Tomato or potato blight? Or both? The RHS page might help a bit http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=217
It is mostly air borne in wet conditions, and hot dry weather is the best thing to combat it. This year almost all of our outside tomatoes were wiped out in July with the wet weather, but four or five survived from which I removed the affected foliage. With warm dry weather since, these plants have completely recovered as if never affected, though a bit late to set a useful amount of fruit. I'm planning to collect seed from these in the hope the offspring might have a degree of resistance.
Blight does not survive in the soil but it CAN survive in the tubers of affected potatoes and when these start to regrow next spring, the blight spores can be spread to other plants, including tomatoes. That is why it is vital NOT to allow potato 'volunteers' to regrow each year but always use fresh seed potatoes.