Looks like septoria leaf spot beginning. I'd remove the affected leaves immediately, and begin a spraying regimen weekly with a fungicide. Suggest either Daconil, or copper, or both. I use both and alternate. You can defeat it if you catch it early.

Is that only one leaf that has the spots?

I agree that it looks like Septoria. It blows in on the wind around here. I always remove the infective leaves, and my plants usually make it through to the end of the season.