I hope I am wrong but the only thing that looks like this and is highly contagious to other types of plants is bacterial canker. Without being there to actually look at the plants, in these circumstances I could very well be wrong. There is another disease that looks very much the same called Tomato Pith Necrosis but is not nearly as bad as bacterial canker. It is normally found in greenhouses but has also been found in field tomatoes. It is caused by synthetic high nitrogen fertilizers. Hope it is the pith necrosis as the necrosis is not usually fatal. I would take the plant to the county extension agent to make sure just what it is.