Rust is a common disease. It can pop up in any lawn (newly seeded lawns frequently experience worse rust than established lawns), but it doesn't necessarily mean you have a problem that needs solving. Because rust is not terribly harmful (it just looks bad), and because fungicides are expensive (and may require sequential applications to work well), relatively few homeowners choose this approach. If the right fungicide is used, it can cut down on the rust, but the best way to prevent rust is to keep the lawn healthy, well watered, and fertilized. In seasons when the grass grows rapidly, it will outgrow the rust, which can simply be mowed off.