Colletotrichum trifolii is a fungal plant pathogen of alfalfa, causing the disease alfafa anthracnose.

It is a biotroph, obtaining nutrients from the living plant cells before forming asexual spores.

The visual symptoms include:[3] Once parts of the plant die parts of the leaves will turn tan making the black fruiting bodies of Colletotrichum trifolii especially visible to the observer.

[3] Anthracnose from Colletotrichum trifolii most severely affects crops east of the Mississippi River and south of Wisconsin.

It is also found in California and southern Arizona and in less severe cases, Colletotrichum trifolii has a moderate pressure across the whole United States except around the Rocky Mountains.