Tomato doctor needed to identify possible pestilence

It's more about their smaller root systems being able to cope with drawing up enough water to stop them wilting. A tomato seedling is pretty small and with the lovely sunny weather we have been getting recently our garden soil dries out very quickly. A seedling just cannot get it's roots down far enough to find moisture and will therefore frazzle up. As they get older their root systems develop more in proportion so they can withstand extremes.

Only thing you can do is to shade them in the midday fierce sun - drape garden fleece over them or as I do at the moment (with them being in pots) move them to a shadier spot.

 

Generally I re-pot tomatoes into another slightly bigger pot. But occasionally when my plants have got really leggy I've used rose pots which are deep and narrow:



(Stock image I found on google)

The main reason I used those is because they can be packed together closely overnight i.e. less space). I think it is a good idea to encourage extra roots. You can strip off lower leaves up to the first truss in order to maximise the stem root depth.