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.
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.