Yellow grass on a lawn can indicate injury, stress, nutrient deficiency or disease. Because many different issues cause the same symptom, determining the exact cause of yellowing grass is difficult.
Injury to grass generally results in yellow spots only in small areas. Foot traffic, dog urine and chemical spills result in yellow grass in a very specific area. Stress affects the entire lawn. Extreme temperatures and incorrect amounts of water or fertilizer result in stress. Nitrogen and iron deficiencies also affect the entire lawn and generally occur slowly. Taking a small sample of the lawn to a nursery can help determine the presence of disease if no other cause is suspected.