"When all you have is a hammer, everything looks like a nail." I think that came out wrong. I was not hinting at anything you said. I was setting up for what I was about to say. My hammer is corn meal, which in the organic world is recognized as a fungal disease preventer and cure. The nail is your yellow grass. When I see the yellow grass, I assume it is a fungal problem. I can see where that could easily be misinterpreted. Sorry. YES, you have to wait until late summer to reseed. Why? Well you don't have to but if you want good results, wait. Seeding now will give you a yard full of crabgrass right away and later on, any grass that did germinate will die from the heat allowing the crabgrass to fully take over. If you want to fix that for the future, you could seed in some Kentucky bluegrass in the late summer. It will spread to fill in weak spots where fescues do not spread.

Yep, that looks like Dollar Spot, and it appears to be coalescing together in one area. As you are mowing, you are spreading the mycelium to other parts of your yard. You also need to make sure you wash off your mower after each use. Go get some Immunox from Lowes or Home Depot and apply according to label directions. I would suggest mowing the afternoon or evening before you apply, then apply Immunox early the next morning. Then do not water or mow for 24 hours. A repeat application will probably be needed 14 days after the first application. As I have stated on other threads, whenever I see Dollar Spot, I immediately think of a lack of Nitrogen, especially this time of year. Another way to tell if it is Dollar Spot is to go out to those areas early in the morning and see if they have a cottony, web-like appearance.

I can't tell by the pics. It just looks like brown spots. It might help if I could see really close pics to examine the leaf blades. Hopefully, someone else will be more helpful. Or to know for certain, you can take samples to your nearest University of Wisconsin Cooperative Extension Service for diagnosis. Call them for sample kit and instructions.

OK! Right plant, wrong species! That's why I asked where you were located. Maianthemum dilatatum is exclusive to the west coast. Wasn't aware there was an eastern version :-) And this is a wildflower - not necessarily a garden escapee. The tarp could help but it will also smother the grass. And it is not that effective for plants that spread rhizomatously (via underground stems). The rhizomes just keep growing to where the tarp doesn't cover and then pop up. Depending on how shady the area is, you might just want to encourage it and skip the lawn. Lawn grasses are difficult to encourage to lush growth in very shady locations and a shade loving groundcover - like this - is often recommended as a suitable replacement. Otherwise, containment would involve some sort of root barrier/edging at the edge of the lawn and some targeting herbicide applications. Any broadleaf weed killer for lawns should work well - the stuff can't tell between a 'weed' or any other type of broadleaved plant. Just spray individual leaves directly and be sure none gets on anything more valuable.