I've included a picture of a Mortgage Lifter leaf below. I'm hoping someone can identify what is causing this and tell me what, if anything, I can do to save the plants.

Some background: I started Mortgage Lifter, Yellow Pear, and Sweet 100 seeds in Jiffy Mix at the end of March. Plants have been doing very well (8-10" tall, nice green color, etc) under 4 4ft fluorescent lamps until 3 days ago. The lower leaves (first "true" leaves) on several plants have curled up, dried up, and then fallen off. Most leaves have gone from green to brown/black, but a few have yellowed before falling off. Top of plants still look nice and green, but the problem is moving up to the next set of leaves. I was careful to sterilize my pots before planting, and as it's a cold spring these plants haven't left the basement. I have bottom watered using a liquid 6-4-4 mixed at half strength every third watering with plain water in between. The lights are on 15 hours a day along with a 12" fan. I gave them a shot of 15-30-15 Miracle Grow mixed at half strength to up the phosphate. Thought about trying some Epsom salt for magnesium, but my liquid 6-4-4 concentrate already has 0.6% magnesium so I held off. I couldn't see any critters on the bottom of the leaves, but I'm not exactly sure what I should be looking for, either. Is there anything I can do to save these, or is it time to start over from seed?

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