This looks like tomato blight, caused by warm and damp summers. The only real way to fight it is by spraying. You can delay it in the short term by removing any leaves showing signs of the disease as soon as they are affected, but if the weather continues to be damp, the disease will win in the end. Look out for varieties that have more resistance to it

its blight my friend tomatoes and potatoes are all the one family i had it 3 years ago in my polytunnel 250 plants infected in a week , you could spray the ones not infected with dithane its copper based it keeps blight away, the ones that are infected rip out and burn ,the blight comes in the wind little spores when its damp and humid its a slow killer if it ever catches sorry for your troubles.