Could be fertiliizer burn, spray burn, or spider mites. If spider mites are bad enough to kill leaves, their webbing should be visible. Leaves that are still greenish will usually show yellow stippling on the top surface, and the underside looks dirty and dusty. Mite damage is worst on lower leaves.

Thanks for all the great advise. Simple fix - I've inserted slow release 10-10-10 bonsai fertilizer pellets into the soil and AWAY from the clumps of moss. So far, for the past 10 days, the moss is doing great, and the fertilizer is reaching my trees that are slowly waking up from their winter nap. As long as the moss is spread out in clumps with open spaces for pellet fertilizer, everything should be fine. This way the pot won't get saturated with moisture, and the trees roots can breathe and get fertilized. The type of moss I use doesn't like the grainy bonsai soil, so I've put muck dirt under each clump. With the cold damp 35 F degree nights, and the warm 50 degree days, my moss is thriving like crazy (as long as I spray-mist it with water every day). I've collected about 5 square feet from our property.....I found so many new species of moss that I never knew they grew around here. What I'm doing is using slabs of 24" x 12" slate, and creating micro-environments with some bonsai, driftwood, rocks, various soils, and the most important part - MOSS. I'll post some pictures in the gallery by this weekend. David in zone 5 b

so tiny that not a problem unless, of course, they become much larger.

That's KBG, right? My understanding is that summer patch shows up later in the season, like July/August, but the infection is in the soil through the winter and starts back up in the spring, working on infecting the roots. I'm surprised it has progressed so soon. So maybe it's not summer patch, or the other possibility is that I don't understand it well enough myself. Below is a photo of my lawn last year, which looks an awful lot like yours. This year I'm mowing a little lower (down to 3 inches from 4) and have been applying the fungicide Headway G. It is a little pricey but it goes a long way. You can apply the fungicide and it might minimize the damage. It is very likely that the damage is already done and you'll have to live with whatever happens by the end of the summer. If I were you I would immediately take a sample of the border between the diseased area and good grass and send it to a local lab who can tell you exactly what you have going on. They will tell you how to sample it. It might cost you $100 or so but, IMO, having absolute certainty in what you are dealing with is worth it. If I get another out break this year that's exactly what I am going to to. The treatment for all bad fungi is not the same. Sometimes you need to feed N and sometimes you need to starve it of N. It would be helpful if you could take a couple of photos of the grass from 3 or 4 inches away and not in full sun. A cloudy day or closer to sunset is best. We might be able to tell something from what the lesions on the half dead grass blades look like.