Large areas of my lawn have turned orange. Why?

FAQ
Question: 

Large areas of my lawn have turned orange. Why?

Answer: 

The orange color is likely due to rust.  Rust is a fungal disease.  It can be diagnosed by walking across the lawn in white shoes.  As you walk across the lawn, bright orange spores of the rust fungus will rub off onto the shoes, giving them an orange appearance. 

Rust most often occurs in mid to late summer.  Warm days, moderate night temperatures, high humidity, and heavy dews provide favorable conditions for rust infections.  Rust is most severe when lack of water, low fertility, or soil compaction slows turfgrass growth. 

Rust is best managed with a balanced fertility program and regular mowing.  If possible, irrigate early in the day to reduce drought stress.  Core aerate if soil compaction is a problem.  In most situations, fungicide sprays are not necessary.