.. on water-soaked Betula wood lying in a small stream. The minute perithecia remind me of specimens I determined as "Nectria" viridescens or as N. rishbethii in earlier times. I know a lot has changed since. When I compare Cosmospora viridescens I find the spores of my specimen to be quite large. Who can say more?
For sure this is a Nectriaceae, because the perithecial wall stains deeply magenta-violaceous in KOH. I think Cosmospora could be the right genus (?).
The spores are brownish at maturity and are slightly warted-rough, they meaure about 13-15/6-7 µm.
Best regards, Lothar