This name is a synonym of Plantago tenuiflora Waldst. & Kit..
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 100372966) with original publication details: 1836.
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 Plantago pusilla in version 1 of The Plant List.