That looks like a fungal disease. As you noted, it's rained a lot.

While unusual for October, newer lawns are certainly going to be the ones that get them.

I just had pretty much the same issue on another lawn, much further south, but your problem is much worse, you don't have much time left on the clock, and so I'm not going to copy that advice here.

At that point, try DiseaseEx from the Home Depot and ignore anything any salesperson says. It's a wide-spectrum curative, will prevent fungus for 2 weeks, and that should give you enough time to start moving into winter.

Expect losses with that much damage, on a young lawn, and with having to apply a fungicide (although I chose the most gentle one I could with a wide spectrum) both on a young lawn and just before a Michigan winter. You did catch this about as early as you could, it's just that this went pretty far pretty fast.

Strictly secondary, but it's more likely that your ion balance is out of whack. We have a saying around here that basically reads, "You don't have clay soil." :-) Although about 2% of people actually do, it's exceedingly rare, and even then, it's usually a clay loam, which is not clay.

If it drains that poorly, a soil test from Logan Labs would be in order, as the ion balance is probably quite poor (as I noted). That can be fixed.

Great pictures. I agree with the fungal disease diagnosis.

In addition to Mother Nature, what is your watering schedule? How often and for how long do you water?

You mentioned Ryegrass, any idea if it's annual or perennial? Because the grass blades in the images you provided look too broad for perennial. In general mixing annual Ryegrass for a permanent lawn is a bad idea.

Neither you nor the hydroseed company is responsible for rain. They will fight you on that. As for thin spots, they should work with you.

Are you still watering after the rain?

@adel - The Hydroseed company gave me an extra large bag of the following:

25% - SR2100 Kentucky Bluegrass

20% - Fiesta 4 Pernnial Ryegrass

20% - Wild Horse Kentucky Bluegrass

20% - Fielder Kentucky Bluegrass

15% Sideways Perennial Ryegrass

No idea what it could be if it's not the Bluegrass or Ryegrass. Unless the extra seed bag mixture they gave me is different from the initial hydroseed. I figured they used the same mixture as the bag they gave me.

With that much damage, it's hard to tell, frankly. My guess, given the tips, is ryegrass. Perennial. There are plenty of cultivars in that broadness range if I'm reading the lengths correctly.

Annual PR is rougher than that.

As a second opinion, and since you live in Michigan, I suggest reaching the MSU Extension as soon as tonight.

Also, I found this MSU Home Lawns page to be very informative and suggest looking over "Performance of Kentucky bluegrass cultivars in Michigan", "Mow at the highest setting" articles; quote from the former, "This grass species does not tolerate poorly drained soils." You should consider aeriation in the near future after your turf get well established.

Regarding the seed mixture, and even though this is most likely what the company used, still it won't hurt to verify! And don't forget storing the bag in a cool place, not the garage, and maybe use it as soon as possible before the seed is forgotten in storage and gets too old.

Earlier you said, "I also fertilized at the 4 week mark using the recommended 16-32-3 (Starter Plus) mix." Now I am wondering if you also added a starter fertilizer before seeding, which in this case would be too much and might have contributed to the problem.

Update: Turf flooding is another interesting read.

I'd put down some more fertilizer. If it were me, I'd put down Scott's starter with pre-emergent because you still are under threat of weeds that mesotrione can mitigate. Hopefully some of the affected grass is just the perennial ryegrass. You don't want that crap anyways, just as a nurse grass. I feel like any fungal issue needs to be dealt with for a new lawn--even more benign stuff like rust and powdery mildew.