This name is a synonym of Avena volgensis (Vavilov) Nevski.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 25559290) with original publication details: Trudy Prikl. Bot. 16: 92, 94, 210 1926.
This name record (from Tropicos) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Avena diffusa in version 1 of The Plant List.