From Middle High German esche f, asch m, from Old High German asca f, asc m, from Proto-West Germanic *ask.
The umlaut can be explained here from the plural of the masculine noun, though it may also be due to the secondary umlaut triggered by following -sch- which is found throughout western dialects of High German (compare Äsche, where this is probably the cause of the umlaut).
Esche f (genitive Esche, plural Eschen)
- ash (tree)
Declension of Esche [feminine]
Esche n (proper noun, genitive Esches or (optionally with an article) Esche)
- Eš (a village in the Czech Republic)