessive overhead irrigation. The disease occurs sporadically in Arizona.
Stem lesions are elongate or irregular shaped and tan to black in color (photo 1). When there is ample moisture, reproductive spores are produced in the lesions in sunken structures called acervuli. Within the acervuli are hair-like structures called setae that can be seen with a hand lens and are diagnostic for the fungus. The spores are disseminated by rain, splashing water and wind.
There is no chemical treatment for anthracnose. Alfalfa varieties with tolerance may be available. If disease occurs in fields with overhead irrigation, reduce the frequency of irrigations as much as possible. Cutting hay may help reduce disease by reducing canopy and the amount of spores.
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July 13, 2006