Grass Rust fungus is a common problem.  This occurs most often when there are dry periods of weather or the grass is low on nitrogen.  The spores of the Rust fungus are formed when the grass is not allowed to dry properly. This is caused by dew not drying on humid days.  It can also form when the thatch is too thick in the yard. Rust fungus makes grass more susceptible to pests and other diseases as it weakens the grass. There are some ways to treat and help prevent Rust fungus. Mow frequently at a medium height, and when mowing make sure you rinse off your lawn equipment to help prevent the fungus from spreading.  Make sure you remove any excess dead grass, or make an appointment to have your yard de-thatched.  Make sure you water early in the day so that your lawn can dry out before the strong heat at mid-day.  Finally, September is the best time to fertilize your lawn, check your lawn and see if you need to add nitrogen to the fertilizer.  Rust fungus is not harmful to humans and pets, but can be easily spread by them.