the paraphyses contain very refractive VB-guttules which are unknown in Monilinia. Could you please check the iodine reaction and photograph the apical ring if present? I think the spores are uninucleate.

My guess is that it is a Hymenoscyphus.

Some more pictures. Asci with Lugols solution, spores with pushed contrast.

Yes, clearly Hymenoscyphus-type and spores uninucleate.

Further necessary data include asus base (croziers) and excipulum structure (prismatica?).

I know a Hymenoscyphus species on Cornus seeds, but remember narrower spores for that.

Ascus on croziers. Excipulum textura prismatica indeed, sometimes almost globulosa. Habitus is not much like Monilinia at all when I compare pictures...

Clearly with croziers. 

I find noting in my database on seeds/fruits of Prunus that might fit. Ciboria aestivalis is the only one, but it is sclerotiniaceous, obviously, when checking data in GenBank.

But I remembered wrong: The Hymenoscyphus on Cornus seeds fits yours in spore size (6-10 x 3-4). I have it as H. corni-maris in my folder Hymenosc Cudoniella. In another folder ("cornicola Asia") is a similar species differing in being without croziers. This folder contains also "Lanzia" glandicola, which might be the same.

Could it be that your seeds belong to Cornus rather than Prunus? - No, sorry, I did not remember your clear photo. We actually have a sequence of the Cornus fungus (ITS), so a comparison with yours would be interesting.

I compared this aggregate also with Gloeotinia and Symphyosirinia, and the latter was suggested to me also by Peter Püwert regarding your sample.

Zotto

At some time I intend to clarify this Cornus fungus, but at the moment it is impossible. Various people have the opportunity to do such molecular work, but it is a bit of inflationary today, how many species and samples need sequencing. Important is to keep the dried sample.