Orange colorations indicate rust fungus, and they can affect the stems, leaves, and crowns of your lawn. Low temperatures will arrest its development. Spread and infection is slight during late spring and early summer when your grass is growing rapidly. But symptoms become more evident in the late fall when the growth of your lawn is slower. The ultimate environment for stem rust is 8 to 16 hours a day of high light intensity with air temperatures of 85° to 95°F.
Stem Rust occurs when Kentucky bluegrass is grown. The cultivar Merion is extremely susceptible.
Leaf Rust is widespread, generally found throughout the summer. It's usually of minor importance. In heavily infected years, the leaf blades are distinctly yellowed.
Crown Rust is a common disease of Italian ryegrass, perennial ryegrass and fall fescue. The bright yellow fungus is often found wherever those turf varieties are grown.
Rust can become a major nuisance turning 'Everything Orange'. Rust can be reduced by core aeration and overseeding more resistant grass types or by use of a fungicide.