There has been a lot of calls in the last couple of weeks regarding Tomato fruit rotting prior to harvest. This is called Blossom end rot and occurs every year to varying degrees. This problem is associated with the fluctuation of wet and dry conditions in the soil. Sprays after the fact are not affective for this disorder and actually sprays are not effective prior to this problem. The key to this situation is to maintain soil moisture and not allowing the plants to dry out.
The following link is an article written by Dr. William Nesmith and Dr. Paul Vincelli. http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-VG-2.pdf
Take a look I think your questions will be answered.