Hard to see in your picture but the cup you have it in is sure pretty small. No question that tomato would be highly root-bound if it was in soil. Not quite sure how hydro relates to container size, but ...

There is a separate forum for hydroponics. This question might be more appropriate there.

TMV is almost non-existent in the US but don't know where you are located. I'd say it is far more likely Septoria leaf spot which is VERY common, especially in cooler Spring weather. Remove those leaves. Never leave damaged or possible infected leaves on the plant. and continue to monitor. The only prevention is early application of fungicide of your choose before plant is exposed.

Agree with Dan that just based on the scale of the plant to the cup in the photo the container is awfully small for it.

Dave

Look again guys, it's an optical illusion. I too thought I saw a cup, then I reread the OP and started looking for the hydroponic aspect.

I won't give it away except to say... look for the rockwool.

Oh, I thought it was a little puddle of hydroponic juice in a cup with rock wool. It's a hole. No, this question belongs in another forum. To the extent that the problem is about nutrients, we can't have a clue.

The fact that they have recently been exposed to the out of doors makes me think it might be environmental although it doesn't have the classic look of it. That would be very fast to pick up and present disease as well. Might be a combination of sun and or wind damage and nutrient problem but you would have to consult someone that uses that method.

"They have mostly been indoors under a T5 fluorescent and LEDs, but now we are giving them outdoor weather - natural sun and wind."

I was thinking the same thing as hoosier but as was mentioned this doesn't really have the typical appearance of sun or wind burn. I would have expected more widespread damage whereas this seems rather localized.

Rodney

Like many said, looks like a variation of exposure. Looks like a petiole with some leaflets too close to a light bulb and damaged indoors, just to be taken outdoors and face the elements and cause that.

Whatever mold is probably there is just an afterthought and fairly benign growing on the necrosis caused by that harshness. Maybe this longest leaf got a double whammy by growing further away from the bulb after the hot exposure, and then developed poorly without receiving necessary light intensity after its intense time close to the bulb. With hydroponic nutrients it is easier to create the conditions for some fertilizer burn, too, in such compromised leaves, adding more gasoline to the fire...

I would leave it all, mind the humidity and avoid leaf wetness, and watch if the leaves naturally dried out on their own till my plant had more leaf area. Then, possibly cleanly remove the tip or all of that 'branch' before it may abscise, though if it proceeds naturally by drying out I'd rather leave it to its own.

PC