Bonjour tous
Aesculus hippocastanum (certainly in the UK) appears to have very few 'specialist fungi'. Ellis and Ellis has only four species as direct associates (although Erysiphe flexuosa has arrived since then and is spreading rapidly). Perhaps the situation is the same across Europe - ASCOFrance has only two records with Aesculus as an associate in "la Base de données"

I recently collected a pyrenomycete growing on dead attached twigs and branches of a young Aesculus hippocastanum tree. The perithecia occur extensively, immersed, mostly in ones or groups of 2 (rarely more), and appear to be restricted to the bark.
Asci are long-cylindrical, J-, with uniseriate spores (reminiscent of Phomatospora). The spores are ellipsoid, non-septate and have two large guttules (again somewhat reminiscent of Phomatospora); they measure 8.5-9.2 x 3.4-4µm.

The big surprise for me came when I examined the spores (living in water) under oil immersion at x1000 and saw that they have distinctly verrucose walls, such as one might expect in operculates. I am struggling to think of an order, let alone a genus, in which to place this collection.

Any suggestions would be very welcome.

Cordialement
Chris