The nominees for the dead spots are:
1) Dry spring. I started watering too late and some patches never recovered. But I never watered the back lawn (shady) and it has recovered nicely after the recent rains.
2) Fertilizer burn. Perhaps, but the telltale bright outer ring is missing and no amount of watering has helped this. Also, the same dufus (moi) fertilized the back and front lawns.
3) Crappy sprinkler coverage. We had one impact sprinkler that I moved around and there is no question that it misses spots. But this has not been an issue in other years.
4) Mould or Fungus. I have no experience with this.
5) Grubs. This seems to be the opinions of neighbours and others, but the Skunks and Raccoons do not concur. Also, I have excavated two 1' square patches, one in the dead zone (3 weeks ago) and one outside the dead zone a few days ago. I found ZERO grubs. Perhaps I waited too late to dig up a test patch, but we have had no Skunks or Raccoons digging up the lawn like other years. We have also had the lawn treated with Merit several years in a row, although not last year.
Your advice would be appreciated. I'm leaning toward a combination of 1 & 2, but I have no experience with Mould.
We recently had REAL sprinklers installed, as we plan to put some soil in the dead spots and seed them. But I'd like to solve any underlying problems before I do this.
I'm in Zone 6A (Southern Ontario)