See pics

Any fungicide you recommend? Also, do I reseed after that?

Scotts disease ex or bayer advanced.

Reseed in September.

Have you been getting a lot of rain like the folks to the south are reporting? Or are you watering more than once per week??

Yes, non stop rain all Spring. I did a bit more research myself. I don’t think it’s grubs but snow mold images on the net seem like a closer match.

Good, in a way. At least you can blame Mother Nature for the over watering.

Here are more pics. Didn’t see any grubs.

Are those patches still growing? It actually looks like the disease may have burned itself out already--I see no signs of active rhizome growth.

That doesn't mean you're in the clear, just that I don't currently see it. :-) However, if there's no active disease and the grass isn't dying, there's no current need (or reason) to treat for the disease. It's not active and not there to kill.

Yeah, the patches are dead. So in the future how do I prevent this? Do I take out the dead grass and reseed?

I actually meant--are the patches of dead increasing in size, or is the dead area now stopped and no longer increasing?

Depending on your grass, it may re-invade on its own. Bluegrass won't have any problem filling in holes like that this fall. Fescues don't fill in holes well and will require reseeding in August, but you don't have to remove the dead grass in either case.

What I'm seeing looks like a random mix of grasses, but there's no good closeup of the grasses themselves and I'm lousy at ID-ing older cultivars anyway.

Ok, the patches are not expanding in size. I think I used Scott’s northeast mix so p-rye, fescue and kB. So for next year, any steps to prevent?

If you see it coming on, a preventative fungicide if we can ID what it was! If this was something that materialized overnight, then I'd simply apply a preventative next year a couple weeks before this year's onset date (again, if we can ID what it was).

I wouldn't necessarily object to a wide-spectrum fungicide if we can't ID the disease specifically. Just for that one year.

This "fall," I'd overseed if the holes don't fill in (for you, in ME, the weather is likely to be compatible around August first to August 10th--I used to vacation in Casco Bay off Portland, and by Labor Day, it was actually getting too cold).

I might be inclined to go with a different but similar-looking mix just to get some different cultivars in there that might be resistant to this particular disease. It's not a good bet but...worth a shot. In my lawn, where I specifically chose my own cultivars and ordered individually, one consideration was disease resistance and that each chosen grass had different sets of susceptibilities so that any given disease didn't have a completely clear field to wipe out.

Thank you for the detailed response; some great points like mixing up the grass seed. August sounds like a plan for reseeding.