I believe the main culprit is the high humidity which makes it difficult for foliage to stay dry or dry off after rain or morning dew. I remember a few summers when we had unusually low humidity and foliage diseases were nearly nonexistent.
Because of the humidity in the Gulf Coast region I live in I have changed the way I support my tomatoes and the way I manage their growth and the types I try to grow. I used to grow mostly determinate types that I would stake or cage. The problem with this is the heavy bushy growth allows little air flow and the leaves remain wet too long. This leads to devastating foliar disease starting in the densest area and quickly spreading. I now only plant determinates very early for a quick crop and even prune them to keep them more open so they will remain healthy long enough to make.
I now plant mostly indeterminate types and grow them on a trellis so they can spread out more. I also prune to keep the foliage under control so air and sun can reach most of the leaves. Despite unbelievable rainy wet conditions last year I had the best tomato season in over 30 years of gardening. I just wish I had started doing this many years ago.
As for tilling in the winter cold to help control nematodes; if you do it during a long cold spell you don't have to worry about killing too many beneficial worms because they have already moved deep to avoid the cold.
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