Answer: Yellow spots will slowly spread, but they won't kill your entire lawn in a week's time. So, that leaves the fertilizer application as the culprit. You probably applied too much at one time and what you've ended up with is fertilizer burn. With any luck, the fertilizer only burned the leaf blades and not the roots of the plants. If this is the case, the grass will recover if you keep it well watered. (The water will help rinse the excess fertilizer off your lawn.) If the fertilizer worked its way down to soil level, the excess salts may have killed the roots, as well. There's not much you can do in this instance other than rake out all the old dead grass and roots and reseed with fresh grass seed. I know it's too late for you this time, but it's really important that you follow the application rates listed on the package label. As you have learned, more is not necessarily better when it comes to feeding your lawn.
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