There are several different rust fungi that cause rust. The most common one on Kentucky bluegrass, annual bluegrass, fescues, and ryegrasses is Puccinia graminis . A separate species, Puccinia zoysia can infect zoysia grass. Rust becomes a problem when grass plants are growing slowly. When grass plants are growing fairly rapidly, leaf tissues are removed by mowing at relatively frequent intervals, and the disease does not become apparent.
Rust fungi are obligate parasites.
That is, the fungi can reproduce only in living tissues. When leaf tissue is removed, the mycelium of the fungus in the removed tissue dies. These spores are then wind-blown or splashed by rain or irrigation to other leaves where new infections can occur. Consequently, the disease can become very severe when certain weather conditions occur when coupled with slow grass growth. Rust infections first appear as minute yellowish flecks in infected leaf blades.