Tomato Leaf Discoloration

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!
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Suzanne said:

A few hours in a sunny spot? That might be enough to sunburn some tender leaves. The seedlings need to be gradually acclimatized to full sunlight. I usually place them where they will get indirect light or dappled shade the first time they go out, then place them farther and farther ahead of the shadows.
Indoor plants will also need shelter from the wind when they first go out. I run a fan on my seedlings while they are indoors. The air circulation is good for them and the breeze helps toughen them up a bit.
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Pat in Plymouth MI ('someplace.net' is comcast)

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Were they on the outside of the flat?

I think it's just plain sunburn. If it was a windy day, the leaves could have been battered some, too. Pat's advice is spot on, and pretty much what I do, too. Filtered sunlight for a few days, then slowly moving the plants out to where they get a little bit more sunlight each day.
If you don't have a handy tree, try making a low tent out of a light colored sheet. Just be sure that the plants are positioned so that as the sun moves across the sky, they don't get any direct sunlight. After a few days of this bright indirect sunlight, you can move the plants to where they get a little bit of morning sun, but are protected by nine or ten o'clock, depending on your location. Then each day let them get a little bit more sun.
Good luck!
Penelope
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The blotches that you found on your tomato were porabably sun burn, you said that they were under shop lights, that plants that are grown inside are not yet used to real direct sun light, once you put them under the sun it will effect them with a sunburn, this should go away in couple of days, as the plants get used to the real sun.
Good luck
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To diagnose, check out:
http://aggie-horticulture.tamu.edu/tomatoproblemsolver/index.html
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Warm Regards,

Claire Petersky
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I/ve noticed the same thing with my seedlings grown under lights - a lighter green patch in the leaf/s center. I/ve sen it happen with Bell peppers, too.
Not to worry. When all is said and done, it won/t be a problem.
Harden off your plants for about a week when min temperatures average ~55F, set them in the garden, and you/ll be fine.
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Thank you everyone for your advice. I removed the affected leaves and the plants seem to be doing fine.
Does anyone have a good fertilizer suggestion? There seems to be too many options.
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I water seedlings with fish oil emulsion, then side dress with 5-10-10 fertilizer about a week after setting them in the garden. Once a month I foliar feed with a 2-3-1 seaweed solution. Avoid high nitrogen fertilizers or you/ll end up with all vine and no fruits.
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TQ






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Suzanne said:

Well, for seedlings I like the fertilizer sold by Gardener's Supply Company:
http://www.gardeners.com/Plant-Health-Care-for-Seedlings/Houseplants-8-O z/default/31-279.prd
without line wrap: http://tinyurl.com/rmusn
In the garden I use Espoma's Tomato-tone and Maxicrop for a foliar spray. (Maxicrop is made from kelp.)
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