If the leaf blades of your lawn turn yellow at the top of the leaf, your lawn may have an iron deficiency. Mowing at too low a level can cause this problem. The roots of a scalped lawn cannot provide enough iron to the rapidly growing leaves. Don't confuse an iron deficiency problem with a nitrogen deficiency problem. Adding more nitrogen to an iron-deficient turf increases growth and puts even more stress on the roots. Instead, add an iron supplement or aerate yellow areas, and apply a 1/3-inch layer of compost.
Rhizoctonia Yellow Patch
When spotty areas of yellow appear in a Kentucky bluegrass lawn, you might have to address a problem called Rhizoctonia yellow patch. The fungus Rhizoctonia cerealis causes the disease and typically attacks in cool, wet weather, advises North Dakota State University. Compacted soils also increase a turf's vulnerability. The area typically turns light brown after yellowing. Cultural controls includes aeration, dethatching and reducing excessive moisture. No effective fungicide for this disease currently exists.
Certain insects attack lawn turfs and cause yellowing. One prominent pest of turf lawns, the chinch bug, hibernates in lawns during the winter, then lays its eggs near the base of the grass blades in the spring. The signs of chinch bug damage, small yellow areas that increase in size as the bugs continue to attack the lawn, usually occur during hot, dry conditions. Natural predators typically control chinchbug populations, but for severe outbreaks, you can apply an appropriate pesticide. Follow label directions.
St. Augustine Decline
St. Augustine decline, as the names implies, attacks St. Augustine grass. The first sign is a yellow mottling appearing amid the green of the leaves. This viral disease does not not kill the lawn but severely weakens it. A few seasons after the symptoms first appear, the grass might not have the ability to resist an invasion by Bermudagrass. To solve the problem, plug a resistant variety of St. Augustine into the affected location.
Lawns need nitrogen for healthy growth. When they don't get enough of this vital nutrient, yellowing can occur. The yellowing typically shows up on the leaves closest to the soil. Adding nitrogen, one of the primary ingredients of most commercial fertilizers, should return your lawn to a healthy green state. Spread the fertilizer over the affected area in the amount suggested on the packaging.