Bentgrasses are usually not affected. Rust fungi are obligate parasites and infect only living grass plants. Two or more rusts may attack the same grass plant at the same time. Grass plants are most easily infected under stressful growing conditions.
Rusts are most severe when water and fertility are less than adequate for good growth.
Most rust problems occur on Kentucky bluegrass, perennial ryegrass, tall fescue, and zoysiagrass. These diseases occur throughout the United States wherever susceptible grasses are grown. Most rusts do not usually become a growth-limiting problem until mid to late summer during extended, warm to hot, humid, but dry periods when grass grows slowly or not at all and nights are cool with heavy dews. Click on images for larger version Figure 1. Leaf rust on bluegrass showing advancing stages of infection.