the similarity is superficial; you will see from Index Fungorum that “Rhytisma ulmi” has been used by three different authors – Fries first and then by two others (these latter two being invalid, as – nomenclaturally at least – they are illegitimate synonyms). What Fries and the others intended is uncertain so they’re effectively useless names.
Many thanks for the correction and identification Chris – I just grabbed a couple of photographs while searching for leaf miners but didn’t collect a leaf for closer examination
Chris wrote a really informative piece of this a few years ago on WAB: IIRC in response to a query of mine very similar in tenor to yours. One of the biggest losses are many of these in-depth answers from Chris.