I did a quick google search and it almost seems like an example of magnesium deficiency. They need potted up or put in the ground, so I could easily see that coming on. Hmm

I see that occasionally, and my bed tests out as "High" for magnesium. So I suspect it is something else. I think there are some fungal infestations that put white spots on tomato leaves. Powdery mildew is another possibility.

Did you just recently put the plant outside?

It's lived in a low tunnel all its life but lately I've had it outside. It is well hardened.

Hmm- it really doesn't look like powdery mildew. Is it really warm enough for fungi yet? The humidity has been consistently around 30% lately with temps in the 70's during the day and 40's or 50's at night.

Is there any chance that maybe it's slight tissue damage from being exposed to temperatures in the mid 40's at night, with no cover? It has started just recently after I've exposed them to that.

I will try and spray them with a fungicide. It would be a good idea for preventative spraying to be started anyhow. Couldn't hurt.

Can you get a better photo of the damage? Closer up and in focus?

Let me add that after this has started, I also started seeing little black/red bugs on it. On a few, and I didn't think much of it. Coincidence?

I have no idea what the bugs are. If I find it again, I'll post a pic.

That's a lot of plant in a small pot.

Likely ran short of water, briefly.

choose your strategy: Water more often: pot into large r container; plant into garden.

I can't get a clearer photo with my phone. I will get out a camera tomorrow and try to get a better one.

Jean, I've had a look at the root system and they are ready to go out or be potted up, but they're not rootbound. The pots did happen to dry out a few times this week in the sun, but that has happened many a time before, and this is the first time I am seeing this.

After viewing that article, I have not seen the examples of a thrip larva and adult shown, BUT the damage they show looks almost RIGHT ON target with what this looks like. I will get you a clear picture tomorrow- but if it is thrips, how would I treat that?

Not thrips damage. Look closely; margins of white areas are different.

Should not wait until rootbound to pot on.

My first thought was edema exposed to the sun and/or sunscald. Whatever it is, it looks like it's on the old growth. If the newer leaves eventually get it, too, if you don't put it back in the high tunnel (where edema might be a bigger risk), I might be concerned, but it doesn't look like much to worry about to me.

Edema and sunscald are both pretty harmless. My guesses could both be wrong, though.

So, I actually had a friend of mine have a look at it, and she said it's environmental damage, which makes sense. Around here it's been very windy this year, drying out these pots before I can water them in time. Then the wind dries out the leaf cells when the pot is dry and causes what I'm seeing. I've been keeping on top of watering this past week and haven't seen it on any other plants, so I'm thinking that's the issue- similar to what Jean suggested.

Yes, they either need potted up or put in the ground. I'd really originally planned to pot up now and put them in after Mother's Day, but we will be leaving on a trip, and I'm thinking of getting them in the ground as soon as possible, as there's no way I can keep them watered during the trip in pots.

Yeah, I've been keeping them out of the low tunnel at least during the day now. It's impossible to keep below 100, and likes to average 120-130 with the sun on it, so...(and that's with it at 70 outside!)

Thanks everyone

Could also be leafhopper. They make similar damage, and they're so mobile you rarely see them on the plants feeding.

It is not leafhopper damage nor thrip damage.

I'd put money on it that it is some sort of chemical burn, probably from liquid fertilizer.