The Collaborative International Dictionary of English v.0.48:

Wheat rust \Wheat rust\
   A disease of wheat and other grasses caused by the rust
   fungus Puccinia graminis; also, the fungus itself.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

wheat rust
    n 1: rust fungus that attacks wheat [syn: wheat rust,
         Puccinia graminis]