Many cool-season grasses, for example, bluegrass, bentgrass, ryegrass and others are susceptible to attack by a rust fungus. Within these grass groups, there are cultivars with different levels of resistance to attack. Therefore, you will probably note that your lawn is not uniformly infected, as most lawns are a mixture of grass species and cultivars. The rust fungus survives year-to-year in your lawn. It is usually in spring that temperature and moisture conditions are favorable for the rust to grow, reproduce and infect new tissues causing the yellowing.
If you want to control the rust, a spray treatment will suppress the fungus. One spray, at the most a second spray, applied 10-14 days apart should provide adequate control. Most lawns will survive rust attack without treatment. It is suggested that you fertilize to stimulate vigorous growth and then mow the lawn, catch and dispose of the clippings. In this way the grass will often outgrow the fungus, which will go dormant as soon as warmer, drier weather occurs.