Rusts and Smuts

Rusts and smuts are fungi belonging to the orders Urediniales (rusts) and Ustilaginales (smuts) which are basidiomycete fungi. The rusts have complicated life cycles which involve the infection of two different plant species. The most well-known members of these groups are wheat rust (Puccinia graminis tritici) and corn smut (Ustilago myadis). Rust fungi attack plants such as ferns, gymnosperms, and flowering plants. When a wheat plant is infested by Puccinia graminis tritici, the infestation may become obvious during the summer growing season when rust colored growth appears on the stems of infected plants. Fungal hyphae are composed of groups of spore generating structures (sporangia) called uredinia that rupture the stem and become visible. It is the spores released from the uredinia (called urediniospores) that infect new wheat plants and spread the disease. In the fall, Puccinia produces black sporangia (called telia) and the infected wheat plants have distinct black patches on their stems. Spores from the telia (called teliospores) do not attack other wheat plants but instead infect barberry plants. Teliospores which land on barberry leaves germinate and form small cup-shaped structures called spermagonia. Each spermagonium produces long filaments called receptive hyphae which extend above the spermagonium and spermatia, which are sexual gametes. The spermagonium also produces a nectar-like substance which is attractive to flies. Spermatia are mixed with this nectar and flies transfer the spermatia from adjacent spermagonia as they feed. New fungal mycelia, resulting from the union of the spermatia with the receptive hyphae of spermagonia of different genetic strains, grow on the underside of the barberry leaf. There, the mycelium produces a larger bell-shaped sporangium called an aecium, which generates aeciospores which in turn infect new wheat plants.

Smut fungi differ from rust fungi in several ways. While rust fungi require two different hosts to complete their life cycle, smut fungi may complete their life cycle on only one host, which is always a flowering plant. Another difference between rust and smut fungi is seen in the way that they infect their host plants. Infections from rust fungi are localized to that part of the plant close to where a germinated urediniospore, aeciospore, or teliospore becomes established. Smut fungi spread to infest the entire plant from a single initial infection site, often targeting specific organs. This is exemplified by the smut fungus Ustilago violacea which attacks plants of the genus Silene. Ustilago violacea infests the entire plant but its presence within the plant is only apparent where mycelia grow within the anthers of the plant. There, hyphae divide to become teliospores and these take the place of pollen grains. Pollinating insects then carry the teliospores from infected Silene plants to uninfected ones. Teliospores mature along with the Silene flower and fall to the ground along with seeds of the host Silene plant. When the seeds germinate, the smut fungus teliospores germinate along with them and immediately infect the Silene seedlings. Ustilago myadis, is a well-known smut fungus that infects corn, where its immature teliospores are enclosed in sacs which replace the kernels of corn. When these sacs burst, U. myadis spores are released and cling to normal corn kernels. When these kernels are planted, teliospores are planted along with them, infecting new corn plants when they germinate.

Rust and smut fungi are both of great economic importance due to their destruction of cash crops. An effort to eliminate Puccinia graminis tritici by the eradication of barberry was not successful. This rust fungus is now controlled by selection for genetically resistant wheat plants, but rust fungi frequently mutate and override Smut on corn. © Leonard Lee Rue III, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission. wheat resistance, so an ongoing genetic selection program for wheat is necessary. Another economically important rust fungus is Gymnosporangium juniperus-virginiae, which has as its two plant hosts the common juniper (Juniperus virginianus) and the domestic apple, and other species of the rose family. This fungus produces large orange colored spore-generating structures on juniper trees, which then infect apple trees, causing the tree to produce deformed and unmarketable apples. The best way to avoid ruined apples is to keep apple trees away from juniper trees and to remove all infected juniper trees in the area.

One way that humans have reduced infection of corn by smut has been to wash away any clinging fungal spores from the kernels of corn. In the southwestern United States and in Mexico immature corn smut sacs are fried and eaten as a delicacy.