noun: A genus of uredineous fungi having 4 spore forms (spermagonia, æcidia, uredo- and teleutospores), all produced on the same host. The æcidia are without peridia and paraphyses, and the teleutospores form a dark crust on the surface of the host. The species are numerous and common, causing rusts of various plants. M. farinosa is usually abundant on the leaves of various willows.