In the photos above, puccinia grammis (the rust) on upper leaves of barberry from eye sight and under microscope are shown. It is a parasitic Basidiomycete that will live on two hosts throughout its whole live cycle - barberry leaf (1st host) and wheat (2nd host). According to the microscopic image, we can see that the small, round spermatia are still enclosed within the barberry leaf epidermis. I guess the basidiospores should have just arrived this host, at least not for long enough such that they could break through the epidermis and develop into further stages.

This is not a rust but a water mold, Albugo.