noun
1.
a. : the uredinial stage of a rust (as of a cereal grass)
b. : the diseased condition produced by such fungi
2. : an infestation of plants by red spiders
3.
a. : a disease of the leaves or twigs of tropical or subtropical plants (as tea and citrus) characterized by a rusty appearance and caused by a parasitic green alga ( Cephaleuros virescens ) of the family Trentepohliaceae — called also algal disease
b. : the alga producing red rust of tea