Alternaria alternata

From ISTA's Common Laboratory Seed Health Testing Methods for Detecting Fungi:

Brown to black growth consisting of conidia in long chains is characteristic of the species. The chains are usually simple, but can also be branched. Conidial beaks (between the conidia) can sometimes be seed at higher magnifications. In most cases, growth of the fungus covers the entire seed surface.

Conidia polymorphous, short to long, olive brown to dark brown, highly variable in shaoe (oval to cylindrical), size and # of septation (transverse, longitudinal and oblique), 10-58 x 7-19 μm. Conidial beaks also show variation, short to long, mostly of the same colour as the main body or a little lighter, 2-19μm in length. Total length of conidia with beak 10-71μm.