Euphorbia characias subsp. wulfenii
When wild plants are found growing over a wide geographic area, populations of these plants may acquire slightly different characters. These can be distinguished as subspecies within one species. The word subspecies is usually abbreviated to subsp. but should not be written in italics, although the scientific name of the subspecies is written in italics.
So for example, the scientific name of the pedunculate oak, a subspecies of Quercus robur native to parts of Europe and west Asia is written :
Quercus robur + subsp. pedunculiflora = Quercus robur subsp. pedunculiflora