My spots are more orange than yellow, almost a rusty color, but I know that color is subjective. I have read the feedback and still haven't found a solution, just that many other folks had this problem. I'm in zone 7 and planted some seedlings as I'd arrived too late to plant from seed. They were doing great but the lower leaves of 2 out of 4 plants have these dry looking orange spots. Any ideas of what to actually do about it, what it might be? Thanks!!

I'm no expert, but if they were my plants I'd blame the rain, but keep an eye on them. Now, I'm assuming you've had quite a bit of rain. Or perhaps only a few inches, but the tomatoes are in a low-lying area, or your soil doesn't drain as well as you'd like. Anyway, have a look at the thread linked below. Here is a link that might be useful: http://forums.gardenweb.com/forums/load/tomato/msg0621241916357.html

I'm not finding photos of necrotic blight, but other blights show darker spots with a circular pattern not seen on my leaves. Also my fruit is unaffected. Septoria Leaf Spot sounds and looks similar to what mine have. Either way, the treatment is removal of affected leaves and a copper spray. I will do both as well as a preventative spray on my other tomatoes. Thanks, Omni.

Yes, atleast the packaging says so. The current one is 13-40-13 plus micro elements such as boron, iron, manganese, molybdenum, zinc, copper. They were transplanted recently and I give them more phosphorus because of weak roots and thin stems. A friend of mine thinks it's either magnesium deficiency or leaf mold due to the high humidity inside the greenhouse however I regularly open the windows for air flow and seal it for the cold nights only.