DOES YOUR LAWN HAVE SNOW MOLD?? Are you are seeing irregularly shaped damaged areas in your lawn? Look closer at them. Are they matted down with patches that look slimy or crusty, yellowing or browning, possibly with a slightly red or gray coloration? If so, you have snow mold. Snow mold is a fungal disease, so named because it most often occurs under the cover of snow (although not always) such as we had in January.
So what do you do? Temperatures have risen so it is unlikely you need a fungicide application to stop further damage. Ideally, you can 'fluff up' these damaged areas with a leaf blower. This greatly aids the recovery of your turf by promoting air movement and breaking up the crusty areas and removing any debris, allowing the grass to recover more quickly. If you don't have a leaf blower you can lightly rake these areas, preferably with a grass specific rake. While not as effective as using a blower, it will still be helpful in the recovery process.
Your lawn care specialist will advise if he sees it in your lawn and recommend any additional measures (such as fungicide) that might be necessary. The fertilizer he is applying will also be very helpful in the recovery process.