Tomato Seedling Trouble, Again

Sigh...
Last year I had some trouble with my tomato seedlings (Hardening Off & Tomato Problems).  Then later in the year all of my tomato plants - and my cucumbers as well - got sick.  This year all of my tomato seedlings were doing wonderfully until a few weeks ago, when I started to notice that some of the leaves were yellowing, turning black in some places, and even falling off.  I tried fertilizing, and picked off all the yellowing leaves promptly, but I think they have some kind of blight.  I'm pretty upset, since I thought the problem last year was a disease and went to great lengths not to allow any kind of contamination (new pots, new soil, disease resistant seeds, no tomatoes or cukes in the home garden this year, etc.)  It's also just disappointing to watch them growing so strong and healthy and then have to trash them.  But, it's better than watching them grow for months and only getting about 10 tomatoes, which is what happened last year.

I tried keeping them alive in the basement and just letting them go to see if it would make the problem easier to identify, but I never could see any "concentric rings" that would indicate early blight.  So either it is a fungal problem and the spores are in my basement (totally possible and I think I may have to move my whole operation upstairs next year) or it is a disease (in some of the seeds left over from last year?) and was spread from plant to plant by the marauding critter that's been nibbling down there.  Whatever the cause, I decided to buy my tomato seedlings this year from a reliable source.  At least I did get a lot of other things off to an early start, so I don't feel like my seed-starting was a waste.