DISEASES OF SMALL GRAINS - RUSTS

Rusts are classic diseases of small grains that are readily identifiable. Rusts are characterized by the production of rusty-orange spore masses produced in fruiting bodies (pustules) that erupt from the surface of leaves and stems. The spores (urediospores) are readily removed by rubbing.

In this photo, two rusts of wheat are shown. On the right is leaf rust, and on the left is stem rust. Note that both rusts can occur on both the leaves and the stems, although, as their names imply, leaf rust tends to concentrate more on leaves, while stem rust tends to occur on both stems and leaves.

Stem rust has an interesting life cycle. It spends part of its life on wheat, and another phase of its life cycle on barberry. Spores (aeciospores) produced on the barberry plant infect wheat plants. The rusty-orange spores (urediospores) produced on wheat plants reinfect wheat. Late in the growing season, the fungus produces darker-colored resting spores (teliospores) on wheat that germinate the following growing season to produce yet another spore type (the basidiospore) that infects barberry.

At one time, a concerted effort was made to eradicate barberry plants. It was thought that removal of the alternate host for stem rust would lead to a disruption in the rust’s life cycle and thus lead to disease control. Unfortunately, after the effort was made, researchers discovered that infections could still occur because urediospores migrate to the northern US from areas such as Texas and Mexico where the fungus survives all year on wheat. Barberry eradication is still important however, because sexual reproduction occurs on this host. Limiting this kind of reproduction reduces the chances of genetic recombination in the fungus and thus limits the development of new rust races (subgroups within the rust that are able to overcome resistance in grain varieties).

Current control of cereal rusts relies heavily on the use of resistant grain varieties. Unfortunately many varieties tend to have resistance to only certain rust races in the environment. As varieties are deployed that are resistant to the predominant rust races, other races proceed to become more prevalent in the overall rust population and thus new varieties of rust-resistant grains must be developed and deployed. Other varieties have been developed however that have a broader base of resistance and tend to be more stable.