This could be any number of fungal diseases: Snow mold...but the solution is proper maintenance and management practices of your lawn. Chemicals for 'Turf Building' are not the answer.
Please send or tell us the product more closely, what it says on the product bag...this is most certainly a fungus. You don't want to use band aids on this problem.
Mow your lawn HIGHER. 3inches plus no lower. Train your grass to grow deep roots and to only need once per week watering...1 inch of water per week. Fungus will only grow if you are watering daily, watering at night. Let your lawn dry out.
Fungus is easily spread by shoes, mowing (please bag your clippings). Fungicide is rarely helpful.
Other practices include; SHARP blades, clean the blades before and after each and every mow, proper fertilization (we can discuss this later), aeration by pulling plugs of grass and soil and leaving those plugs on your lawn at least once per year...mow no lower than 3"...I can explain how to measure the water you apply and how to train your grass to be drought tolerant. Moisture control is the best way to get rid of fungus! Fungus needs constant moisture. The other wrong management practices that cause lawn grasses to be weak and vulnerable are; too short, too little chemistry available for photosynthesis or even too much nitrogen going into the winter or dormant phase. This sets your lawn up for fungus amungus. Fungicide after the fact is fairly worthless.
Please explain what your management practices have been. Where it is you live. How you do your watering, your fertilizer. How often you mow and how often you sharpen your blades. How large is your lawn?
Edit: Did you use any 'grub' control last year? Actually it is a non specific insecticide that kills all insects in the soil. Including those involved with grub control. The next year you lose your lawn completely.
If you are thinking grubs (these are very season specific), try taking a 5 gallon bucket of soapy water, pour it on the soil in one spot. If you have grubs, like cranefly larvae they will come to the top and you can count them. Count how many you have in one square foot.
If you did use pesticide last year, that is why you have a problem. Do not do that again. You WANT the soil to be full of insects, soil organisms. Otherwise you will have to put pesticide on every year and your grass needs that life in your soil to be vigorous.
I would get a sod cutter and take out the damaged sod and replace. That is what we had to do every time a client killed all the life in the lawn bed. Replace the lawn and add decomposed organic matter to promote the soil life that relies on decomposed organic matter for energy.
Since you are thinking grubs, I think that you might have had to deal with this last year and used a non specific insecticide. Please tell me I am wrong. If not this is a fungal infection most likely because of too much Nitrogen right before winter or the opposite, too little chemistry before the winter.