Plant rusts are fungi, organisms that feed on other organisms and reproduce using spores. Plant rusts belong to the same classification as many types of mushrooms and have one of the most complicated life cycles of any living thing. Most plant rusts require two different plant species in order to reproduce successfully and some, like cedar-apple rust, have as many as five life stages.
Rusts are known as obligate parasites, meaning that they must feed on a host organism in order to survive. Three of their spore types require host tissue in order to survive and proceed to the next stage in the life cycle. Rusts develop filamentous structures that puncture plant cells and withdraw nutrients, causing damage to the plant.
Plant rusts are among the most damaging agricultural diseases because some varieties attack grain crops, reducing a field of wheat to a field of useless, black kernels. Rusts have been found on wheat grains excavated from 3,000-year-old archeological sites and earn mention in the Bible and by ancient Greek and Roman writers. Rusts such as wheat rusts have caused famines in the past, making them of interest to agriculturalists today. Interrupting the life cycle at one of its points is one way to eradicate the disease.
Plant rusts require two plant hosts and undergo two reproductive stages, both sexual and asexual reproduction. The sexual reproduction stage occurs on the alternate host, producing a type of spore that is carried by the wind to the primary host. Here, the spore develops into a rust that damages the host, undergoing asexual reproduction to produce another type of spore that infects the alternate host.
Spores are similar to seeds in that they germinate to produce a new organism. In rusts, spores are produced via sexual and asexual reproduction, and there may be as many as five types during the life cycle. Rusts produce, in sequence, basidiospores, spermogonia, aeciospores, urediospores and teliospores. Each plays a different role in the life cycle.
At the beginning of the cycle, which occurs in early spring for many rusts, basidiospores are borne on the wind to the alternate host plant. On the alternate host's leaves, they form spermogonia, another set of spores that produce the male and female sex cells in addition to a sweet substance that attracts insects. The insects carry the sex cells to each other, where they fuse to produce aeciospores, the third spore type, that carries on the wind to infect the primary host plant. Aeciospores invade the cells of the host plant, withdrawing nutrients, and in turn produce urediospores, which appear as the actual rust disease. Throughout the growing season, the wind may transport urediospores to infect other primary host plants. As winter approaches, the aeciospores produce the fifth type of spore, the teliospore, which has a thick enough wall to withstand the cold winter. When winter passes, the teliospore produces basidiospores, which when carried to the alternate host, begin the disease process again.