This is my first time growing tomato seedlings. They are 3 weeks old and growing large. All was going well until today I noticed some of the older leaves on the Brandywine plants are begining to discolor. It almost looks like the green is rubbing off in a large blotch in the center of the leaf. The color is paler that the rest of the leaves. I haven't been able to find any photos of diseases that look similar.
It is strange that I found this on 3 Brandywine plants that are located far from each other with plenty of healthy-looking plants in between.
Is it possible that this discoloration is sunburn from water dripping on the leaves (I have heard that it is bad to get water on the leaves)?
Another thought is that I took the plants outside yesterday and today for a few hours. They have grown under shop lights exclusively but I thought they might enjoy the nice weather. Would this have anything to do with their problem?
Or if this is a fungal problem, how can I treat it?
Thank you for your suggestions!