Grass fungus is a term used to describe two different types of fungi that can affect grasses. The first is SMUT, which is a type of fungal disease that causes dark spots on the leaves and stems of grasses. The second is ERGOT, which is a type of fungus that produces alkaloids that can be toxic to animals and humans if ingested. Both SMUT and ERGOT can cause significant damage to grasses, leading to reduced yields and poor quality of grasses.