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Tomato blight rapidly destroys affected plants. For tomatoes to be affected the fungus must be present in the air (nothing you can do about that) and the leaves have to be wet. Dry leaves cannot be affected.
Here in Europe blight is caused by phytophthora infestans and is worse in cool, damp weather. This is known as late blight in the US. It's the same blight that attacks potatoes and most of the information available is for potatoes, not tomatoes, and it behaves differently in tomatoes.
It starts with some damaged leaves (they go yellow and brown then dry up and fall off). The plants get brown blotches on the stems, flowers dry up and fall off and any fruit that has formed turns brown and leathery. The brown areas on the stem lose their strength and bend and break easily. If you collect undamaged fruit to ripen off the plant, it usually dvelops the brown leathery fungal damage and cannot be eaten.
You can save seed from blighted tomato plants as long as you ferment the seed for about three days before drying it. This fermentation destroys the fungus.
Some people spray with copper sulphate, some with marestail infusion (equisetum arvense or e. sylvaticum) but these need to be applied before attack to be any good.
DO NOT put diseased plants in your compost heap (unless it is a really hot one) and make sure you collect any tomato leaves/flowers/fruits that have fallen onthe soil as the fungus can survive on these for up to a year.
DO NOT plant potatoes in the same ground the following year.
