Although as many as 95% of the Pucciniomycotina are members of the Pucciniales there is nevertheless a diversity of forms that are not rusts. These fungi are so diverse that it is difficult characterize them other than to re-state that they are not rusts. Therefore the most convenient way to approach them is to define a rust.

A complete rust life cycle

Rust life cycle

The numbered views above were photographed from microscope slides used for an introductory course in mycology. These slides are readily available from companies selling supplies for science classes. Students often find them dull and detached from the realities of the biological world, but in fact these slides have been professionally prepared from competently collected materials and clearly illustrate structures that would be difficult for a student to observe first-hand.

The fungus used for the slides is Puccinia graminis, the organism causing stem rust of wheat and many other grasses. It is a useful fungus for study because it is a rust having all of its stages and structures intact. Many of the Pucciniales have heteroxenous life histories, that is, they are parasites requiring more than one host species to complete their life cycle. The Pucciniales are a large enough group that specialists have developed a terminology for them separate from that used for other parasitic plants and animals. For their life histories uredinologists (rust specialists) have come to use the term heteroecious in place of heteroxenous, and autoecious instead of homoxenous. Puccinia graminis has two hosts, barberry (Berberis vulgaris) and wheat (Triticum aestivum) and must have both in reasonable proximity to complete its life cycle. In order to accomplish this cycling between two hosts it uses five different kinds of spores, traditionally designated with the numeric ciphers 0, I, II, and III. The Roman numeral IV has been used for basidiospores but this has never been widely accepted. Stages 0 and I occur on barberry while II, III and basidiospores are on wheat. Of course there is a special terminology associated with the numbered stages:

The life cycle of P. graminis is a complete one, involving two hosts and five spore types. However, one of the most interesting features of the rusts is that many species exhibit modifications of this basic type, including the loss of one host and one or more spore types. The patterns that these modifications display and the ecological factors that influence them present a fascinating field of study. Although a great variety of modifications exist it is always possible to recognize a member of the rust fungi because at least one of the basic stages persists. No other fungi have these stages, which is the basis for saying that the best way to recognize one of the other members of the Pucciniomycotina is to see that it has none of the characteristic rust spore states. A rather deductive way to make an identification!