Rust diseases are caused by highly specialized fungi. Some complete their life cycle on one plant (for example, hollyhock rust, raspberry rust, and asparagus rust). Other rusts require two different and alternating hosts (such as white pines and currants or junipers and apples).
Rusts are not reddish discolorations of the leaf but are, instead, the rust-colored spores present in powdery pustules or gelatinous-like horns. The rusts appear on the leaves, twigs, or fruits, causing the leaves to deform or drop off prematurely.
Control by repeated application of recommended fungicides, beginning at the first sign of rust. Destroy alternate hosts that you don't need, and you may be able to stop the cycle. Plant resistant varieties when available.
