Early scientists have observed Claviceps purpurea on other Poaceae as Secale cereale.
1855, Grandclement described ergot on Triticum aestivum.
During more than a century scientists aimed to describe specialized species or specialized varieties inside the species Claviceps purpurea.

Claviceps microcephala Tul. (
1853)
Claviceps wilsonii Cooke (1884)
Later scientists tried to determine host varieties as 
Claviceps purpurea var.
agropyri
Claviceps purpurea var.
purpurea
Claviceps purpurea var.
spartinae
Claviceps purpurea var.
wilsonii.

Molecular biology has not confirmed this hypothesis but has distinguished three groups differing in their ecological specificity.

G1—land grasses of open meadows and fields;
G2—grasses from moist, forest, and mountain habitats;
G3 (C. purpurea var.
spartinae)—salt marsh grasses (Spartina, Distichlis).

Morphological criteria to distinguish different groups:  The shape and the size of sclerotia are not good indicators because they strongly depend on the size and shape of the host floret.
The size of conidia can be an indication but it is weak and it is necessary to pay attention to that, due to osmotic pressure, it varies significantly if the spores are observed in honeydew or in water.
The sclerotial density can be used as the groups G2 and G3 float in water.

The compound of alkaloids is also used to differentiate the strains.
Early scientists have observed Claviceps purpurea on other Poaceae as Secale cereale. 1855, Grandclement described ergot on Triticum aestivum. During more than a century scientists aimed to describe specialized species or specialized varieties inside the species Claviceps purpurea.
Claviceps microcephala Tul. (1853)
Claviceps wilsonii Cooke (1884)
Later scientists tried to determine host varieties as 
Claviceps purpurea var. agropyri
Claviceps purpurea var. purpurea
Claviceps purpurea var. spartinae
Claviceps purpurea var. wilsonii.
Molecular biology has not confirmed this hypothesis but has distinguished three groups differing in their ecological specificity.
G1—land grasses of open meadows and fields;
G2—grasses from moist, forest, and mountain habitats;
G3 (C. purpurea var. spartinae)—salt marsh grasses (Spartina, Distichlis).
Morphological criteria to distinguish different groups:  The shape and the size of sclerotia are not good indicators because they strongly depend on the size and shape of the host floret. The size of conidia can be an indication but it is weak and it is necessary to pay attention to that, due to osmotic pressure, it varies significantly if the spores are observed in honeydew or in water. The sclerotial density can be used as the groups G2 and G3 float in water.
The compound of alkaloids is also used to differentiate the strains.
Claviceps purpurea