Both grass rust and red thread are lawn diseases caused by a fungus, and they both tend to affect the same kinds of turfgrasses.
However, grass rust produces a yellow, orange, or brown powder that will come off on your shoes as you walk through your lawn. Red thread, on the other hand, will cause isolated patches of brown, dying grass, and if you look closely, you’ll be able to see red or pink fungal threads. Additionally, red thread tends to affect lawns during mild spring weather, whereas lawn rust prefers the warm, humid weather of summer and early fall.