Ferrugo

from The Collaborative International Dictionary of English v.0.48

Ferrugo \Fer*ru"go\ (f[e^]r*r[udd]"g[-o]), n. [L., iron rust,
   fr. ferrum iron.]
   A disease of plants caused by fungi, commonly called the
   {rust}, from its resemblance to iron rust in color.
   [1913 Webster]