A powdery mildew found on Horse Chestnut.  The chasmothecia have twisted and coiled appendages.

Horse Chestnut may also have powdery mildews of differnt species - see Woods et al (2024) for more detail. 

host/microscopy

Photogrpah the mildew on the leaf, and not host plant species.  Microscopic exmaination of chasmothecia will be needed for verification - proivde a photograph of these, or description

Horse Chestnut and other Aesculus species