Posted: 13/07/2012 at 22:23
Brandywine Pink? It is a Potato Leaf? With the almost smooth, slightly scalloped edges? As distinct from the traditional saw-tooth edge of a tomato leaf? Glorious tomato. Possibly the best I've ever tasted. Only Marianna's Peace comes close, I think.
It's so hard to know without actually seeing anything. Did you notice whether the problem developed from top to bottom or vice versa? The brown underneath hints that it might have developed on the underside. Septoria Leaf Spot does that and it, along with Early Blight, are the most common fungal problems in the home garden. They both tend to start from the bottom of the plant - the older leaves - up. EB is more apparent on the top of the leaf and pretty quickly develops a "halo" around the spot.
You might not want to, but I'd be inclined to leave one leaf in situ and watch it. If it's SLP, you'll see little pin-head type mini-bumps develop inside the brown patch. If it happens to be SLP it won't terminally damage the plant if you contain it. None of the fungal problems do massive instant damage - except Late Blight, which will kill a plant in weeks - and the onset of cold weather usually terminates things before the fungal problem does.
Sorry I can't be more specific. Without an image, it's sort of guess work.
I overlooked your earlier query about doors and windows. Air circulation is an absolute priority in greenhouses. A closed environment can be an incubator for all sorts of nasties.