I've been growing the San Marzano variety of plum tomato for many years and have nearly always had the same problem. The inside of the fruits often turn black with what looks like a mold of some sort. Eventually, the whole bottom of the tomato turns black. I can out the bad part, but it's a nuisance when trying to process a lot of tomatoes. Many are so bad that I just throw away the whole tomato. I don't have the same problem with other varieties such as Roma or Debarao.
Does anyone know if there's anything I can do to prevent the problem, or should I just give up on San Marzanos? Not all of the fruits are affected, but I'd guess it's about 50%. I'm wondering whether it's from too much or too little water or perhaps humidity-related. I'm reluctant to use fungicides; I'd rather just grow a variety that doesn't require their use.