I stopped growing modern paste tomatoes such as San Marzano a few years ago, when I began finding furry mold growing in fruits that appeared perfect on the exterior. So, while talking about preserving tomatoes to the Multnomah County Master Gardeners the other night, I asked if others had had the same experience. Indeed they had: At least three people in the audience had found mold inside their paste tomatoes. The mold was usually black, but one person had found white mold as well.
I would guess that extended heating in a boiling-water bath would kill the mold, but I’m not sure. What I’m sure about is this: I would never recommend preserving tomatoes of this type without cutting them open first.
2024 update: I’m sorry to say that the photos of moldy tomatoes have been lost. If you find a paste tomato with mold inside, please send me a picture.