Probably a fungus like leaf spot. I've never done it, but you could probably treat it with a fungicide if it really, really bothers you.

I see it lots. It seems to me to be a good barometer of the stress that the plant is going through. For me, if the plant is not stressed I see very little of it. And so if I see it, I try to find the source of the stress.

Being in the ground for only a month is pretty stressful for a plant.

Good Luck. Hay.

Could it be that they are getting burned from the sun? It was very hot where I live during the month of June. Lots of days in the 90's. I planted nine Endless Summers the beginning of June. Some of the plants have some burned leaves now (brown and dried spots). I may have to move them, but I think I'll wait until next year to see if it was just the extreme temperatures and the plants being so young.
Good Luck!