The plant rusts form one of the largest natural groups of plant parasitic fungi (6,000-7,000 species), many causing severe diseases of our most important crops. Some examples are: the rusts of wheat, corn and other cereals; forage and range grasses and sugar cane; beans, soybeans, peanuts, and other legumes; various fruits and vegetables; coffee, and forest and plantation timber and pulp trees. Rusts probably attack more different kinds of wild and domesticated plants than any other natural fungus order. Because a rust species is usually highly host specific, i.e. attacking only one or a few closely related plant species, and may have up to six different and dissimilar spore forms and two unrelated hosts while completing its life cycle, the rusts are among the most complicated microorganisms.

Rusts are of great scientific interest because of their close evolutionary relationships with their host plants, their complex life cycles, and their numerous biological adaptations that permit them to thrive on all the continents (except Antarctica) under great extremes of environments. Present day rusts represent a very ancient group of organisms whose ancestors were well established parasites on the primitive ferns and fern-like plants of the carboniferous age some 250-300 million years ago. As new kinds of plants evolved, culminating in the numerous species of flowerings plants that now dominate and clothe the earth making human civilization possible, the rust fungi also evolved hand in hand with their new hosts, producing the great diversity of rusts that we know today.

http://www.btny.purdue.edu/Herbaria/Arthur/