I attended a webinar about late season blight. It is caused by a fungus that can affect both tomatoes and potatoes. The presenters at the webinar, all agricultural researchers, agreed on several statements:
Agricultural scientists who conducted a webinar I attended said that the most likely way for spread of the fungal spores is via over-wintering potatoes who had blight the previous season. I believe it is also possible for spores to travel on the breezes from one blight-stricken garden to another.
Agriculture scientists say not much can be done after blight appears. However, if it is late enough and fruit is getting close to ripe enough to take inside, I do the following:
In our experience, once the blight gets to the trunk and stems of a tomato plant the game is over. Soon any fruit will be blighted also and even heroic efforts to save the fruit will have minimal effects. I have attempted heroic measures with very little success.
This wasn't a bad idea - it just didn't help. The extended cool and cloudy weather in late August took out all of the tomatoes with blight in a really ugly fashion.