Old topic but wanted to share my input. I'm growing hydroponic tomatoes (various strains) at university in a lab setting (indoors under HID lights) and outdoors in a greenhouse. In one of my greenhouse NFT systems, i recently had a similar problem on 1 tomato plant (total 9 plants that share the same res and tubes). But im gonna try to diagnose your issue by mentioning what others missed out on. 1 major cause of leaf curl is compaction of root system (i.e. roots are compressed or dont have enough space to expand). This risk is exacerbated in hydroponic systems that use tubes. This was the problem i had and had to replace tubes with square tubings that have a liftable top. Once you have access to roots, you must seperate them strand by strand (i.e. so they arent sticking together). In your setup, this risk will further be exacerbated because you filled the entire pvc tube with hydroton (right?). Remove hydroton from tube and place the plant in a net cup with hydroton. Nitrogen burns usually cause browning/yellowing of leaves (similar to sun/heat burn) and not purpling. Test your solution for each chemical if possible, or use a commonly employed aggregated measuring device that shows total concentration in ppm (dont let it exceed 2500). In my system, only one plant was affected. Coincidentally, it was the largest of all plants with the most root mass. I let another normal one grow to its size to see whether it would undergo the same problem, and it did. Viral and bacterial infections that cause leaf roll usually cause it to roll up. Same with the physiological roll. If your leafs roll down AND if your stems bend, this is caused by a diminishing turgor pressure (read: not enough water reaching afflicted area). Mist several times when lights are off (no photosysnthesis) and observe increasing turgor pressure (unbending). Or, untangle roots and remove hydroton from from pipes. I tested various growing methods including fully and partially submerged roots. They all work but a submerged root cannot intake oxygen; it has to borrow oxygen from another part of the plant (i.e. a root part that isnt submerged or above ground stem). This requires energy for transportion and isnt the most efficient (optimal) way for growth and cellular processes. Lastly, an airstone in the reservoir is useless unless the reservoir is completely airtight all the way up to the plant holes. Better to keep small airstones in the pipes right under the plant roots. Those that say aerating the res prevents bacterial development.. is false. If you dont have air in res, you will get anaerobic bacteria (they grow/accumulate slowly but acidify the solution); if you have air in res, you get aerobic bacteria (not acidifying but grow extremely fast). Most bacteria prefer aerobic environments for optimum growth.

Do your leaves have some purple on the edges? Just curious, and did the leaves start out curling, or any yellowing spots?? Does it happen to the bottom of plant, upper or?? Could be both but without seeing any leafspot, I can't say. You said it got a lot of watering! DARN THAT GARDEN SITTER, same thing happend to me, except, they didn't turn off the water, and the soaker hose stayed on for 3 days. MY BILL WAS HUGE! If the leafs curl upwards and tend to get kind of thickened and leathery,could be physiological leaf roll. It is a protective mechanism the plant has under stress, e.g. from cool rainy weather, severe pruning, going back and forth between very wet and very dry. http://cru.cahe.wsu.edu/CEPublications/PNW616/PNW616.pdf

"I'm guessing the pot is so large that I've been overwatering the tree and also not flushing it when I water. Is this he reason the leaves are turning brown?" It sounds like the foliage that's in the process of being shed is part of the plant's drought response due to too much water in the soil. Too much water = not enough oxygen to drive the O2 dependent process of water uptake = not enough water getting to distal parts of the plant (leaves - particularly tips and margins) = shedding in response. The yellowing is a result of the lack of chlorophyll that is being salvaged (along with mobile nutrients and other bio compounds) by the plant from leaves being shed. "..... what should the dowel rod look like when it needs to be watered? Should it be completely dry at the bottom of the rod and not pick up any of the dirt?" When the dowel first comes out clean and dry, it's time to water. I'd work toward getting watering under control, then follow the instructions in the thread I left a link to above to turn the plant around. There are some things you can do to help avoid limitations associated with saturated media. You're sure to have other questions, so don't be shy about voicing them. We all want you to have the best growing experience you possibly can. Al

Based on the browning of the vascular tissue, that looks like some kind of vascular wilt disease or pith necrosis, i.e., soilborne. This would be very rare in container plants grown in fresh commercial medium, but there's always a chance you can get unlucky with your potting mix, one of the amendments, your nursery plants or seeds, etc. Could maybe also be one of the root rots. Is there any sign of rot or girdling on the main stem right at the soil line? Unfortunately, if any of the above is the case, there's really nothing you could have done/can do to save the plant. The good news is, unless you're contaminating the soil in other containers with soil from this one, there's no real risk to any of your other plants. You're not seeing any spots or any other lesions or symptoms on the developing fruit, correct? Because if so, it could be bacterial canker, which is something that you can spread to your other plants on pruners, etc. Your reported symptoms don't really line up with canker, but diseases don't always present according to textbook. If you suspect it's canker, separate the plant a bit from the others and always clean your hand, pruners, and other tools before handling other plants. If the plant is still producing and growing and has healthy new growth, you can see whether it will live long enough to mature the fruit you have and hope that the fruit is of decent eating quality (there's no health danger to you--it's just that fruit from diseased plants sometimes doesn't develop good flavor and texture). Don't use any fruit in canning, as diseases or pest-affected fruit can have altered pH. You can always contact your local extension to try to get a more definite diagnosis. Regardless of what it is, I would make sure to not re-use the potting medium in that container, clean the container and supports thoroughly with bleach at the end of the season, do not compost the plant, and do not save any seeds from the fruit.