Answer: Often, dried out edges on leaves indicates overwatering. I would try cutting back a little on the watering--keep them slightly moist, but at no time soggy. Also, try putting a fan on low in the room, to create some air circulation. It is possible that they have some type of leaf disease, but if you used a sterile seed-starting mix, and none of the others are affected, I doubt that's the problem. Purple coloring can indicate a phosphorus deficiency--but many tomato seedlings do have a purplish cast. If you have been feeding them with a balanced fertilizer solution at the recommended rates, then nutrients probably aren't the problem either. (Be sure you are not over-fertilizing with too strong a solution--concentrated fertilizers can "burn" the leaves.) Try giving them less water, and more room to "breathe" and see what happens. To be on the safe side, you might also try starting more seeds.
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