Many times the under lying problem can be within the soil. You may be topically helping your plants but its actually the soil needs to be fixed.
Once you've pulled your old plants out, give this tip a try...
Rotate tomatoes with rocket.
Rocket is a member of the brassica family. Their roots are rather special because they produce a compound called glucosinolates, which is where rocket gets its mustard like smell from. Their deep roots get down far enough.
Glucosinolates inhibit many soil pathogens such as nematodes and fungi. It's a natural fungicide.
This effect is not limited to just tomato plants, but tomatoes do give the best indication as to how balanced our soil is, thus more likely to show disease during its growing season.
Give it a try!