Yellow color in the lawn
I am often asked how to green up a cool season lawn that has turned yellow at this time of the year. The answer to the question will depend upon the cause. Although a yellow cast to a cool season lawn can be due to several reasons, I usually find that during June and July, the cause is a disease.
If your fescue lawn looks yellow, or has a yellow cast to it, go out early on a morning when there is a lot of dew, when no rain fell the night before, and before the sun hits the area you are looking at. Then get on our hands and knees and look at areas where there is a transition between yellow and green. Look for very fine "cobwebs" at or near the soil surface. If you find these "cobwebs", your lawn is probably suffering from dollar spot or brown patch, both of which are lawn fungi.
Brown patch will show up as irregular shapes on fescue lawns. It very seldom is found on bluegrass or rye lawns. Dollar spot will start out as small round circles, about the size of an old silver dollar. As the disease progresses on fescue or bluegrass lawns, these silver dollar circles will get larger, and finally coalesce.
If you find these "cobwebs" in your lawn, then inspect individual leaves of grass. Brown patch will show up as brown areas in the shape of an hourglass on the leaves of fescue. The edges of the brown areas will have a darkened margin along the brown area. Dollar spot will show up as brown area on the ends of the leaves. It too will have a darkened margin between brown and green areas.
If you have a bluegrass lawn, the leaves may have brown spots on the leaves, encircled by a darkened margin. This disease, called leaf spot, usually starts out the early spring. As the season progresses, the disease begins to work itself down into the crown, and plants here and there begin to dye. This is called the melting out stage of leaf spot.
If you find none of these of these "cobwebs" or leaf disease configurations, your lawn could be suffering from lack of nutrition or from an insect infestation.
If your lawn is suffering from a disease, there are several fungicides on the market that you can use, such as bayleton or daconil. Make applications of these products about every three weeks during the time of hot and humid weather conditions. Do not apply fertilizer or water your lawn a lot because you can actually encourage the growth of the fungus. You can also wait for weather conditions to change.
If you find that your lawn suffers from disease on a yearly basis you may want to overseed your lawn this fall with a combination of both bluegrass and turf fescues. Each species of grass is susceptible to different diseases. For example, when fescue is attacked by a fungus, the unaffected bluegrass will look good. When bluegrass is attacked by a fungus, the fescue may look good. In effect, you are masking the appearance of the diseased plants. Although you have not prevented the infection of your lawn, you have, in effect, kept the appearance of a good lawn.