Healthy lawns generally have a consistent colour across their entire surface. Light green spots in the lawn indicate a few types of lawn diseases, which often originate when a gardener does not know how to care properly for a lawn.
Dog urine or chemical spills can also cause patches of lawn to die off, and they may turn light green during this process. Nitrogen and iron deficiencies can cause lawn lightening, and lawns sometimes also become lighter in areas of high traffic.
Dollar spot fungus shows up as blotchy areas of lighter colouring on the lawn, and cobweb-like fungus often shows up in the morning dew. If the lawn has rings of lighter colouring, it may have necrotic ring spot fungus.
Cornell University recommends preventing lawn diseases through proper lawn care rather than trying to get rid of diseases after they show up. To prevent the moist environment that fungi love, water grass during the morning to allow time for the water to evaporate during warmer daytime temperatures. To fix nutrient-based light green spots, send a soil sample to a local nursery or lab for nutrient testing and add more or fewer fertilisers based on their recommendations.