He was appointed to an academic post at
University of Berlin in 1972 but in 1973 he moved to
Munich to take up the post of curator at the
State Herbarium in Munich. He became the provisional director from 1985 to 1992 and was then confirmed as Director and remained until his retirement in 2004. His specialist area was the taxonomy of lichens, and especially the genus
Lecidea that he had first studied for his doctorate. He and students that he supervised brought order and a critical review of the 1000 accepted and 4000 published names within the genus in the early 1970s. and
Lecideaceae exsiccatae. The State Herbarium in Munich contains the lichen collection of
Ferdinand Christian Gustav Arnold, 100,000 - 150,000 specimens collected in the nineteenth century. Hertel made a particular effort to transcribe the detailed labels from Arnold's handwriting that was difficult to read and ensured this was published so that the collection could be more easily used by others. ==Publications==