Another collection from more wet place and cross section of Acer negundo rachis to compare.

Dear Zotto,

the first set of photos is from Horka nad Moravou, near the Podebrady lake, under Acer negundo, Fraxinus excelsior, F. pennsylvanica, 200–220 m, 49°37,407' 17°13,693', leg. 2. IX. 2020.
The second set of photos is from Olomouc-Cernovir, "Cernovirsky les" forest, near a spring (Fe3+ mineral water), under Acer negundo, Ulmus, Rubus fruticosus, ca. 220 m, 49°37,300' 17°16,240', leg. 18. IX. 2020. 

The Fraxinus petioles hosting Rutstroemia longipes were also often reduced to just the xylem and very little of phloem. 

That brown parts below sclerenchym are actually full of hyphae (normally there are large isodiametric cells). There was some anamorph on the surface too, but I haven't seen any konidia and don't know whether it belongs to the same fungus. 

Viktorie 

Thanks. Looks really black. The empty zones (holes) are probably the consumed phloem.

Sorry for disturbing the Hymenoscyphus discussion, but if you mean the South Bohemian collection of Ombrophila limosella, the data are as follows:
Czech Republic - South Bohemia,  E of Cerná v Pošumaví, on soil with small mosses in a Picea forest, ca 750 m a.s.l., 30 September 2020, leg. Z. Sochorová

Thank you Zuzana, it is not the first time I confused you :-(

No problem :-)