Rusts are among the most destructive of plant diseases with many being highly specialized parasites attacking only certain host genera. For example, one Puccinia graminis race attacks only wheat while another Puccinia graminis race attacks only barley. Within each of these races, however, are so-called physiological races that attack only certain varieties within the species. In other words, one physiological race may attack one variety of wheat exclusively and not other wheat varieties. Rusts are known to be especially destructive on grain crops such as wheat, oats and barley with shortages causing famines and ruining the economies of entire countries.
Rusts also attack vegetables, cotton, soybeans, flowers , coffee, apple and pine trees.
With 4000 species of rust fungi, the potential destruction caused by this type of disease organism can not be under-estimated. The various stages of rust development produces different types of spores. Some rusts produce up to five different spore forms. Heteroecious rusts spend part of their life stages on one host and the remainder of their life on another host. In other words, they alternate between hosts.