I fear this cannot be answered easily. Groves did not study a type of D. prunastri when he reported such wide conidia for that species. His description recalls for me D. padi, whereas his description of D. padi recalls D. prunastri as I interpret these species. I observed a high host specificity and suspect that your collection was on Prunus spinosa, right?

Groves gives several Prunus species for D. padi and merely "Prunus spp." for prunastri.  For D. padi he says that the spores are sickle-shaped to almost straight, 2.5-4 µm wide, and for prunastri +/- straight, mainly 5-7 µm wide.

One of the species  studied by Groves (Roumeguere) was named Cenangium prunastri but Groves identified it as D. padi. 

These old taxa described by Albertini & Schweinitz and Persoon and Fries are probably not documented by exsiccata and surely lack microscopic data in their description. So I think a revision is needed which defines neotypes. These should be collected in Europe, maybe in America different species exist.

The choice of the names prunastri and padi by us might have followed simply the host specificity which leads to padi for the species on Prunus padus.

I see that besides P. spinosa I have seen also a sample on Prunus domestica with narrow falcate conidia.

Zotto

What is right and what is wrong needs to be brought out by a detailed study. First we must look whether a host species is indicated in the protologue, I did not look up so far, and I just se it is difficult.
 Sometimes the identity of a name depends on the epitype that is proposed.