Check the TAMU tomato problem solver site to see if any of the pictures match your problem. It sounds like either a fungus or a virus wilting your plants. If your others are so far unaffected--depending on how far the original infection has gone, I'd consider sacrificing your plum for the sake of your other plants, and spray the others with a fungicide of your choice as a preventative. As far as tomatoes getting brown at the end, that could be blossom end rot or the same fungal disease--it's really hard to tell without pictures. Good luck picking it out. If you can't find the TAMU site, check for the tomato problem solver thread in here--the link is in there.

You wrote: "tomoatoes at the bottom end are getting round brown spots. "

Check the FAQ for details about blossom end rot.

The blossom end rot is a symptom of the overall problem, I think, not the classic watering issue.

Wilting at the top says to me that the plant isn't able to get water up from the roots, and the cause is probably the browning of the stem. BER would result, too, since it wouldn't be able to get water to the fruit, either.

I'm guessing (without a picture, it really is a guess) that it's a stem canker of some sort, either Alternaria canker (if dark) or even Southern Blight (if it's starting to grow white threads around it). Take a cutting of the top if it's not totally gone yet (noting the date of your post), then root that in water and toss the rest of the plant in the garbage, dirt and all. Sterilize the pot with bleach, buy new soil, stick the (rooted) cutting in the new setup, and you should be okay.

Don't use tools, like pruners, on the uninfected plant after using them on the infected one... and wash your hands when switching from one to the other, too.

--Alison