Stem rust of Oats
Puccinia graminis f. sp. avenae

Dark reddish brown, oval to elongated pustules, full of powdery spores appear on both sides of the leaves, sheath, heads and stems. Leaf and stem surfaces appear torn from ruptured pustules.
The red brown spores will rub off the leaf onto your finger rubbed. The black spores at the end of the season don't rub off.